Hysteresis and Thermomagnetic Characteristics of Soils and Rocks in Eastern Botswana

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MOIDAKI, Rubeni. T. RANGANAI, James. G. KING This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5608272/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Soil magnetic properties are important for many applications including palaeomagnetism and (plate) tectonics, geological interpretation, geoexploration, mining, forensics, geotechnical engineering, and industry. The results of this work are a continuation of previous work on magnetic susceptibilities of soils in eastern Botswana. The magnetic properties measured include Curie temperature (Tc) determined in nitrogen gas atmosphere and hysteresis parameters (Hc, Hcr, Mr, Ms). Thermomagnetic curves were measured from room temperature up to 700 o C, enabling determination of Curie temperature which has supported the classification of the magnetic granulometry and mineralogy. The thermomagnetic curves revealed the existence of magnetic minerals such as hematite, magnetite and pyrrhotite, and their Curie temperatures were found to be 600 °C, 580 °C, and 200 °C, respectively. The presence of magnetic materials is also revealed by the hysteresis loops of some soil samples. The hysteresis and thermomagnetic curves both indicate that the main contribution comes from magnetite-like phases. Thermomagnetic curves of soil samples collected along the Ramatlabama to Ramokgwebana, which traverses through the Kaapvaal Craton, Limpopo belt and Zimbabwe Craton could be used to differentiate the boundaries of the cratons and the orogenic belt. Airborne magnetic data covering the study area are being interpreted with particular reference to some of the measured magnetic properties and determined soil characteristics thereby increasing confidence in the geological interpretation. magnetic hysteresis Curie temperature magnetic mineralogy ferromagnetic minerals Botswana Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction Rocks and soil magnetic parameters are extensively used to study a variety of geological and environmental processes, including rock-forming and altering geological processes (Abdel Aal et al., 2014 ; Ayoubi et al., 2018 ; Mello et al., 2023), palaeomagnetism and (plate) tectonics (Muxworthy, 2001 ), ore genesis (geoexploration) (Alva-Valdivia and Lopez-Loera, 2011), geomorphology (Da Silva et al., 2015 ), magnetic mineralogy (Clark, 2016 ; Szuszkiewicz et al., 2021 ), archaeology and forensics (Beatrice et al., 2008 ; Sanchez-Roda et al., 2022 ; Shirzaditabar and Heck, 2022 ), and anthropogenic and environmental activities/factors (Hanesch et al., 2007 ; Da Silva et al., 2015 ; Ranganai et al., 2015 ). Recent advancements in low-temperature magnetic measurements, hysteresis loop parameters, magnetic susceptibility dependence on temperature and frequency, and M ssbauer effect, spectrometry, have made it possible to accurately classify magnetic mineralogy and particle size (Zhao et al., 2006 ). Interpretation of magnetic parameters requires a clear understanding of the underlying processes that influence the presence, distribution, preservation, or neoformation of magnetic minerals in soil and rock (e.g., Fialova et al., 2006; Abdel Aal et al., 2014 ). Magnetic properties are dominantly controlled by the presence and volumetric abundance of iron and its oxygen fugacity. The grain size distribution, domain size distribution, amount and presence of titanium, and other factors may also be important. Until recently, there have been no systematic studies of the magnetic properties of rocks and soils in Botswana. In particular, little attention has been devoted to thermomagnetic properties, which are important in a variety of applications in geo-exploration, geotechnical engineering, and industry in a developing economy. This study expands on the work of Ranganai et al. ( 2015 ), which focused on soil colour determination, low temperature magnetic susceptibility measurements at low and high frequencies by adding thermomagnetic, hysteresis properties of soils, and determination of Curie temperatures.. The eastern side of Botswana is the only region in the country with detailed combined airborne magnetic and radiometric survey (e.g., Ranganai et al., 2006 ). However, the lack of further analysis and publication of the data has resulted in its limited utilisation to date, and the current work is important in that regard. The purpose of this study is to report on Curie temperature as observed in thermomagnetic curves, and magnetic hysteresis parameters such as: saturation remanence magnetisation (Mrs), saturation magnetisation (Ms), coercivity of remanence (Hcr), and coercive force (Hc). Thermomagnetic curves involve measuring the magnetic susceptibilities of soil samples at different temperatures. This can help identify the types of magnetic minerals present and their thermal stability. Hysteresis parameters, on the other hand, involve measuring the magnetic properties of a soil sample as an external magnetic field is applied and then removed. This can provide information about the size, shape, and distribution of magnetic particles in the soil. By analyzing the magnetic properties of soil samples from a specific region, researchers can gain insights into the geological history and environmental conditions of that area. For example, certain magnetic minerals may be indicative of past volcanic activity or changes in climate. This information can be useful for a variety of applications, such as geologic mapping, mineral exploration, and environmental monitoring. 2. Regional Geological Setting Botswana is a land-locked country covering approximately ~ 583 000 square kilometres with a population of about 2.2 million (Fig. 1 ). The country is relatively flat with almost 75% covered by superficial Kalahari sand deposits with depths ranging from few meters in the east to more than 200 m in the west (Meixner and Peart, 1984 ; Key and Ayres, 2000 ). Almost 80% of the population reside in the southern and eastern part of the country, (the study area), where favourable conditions exist for agricultural activities like cattle ranching and subsistence crop farming (e.g., DLFRS, 1985; De Wit and Nachtergaele, 1990 ). The study area is underlain by geologic terranes comprising the Archaean Zimbabwe, Kaapvaal Cratons, and the Limpopo mobile Belt (Fig. 1 ) (Key and Ayres, 2000 ). The soils in this region have a ustic moisture regime, while the rest of the country is aridic (De Wit and Nachtergaele, 1990 ). The main geologic terranes in Fig. 1 consist of the Kaapvaal craton (3.5-2.5Ga) and the Zimbabwe craton, which were formed during two major tectonic cycles that involved the formation of greenstone terranes and gneisses, with the rocks in the Kaapvaal craton being slightly older. The Limpopo Belt was formed by the collision between the two cratons and has high-grade metamorphosed rocks, e.g., granitoid gneiss (Key and Ayres, 2000 ). It contains numerous mineral occurrences such as copper and nickel hosted in ultramafics (BGI, 2018 ) and diamondiferous kimberlite pipes (de Wit, 2017 ). A Uranium mining prospect at Serule within (Lower Karoo) Ecca Group mudstone (A-Cap, 2015) is currently under development. Other concerns include air and soil pollution, as well as industrial pollution related to mining activities (e.g., Chimidza and Moloi, 2000 ), The magnetic properties of rocks and soil are mostly influenced by the lithology of the underlying bedrock and soil-forming processes. These magnetic properties can thus assist in broad geological mapping and interpretation (e.g., Reynolds et al., 1990 ; Lu et al. 2008 ) as well as environmental assessment (e.g., Fialova et al., 2006; Hanesh et al., 2007). 3. Materials and Methods 3.1. Sample Collection and Preparation Soil samples were collected in the eastern side of Botswana along the major road network while rock samples used are mainly from the Water Utilities Corporation (WUC) North-South Water carrier (NSWC) excavations (e.g. Figure 2 ) between Gaborone and Selebi Phikwe (Fig. 1 ), with a few taken from outcrops and quarries. Excavation on average reached 2 m depth, which ensured relatively fresh (unweathered) samples (Kedisang, 1999 ). Soil sampling profiles (P1 to P3, Fig. 1 ) and sample preparation are as discussed in Ranganai et al. ( 2015 ), and only a brief summary is presented here for completeness. Samples were collected at 10 km, 5 km and/or 1 km intervals using the vehicle odometer and about 100 m from the main road to minimize anthropogenic effects and were kept in diamagnetic plastic containers. This distance from the road helped to avoid transported material and contamination from metal debris left during road construction and from vehicle exhaust fumes (cf Hoffmann et al., 1999 ; King and Ranganai, 2000 ; Kim et al., 2007 ; Shirzaditabar and Heck, 2022 ) while plastic shovels were used to avoid ‘mineral’ contamination. Samples were obtained during the dry season and thus reducing moisture problems (cf Schibler et al., 2002 ; Maier et al., 2006 ) and at a depth of 5–30 cm (topsoil), while avoiding compost (organic) material (cf Shi and Ciopaa, 2006). Laboratory preparations involved air-drying samples at room temperature (< 30 ºC) for a duration of 48 hours, to minimize chemical alterations (Ranganai et al., 2015 , Maier et al., 2006 ). The eastern part of Botswana is diverse in terms of topography and physiography and is divided into four main geomorphological zones: alluvial deposits, sandveld, hardveld, and lacustrine deposits. The primary source of the parent soil material is mostly the hardveld, which has an Archaean age (> 2.500 Ga). Profile P1, which is more than 650 km long, consists of soil samples collected 50 m from the tarred road between Ramatlabama and Ramokgwebana, crossing the Kaapvaal Craton in the south, the central zone of the Limpopo belt, and the Zimbabwe Craton in the north. Profile P2 passes over the hardveld, which is underlain by the Kaapvaal Craton, through the central zone of the Limpopo Belt and the Zimbabwe Craton. Profile P3 crosses the Sandveld as it moves towards the Kalahari Desert in the west. 3.2. Thermomagnetic Curves and Curie Temperature Thermomagnetic curves were used to determine the composition of mineral phases in rocks and soils (e.g., Dunlop and Ozdemir, 1997; King and Ranganai, 2001 ; Lu et al., 2008 ). The Curie temperature (Tc), a critical measure of the magnetic mineralogy, was obtained by heating samples to 700°C after performing the empty furnace correction (Ranganai et al., 2015 ). The measurement of room temperature magnetic susceptibility and generation of thermo-susceptibility (χ-T) curves was carried out using a water cooled MS2W sensor and MS2WF furnace. The MS2W has an operational frequency of 696 Hz, a range of -200 o C to 900 o C and a precision of 4πx10 − 6 SI per 10 − 5 m 3 . The samples were heated and then cooled at a rate of 20°C/min to enhance any mineralogical transformation, in an inducting field of 300 A/m (0.38 mT) (cf Jordanova and Jordanova, 2016 ). To minimise oxidation, the samples were heated in an inert nitrogen atmosphere (99.99% purity) and a straightforward graphical method was used to measure the Curie temperature (Tc) (Zhao et al., 2006 ). However, this method underestimates Curie temperature compared with others (e.g., Petrovsky and Kapicka, 2006; Tauxe et al., 2018 ). Some magnetic materials which are metastable crystallize at low temperatures undergo chemical changes when heated in air (Piper, 1987 ). These changes can result in the formation of new minerals with either higher or lower magnetic susceptibilities, leading to dissimilar heating and cooling curves. For instance, heating can cause the oxidative formation of antiferromagnetic hematite, which has magnetic susceptibilities that are several orders of magnitude lower than those of magnetite or maghemite (Dunlop and Ozdemir, 1997). 3.3 Magnetic Hysteresis (and Day plots) Hysteresis loops provide important information about the coercivity spectrum and domain state of ferrimagnetic materials, helping to characterise their intrinsic magnetic behaviour in rocks and soils. The study of hysteresis parameters helps to understand the origin of remanence (Day et al., 1977 ; Dunlop, 2002a , b ; Dunlop and Özdemir, 1997 ; Zhao et al., 2006 ). The domain state of ferrimagnetic materials changes as grain size increases, transitioning from superparamagnetic (SP) to stable single domain (SSD/SD), then to pseudo-single domain (PSD), and finally to multidomain (MD) (Dunlop and Ozdemir, 1997; Dunlop, 2002a , b ; Zhao et al., 2006 ). Hysteresis parameters of selected samples were determined using a Princeton MicroMag 2900 vibrating sample gradient magnetometer (Princeton Measurements Corp., USA) linked to a microcomputer capable of resolving magnetic moments as small as 5.0x10 − 8 emu (Zhao et al., 2006 ). Samples (a few milligrammes) were attached to the probes using wax (crystal clear nail polish) and paper, which have a negligible magnetic susceptibility (cf King and Ranganai, 2000 ). The coercive force (Hc), coercivity of remanence (Hcr), saturation magnetisation (Ms) and saturation remanent magnetisation (Mrs) were measured with a magnetic field applied to a maximum ('saturating') of 1.0 T, suitable for most types of minerals. The room-temperature hysteresis parameters determined from selected samples were then used in the classification of domain states based on the SD-PSD-MD boundaries of Dunlop ( 2002a , b ) using Hcr/Hc and Mrs/Ms ratios (Day et al., 1977 ). The hysteresis ratios in soil and rock magnetic studies provide important information on the grain size distribution of magnetic carriers in samples (Day et al., 1977 ; Lu et al., 2008 ). The shape of the hysteresis loops reflects the degree of pedogenesis, which mainly involves ferromagnetic mineral neoformation and dissolution (e.g., Dearing et al., 1996a ; Lu et al., 2008 ). However, it is worth noting that the ratios have limitations compared to using χfd % as they cannot differentiate between MD and SP grains (Lu et al., 2008 ). Magnetic parameters can also be determined from Day plots (Day et al., 1977 ; Dunlop, 2002a , b ) using Rock Magnetic Analyzer 1.0 software. 