Spherulitic orthopyroxene–bearing metaperidotite in Ray-Iz ophiolites (Polar Urals): high-pressure deserpentinization in a mantle wedge

Spherulitic orthopyroxene–bearing metaperidotite in Ray-Iz ophiolites (Polar Urals): high-pressure deserpentinization in a mantle wedge | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Spherulitic orthopyroxene–bearing metaperidotite in Ray-Iz ophiolites (Polar Urals): high-pressure deserpentinization in a mantle wedge Satoko Ishimaru, Rie Sonoda, Makoto Miura, Vladimir R. Shmelev, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4210973/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 11 Apr, 2026 Read the published version in Mineralogy and Petrology → Version 1 posted 4 You are reading this latest preprint version Abstract The Ray-Iz (or Rai-Iz) peridotite massif, the main part of the Paleozoic Ray-Iz ophiolite in the Polar Urals, comprises harzburgite, lherzolite, and dunite lenses, which contain abundant chromitite pods. The metaperidotite zone occurs as the ENE-WSW linear zone (width: ~4 km) at the center of the Ray-Iz peridotite massif. We sampled peculiar orthopyroxene-rich peridotites from the metaperidotite zone that contain spherulitic aggregates of orthopyroxene. This investigation provided insights into the key petrologic nature of the Ray-Iz peridotite massif, particularly its P-T history. The orthopyroxene-rich rocks and their surrounding metaperidotites contain various metamorphic minerals, such as tremolite, chlorite, serpentines (lizardite and chrysotile), and talc. Furthermore, the olivine and orthopyroxene grains in these lithologies contain minute inclusions of opaque spinels (ferritchromite and Cr-rich magnetite) and metamorphic minerals (tremolite, chlorite, serpentine minerals, etc.). The spinel occurring both as discrete grains and inclusions in the orthopyroxene-rich rocks exhibits low Al and high Fe 3+ concentrations, and the contents of Ca, Al, and Cr in orthopyroxene are extremely low. The presence of high-Fe 3+ spinel inclusions in olivine and orthopyroxene, combined with the presence of spherulitic aggregates and the extremely low Ca and Al concentrations in orthopyroxene, indicates deserpentinization as the origin of the metaperidotites, including the orthopyroxene-rich rocks. Contact metamorphism appears to be unbefitting as the cause of the deserpentinization because of the lack of a heat source close to the Ray-Iz massif. Instead, we propose that a hydration–dehydration path resulted from the subduction and exhumation of the forearc mantle wedge peridotite. opaque inclusion ferritchromite deserpentinization metaperidotite Ray-Iz ophiolite Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction The Ray-Iz (or Rai-Iz) ophiolite comprises mainly peridotites, which is considered a residual mantle formed by multistage partial melting under different tectonic settings (beneath a mid-ocean ridge and a suprasubduction zone), based on the mineral chemistry of the residual peridotites (Shmelev 2011; Shmelev et al. 2018, 2019). The Ray-Iz peridotite massif underwent metasomatism (via an interaction with an arc-type melt or a hydrous fluid) (Glodny et al. 2003; Ishimaru et al. 2015; Meng et al. 2018) and retrograde metamorphism during exhumation (Shmelev 2011). The Ray-Iz ophiolite has received significant attention, attributed to the occurrence of diamond and other unusual minerals in the podiform chromitites residing in peridotites (Yang et al. 2015). The peridotites in the Ray-Iz ophiolite locally contain abundant metamorphic minerals, such as tremolite, talc, and chlorite, as well as minute inclusions of opaque minerals (ferritchromite, Cr-rich magnetite, and various sulfides) in olivine and orthopyroxene (OPX) grains. The presence of these inclusions indicates that the peridotites were formed through serpentinization and deserpentinization of primary peridotites (Arai 1975; Moody 1976). As shown in Fig. 1a, spherulitic OPX-bearing harzburgite occurs in the “metaperidotite zone” (within the center of the Ray-Iz peridotite massif; Shmelev et al. 2014). Notably, the occurrence of spherulitic OPX in metaperidotites has been reported elsewhere (Arai, 1974, 1975; Evans and Trommsdorff 1974; Frost 