4. Results 4.1. Magnetic Susceptibility at High Temperature (Thermomagnetic Curves) Thermomagnetic curves were generated to determine Curie temperatures (Tc), which indicate mineralogical and chemical compositions. The graphical intersecting tangent or inflection point method was used to estimate Tc by fitting the paramagnetic part of the inverse susceptibility versus temperature relation with a hyperbolic function (as described in Dunlop and Ozdemir, 1997, and Petrovsky and Kapicka, 2006). A Hopkinson effect, characterised by a peak in magnetisation (Hopkinson peak) before a decrease in susceptibility, can precede the Curie temperature (Tc) (as noted by King and Ranganai, 2001). This peak can be used to calculate Tc (as per Petrovscky and Kapicka, 2006). Most of the soil samples exhibit nonreversible behaviour, with the cooling curve being higher than the heating curve, and a Curie temperature (Tc) of nearly pure (stoichiometric) magnetite at 585 °C. Some plots (e.g. Figures 3, 4 and 5) show a Hopkinson peak for magnetite before the Curie temperature, but this disappears during cooling. This behaviour is likely due to the presence of small PSD grains. The high room temperature susceptibility (χ) indicates the initial presence of magnetic minerals. Furthermore, most of the samples show varying degrees of difference between the heating and cooling curves (at least χrtc=2χrth), which suggests that low-temperature oxidised titanomagnetite is the dominant magnetic mineral in many samples. The sample for Figure 3 A was heated in the presence of nitrogen, which reduces the chemical reactions taking place in the soil sample. The heating curve shows a small hump (Hopkinson peak; susceptibility enhancement at ~555 o C) as the temperature increases, which indicates the presence of single-domain magnetite grains. There is a slight indication of a smaller secondary peak, which may be due to mineralogical changes during the heating and suggests inversion or unmixing. The spinel structure of titanomaghemites becomes unstable during inversion and changes to rhombohedral, or to intergrown iron-rich spinel and Ti-rich rhombohedral. In this sample, the Curie temperature is estimated to be 585 o C, for both the heating and cooling curves. The mineral is likely to be magnetite (Ti-poor) with a Curie temperature of 585 o C. The sample susceptibility approaches zero as the temperature reaches 650 o C, indicating complete destruction of magnetic order (conversion of magnetite to hematite). The cooling curve also reflects the presence of magnetite with alteration of some magnetic minerals that enhances magnetization. The room temperature k on cooling is roughly twice/double that for heating (cf Jordanova and Jordanova, 2016). The Curie temperature of about 570 o C suggests a ferrimagnetic component as the principal magnetic mineral. In Figure 3 B , the heating curve illustrate that more than one mineral is present in the sample; suggestive of several magnetic phases. There is a slow increase of k with temperature up to an observed peak at T N = 300 o C where T N is the Neel temperature and there is a gradual decrease thereafter until another small peak at ~550 o C (probably T H ). This behaviour is attributed to thermally induced alteration of metastable cubic maghemite to weakly magnetic rhombohedral hematite (Dunlop and Ozdemir, 1997; Deng et al., 2001; Lu et al., 2008). The inflection point on this curve occurs at a temperature of 570 o C, which is the Curie temperature of near-pure magnetite. Curve reaches a minimum of ~20x10 -5 SI; indicating some magnetisation remains as sample is heated to 700 o C, which is the temperature that was set for all the samples. The cooling curve also shows the presence of additional magnetite. The magnetic susceptibility at room temperature is reasonably high and leads to the conclusion that the soil sample is dominated by paramagnetic or ferrimagnetic minerals. The peak at 300 o C is likely caused by presence of ferrimagnetic ilmenite that is reported to have Curie temperature in the range of 50-300 o C (Piper, 1987). The Curie temperature of 300 o C is also compatible with titanium-rich titanomagnetite (such as TM60) or low-temperature oxidised titanomaghemites (Dunlop and Ozdemir, 1997; Zhao et al., 2006). In Figure 4 A , the room temperature magnetic susceptibility observed from the heating curve is very low ( SI) which might mean that the soil sample is mainly dominated by paramagnetic minerals. On cooling the magnetic susceptibility increases as the temperature is decreased, with a strong peak at around 300 o C and a room temperature susceptibility of SI (with almost c rtc = 14c rth ). The observed peaks correspond to Neel temperature of magnetic minerals like pyrrhotite and ilmenite. The sample was heated in air and this leads to a great difference in the magnetic susceptibility at room temperature magnetic susceptibility (~ SI) since the chemical reactions were not minimised. The susceptibility difference at room temperature suggests mineral alteration and hence the formation of new minerals as a result of temperature changes. This type of curve is typical of hematite-ilmenite series (Bohnel et al., 2002; Peters and Dekkers, 2003). For Figure 4 B , the heating and cooling curves are reasonably close, the former showing a Hopkinson peak suggesting the existence of multiple magnetic phases. Both curves show high room temperature magnetic susceptibility ( SI and SI, respectively), representing the presence of magnetic minerals. The plot shows a discernible hump (or peak/dome) at 270-300 o C, which is followed by a clear drop in susceptibility and then a Hopkinson peak before the Curie temperature. The peak at 300 o C may correspond Curie temperature of pyrrhotite, or alternatively titano-maghemite (e.g., Shi and Cioppa, 2006). Pyrrhotite is converted to magnetite as the temperature increases. The observed Curie temperature from this curve is around 585 o C, which is the Curie temperature of magnetite. The thermomagnetic signatures show the transformation of titano-maghemite (the low Curie temperature phase) to a strongly magnetized magnetite, as shown by the irreversible cooling curves. The dominant mineral in the cooling curve is multidomain magnetite with Curie point at 585 o C. The curve reaches a minimum of ~ SI; indicating significant magnetisation remains as the sample is heated to about 700 o C. Figure 5A: shows room temperature susceptibility very close to that for heating and moderately high ( k =96*10 -5 and 104*10 -5 SI), indicating the initial presence of magnetic minerals. The heating curve has a Curie temperature of 585 o C, which correspond to the Curie temperature of magnetite; the hump is a typical feature of a single-domain magnetite mineral. When the sample is cooled, a different mineral is formed with a Neel temperature of 300 o C. The mineral formed could be ilmenite with a Curie temperature of ~300 o C. Alternatively, this could be titanium-rich titanomagnetite (such as TM60 or TM45) or low temperature oxidised titanomaghemites (Dunlop and Ozdemir, 1997; Zhao et al., 2006; Lu et al., 2008). The heating curve runs below the cooling curve up to ~450 o C indicating that there is no formation of new magnetic phases initially upon heating or a phyrrhotite bearing sample (cf Bohnel et al., 2002). However, because some susceptibility values are higher after cooling, new magnetite should have been formed as well. In Figure 5B, a progressive increase of k with temperature up to 500 o C is observed and suggests a significant contribution of single domain (SD) and/or pseudo-single domain (PSD) magnetite particles. This may be due to gradual unblocking of fine-grained (near the SP/SD boundary) ferromagnetic particles (Lu et al., 2008). Also, the hump/bulge is a typical feature of a single-domain magnetite mineral (Hunt et al., 1995; Dunlop 2002). There is relatively fast drop of c starting at ~520 o C, with room temperature c again high (125x10 -5 SI), indicating the presence of magnetic minerals; The heating curve has a Curie temperature of 500 o C, which corresponds to the Curie temperature of Ti-rich magnetite; the hump is a typical feature of a single-domain magnetite mineral. On cooling, a different mineral is formed with a Curie temperature of 350 o C. The mineral formed on cooling could be ilmenite with a Neel temperature at ~300 o C. The cooling curve is initially nearly reversible but there is a dramatic parting of the heating and cooling curves below the cross-over at around ~500 C, implying complex alteration. This shows thermal enhancement of magnetic fabric and different magnetic phases formed, or grain-size re-distribution as a result of heating. This is a relatively frequent case (Fig. 3) with the creation of new magnetite from weakly magnetic phases as a result of heating. 4.2. Magnetic Hysteresis Results Some samples contained sufficient magnetic material, i.e. detectable ferrimagnetic contribution, to yield hysteresis parameters (Figure 6). The form of the loop and numerical values of Mr/Ms and Hcr/Hc depend on the microstructure, primarily the grain size and shape and domain structure (Day et al., 1977; Lu et al., 2008). Further, the presence of high coercivity component Hcr/Hc = 1.02-1.17 for MD is a clear indication of hematite, goethite or SD maghemite, Hcr/Hc = 1.45-1.62 for SD hematites (e.g., Dunlop and Ozdemir, 1997; Peters and Dekkers, 2003; Ozdemir and Dunlop, 2014). The hysteresis loops of representative samples are shown in Figure 6. Hysteresis loop measurements show that samples reached a saturation magnetization of 250-300 mT field strength, and most curves are rather symmetrical. Near the origin, no potbellied and wasp-waisted behaviour (Tauxe et al., 1996) was observed/detected, but overall shapes show significant paramagnetic contributions which probably reflects the mineralogical phase changes. Soil samples from Nata show a narrow hysteresis loop and samples near Francistown show intermediate while the samples near Maitengwe display wide hysteresis loops. The hysteresis loops are closed at about 200 mT for the Maitengwe sample, which is consistent with the existence of a dominant ferrimagnetic phase. The hysteresis loop of the Trip3_46 sample closes at a higher field than those of the Trip2_70 sample. The high coercivity in the Trip2_70 sample is due to the collective effects of antiferromagnetic phases (e.g. haematite and/or goethite) and low-temperature oxidized coarse-grained magnetite. Most magnetic parameters are dominated by low-coercivity ferrimagnets (magnetite or maghemite). Antiferromagnetic minerals like hematite and goethite are the dominant remanence carrying component by mass (Dunlop 2002). Hcr/Hc values range 1.02-1.17 and Mrs/Ms ratios range from 0.5 to 0.9 for MD hematites which basically correspond to pseudo-single-domain (PSD) grain size region according to Day et al. (1977) and/or Dunlop et al. (2002a, b). 5. Discussion Various magnetic properties of rock and soil have been determined to evaluate magnetic carriers and domains, calculate Curie Temperatures, and provide constraints for the interpretation of aeromagnetic data (e.g., Alva-Valdivia and Lopez-Loera, 2011) and magnetic modelling (e.g., Reynolds et al., 1990 ). A combination of magnetic parameters was used to quickly measure changes in the concentration and grain size of lithogenic and pedogenic magnetic components. Changes in mass-normalized magnetic susceptibility (χ) were used to monitor changes in the concentration of ferrimagnetic minerals, such as titano-magnetite or maghemite (Ranganai et al., 2015 ). Thermomagnetic curves were used to estimate the Curie temperature (Tc), and hysteresis properties have been interpreted for remanence and domain states. Hysteresis loops provide valuable information on parameters such as retentivity, coercivity, permeability, and susceptibility and are used in selecting appropriate materials for specific purposes, i.e. the so-called soft and hard magnetic materials. Magnetic susceptibility measurements have indicated that the Kaapvaal Craton, Zimbabwe, and Limpopo Belt have distinct magnetic signatures. The Kaapvaal Craton was observed to have low magnetic susceptibility measurements ranging from \(\:24*{10}^{-5}\) SI to \(\:511*{10}^{-5}\) SI (Ranganai et al., 2015 ). On the other hand, the magnetic susceptibility measurements in the Limpopo Mobile Orogenic Belt ranged from \(\:64*{10}^{-5}\) SI to 220x10 − 5 SI and in the Zimbabwe Craton, they ranged from \(\:98*{10}^{-5}\) SI to \(\:878*{10}^{-5}\) SI. According to a study by Moidaki ( 2001 ), a considerable number of soil samples had volume susceptibilities ranging between \(\:0-50*{10}^{-5}\) SI, as revealed by the low-frequency magnetic susceptibility histograms. These soil samples were considered to be weakly magnetic and their magnetic properties were controlled by paramagnetic minerals such as pyrrhotite and ilmenite (Dearing, 1997). Goethite and hematite, which are paramagnetic and antiferromagnetic iron oxides, play a minor role in determining the magnetic character of soil (Maher, 1986 ; Dearing et al., 1996a ). The presence of sedimentary rocks with volume susceptibilities mainly controlled by paramagnetic and diamagnetic minerals, as well as the overlaying Kalahari sands, is responsible for the low magnetic susceptibility (Ranganai et al., 2015 ). On the other hand, some showed a significant amount of paramagnetic and ferrimagnetic minerals, leading to higher magnetic susceptibility values upon heating. Soils derived from igneous and ultramafic rocks have high magnetic susceptibility values, while soils derived from basaltic rocks exhibit very strong magnetic signatures in many cases (Lu et al., 2008 , Dearing et al., 1996b ). Previous research has found that high values of magnetic susceptibility and frequency dependence of magnetic susceptibility are common in tropical soils (Hendrickx et al., 2005 ; Hanesch et al., 2007 ; Lu et al., 2008 , Ranganai et al., 2015 ). The magnetic behaviour is believed to be caused by the substantial concentrations of ferrimagnetic iron oxide minerals, such as magnetite, maghaemite, and pyrrothite, present in basaltic rocks (Zhao et al., 2006 ; Lu et al., 2008 ). These minerals are known to be the most magnetic of the iron oxides (Cornell and Schwertmann, 2003 ; Dearing et al., 1996a ; Hendrickx et al., 2005 ). The difference between magnetic susceptibility measured at low and high frequency indicates the presence of ultra-fine superparamagnetic minerals that occur as crystals produced by bacteria or chemical processes in the soil (Dearing, 1996a, 1997). 