1975). The Ray-Iz spherulitic OPX-bearing metaperidotite has been described as a sagvandite (a magnesite-bearing orthopyroxenite), which implies that a CO 2 -rich fluid was involved in its formation (Shmelev et al. 2014). However, we did not observe any enrichment in carbonate minerals or the presence of a heat source close to the spherulitic OPX-bearing harzburgite block (Fig. 1a). We present detailed petrological descriptions of several peridotite samples, which are employed to discern the P-T history of the Ray-Iz mantle slice. The OPX-bearing harzburgite block records various metamorphic events related to the subduction/obduction of the Ray-Iz peridotite massif. Geological background The Ural Mountains represent a linear mid-Paleozoic orogenic zone that was formed during the closure of an ocean basin–island arc system between the European Plate to the west and the Siberian Plate to the east (Savelieva and Nesbitt 1996; Brown et al. 2006). U–Pb dating of zircon in an eclogite from the Marun–Keu complex in the Polar Urals indicates that the collision occurred at 360–355 Ma (Glodny et al. 2004). The Ray-Iz ultramafic massif is located in a long ophiolitic belt in the Polar Urals and is associated with the Voikar and Syum-keu ultramafic massifs. Together, these three massifs are interpreted to represent a single large-scale ophiolite (Bogatsky et al. 1996) that was thrust over the Paleozoic sedimentary rocks of the East European Platform (Shmelev et al. 2014). Sm–Nd dating indicates that partial melting and the formation of residual mantle peridotite occurred at 387 ± 34 Ma (Sharma et al. 1995). The ophiolite massifs are accompanied by high-pressure metamorphic rocks, including eclogites, blueschists, amphibolites, and albite–lawsonite-facies rocks (Yang et al. 2015). The Ray-Iz ultramafic massif comprises mainly lherzolite–harzburgite, dunite–harzburgite, and metaperidotites (Fig. 1a), and the residual features indicate that they were formed by at least two partial melting events: the former occurring in a mid-ocean ridge setting and the latter in a suprasubduction zone setting (Shmelev 2011; Shmelev et al. 2014, 2019). The Moho transition zone is located at the southern end of the Ray-Iz massif, and the peridotites are in tectonic contact with a hornblende gabbro-diorite–plagiogranodiorite complex (Sob complex) to the south (Fig. 1a) (Shmelev and Meng 2013; Shmelev et al. 2014). U–Pb dating of zircon obtained from the gabbroic rocks yielded a crystallization age of 418 ± 2 Ma (Shmelev and Meng 2013). The Ray-Iz peridotite massif underwent polyphase metamorphism and deformation, and the peridotite shows signs of undergoing variable serpentinization (Meng et al. 2018). Corundum-bearing felsic veins, formed through interactions between slab-derived fluid/melt and peridotite (Glodny et al. 2003; Ishimaru et al. 2015), occur in the central dunite zone (Fig. 1a). The 40 Ar/ 39 Ar dating of phlogopite and the U–Pb dating of zircon indicate that these veins were formed at ~380 Ma (Meng et al. 2018), which is similar to a Rb/Sr isochron age (373.1 ± 5.4 Ma) (Glodny et al. 2003). The metaperidotite zone at the center of the Ray-Iz ophiolite (Fig. 1a) comprises mainly harzburgitic metaperidotites that commonly contain large proportions of chlorite and amphibole (Figs. 2a, b). Within the zone lies a block of spherulitic OPX-bearing harzburgite (Figs. 2a, b) that was interpreted to have been produced during the infiltration of a CO 2 -rich fluid (Shmelev et al. 2014). We observed large spherulitic OPX aggregates (diameter: <10 cm) in a small block (20 × 20 × 10 m) of OPX-rich harzburgite located at N66°53.547¢, E65°11.821¢. A small amphibolite block was enclosed in the metaperidotite at the northern end of our sampling points (RY26-08 in Fig. 1b). Sample descriptions We collected eight samples from the metaperidotite zone: three from the OPX-rich harzburgite (RY26-01, RY26-03, and RY26-04), four from the surrounding metaperidotite (chlorite- and talc-bearing harzburgite/lherzolite; RY26-02, RY26-05, RY26-06, RY26-07, RY26-10), and one from an associated garnet amphibolite block (RY26-08) (Fig. 1b). Hereinafter, we refer to the metaperidotite containing spherulitic OPX and