5.1. Thermomagnetic Curves and Curie Temperature The thermomagnetic curves obtained by plotting magnetic susceptibility versus temperature show that the soil samples under investigation contain a range of minerals that behave differently. The thermomagnetic curves reveal the presence of the magnetite-ulvospinel series (Fig. 3 , 4 B and 5 ), which are characterized by Curie temperatures ranging from 100 o C to 600°C. Curie temperatures ranging from 550 o C to 585°C (Fig. 3 A and 4 B) suggest that a ferrimagnetic mineral similar to magnetite, with probably minor Titanomagnetite, dominates the magnetic properties of the soil (e.g., Geiss et al., 2006). The mineralogical constituents from soils samples obtained in this study are consistent with the geology of the craton area since the Limpopo Central Zone metamorphism is associated with 790 to 890 o C (Tsunogae et al., 1992 ; Millonig et al., 2010 ). The hematite-ilmenite series is also a characteristic feature of some soil samples (Fig. 5 A). Hematite is a component of many igneous and sedimentary rocks and may be formed by the dehydration of goethite or by the weathering of Fe2 + in the lattices of clay mineral surfaces (Piper, 1987 ). Pure hematite is already completely oxidized, but ilmenite, the other end member of the series, undergoes oxidation at temperatures above 500°C (Piper, 1987 ). As a result of the varying magnetic minerals that behave differently when heated, the boundaries of the major craton cannot be delimited accordingly. Maghemite has the same chemical formula as hematite. Its similarity in structure to magnetite gives it comparable magnetic properties and saturation magnetization (Piper, 1987 ). Maghemite in soils can be produced through pedogenic and/or lithogenic processes (Cornell and Schwertmann, 2003 ). In the latter, maghemitization is a low-temperature oxidation usually obtained through weathering processes (Dunlop and Ozdemir, 1997). Maghemite is formed by the low-temperature oxidation of magnetite (maghemitization) in both subaerial and submarine environments (Piper, 1987 ). Some of the soil samples reveal the presence of maghemite, which could mean that the soils in that area were once covered by water. It indicates processes of low-temperature oxidation which commonly occur in soils at normal (room) temperatures. Maghemite can also form in soil through the action of burning, where oxides and hydroxides of iron are first reduced to magnetite and may subsequently be oxidized to maghemite. The difference in room temperature magnetic susceptibility supports the idea that bush fires can affect the magnetization of topsoil minerals by creating highly magnetized magnetic minerals (e.g., Clement et al., 2010 ). Wildfires are common during the pre-summer season (autumn) due to land use (e.g., Dube, 2013 ), which could affect soil magnetic susceptibility. Goethite (limonite) is a magnetic mineral found in most soils, although the thermomagnetic curves discussed above do not show Curie temperatures corresponding to goethite (Tc ~ 110\ °C to 120°C). Goethite and other hydrated iron oxides are formed in weathering environments and appear as yellow to brown coloured phases in weathered rocks and soils (Piper, 1987 ; Ranganai et al., 2015 ). The Curie temperature (also known as the critical or ordering temperature) of a magnetic mineral is the temperature at which rock and soil samples lose their typical ferromagnetic properties and become paramagnetic. It is a critical point (Kiss et al., 2005 ) that indicates the mineralogical and chemical composition of rocks and soils (Dunlop and Ozdemir, 1997; Zhao et al., 2006 ; Lu et al., 2008 ). For antiferromagnetic minerals, this change occurs at the Neel temperature. The source of variation in magnetic properties with temperature is the disruption of the alignment of molecular magnetic moments due to thermal motion of the atoms (Dunlop and Ozdemir, 1997). Ferromagnetic and ferrimagnetic compounds, and some paramagnetic compounds, show a decrease in magnetic susceptibility with increasing temperature (Telford et al., 1990 ; Hunt et al., 1995 ). However, the overall plot of magnetic susceptibility versus temperature shows a different variation with temperature for the former (e.g., Thompson and Oldfield, 1986 ). For example, there is a low steady increase from room temperature with a susceptibility enhancement up to 30 times (Hopkinson peak), followed by an inverse drop to small values (Hunt et al., 1995 ; Kiss et al., 2005 ). The minerals that show the Hopkinson effect the most are magnetite, titanomagnetite, pyrrhotite, and other earth materials such as cataclasites (fault rocks) (King and Ranganai, 2001 ; Kiss et al., 2005 ). Most of the samples exhibit non-reversible thermomagnetic behaviour, with a strongly increased magnetization after heating (cf. Bohnel et al., 2002 ), and a Curie temperature (Tc) of magnetite at 585°C. Several plots display a Hopkinson peak at around 500°C before the Curie temperature (e.g., Fig. 4 B and 5 ), which is common for natural titano-magnetite, and their room temperature magnetic susceptibility (κ) is invariably high, suggesting the initial presence of magnetic minerals. The different shapes of the Hopkinson peaks may be indicative of different grain-size distributions in the studied samples, since the Hopkinson effect is best detected in samples with a narrow grain size range (Dunlop and Ozdemir, 1997), and King and Ranganai ( 2001 ) have used the susceptibility enhancement factor (SEF) of the Hopkinson peak to determine grain size in some rocks. There are three types of heating and cooling curves: (i) the simple case, where the curves are nearly identical or reversible, which is common in rocks; (ii) the most frequent case, where χ for cooling is much higher than for heating, but the trends are generally similar; and (iii) the relatively infrequent and difficult case, where χ on cooling is lower than heating (Zhao et al., 2006 ; Lu et al., 2008 ). Our curves fall into category 2 and usually characterize the situation when a new and strongly magnetic phase (typically magnetite) is created from less magnetic phases during heating. The moderately large difference between heating and cooling of some samples suggests that a low-temperature oxidized titanomagnetite is the main magnetic mineral. The substantial peak around 500°C is caused by the neoformation of magnetite through the conversion of iron-containing silicates/clays (Shi and Cioppa, 2006 ; Lu et al., 2008 ), and a relatively sharp decay around 580 \(\:\:^\circ\:C\) -600 °C indicates that substitution for iron (Fe) is very low. The presence of marked Hopkinson peaks in many samples indicates that their magnetic phases are dominated by single-domain (SD) and/or pseudo-single-domain (PSD) magnetic grains (e.g., Dunlop and Ozdemir, 1997; Deng et al., 2001 ), which is further supported by the hysteresis Hcr/Hc and Mrs/Ms ratios. The Hopkinson peaks are generally broad, suggesting the presence of magnetite particles possessing a range of grain sizes, probably due to pedogenesis. Within the non-reversible thermomagnetic curve category, three general groups of behaviour types were identified in the magnetic measurements. The first group involves samples which show the presence of a single phase of Ti-poor titanomagnetite, with Curie temperatures ranging from 540 \(\:\:^\circ\:C\) to 580 °C. Pure, Ti-free magnetite has a Curie temperature close to 580°C, while the content of Ti in titanomagnetites (Fe3 − xTixO4, 0 ≤ x ≤ 1, represented as TM0–TM100) decreases the Curie temperature (Dunlop and Özdemir, 1997 ; Kiss et al., 2005 ; Zhao et al., 2006 ; Lu et al., 2008 ). The second group B has lower Curie temperatures (300 °C), typical of titanium-rich titanomagnetite (such as oxidized TM60) or low-temperature oxidized titano-maghemites (Dunlop and Ozdemir, 1997). While the third group samples are characterized by the presence of two Curie temperatures, indicating the existence of multiple magnetic phases. The thermomagnetic curves show a magnetic phase with a Curie temperature in the range of 330 °C -380 °C upon heating, which is most likely titano-maghemite. A second-high Curie temperature phase is observed at 550 °C -580 °C. The large difference between the heating and cooling temperatures suggests that the main magnetic mineral is likely to be a low-temperature oxidized titanomagnetite. The low temperature phase is thought to be maghemite or Ti-rich magnetite (such as TM60), while the high temperature phase is probably Ti-poor magnetite or magnetite-rich titanomagnetite (as described in Dunlop and Ozdemir, 1997, Zhao et al., 2008). 5.2. Magnetic Hysteresis Curves and Parameters The hysteresis and thermomagnetic curves both suggest that the main contribution comes from magnetite-like phases (as reported in Dunlop, 1986 ; King and Ranganai, 2001 ; Peters and Dekkers, 2003 ). The Curie temperature and saturation magnetization are intrinsic properties that are dependent on the chemical composition and crystal structure, while both hysteresis and remanence are highly influenced by grain size (as discussed in Day et al., 1977 ; Dunlop, 1986 ; Hunt et al., 1995 ; Zhao et al., 2006 ). The coercive force (Hc) was found to range from 8 mT to 13.5 mT and the ratio of saturation remanence (Mr) to saturation magnetization (Ms) ranged from 0.1518 to 0.2247, which suggests that single-domain (SD) sized magnetite is likely the dominant magnetic carrier mineral (as stated in Day et al., 1977 ; Dunlop et al., 2002a,b). The results of a few samples also confirmed the presence of pseudo-single-domain and multidomain magnetite grains as the values of Mr/Ms ratio and coercive force (Hc) corresponded well with the values observed by Thompson and Oldfield ( 1986 ). 5.3 Magnetic Measurements and Mineralogical Signatures The magnetic measurements and thermomagnetic curves in this study provided valuable insights into the mineralogical composition of the subsurface materials. The observed room temperature magnetic susceptibility (χ) levels indicated the initial presence of magnetic minerals in the samples. The significant drop in susceptibility at the Curie temperature of 585°C during heating strongly suggested the dominance of magnetite, a common magnetic mineral. However, the subsequent formation of a different mineral with a Neel temperature of 300°C upon cooling pointed to the potential presence of ilmenite or titanium-rich titanomagnetite. Notably, the curves' behaviour during heating and cooling indicated complex mineral alteration processes. In the case of another set of samples, the progressive increase in susceptibility with temperature up to 500°C suggested the contribution of single domain (SD) and pseudo-single domain (PSD) magnetite particles, possibly due to the unblocking of fine-grained ferromagnetic particles. 6. Conclusions The major geological terrains in the study area exhibit varying soil magnetic signatures due to the different rocks that span significant geological time periods. The Curie temperature determinations performed in a nitrogen atmosphere showed that the magnetic minerals have variable particle sizes, primarily falling within the pseudo-single-domain and multi-domain range (0.2–14 µm), which implies inheritance from parent rocks or materials and weak weathering. Thermomagnetic curves and room temperature hysteresis suggest that the main contribution comes from magnetite-like phases. The non-reversible temperature dependence of the low-field susceptibility exhibits a Hopkinson peak at around 550°C and is characterized by a significant decrease near 580°C, with susceptibility nearly zero at about 650°C. A small bulge or hump is observed around 300°C on the heating curve, indicating the presence of low-coercivity ferromagnetic minerals, primarily magnetite, with possible traces of iron sulfides. These thermal properties of soil collected from Ramatlabama to Ramokgwebana, which goes through the Kaapvaal Craton, Limpopo belt and Zimbabwe Craton could be used to differentiate the boundaries of the cratons and the orogenic belt. On the other hand, airborne magnetic data covering the study area are being interpreted with particular reference to some of the measured magnetic properties and determined soil characteristics thereby increasing confidence in the geological interpretation. This study demonstrated the potential for linking magnetic property variations with lithogenic, pedogenic, and anthropogenic factors and the type of magnetic minerals in terms of grain size, domain structure, and magnetic parameters. It opens up opportunities for long-term multidisciplinary research on various rock and soil environmental characteristics, providing a fundamental understanding of soil physical properties and processes, and addressing practical problems related to exploration, the environment, hydrology, land use, and agriculture. Mass specific susceptibility measurements of samples in powder form lead to unbiased volume and density calculations of soil samples. Furthermore, it is recommended that X-ray diffraction (XRD) and scanning electron microscopy (SEM) supportive studies could be performed, as well as Mossbauer effect investigation to provide another means of identifying sources of magnetic parameters. Vertical sampling in trenches where possible to check lithogenic and anthropogenic contributions to the magnetic susceptibility is another possibility. Declarations Author Contribution Author statementMM: Data curation, Writing-original draft, analysis and interpretation of data, and editing. RTR: Data curation Methodology, Conceptualization, Reviewing and Editing, Final approval. JK: Data curation, Methodology, Conceptualization, Reviewing and Editing Acknowledgements Many students were involved in the project over the years and their efforts and contributions are greatly appreciated. We thank the reviewers and the editor for constructive comments that improved the paper. References A-Cap Resources, 2015. Letlhakane Uranium Project- Botswana. Botswana Resources Conference, Gaborone. Abdel Aal, G. Z., Atekwana, E. 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Metamorphic P–T profiles from the Zimbabwe Craton to the Limpopo belt, Zimbabwe, Precambrian Research, 55, 303–319. Zhao, X., Riisager, P., Antretter, M., Carlut, J., Lippert, P., Liu, Q., Galbrun, B., Hall, S., Delius, H., Kanamatsu, T., 2006. Unraveling the magnetic carriers of igneous cores from the Atlantic, Pacific, and the southern Indian oceans with rock magnetic characterization. Physics of the Earth and Planetary Interiors, 156, 294–328. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5608272","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":403944303,"identity":"2bb82c8e-72cd-4c50-b76e-b66d639da800","order_by":0,"name":"Moikwathai. MOIDAKI","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAuUlEQVRIiWNgGAWjYFACxgYJhgMScgwSYN4B4rUYk6KFAaj4AENiA9FadNubG2/8OGORvuF2A+OHHwx3EhsIaTE7c7DZsueGRO6GOweYJXsYnhGh5UZimwTPB6CWGwkM0gwMh3OJ0iL554NEusGNBObfRGuR5rkhkQDUwkakLUC/WMuckTCceedgm2WPwbN6wlqOtz+8+eZYnTzf7ebDN35U3DEmpAMZMALNNyBFwygYBaNgFIwCnAAAor9GvdRkH+EAAAAASUVORK5CYII=","orcid":"","institution":"University of Botswana","correspondingAuthor":true,"prefix":"","firstName":"Moikwathai.","middleName":"","lastName":"MOIDAKI","suffix":""},{"id":403944304,"identity":"6003defa-8041-4aca-b453-1220a30f941d","order_by":1,"name":"Rubeni. T. RANGANAI","email":"","orcid":"","institution":"University of Botswana","correspondingAuthor":false,"prefix":"","firstName":"Rubeni.","middleName":"T.","lastName":"RANGANAI","suffix":""},{"id":403944305,"identity":"38861611-1e60-4012-82a2-2f74a7039c28","order_by":2,"name":"James. G. KING","email":"","orcid":"","institution":"University of Botswana","correspondingAuthor":false,"prefix":"","firstName":"James.","middleName":"G.","lastName":"KING","suffix":""}],"badges":[],"createdAt":"2024-12-09 10:53:17","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5608272/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5608272/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":74521013,"identity":"48ebdb12-093d-45a8-ac76-b20742c2b7ba","added_by":"auto","created_at":"2025-01-23 05:59:30","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":514298,"visible":true,"origin":"","legend":"\u003cp\u003eSoil sampling profiles are as discussed in Ranganai et al., 2015 and an insert \u003cem\u003eon the geology of Botswana, showing major tectonic provinces in the eastern parts of the country (after Singletary 2003).\u003c/em\u003e\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5608272/v1/50f0771647d0fdba60f0dfea.png"},{"id":74523041,"identity":"736db0c0-e373-4495-91ab-72d0c565e9f2","added_by":"auto","created_at":"2025-01-23 06:15:31","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":743495,"visible":true,"origin":"","legend":"\u003cp\u003eRock sampling at one point in the NSWC pipeline trench, partly showing the soil profile (Photo credit: R.T. Ranganai).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5608272/v1/40dd37da694f8001d274c124.png"},{"id":74521011,"identity":"b7483386-bfa4-458b-b4e5-3e2e30e6ce7a","added_by":"auto","created_at":"2025-01-23 05:59:30","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":61665,"visible":true,"origin":"","legend":"\u003cp\u003eA: Thermomagnetic curve for sample # 2 from profile P1 which traverses the Zimbabwe craton (Sample coordinates: 20°35'S; 27°38'E). The sample was heated in a nitrogen environment. C and H denote cooling and heating respectively. The plot indicates the following: Curie temperature when heating: 585 \u0026nbsp;\u003csup\u003eo\u003c/sup\u003eC; Curie temperature when cooling: 585 \u003csup\u003eo\u003c/sup\u003eC\u003csup\u003e .\u003c/sup\u003e Room temperature magnetic susceptibility before heating: k=26x10\u003csup\u003e-5\u003c/sup\u003e (at ≈ 25 \u003csup\u003eo\u003c/sup\u003eC) Room temperature magnetic susceptibility after heating: k=58x10\u003csup\u003e-5\u003c/sup\u003e (at ≈ 25 \u003csup\u003eo \u003c/sup\u003eC). B: Thermomagnetic curve for sample #3 from trip1 (Profile P1) (Sample heated in Nitrogen); C: cooling Curve, H: heating curve. Sample coordinates: 20°40'S; 27°37'E. Soil sample collected in the Zimbabwe Craton. The plot indicates the following: Curie temperature when heating: 570 \u003csup\u003eo\u003c/sup\u003eC and Hopkinson temperature: 300 \u003csup\u003eo\u003c/sup\u003eC. Curie temperature when cooling: 570 \u003csup\u003eo\u003c/sup\u003eC and 250\u0026nbsp; \u003csup\u003eo\u003c/sup\u003eC. Room temperature magnetic susceptibility before heating: k= 54x10\u003csup\u003e-5\u003c/sup\u003e (at ≈ 25 \u003csup\u003eo\u003c/sup\u003eC), Room temperature magnetic susceptibility after heating: k= 105x10\u003csup\u003e-5\u003c/sup\u003e (at ≈ 25 \u003csup\u003eo\u003c/sup\u003eC)\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5608272/v1/56b6a9ba287e84a2a6be1fec.png"},{"id":74521017,"identity":"f5e6a3ac-717d-45b8-934b-1af458cd7376","added_by":"auto","created_at":"2025-01-23 05:59:31","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":56849,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eA\u003c/strong\u003e: Thermomagnetic curve for sample # 57 from trip1 (Profile P1) (Sample heated in Air); C: cooling curve, H: heating curves. Sample coordinates: 24°55 S; 25°46 E. Soil sample collected on the Kaapvaal Craton. The plot indicates the following: Curie temperature when heating: 350 \u003csup\u003eo\u003c/sup\u003eC, Curie temperature when cooling: 300 \u003csup\u003eo\u003c/sup\u003eC, Room temperature magnetic susceptibility before heating: k= 11x10-5 (at ≈ 25 \u003csup\u003eo\u003c/sup\u003eC), Room temperature magnetic susceptibility after heating: k= 153x10\u003csup\u003e-5\u003c/sup\u003e (at ≈ 25 \u003csup\u003eo\u003c/sup\u003eC). \u003cstrong\u003eB\u003c/strong\u003e: Thermomagnetic curve for sample # 70 from trip2 (Profile P3) (Sample heated in nitrogen); C: cooling Curve, H: heating curve. Sample coordinates: 21º52.1’S; 27º27.8'E. Soil sample collected on the Zimbabwe Craton. The plot indicates the following: Curie temperature when heating: 585 \u003csup\u003eo\u003c/sup\u003eC and Hopkinson temperature: 300 \u0026nbsp;\u003csup\u003eo\u003c/sup\u003eC, Curie temperature when cooling: 585 \u003csup\u003eo\u003c/sup\u003eC, Room temperature magnetic susceptibility before heating: k= 179x10\u003csup\u003e-5\u003c/sup\u003e (at ≈ 25 \u003csup\u003eo\u003c/sup\u003eC), Room temperature magnetic susceptibility after heating: k= 214x10\u003csup\u003e-5\u003c/sup\u003e (at ≈ 25 \u003csup\u003eo\u003c/sup\u003eC).\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5608272/v1/923e6c0e1a2f639270a86eff.png"},{"id":74523040,"identity":"dc1d2fa3-f192-4706-b6e9-e13b061bc47c","added_by":"auto","created_at":"2025-01-23 06:15:31","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":59285,"visible":true,"origin":"","legend":"\u003cp\u003eA: Thermomagnetic curve for sample # 42 from trip 2 (Profile P3) (Sample heated in nitrogen); C: cooling Curve, H: heating curve. Sample coordinates: 22º15.0'S 26º46.4'E. Note the cross-over of the heating and cooling curves at ~450 \u0026nbsp;\u003csup\u003eo\u003c/sup\u003eC, indicative of the complex mineral alteration of the magnetic fabric. The plot indicates the following: Curie temperature when heating: 585\u003csup\u003e o\u003c/sup\u003eC,\u0026nbsp; Curie temperature when cooling: 300\u003csup\u003e o\u003c/sup\u003eC, Room temperature magnetic susceptibility before heating: k=96*10\u003csup\u003e-5\u003c/sup\u003e (at ≈ 25 \u003csup\u003eo\u003c/sup\u003eC), Room temperature magnetic susceptibility after heating: k=104*10\u003csup\u003e-5\u003c/sup\u003e (at ≈ 25 \u003csup\u003eo\u003c/sup\u003eC). B: Thermomagnetic curve for sample # 16 from trip2 (Profile P3) (Sample heated in nitrogen); C: cooling Curve, H: heating curve. Sample coordinates: 23º36.5'S 25º49.2'E Kaapvaal Craton /Limpopo belt. Note the cross-over of the heating and cooling curves at ~500 \u003csup\u003eo\u003c/sup\u003eC, suggesting thermal enhancement of the magnetic fabric.\u0026nbsp; The plot indicates the following: Curie temperature when heating: 500 \u003csup\u003e\u0026nbsp;o\u003c/sup\u003eC, Curie temperature when cooling:\u0026nbsp; 350 \u003csup\u003e\u0026nbsp;\u0026nbsp;o\u003c/sup\u003eC, Room temperature magnetic susceptibility before heating: k=125*10\u003csup\u003e-5\u003c/sup\u003e (at ≈ 25 \u003csup\u003e\u0026nbsp;o\u003c/sup\u003eC), Room temperature magnetic susceptibility after heating: k=195*10\u003csup\u003e-5\u003c/sup\u003e (at ≈ 25 \u003csup\u003eo\u003c/sup\u003eC).\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5608272/v1/676a8d139b9ad1cbf28a05ab.png"},{"id":74521916,"identity":"9f570a5c-6573-4985-bb96-54ac1210168e","added_by":"auto","created_at":"2025-01-23 06:07:31","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":62648,"visible":true,"origin":"","legend":"\u003cp\u003eA: Hysteresis Trip2_70; Low frequency magnetic susceptibility =839*10\u003csup\u003e-5\u003c/sup\u003e SI. Soil sample near Francistown (see Figure 3-6). Coercive force is 8.015 mT (Hc=80.15Oe and Mr = 176.4uemu), comparable to the observed value of 10mT for single domain magnetite (Thompson and Oldfield, 1986). The ratio of Mr/Mrs is found to be 0.1518. B: Hysteresis Trip3_46; Low frequency magnetic susceptibility = 16x10\u003csup\u003e-5\u003c/sup\u003e SI soil sample collected 30 km from Maitengwe towards Nata. The coercive force is 10.53 mT (Hc =105.3 Oe and Mr = 730 nemu), close to the observed value of 10mT for single domain magnetite (Thompson and Oldfield, 1986). The ratio of Mr/Mrs was found to be 0.195. C: Hysteresis Trip3_65; (Hc=32.30, Mr=10.91uemu, Ms= 94.92, Mr/Mrs= 0.1150) Low frequency magnetic susceptibility = 38x10\u003csup\u003e-5\u003c/sup\u003e SI soil sample collected 60 km from Nata towards Francistown. The value of the coercive force was found to be 3.23 mT, close to the observed value of multi-domain magnetite (Thompson and Oldfield, 1986). The ratio of Mr/Mrs was found to be 0.115. D: Hysteresis Trip3_50; Low frequency magnetic susceptibility = 3x10\u003csup\u003e-5\u003c/sup\u003e SI soil sample collected 60 km from Maitengwe towards Nata. The value of Hc was found to be 13.72 mT (Hc=137.2 Oe, Mr=1.673uemu), comparable to the observed value of 10mT for single domain magnetite (Thompson and Oldfield, 1986). The ratio of Mr/Mrs was found to be 0.2247.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-5608272/v1/84f6b93ff97a0e387bd7dafe.png"},{"id":74523792,"identity":"7dd813d0-05e6-42d6-b2c3-e3f6e31df399","added_by":"auto","created_at":"2025-01-23 06:23:39","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2255850,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5608272/v1/a8a2bd20-0f00-4737-baac-c09021535a48.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Hysteresis and Thermomagnetic Characteristics of Soils and Rocks in Eastern Botswana","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eRocks and soil magnetic parameters are extensively used to study a variety of geological and environmental processes, including rock-forming and altering geological processes (Abdel Aal et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Ayoubi et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Mello et al., 2023), palaeomagnetism and (plate) tectonics (Muxworthy, \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2001\u003c/span\u003e), ore genesis (geoexploration) (Alva-Valdivia and Lopez-Loera, 2011), geomorphology (Da Silva et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), magnetic mineralogy (Clark, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Szuszkiewicz et al., \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2021\u003c/span\u003e), archaeology and forensics (Beatrice et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Sanchez-Roda et al., \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Shirzaditabar and Heck, \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), and anthropogenic and environmental activities/factors (Hanesch et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Da Silva et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Ranganai et al., \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2015\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eRecent advancements in low-temperature magnetic measurements, hysteresis loop parameters, magnetic susceptibility dependence on temperature and frequency, and M\u003cspan class=\"InlineEquation\"\u003e\u003c/span\u003essbauer effect, spectrometry, have made it possible to accurately classify magnetic mineralogy and particle size (Zhao et al., \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Interpretation of magnetic parameters requires a clear understanding of the underlying processes that influence the presence, distribution, preservation, or neoformation of magnetic minerals in soil and rock (e.g., Fialova et al., 2006; Abdel Aal et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Magnetic properties are dominantly controlled by the presence and volumetric abundance of iron and its oxygen fugacity. The grain size distribution, domain size distribution, amount and presence of titanium, and other factors may also be important. Until recently, there have been no systematic studies of the magnetic properties of rocks and soils in Botswana. In particular, little attention has been devoted to thermomagnetic properties, which are important in a variety of applications in geo-exploration, geotechnical engineering, and industry in a developing economy.