the harzburgite from the metaperidotite zone as OPX-rich rock and meta-harzburgite, respectively (cf. Figs. 2 and 3). In addition, we collected samples of harzburgite and dunite from the main body of the Ray-Iz peridotite massif for comparison and conducted a major-element mineral analysis of one harzburgite (RY14-02). The OPX-rich rocks comprise typical peridotite minerals (olivine, OPX, and spinel) and hydrous minerals (chlorite and talc ± amphibole). Only one sample (RY26-04) contains carbonate (≤5%) in addition. Although the modal amount of serpentine in the OPX-rich samples is typically small, one OPX-rich sample (RY26-01) contains a large amount of serpentine as veinlets that cross-cut other minerals, as well as locally serpentinized spherulitic OPX aggregates (Figs. 3a, 4a,b). We observed two OPX morphologies: elongated prisms and rounded grains displaying wavy extinction and kink bands (Fig. 3b). Abundant opaque grains occur as inclusions in the relict olivine and OPX grains (Figs. 4c,d). Amphibole is heterogeneously distributed in the OPX-rich rocks, except for sample RY26-01. The amphibole grains show a prismatic or euhedral shape and were locally replaced by carbonate and talc (Fig. 4e). Carbonate is associated with chlorite and amphibole, and it locally replaces OPX porphyroblasts (Fig. 4e). However, no clear replacement relationships with other minerals were observed. Chlorite forms lath and tabular grains and occurs either as inclusions in olivine and OPX or as discrete aggregates (Figs. 4c,e). There is no obvious association with spinels (ferritchromite and Cr-bearing magnetite) (Fig. 4c), except for sample RY26-01. The chlorite grains in sample RY26-01 are abundant and always enclose magnetite in a serpentine matrix. Talc is closely associated with OPX and olivine. The minute inclusions in olivine and OPX were identified as talc and minor low-temperature serpentine. No fluid phase was observed as inclusions in minerals. The surrounding meta-harzburgite displays the same mineral assemblage as the OPX-rich rock (Fig. 3). One sample (RY26-05) additionally contains carbonate (<5 vol.%), which does not exhibit a noticeable replacement texture. However, some olivine, OPX, and amphibole grains were locally replaced by carbonate (Fig. 4f). Serpentine minerals are rare in meta-harzburgites and occur only as veinlets or along olivine grain boundaries with minute concentrations of magnetite grains. Minute opaque inclusions occur in olivine and OPX grains. The petrographic characteristics of the meta-harzburgite (e.g., replacement textures) are similar to those of OPX-rich rocks. The garnet amphibolite associated with the OPX-rich rock is composed of garnet, amphibole, quartz, and plagioclase, with minor titanite, ilmenite, rutile, zircon, epidote, and apatite. Garnet grains appear pale pink under plane-polarized light and contain abundant inclusions of ilmenite, rutile, and titanite. Amphibole displays strong green–yellowish brown pleochroism. The chlorite- and amphibole-bearing harzburgites are the products of polyphase metamorphism (Meng et al. 2018) and are common within and outside of the metaperidotite zone (e.g., close to the Central Chromite Deposit (CCD), where diamond has been observed; Fig. 1b) (Shmelev et al. 2014; Yang et al. 2015). The olivine and OPX grains in the OPX-rich rocks and metaperidotites contain inclusions of opaque minerals (Figs. 4c,d). The harzburgite samples collected from areas close to the CCD are moderately to highly serpentinized and contain abundant chlorite and minor amphibole and talc. Chlorite occurs as dark-green grains displaying weak-to-strong pleochroism and non-pleochroic grains associated with chromian spinel. Chromian spinel exhibits anhedral to subhedral features, and it contains inclusions of chlorite laths. Amphibole grains are elongated, and talc is typically observed surrounding OPX grains. Analytical methods The whole-rock chemical compositions were determined by inductively coupled plasma–optical emission spectrometry (ICP–OES) and inductively coupled–mass spectrometry (ICP–MS) at Activation Laboratories Ltd., (Actlabs), Ancaster, Canada. The samples were also prepared (crushing and pulverization) at Actlabs. The