\u003c/p\u003e \u003cp\u003eThis study expands on the work of Ranganai et al. (\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), which focused on soil colour determination, low temperature magnetic susceptibility measurements at low and high frequencies by adding thermomagnetic, hysteresis properties of soils, and determination of Curie temperatures.. The eastern side of Botswana is the only region in the country with detailed combined airborne magnetic and radiometric survey (e.g., Ranganai et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). However, the lack of further analysis and publication of the data has resulted in its limited utilisation to date, and the current work is important in that regard.\u003c/p\u003e \u003cp\u003eThe purpose of this study is to report on Curie temperature as observed in thermomagnetic curves, and magnetic hysteresis parameters such as: saturation remanence magnetisation (Mrs), saturation magnetisation (Ms), coercivity of remanence (Hcr), and coercive force (Hc). Thermomagnetic curves involve measuring the magnetic susceptibilities of soil samples at different temperatures. This can help identify the types of magnetic minerals present and their thermal stability. Hysteresis parameters, on the other hand, involve measuring the magnetic properties of a soil sample as an external magnetic field is applied and then removed. This can provide information about the size, shape, and distribution of magnetic particles in the soil. By analyzing the magnetic properties of soil samples from a specific region, researchers can gain insights into the geological history and environmental conditions of that area. For example, certain magnetic minerals may be indicative of past volcanic activity or changes in climate. This information can be useful for a variety of applications, such as geologic mapping, mineral exploration, and environmental monitoring.\u003c/p\u003e"},{"header":"2. Regional Geological Setting","content":"\u003cp\u003eBotswana is a land-locked country covering approximately\u0026thinsp;~\u0026thinsp;583 000 square kilometres with a population of about 2.2\u0026nbsp;million (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The country is relatively flat with almost 75% covered by superficial Kalahari sand deposits with depths ranging from few meters in the east to more than 200 m in the west (Meixner and Peart, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e1984\u003c/span\u003e; Key and Ayres, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). Almost 80% of the population reside in the southern and eastern part of the country, (the study area), where favourable conditions exist for agricultural activities like cattle ranching and subsistence crop farming (e.g., DLFRS, 1985; De Wit and Nachtergaele, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e1990\u003c/span\u003e). The study area is underlain by geologic terranes comprising the Archaean Zimbabwe, Kaapvaal Cratons, and the Limpopo mobile Belt (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) (Key and Ayres, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). The soils in this region have a ustic moisture regime, while the rest of the country is aridic (De Wit and Nachtergaele, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e1990\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe main geologic terranes in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e consist of the Kaapvaal craton (3.5-2.5Ga) and the Zimbabwe craton, which were formed during two major tectonic cycles that involved the formation of greenstone terranes and gneisses, with the rocks in the Kaapvaal craton being slightly older. The Limpopo Belt was formed by the collision between the two cratons and has high-grade metamorphosed rocks, e.g., granitoid gneiss (Key and Ayres, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). It contains numerous mineral occurrences such as copper and nickel hosted in ultramafics (BGI, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) and diamondiferous kimberlite pipes (de Wit, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). A Uranium mining prospect at Serule within (Lower Karoo) Ecca Group mudstone (A-Cap, 2015) is currently under development. Other concerns include air and soil pollution, as well as industrial pollution related to mining activities (e.g., Chimidza and Moloi, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2000\u003c/span\u003e), The magnetic properties of rocks and soil are mostly influenced by the lithology of the underlying bedrock and soil-forming processes. These magnetic properties can thus assist in broad geological mapping and interpretation (e.g., Reynolds et al., \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e1990\u003c/span\u003e; Lu et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2008\u003c/span\u003e) as well as environmental assessment (e.g., Fialova et al., 2006; Hanesh et al., 2007).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"3. Materials and Methods","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1. Sample Collection and Preparation\u003c/h2\u003e\n \u003cp\u003eSoil samples were collected in the eastern side of Botswana along the major road network while rock samples used are mainly from the Water Utilities Corporation (WUC) North-South Water carrier (NSWC) excavations (e.g. Figure \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e) between Gaborone and Selebi Phikwe (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e), with a few taken from outcrops and quarries. Excavation on average reached 2 m depth, which ensured relatively fresh (unweathered) samples (Kedisang, \u003cspan class=\"CitationRef\"\u003e1999\u003c/span\u003e). Soil sampling profiles (P1 to P3, Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e) and sample preparation are as discussed in Ranganai et al. (\u003cspan class=\"CitationRef\"\u003e2015\u003c/span\u003e), and only a brief summary is presented here for completeness. Samples were collected at 10 km, 5 km and/or 1 km intervals using the vehicle odometer and about 100 m from the main road to minimize anthropogenic effects and were kept in diamagnetic plastic containers. This distance from the road helped to avoid transported material and contamination from metal debris left during road construction and from vehicle exhaust fumes (cf Hoffmann et al., \u003cspan class=\"CitationRef\"\u003e1999\u003c/span\u003e; King and Ranganai, \u003cspan class=\"CitationRef\"\u003e2000\u003c/span\u003e; Kim et al., \u003cspan class=\"CitationRef\"\u003e2007\u003c/span\u003e; Shirzaditabar and Heck, \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e) while plastic shovels were used to avoid \u0026lsquo;mineral\u0026rsquo; contamination. Samples were obtained during the dry season and thus reducing moisture problems (cf Schibler et al., \u003cspan class=\"CitationRef\"\u003e2002\u003c/span\u003e; Maier et al., \u003cspan class=\"CitationRef\"\u003e2006\u003c/span\u003e) and at a depth of 5\u0026ndash;30 cm (topsoil), while avoiding compost (organic) material (cf Shi and Ciopaa, 2006). Laboratory preparations involved air-drying samples at room temperature (\u0026lt;\u0026thinsp;30 \u0026ordm;C) for a duration of 48 hours, to minimize chemical alterations (Ranganai et al., \u003cspan class=\"CitationRef\"\u003e2015\u003c/span\u003e, Maier et al., \u003cspan class=\"CitationRef\"\u003e2006\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eThe eastern part of Botswana is diverse in terms of topography and physiography and is divided into four main geomorphological zones: alluvial deposits, sandveld, hardveld, and lacustrine deposits. The primary source of the parent soil material is mostly the hardveld, which has an Archaean age (\u0026gt;\u0026thinsp;2.500 Ga). Profile P1, which is more than 650 km long, consists of soil samples collected 50 m from the tarred road between Ramatlabama and Ramokgwebana, crossing the Kaapvaal Craton in the south, the central zone of the Limpopo belt, and the Zimbabwe Craton in the north. Profile P2 passes over the hardveld, which is underlain by the Kaapvaal Craton, through the central zone of the Limpopo Belt and the Zimbabwe Craton. Profile P3 crosses the Sandveld as it moves towards the Kalahari Desert in the west.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2. Thermomagnetic Curves and Curie Temperature\u003c/h2\u003e\n \u003cp\u003eThermomagnetic curves were used to determine the composition of mineral phases in rocks and soils (e.g., Dunlop and Ozdemir, 1997; King and Ranganai, \u003cspan class=\"CitationRef\"\u003e2001\u003c/span\u003e; Lu et al., \u003cspan class=\"CitationRef\"\u003e2008\u003c/span\u003e). The Curie temperature (Tc), a critical measure of the magnetic mineralogy, was obtained by heating samples to 700\u0026deg;C after performing the empty furnace correction (Ranganai et al., \u003cspan class=\"CitationRef\"\u003e2015\u003c/span\u003e). The measurement of room temperature magnetic susceptibility and generation of thermo-susceptibility (\u0026chi;-T) curves was carried out using a water cooled MS2W sensor and MS2WF furnace. The MS2W has an operational frequency of 696 Hz, a range of -200 \u003csup\u003eo\u003c/sup\u003eC to 900 \u003csup\u003eo\u003c/sup\u003eC and a precision of 4\u0026pi;x10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e SI per 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003em\u003csup\u003e3\u003c/sup\u003e. The samples were heated and then cooled at a rate of 20\u0026deg;C/min to enhance any mineralogical transformation, in an inducting field of 300 A/m (0.38 mT) (cf Jordanova and Jordanova, \u003cspan class=\"CitationRef\"\u003e2016\u003c/span\u003e). To minimise oxidation, the samples were heated in an inert nitrogen atmosphere (99.99% purity) and a straightforward graphical method was used to measure the Curie temperature (Tc) (Zhao et al., \u003cspan class=\"CitationRef\"\u003e2006\u003c/span\u003e). However, this method underestimates Curie temperature compared with others (e.g., Petrovsky and Kapicka, 2006; Tauxe et al., \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eSome magnetic materials which are metastable crystallize at low temperatures undergo chemical changes when heated in air (Piper, \u003cspan class=\"CitationRef\"\u003e1987\u003c/span\u003e). These changes can result in the formation of new minerals with either higher or lower magnetic susceptibilities, leading to dissimilar heating and cooling curves. For instance, heating can cause the oxidative formation of antiferromagnetic hematite, which has magnetic susceptibilities that are several orders of magnitude lower than those of magnetite or maghemite (Dunlop and Ozdemir, 1997).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3 Magnetic Hysteresis (and Day plots)\u003c/h2\u003e\n \u003cp\u003eHysteresis loops provide important information about the coercivity spectrum and domain state of ferrimagnetic materials, helping to characterise their intrinsic magnetic behaviour in rocks and soils. The study of hysteresis parameters helps to understand the origin of remanence (Day et al., \u003cspan class=\"CitationRef\"\u003e1977\u003c/span\u003e; Dunlop, \u003cspan class=\"CitationRef\"\u003e2002a\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003eb\u003c/span\u003e; Dunlop and \u0026Ouml;zdemir, \u003cspan class=\"CitationRef\"\u003e1997\u003c/span\u003e; Zhao et al., \u003cspan class=\"CitationRef\"\u003e2006\u003c/span\u003e). The domain state of ferrimagnetic materials changes as grain size increases, transitioning from superparamagnetic (SP) to stable single domain (SSD/SD), then to pseudo-single domain (PSD), and finally to multidomain (MD) (Dunlop and Ozdemir, 1997; Dunlop, \u003cspan class=\"CitationRef\"\u003e2002a\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003eb\u003c/span\u003e; Zhao et al., \u003cspan class=\"CitationRef\"\u003e2006\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eHysteresis parameters of selected samples were determined using a Princeton MicroMag 2900 vibrating sample gradient magnetometer (Princeton Measurements Corp., USA) linked to a microcomputer capable of resolving magnetic moments as small as 5.0x10\u003csup\u003e\u0026minus;\u0026thinsp;8\u003c/sup\u003e emu (Zhao et al., \u003cspan class=\"CitationRef\"\u003e2006\u003c/span\u003e). Samples (a few milligrammes) were attached to the probes using wax (crystal clear nail polish) and paper, which have a negligible magnetic susceptibility (cf King and Ranganai, \u003cspan class=\"CitationRef\"\u003e2000\u003c/span\u003e). The coercive force (Hc), coercivity of remanence (Hcr), saturation magnetisation (Ms) and saturation remanent magnetisation (Mrs) were measured with a magnetic field applied to a maximum (\u0026apos;saturating\u0026apos;) of 1.0 T, suitable for most types of minerals. The room-temperature hysteresis parameters determined from selected samples were then used in the classification of domain states based on the SD-PSD-MD boundaries of Dunlop (\u003cspan class=\"CitationRef\"\u003e2002a\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003eb\u003c/span\u003e) using Hcr/Hc and Mrs/Ms ratios (Day et al., \u003cspan class=\"CitationRef\"\u003e1977\u003c/span\u003e). The hysteresis ratios in soil and rock magnetic studies provide important information on the grain size distribution of magnetic carriers in samples (Day et al., \u003cspan class=\"CitationRef\"\u003e1977\u003c/span\u003e; Lu et al., \u003cspan class=\"CitationRef\"\u003e2008\u003c/span\u003e). The shape of the hysteresis loops reflects the degree of pedogenesis, which mainly involves ferromagnetic mineral neoformation and dissolution (e.g., Dearing et al., \u003cspan class=\"CitationRef\"\u003e1996a\u003c/span\u003e; Lu et al., \u003cspan class=\"CitationRef\"\u003e2008\u003c/span\u003e). However, it is worth noting that the ratios have limitations compared to using \u0026chi;fd % as they cannot differentiate between MD and SP grains (Lu et al., \u003cspan class=\"CitationRef\"\u003e2008\u003c/span\u003e). Magnetic parameters can also be determined from Day plots (Day et al., \u003cspan class=\"CitationRef\"\u003e1977\u003c/span\u003e; Dunlop, \u003cspan class=\"CitationRef\"\u003e2002a\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003eb\u003c/span\u003e) using Rock Magnetic Analyzer 1.0 software.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Results","content":"\u003cp\u003e\u003cstrong\u003e4.1. Magnetic Susceptibility at High Temperature (Thermomagnetic Curves)\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThermomagnetic curves were generated to determine Curie temperatures (Tc), which indicate mineralogical and chemical compositions. The graphical intersecting tangent or inflection point method was used to estimate Tc by fitting the paramagnetic part of the inverse susceptibility versus temperature relation with a hyperbolic function (as described in Dunlop and Ozdemir, 1997, and Petrovsky and Kapicka, 2006). A Hopkinson effect, characterised by a peak in magnetisation (Hopkinson peak) before a decrease in susceptibility, can precede the Curie temperature (Tc) (as noted by King and Ranganai, 2001). This peak can be used to calculate Tc (as per Petrovscky and Kapicka, 2006).\u003c/p\u003e\n\u003cp\u003eMost of the soil samples exhibit nonreversible behaviour, with the cooling curve being higher than the heating curve, and a Curie temperature (Tc) of nearly pure (stoichiometric) magnetite at 585 \u0026deg;C. Some plots (e.g. Figures 3, 4 and 5) show a Hopkinson peak for magnetite before the Curie temperature, but this disappears during cooling. This behaviour is likely due to the presence of small PSD grains. The high room temperature susceptibility (\u0026chi;) indicates the initial presence of magnetic minerals. Furthermore, most of the samples show varying degrees of difference between the heating and cooling curves (at least \u0026chi;rtc=2\u0026chi;rth), which suggests that low-temperature oxidised titanomagnetite is the dominant magnetic mineral in many samples.\u003c/p\u003e\n\u003cp\u003eThe sample for Figure 3\u003cstrong\u003eA\u003c/strong\u003e was heated in the presence of nitrogen, which reduces the chemical reactions taking place in the soil sample. The heating curve shows a small hump (Hopkinson peak; susceptibility enhancement at ~555\u003csup\u003e\u0026nbsp;o\u003c/sup\u003eC) as the temperature increases, which indicates the presence of single-domain magnetite grains. There is a slight indication of a smaller secondary peak, which may be due to mineralogical changes during the heating and suggests inversion or unmixing. The spinel structure of titanomaghemites becomes unstable during inversion and changes to rhombohedral, or to intergrown iron-rich spinel and Ti-rich rhombohedral. In this sample, the Curie temperature is estimated to be 585\u003csup\u003e\u0026nbsp;o\u003c/sup\u003eC, for both the heating and cooling curves. The mineral is likely to be magnetite (Ti-poor) with a Curie temperature of 585\u003csup\u003e\u0026nbsp;o\u003c/sup\u003eC.\u003c/p\u003e\n\u003cp\u003eThe sample susceptibility approaches zero as the temperature reaches 650\u003csup\u003e\u0026nbsp;o\u003c/sup\u003eC, indicating complete destruction of magnetic order (conversion of magnetite to hematite). The cooling curve also reflects the presence of magnetite with alteration of some magnetic minerals that enhances magnetization. The room temperature\u0026nbsp;k\u0026nbsp;on cooling is roughly twice/double that for heating (cf Jordanova and Jordanova, 2016). The Curie temperature of about 570\u0026nbsp;\u003csup\u003eo\u003c/sup\u003eC suggests a ferrimagnetic component as the principal magnetic mineral.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;In Figure 3\u003cstrong\u003eB\u003c/strong\u003e, the heating curve illustrate that more than one mineral is present in the sample; suggestive of several magnetic phases. There is a slow increase of\u0026nbsp;k\u0026nbsp;with temperature up to an observed peak at T\u003csub\u003eN\u003c/sub\u003e = 300\u003csup\u003e\u0026nbsp;o\u003c/sup\u003eC where T\u003csub\u003eN\u003c/sub\u003e is the Neel temperature and there is a gradual decrease thereafter until another small peak at ~550\u003csup\u003e\u0026nbsp;o\u003c/sup\u003eC (probably T\u003csub\u003eH\u003c/sub\u003e). This behaviour is attributed to thermally induced alteration of metastable cubic maghemite to weakly magnetic rhombohedral hematite (Dunlop and Ozdemir, 1997; Deng et al., 2001; Lu et al., 2008). The inflection point on this curve occurs at a temperature of 570\u003csup\u003e\u0026nbsp;o\u003c/sup\u003eC, which is the Curie temperature of near-pure magnetite. Curve reaches a minimum of ~20x10\u003csup\u003e-5\u003c/sup\u003e SI; indicating some magnetisation remains as sample is heated to 700\u003csup\u003e\u0026nbsp;o\u0026nbsp;\u003c/sup\u003eC, which is the temperature that was set for all the samples. The cooling curve also shows the presence of additional magnetite. The magnetic susceptibility at room temperature is reasonably high and leads to the conclusion that the soil sample is dominated by paramagnetic or ferrimagnetic minerals. The peak at 300\u003csup\u003e\u0026nbsp;o\u003c/sup\u003eC is likely caused by presence of ferrimagnetic ilmenite that is reported to have Curie temperature in the range of 50-300\u003csup\u003e\u0026nbsp;o\u003c/sup\u003eC (Piper, 1987). The Curie temperature of 300\u003csup\u003e\u0026nbsp;o\u003c/sup\u003eC is also compatible with titanium-rich titanomagnetite (such as TM60) or low-temperature oxidised titanomaghemites (Dunlop and Ozdemir, 1997; Zhao et al., 2006).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn Figure 4\u003cstrong\u003eA\u003c/strong\u003e, the room temperature magnetic susceptibility observed from the heating curve is very low (\u0026nbsp;\u0026nbsp;SI) which might mean that the soil sample is mainly dominated by paramagnetic minerals. On cooling the magnetic susceptibility increases as the temperature is decreased, with a strong peak at around 300 \u003csup\u003eo\u003c/sup\u003eC and a room temperature susceptibility of\u0026nbsp;\u0026nbsp;\u0026nbsp;SI (with almost\u0026nbsp;c\u003csub\u003ertc\u003c/sub\u003e = 14c\u003csub\u003erth\u003c/sub\u003e). The observed peaks correspond to Neel temperature of magnetic minerals like pyrrhotite and ilmenite. The sample was heated in air and this leads to\u0026nbsp;a\u0026nbsp;great difference in\u0026nbsp;the magnetic susceptibility at\u0026nbsp;room temperature magnetic susceptibility (~\u0026nbsp;\u0026nbsp;SI) since the chemical reactions were not\u0026nbsp;minimised. The susceptibility difference at room temperature suggests mineral alteration and hence the formation of new minerals as a result of temperature changes.\u0026nbsp;This type of curve is typical of hematite-ilmenite series (Bohnel et al., 2002; Peters and Dekkers, 2003). \u0026nbsp;For Figure 4\u003cstrong\u003eB\u003c/strong\u003e, the heating and cooling curves are reasonably close, the former showing a Hopkinson peak suggesting the existence of multiple magnetic phases. Both curves show high room temperature magnetic susceptibility (\u0026nbsp;\u0026nbsp;SI and\u0026nbsp;\u0026nbsp;\u0026nbsp;SI, respectively), representing the presence of magnetic minerals. The plot shows a discernible hump (or peak/dome) at 270-300\u0026nbsp;\u003csup\u003eo\u003c/sup\u003eC, which is followed by a clear drop in susceptibility and then a Hopkinson peak before the Curie temperature. The peak at 300 \u003csup\u003eo\u003c/sup\u003eC may correspond Curie temperature of pyrrhotite, or alternatively titano-maghemite (e.g., Shi and Cioppa, 2006). Pyrrhotite is converted to magnetite as the temperature\u0026nbsp;increases.\u0026nbsp;The observed Curie temperature from this curve is around 585\u003csup\u003e\u0026nbsp;o\u003c/sup\u003eC, which is the Curie temperature of magnetite.\u0026nbsp;The thermomagnetic signatures show the transformation of titano-maghemite (the low Curie temperature phase) to a strongly magnetized magnetite, as shown by the irreversible cooling curves.\u0026nbsp;The dominant mineral in the cooling curve is\u0026nbsp;multidomain\u0026nbsp;magnetite with Curie point at 585\u003csup\u003e\u0026nbsp;o\u003c/sup\u003eC. The curve\u0026nbsp;reaches a minimum of ~\u0026nbsp;\u0026nbsp;SI; indicating significant magnetisation remains as the sample is heated to about 700\u003csup\u003e\u0026nbsp;o\u0026nbsp;\u003c/sup\u003eC.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFigure 5A: shows room temperature susceptibility very close to that for heating and moderately high (\u003cem\u003ek\u003c/em\u003e\u003cem\u003e=96*10\u003csup\u003e-5\u003c/sup\u003e\u003c/em\u003e and 104*10\u003csup\u003e-5\u003c/sup\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eSI), indicating the initial presence of magnetic minerals. The heating curve has a Curie temperature of 585\u003csup\u003e\u0026nbsp;o\u003c/sup\u003eC, which correspond to the Curie temperature of magnetite; the hump is a typical feature of a single-domain magnetite mineral. When the sample is cooled, a different mineral is formed with a Neel temperature of 300\u003csup\u003e\u0026nbsp;o\u003c/sup\u003eC. The mineral formed could be ilmenite with a Curie temperature of ~300 \u003csup\u003eo\u003c/sup\u003eC. Alternatively, this could be titanium-rich titanomagnetite (such as TM60 or TM45) or low temperature oxidised titanomaghemites (Dunlop and Ozdemir, 1997; Zhao et al., 2006; Lu et al., 2008). The heating curve runs below the cooling curve up to ~450 \u003csup\u003eo\u003c/sup\u003eC indicating that there is no formation of new magnetic phases initially upon heating or a phyrrhotite bearing sample (cf Bohnel et al., 2002). However, because some susceptibility values are higher after cooling, new magnetite should have been formed as well.\u003c/p\u003e\n\u003cp\u003eIn Figure 5B, a progressive increase of k with temperature up to 500 \u003csup\u003eo\u003c/sup\u003eC is observed and suggests a significant contribution of single domain (SD) and/or pseudo-single domain (PSD) magnetite particles. This may be due to gradual unblocking of fine-grained (near the SP/SD boundary) ferromagnetic particles (Lu et al., 2008). Also, the hump/bulge is a typical feature of a single-domain magnetite mineral (Hunt et al., 1995; Dunlop 2002). There is relatively fast drop of c starting at ~520 \u003csup\u003eo\u003c/sup\u003eC, with room temperature\u0026nbsp;c\u0026nbsp;again high (125x10\u003csup\u003e-5\u003c/sup\u003e SI), indicating the presence of magnetic minerals; The heating curve has a Curie temperature of 500\u003csup\u003e\u0026nbsp;o\u003c/sup\u003eC, which corresponds to the Curie temperature of Ti-rich magnetite; the hump is a typical feature of a single-domain magnetite mineral. On cooling, a different mineral is formed with a Curie temperature of 350 \u003csup\u003eo\u003c/sup\u003eC. The mineral formed on cooling could be ilmenite with a Neel temperature at ~300 \u003csup\u003eo\u003c/sup\u003eC. The cooling curve is initially nearly reversible but there is a dramatic parting of the heating and cooling curves below the cross-over at around ~500 C, implying complex alteration. This shows thermal enhancement of magnetic fabric and different magnetic phases formed, or grain-size re-distribution as a result of heating. \u0026nbsp;This is a relatively frequent case (Fig. 3) with the creation of new magnetite from weakly magnetic phases as a result of heating.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e4.2. Magnetic Hysteresis Results \u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSome samples contained sufficient magnetic material, i.e. detectable ferrimagnetic contribution, to yield hysteresis parameters (Figure 6). The form of the loop and numerical values of Mr/Ms and Hcr/Hc depend on the microstructure, primarily the grain size and shape and domain structure (Day et al., 1977; Lu et al., 2008). Further, the presence of high coercivity component Hcr/Hc = 1.02-1.17 for MD is a clear indication of hematite, goethite or SD maghemite, Hcr/Hc = 1.45-1.62 for SD hematites\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e(e.g., Dunlop and Ozdemir, 1997; Peters and Dekkers, 2003; Ozdemir and Dunlop, 2014). The hysteresis loops of representative samples are shown in Figure 6. Hysteresis loop measurements show that samples reached a saturation magnetization of 250-300 mT field strength, and most curves are rather symmetrical. Near the origin, no potbellied and wasp-waisted behaviour (Tauxe et al., 1996) was observed/detected, but overall shapes show significant paramagnetic contributions which probably reflects the mineralogical phase changes.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eSoil samples from Nata show a narrow hysteresis loop and samples near Francistown show intermediate while the samples near Maitengwe display wide hysteresis loops. The hysteresis loops are closed at about 200 mT for the Maitengwe sample, which is consistent with the existence of a dominant ferrimagnetic phase. The hysteresis loop of the Trip3_46 sample closes at a higher field than those of the Trip2_70 sample. The high coercivity in the Trip2_70 sample is due to the collective effects of antiferromagnetic phases (e.g. haematite and/or goethite) and low-temperature oxidized coarse-grained magnetite.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eMost magnetic parameters are dominated by low-coercivity ferrimagnets (magnetite or maghemite). Antiferromagnetic minerals like hematite and goethite are the dominant remanence carrying component by mass (Dunlop 2002). Hcr/Hc values range 1.02-1.17 and Mrs/Ms ratios range from 0.5 to 0.9 for MD hematites which basically correspond to pseudo-single-domain (PSD) grain size region according to Day et al. (1977) and/or Dunlop et al. (2002a, b).\u0026nbsp;\u003c/p\u003e"},{"header":"5. Discussion","content":"\u003cp\u003eVarious magnetic properties of rock and soil have been determined to evaluate magnetic carriers and domains, calculate Curie Temperatures, and provide constraints for the interpretation of aeromagnetic data (e.g., Alva-Valdivia and Lopez-Loera, 2011) and magnetic modelling (e.g., Reynolds et al., \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e1990\u003c/span\u003e). A combination of magnetic parameters was used to quickly measure changes in the concentration and grain size of lithogenic and pedogenic magnetic components. Changes in mass-normalized magnetic susceptibility (χ) were used to monitor changes in the concentration of ferrimagnetic minerals, such as titano-magnetite or maghemite (Ranganai et al., \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Thermomagnetic curves were used to estimate the Curie temperature (Tc), and hysteresis properties have been interpreted for remanence and domain states. Hysteresis loops provide valuable information on parameters such as retentivity, coercivity, permeability, and susceptibility and are used in selecting appropriate materials for specific purposes, i.e. the so-called soft and hard magnetic materials.\u003c/p\u003e \u003cp\u003eMagnetic susceptibility measurements have indicated that the Kaapvaal Craton, Zimbabwe, and Limpopo Belt have distinct magnetic signatures. The Kaapvaal Craton was observed to have low magnetic susceptibility measurements ranging from \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:24*{10}^{-5}\\)\u003c/span\u003e\u003c/span\u003e SI to \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:511*{10}^{-5}\\)\u003c/span\u003e\u003c/span\u003e SI (Ranganai et al., \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). On the other hand, the magnetic susceptibility measurements in the Limpopo Mobile Orogenic Belt ranged from \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:64*{10}^{-5}\\)\u003c/span\u003e\u003c/span\u003e SI to 220x10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e SI and in the Zimbabwe Craton, they ranged from \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:98*{10}^{-5}\\)\u003c/span\u003e\u003c/span\u003e SI to \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:878*{10}^{-5}\\)\u003c/span\u003e\u003c/span\u003e SI. According to a study by Moidaki (\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2001\u003c/span\u003e), a considerable number of soil samples had volume susceptibilities ranging between \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:0-50*{10}^{-5}\\)\u003c/span\u003e\u003c/span\u003e SI, as revealed by the low-frequency magnetic susceptibility histograms. These soil samples were considered to be weakly magnetic and their magnetic properties were controlled by paramagnetic minerals such as pyrrhotite and ilmenite (Dearing, 1997). Goethite and hematite, which are paramagnetic and antiferromagnetic iron oxides, play a minor role in determining the magnetic character of soil (Maher, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e1986\u003c/span\u003e; Dearing et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1996a\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe presence of sedimentary rocks with volume susceptibilities mainly controlled by paramagnetic and diamagnetic minerals, as well as the overlaying Kalahari sands, is responsible for the low magnetic susceptibility (Ranganai et al., \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). On the other hand, some showed a significant amount of paramagnetic and ferrimagnetic minerals, leading to higher magnetic susceptibility values upon heating. Soils derived from igneous and ultramafic rocks have high magnetic susceptibility values, while soils derived from basaltic rocks exhibit very strong magnetic signatures in many cases (Lu et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2008\u003c/span\u003e, Dearing et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1996b\u003c/span\u003e).\u003c/p\u003e \u003cp\u003ePrevious research has found that high values of magnetic susceptibility and frequency dependence of magnetic susceptibility are common in tropical soils (Hendrickx et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Hanesch et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Lu et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2008\u003c/span\u003e, Ranganai et al., \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). The magnetic behaviour is believed to be caused by the substantial concentrations of ferrimagnetic iron oxide minerals, such as magnetite, maghaemite, and pyrrothite, present in basaltic rocks (Zhao et al., \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Lu et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). These minerals are known to be the most magnetic of the iron oxides (Cornell and Schwertmann, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Dearing et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1996a\u003c/span\u003e; Hendrickx et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). The difference between magnetic susceptibility measured at low and high frequency indicates the presence of ultra-fine superparamagnetic minerals that occur as crystals produced by bacteria or chemical processes in the soil (Dearing, 1996a, 1997).\u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e5.1. Thermomagnetic Curves and Curie Temperature\u003c/h2\u003e \u003cp\u003eThe thermomagnetic curves obtained by plotting magnetic susceptibility versus temperature show that the soil samples under investigation contain a range of minerals that behave differently. The thermomagnetic curves reveal the presence of the magnetite-ulvospinel series (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eB and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), which are characterized by Curie temperatures ranging from 100 \u003csup\u003eo\u003c/sup\u003eC to 600\u0026deg;C. Curie temperatures ranging from 550 \u003csup\u003eo\u003c/sup\u003eC to 585\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eA and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eB) suggest that a ferrimagnetic mineral similar to magnetite, with probably minor Titanomagnetite, dominates the magnetic properties of the soil (e.g., Geiss et al., 2006). The mineralogical constituents from soils samples obtained in this study are consistent with the geology of the craton area since the Limpopo Central Zone metamorphism is associated with 790 to 890 \u003csup\u003eo\u003c/sup\u003eC (Tsunogae et al., \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Millonig et al., \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). The hematite-ilmenite series is also a characteristic feature of some soil samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eA). Hematite is a component of many igneous and sedimentary rocks and may be formed by the dehydration of goethite or by the weathering of Fe2\u0026thinsp;+\u0026thinsp;in the lattices of clay mineral surfaces (Piper, \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e1987\u003c/span\u003e). Pure hematite is already completely oxidized, but ilmenite, the other end member of the series, undergoes oxidation at temperatures above 500\u0026deg;C (Piper, \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e1987\u003c/span\u003e). As a result of the varying magnetic minerals that behave differently when heated, the boundaries of the major craton cannot be delimited accordingly.\u003c/p\u003e \u003cp\u003eMaghemite has the same chemical formula as hematite. Its similarity in structure to magnetite gives it comparable magnetic properties and saturation magnetization (Piper, \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e1987\u003c/span\u003e). Maghemite in soils can be produced through pedogenic and/or lithogenic processes (Cornell and Schwertmann, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). In the latter, maghemitization is a low-temperature oxidation usually obtained through weathering processes (Dunlop and Ozdemir, 1997). Maghemite is formed by the low-temperature oxidation of magnetite (maghemitization) in both subaerial and submarine environments (Piper, \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e1987\u003c/span\u003e). Some of the soil samples reveal the presence of maghemite, which could mean that the soils in that area were once covered by water. It indicates processes of low-temperature oxidation which commonly occur in soils at normal (room) temperatures. Maghemite can also form in soil through the action of burning, where oxides and hydroxides of iron are first reduced to magnetite and may subsequently be oxidized to maghemite. The difference in room temperature magnetic susceptibility supports the idea that bush fires can affect the magnetization of topsoil minerals by creating highly magnetized magnetic minerals (e.g., Clement et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). Wildfires are common during the pre-summer season (autumn) due to land use (e.g., Dube, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), which could affect soil magnetic susceptibility. Goethite (limonite) is a magnetic mineral found in most soils, although the thermomagnetic curves discussed above do not show Curie temperatures corresponding to goethite (Tc\u0026thinsp;~\u0026thinsp;110\\ \u0026deg;C to 120\u0026deg;C). Goethite and other hydrated iron oxides are formed in weathering environments and appear as yellow to brown coloured phases in weathered rocks and soils (Piper, \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e1987\u003c/span\u003e; Ranganai et al., \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2015\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe Curie temperature (also known as the critical or ordering temperature) of a magnetic mineral is the temperature at which rock and soil samples lose their typical ferromagnetic properties and become paramagnetic. It is a critical point (Kiss et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2005\u003c/span\u003e) that indicates the mineralogical and chemical composition of rocks and soils (Dunlop and Ozdemir, 1997; Zhao et al., \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Lu et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). For antiferromagnetic minerals, this change occurs at the Neel temperature. The source of variation in magnetic properties with temperature is the disruption of the alignment of molecular magnetic moments due to thermal motion of the atoms (Dunlop and Ozdemir, 1997). Ferromagnetic and ferrimagnetic compounds, and some paramagnetic compounds, show a decrease in magnetic susceptibility with increasing temperature (Telford et al., \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e1990\u003c/span\u003e; Hunt et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e1995\u003c/span\u003e). However, the overall plot of magnetic susceptibility versus temperature shows a different variation with temperature for the former (e.g., Thompson and Oldfield, \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e1986\u003c/span\u003e). For example, there is a low steady increase from room temperature with a susceptibility enhancement up to 30 times (Hopkinson peak), followed by an inverse drop to small values (Hunt et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Kiss et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). The minerals that show the Hopkinson effect the most are magnetite, titanomagnetite, pyrrhotite, and other earth materials such as cataclasites (fault rocks) (King and Ranganai, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Kiss et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2005\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eMost of the samples exhibit non-reversible thermomagnetic behaviour, with a strongly increased magnetization after heating (cf. Bohnel et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2002\u003c/span\u003e), and a Curie temperature (Tc) of magnetite at 585\u0026deg;C. Several plots display a Hopkinson peak at around 500\u0026deg;C before the Curie temperature (e.g., Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eB and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), which is common for natural titano-magnetite, and their room temperature magnetic susceptibility (κ) is invariably high, suggesting the initial presence of magnetic minerals. The different shapes of the Hopkinson peaks may be indicative of different grain-size distributions in the studied samples, since the Hopkinson effect is best detected in samples with a narrow grain size range (Dunlop and Ozdemir, 1997), and King and Ranganai (\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2001\u003c/span\u003e) have used the susceptibility enhancement factor (SEF) of the Hopkinson peak to determine grain size in some rocks.\u003c/p\u003e \u003cp\u003eThere are three types of heating and cooling curves: (i) the simple case, where the curves are nearly identical or reversible, which is common in rocks; (ii) the most frequent case, where χ for cooling is much higher than for heating, but the trends are generally similar; and (iii) the relatively infrequent and difficult case, where χ on cooling is lower than heating (Zhao et al., \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Lu et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Our curves fall into category 2 and usually characterize the situation when a new and strongly magnetic phase (typically magnetite) is created from less magnetic phases during heating. The moderately large difference between heating and cooling of some samples suggests that a low-temperature oxidized titanomagnetite is the main magnetic mineral. The substantial peak around 500\u0026deg;C is caused by the neoformation of magnetite through the conversion of iron-containing silicates/clays (Shi and Cioppa, \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Lu et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2008\u003c/span\u003e), and a relatively sharp decay around 580\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\:^\\circ\\:C\\)\u003c/span\u003e\u003c/span\u003e-600 \u0026deg;C indicates that substitution for iron (Fe) is very low. The presence of marked Hopkinson peaks in many samples indicates that their magnetic phases are dominated by single-domain (SD) and/or pseudo-single-domain (PSD) magnetic grains (e.g., Dunlop and Ozdemir, 1997; Deng et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2001\u003c/span\u003e), which is further supported by the hysteresis Hcr/Hc and Mrs/Ms ratios. The Hopkinson peaks are generally broad, suggesting the presence of magnetite particles possessing a range of grain sizes, probably due to pedogenesis.\u003c/p\u003e \u003cp\u003eWithin the non-reversible thermomagnetic curve category, three general groups of behaviour types were identified in the magnetic measurements. The first group involves samples which show the presence of a single phase of Ti-poor titanomagnetite, with Curie temperatures ranging from 540\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\:^\\circ\\:C\\)\u003c/span\u003e\u003c/span\u003e to 580 \u0026deg;C. Pure, Ti-free magnetite has a Curie temperature close to 580\u0026deg;C, while the content of Ti in titanomagnetites (Fe3\u0026thinsp;\u0026minus;\u0026thinsp;xTixO4, 0\u0026thinsp;\u0026le;\u0026thinsp;x\u0026thinsp;\u0026le;\u0026thinsp;1, represented as TM0\u0026ndash;TM100) decreases the Curie temperature (Dunlop and \u0026Ouml;zdemir, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Kiss et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Zhao et al., \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Lu et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). The second group B has lower Curie temperatures (300 \u0026deg;C), typical of titanium-rich titanomagnetite (such as oxidized TM60) or low-temperature oxidized titano-maghemites (Dunlop and Ozdemir, 1997). While the third group samples are characterized by the presence of two Curie temperatures, indicating the existence of multiple magnetic phases. The thermomagnetic curves show a magnetic phase with a Curie temperature in the range of 330 \u0026deg;C -380 \u0026deg;C upon heating, which is most likely titano-maghemite. A second-high Curie temperature phase is observed at 550 \u0026deg;C -580 \u0026deg;C. The large difference between the heating and cooling temperatures suggests that the main magnetic mineral is likely to be a low-temperature oxidized titanomagnetite. The low temperature phase is thought to be maghemite or Ti-rich magnetite (such as TM60), while the high temperature phase is probably Ti-poor magnetite or magnetite-rich titanomagnetite (as described in Dunlop and Ozdemir, 1997, Zhao et al., 2008).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e5.2. Magnetic Hysteresis Curves and Parameters\u003c/h2\u003e \u003cp\u003eThe hysteresis and thermomagnetic curves both suggest that the main contribution comes from magnetite-like phases (as reported in Dunlop, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1986\u003c/span\u003e; King and Ranganai, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Peters and Dekkers, \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). The Curie temperature and saturation magnetization are intrinsic properties that are dependent on the chemical composition and crystal structure, while both hysteresis and remanence are highly influenced by grain size (as discussed in Day et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1977\u003c/span\u003e; Dunlop, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1986\u003c/span\u003e; Hunt et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Zhao et al., \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). The coercive force (Hc) was found to range from 8 mT to 13.5 mT and the ratio of saturation remanence (Mr) to saturation magnetization (Ms) ranged from 0.1518 to 0.2247, which suggests that single-domain (SD) sized magnetite is likely the dominant magnetic carrier mineral (as stated in Day et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1977\u003c/span\u003e; Dunlop et al., 2002a,b). The results of a few samples also confirmed the presence of pseudo-single-domain and multidomain magnetite grains as the values of Mr/Ms ratio and coercive force (Hc) corresponded well with the values observed by Thompson and Oldfield (\u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e1986\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e5.3 Magnetic Measurements and Mineralogical Signatures\u003c/h2\u003e \u003cp\u003eThe magnetic measurements and thermomagnetic curves in this study provided valuable insights into the mineralogical composition of the subsurface materials. The observed room temperature magnetic susceptibility (χ) levels indicated the initial presence of magnetic minerals in the samples. The significant drop in susceptibility at the Curie temperature of 585\u0026deg;C during heating strongly suggested the dominance of magnetite, a common magnetic mineral. However, the subsequent formation of a different mineral with a Neel temperature of 300\u0026deg;C upon cooling pointed to the potential presence of ilmenite or titanium-rich titanomagnetite. Notably, the curves' behaviour during heating and cooling indicated complex mineral alteration processes. In the case of another set of samples, the progressive increase in susceptibility with temperature up to 500\u0026deg;C suggested the contribution of single domain (SD) and pseudo-single domain (PSD) magnetite particles, possibly due to the unblocking of fine-grained ferromagnetic particles.\u003c/p\u003e \u003c/div\u003e"},{"header":"6. Conclusions","content":"\u003cp\u003eThe major geological terrains in the study area exhibit varying soil magnetic signatures due to the different rocks that span significant geological time periods. The Curie temperature determinations performed in a nitrogen atmosphere showed that the magnetic minerals have variable particle sizes, primarily falling within the pseudo-single-domain and multi-domain range (0.2\u0026ndash;14 \u0026micro;m), which implies inheritance from parent rocks or materials and weak weathering. Thermomagnetic curves and room temperature hysteresis suggest that the main contribution comes from magnetite-like phases.\u003c/p\u003e \u003cp\u003eThe non-reversible temperature dependence of the low-field susceptibility exhibits a Hopkinson peak at around 550\u0026deg;C and is characterized by a significant decrease near 580\u0026deg;C, with susceptibility nearly zero at about 650\u0026deg;C. A small bulge or hump is observed around 300\u0026deg;C on the heating curve, indicating the presence of low-coercivity ferromagnetic minerals, primarily magnetite, with possible traces of iron sulfides. These thermal properties of soil collected from Ramatlabama to Ramokgwebana, which goes through the Kaapvaal Craton, Limpopo belt and Zimbabwe Craton could be used to differentiate the boundaries of the cratons and the orogenic belt. On the other hand, airborne magnetic data covering the study area are being interpreted with particular reference to some of the measured magnetic properties and determined soil characteristics thereby increasing confidence in the geological interpretation.\u003c/p\u003e \u003cp\u003eThis study demonstrated the potential for linking magnetic property variations with lithogenic, pedogenic, and anthropogenic factors and the type of magnetic minerals in terms of grain size, domain structure, and magnetic parameters. It opens up opportunities for long-term multidisciplinary research on various rock and soil environmental characteristics, providing a fundamental understanding of soil physical properties and processes, and addressing practical problems related to exploration, the environment, hydrology, land use, and agriculture. Mass specific susceptibility measurements of samples in powder form lead to unbiased volume and density calculations of soil samples. Furthermore, it is recommended that X-ray diffraction (XRD) and scanning electron microscopy (SEM) supportive studies could be performed, as well as Mossbauer effect investigation to provide another means of identifying sources of magnetic parameters. Vertical sampling in trenches where possible to check lithogenic and anthropogenic contributions to the magnetic susceptibility is another possibility.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eAuthor statementMM: Data curation, Writing-original draft, analysis and interpretation of data, and editing. RTR: Data curation Methodology, Conceptualization, Reviewing and Editing, Final approval. JK: Data curation, Methodology, Conceptualization, Reviewing and Editing\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eMany students were involved in the project over the years and their efforts and contributions are greatly appreciated. We thank the reviewers and the editor for constructive comments that improved the paper.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eA-Cap Resources, 2015. Letlhakane Uranium Project- Botswana. 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The Kalahari Drilling Project: A Report on the Geophysical and Geological Results of Follow-up Drilling to the Aeromagnetic Survey of Botswana. \u003cem\u003eGeological Survey Department Bulletin\u003c/em\u003e 27, 224pp. \u003c/li\u003e\n\u003cli\u003eMello, D.C.d., Veloso, G.C., Moquedace, C.M., Oliveira, I.d.A., Oliveira, F.S.d., Gomes, L.C., Souza, J.J.L.L.d., Francelino, M.R., Fernandes-Filho, E.I., Schaefer, C.E.G.R., and \u003c/li\u003e\n\u003cli\u003eDematte, J.A.M., 2023. Radiometric and magnetic susceptibility characterization of soil profiles: Geophysical data and their relationship with Antarctic periglacial processes, pedogenesis, and lithology. Catena, 232, https://doi.org/10.1016/j.catena.2023.107427 \u003c/li\u003e\n\u003cli\u003eMaier, G., Scholger, R., and Schon, J., 2006. The influence of soil moisture on magnetic susceptibility measurements. 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Unraveling the magnetic carriers of igneous cores from the Atlantic, Pacific, and the southern Indian oceans with rock magnetic characterization. Physics of the Earth and Planetary Interiors, 156, 294\u0026ndash;328. \u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"magnetic hysteresis, Curie temperature, magnetic mineralogy, ferromagnetic minerals, Botswana ","lastPublishedDoi":"10.21203/rs.3.rs-5608272/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5608272/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSoil magnetic properties are important for many applications including palaeomagnetism and (plate) tectonics, geological interpretation, geoexploration, mining, forensics, geotechnical engineering, and industry. The results of this work are a continuation of previous work on magnetic susceptibilities of soils in eastern Botswana. The magnetic properties measured include Curie temperature (Tc) determined in nitrogen gas atmosphere and hysteresis parameters (Hc, Hcr, Mr, Ms). Thermomagnetic curves were measured from room temperature up to 700 \u003csup\u003eo\u003c/sup\u003eC, enabling determination of Curie temperature which has supported the classification of the magnetic granulometry and mineralogy.\u003c/p\u003e\n\u003cp\u003eThe thermomagnetic curves revealed the existence of magnetic minerals such as hematite, magnetite and pyrrhotite, and their Curie temperatures were found to be 600 °C, 580 °C, and 200 °C, respectively. The presence of magnetic materials is also revealed by the hysteresis loops of some soil samples. The hysteresis and thermomagnetic curves both indicate that the main contribution comes from magnetite-like phases. Thermomagnetic curves of soil samples collected along the Ramatlabama to Ramokgwebana, which traverses through the Kaapvaal Craton, Limpopo belt and Zimbabwe Craton could be used to differentiate the boundaries of the cratons and the orogenic belt. Airborne magnetic data covering the study area are being interpreted with particular reference to some of the measured magnetic properties and determined soil characteristics thereby increasing confidence in the geological interpretation.\u003c/p\u003e","manuscriptTitle":"Hysteresis and Thermomagnetic Characteristics of Soils and Rocks in Eastern Botswana","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-01-23 05:59:26","doi":"10.21203/rs.3.rs-5608272/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"a7cb8163-6379-40e9-aecf-a3a536805b30","owner":[],"postedDate":"January 23rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-01-23T05:59:26+00:00","versionOfRecord":[],"versionCreatedAt":"2025-01-23 05:59:26","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5608272","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5608272","identity":"rs-5608272","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}