data quality was determined using the analytical results of international standard samples (NIST 694, DNC-1, BIR-1a), and the relative standard deviations were mostly <5% and <10% for major and trace elements, respectively. The major-element mineral compositions were measured in harzburgite, meta-harzburgite, and OPX-rich rock samples using a field emission-scanning electron microscopy (FE-SEM, JEOL JSM-7001F) system equipped with an energy-dispersive X-ray spectrometry (EDS) system (INCA Energy or AZtecEnergy, Oxford Instruments) at Kumamoto University, Kumamoto, Japan. We employed an accelerating voltage of 15 kV, a probe current of 1 nA, and a probe diameter of <1 µm. The analytical precision during the EDS measurement of the NiO concentration in olivine was generally less than ±0.1 wt.% (Fig. 3a). We define Mg# as the atomic ratios of Mg/(Mg + total Fe) and Mg/(Mg + Fe 2+ ) for silicate minerals and spinels, respectively. The concentrations of ferric and ferrous iron in the spinel minerals within peridotitic rocks and in the garnet and epidote minerals within garnet amphibolite were calculated based on their stoichiometry. Parameter Y Fe represents the ratio of Fe 3+ relative to the trivalent cations (Cr, Al, and Fe 3+ ) in the spinels. Results Whole-rock compositions We obtained whole-rock chemical compositions for six samples: two OPX-rich rocks, two meta-harzburgite samples, one meta-lherzolite, and one garnet amphibolite. All peridotitic samples are plotted in the distribution of abyssal peridotite (Fig. 5a), and there is no noticeable relationship between the whole-rock Al 2 O 3 /SiO 2 ratio and their petrographical features, i.e., the presence of carbonate, abundance of OPX and/or amphibole, and calculated normative OPX amount (Table 1; Fig. 5a). The slightly low whole-rock MgO/SiO 2 ratio in one carbonate-bearing sample, RY26-05, is consistent with the calculated high normative OPX amount (42%) (Table 1; Figs. 3, 5a). The whole-rock rare earth element (REE) contents of the studied samples are within the 0.1–1.0 range, relative to the chondrite values (McDonough and Sun 1995), and the shape of the REE patterns is concave upward, with depletion at Eu (Fig. 5b). The pattern is almost the same as that reported in Shmelev et al. (2019) (Fig. 5b). Mineral compositions Olivine generally displays similar compositions of harzburgite (Fo = 90.7–91.3; NiO = 0.25–0.53 wt.%) and meta-harzburgite (Fo = 90.8–91.4; NiO = 0.27–0.62 wt.%) (Fig. 6a). The only exception is sample RY26-05, which contains carbonate minerals (<5 vol.%) and yields higher NiO concentrations (0.50–0.90 wt.%) and similar Fo values (90.6–91.6) (Table 2; Fig. 6a) compared with the values for the harzburgite. The olivine in the OPX-rich rocks exhibits variable NiO concentrations (0.28–0.62 wt.%) and Fo values (90.3–91.8; Fig. 6a; Table 2). Sample RY26-04, which contains carbonate and abundant magnetite (Fig. 4c), exhibits high Fo values (95.3–95.9) and NiO concentrations of 0.35–0.64 wt.% (Fig. 6a; Table 2). The measured NiO concentrations in olivine in our OPX-rich samples and the carbonate-rich meta-harzburgite sample (RY26-05) are higher than those reported previously for metaperidotites (Shmelev 2011). The spinel in the OPX-rich rocks has unusual compositions, characterized by low Mg# values (0–0.26) and very low Al 2 O 3 concentrations (<2.5 wt.%), and the constituent minerals are classified as chromite, magnetite, and ferritchromite (Fig. 6b) (Table 2). Most grains lack compositional zoning; however, some contain chromite or ferritchromite cores surrounded by magnetite rims, i.e., the Cr concentration decreases outward from the grain cores. No clear relationship is observed between the degree of chemical homogeneity and the spinel grain size. Spinel-group minerals contain up to 2 wt.% TiO 2 , with Y Fe values of <0.7. Their MnO concentrations are typically <0.8 wt.% and are weakly negatively correlated with the Y Fe values (Fig. 6c). The only spinel-group mineral in the carbonate-bearing OPX-rich sample (RY26-04) is magnetite, which contains minor Cr (Figs. 6c–d) and abundant NiO (typically >1.0 wt.%) (Fig. 3d) (Table 2). The spinel-group minerals in the meta-harzburgite range from chromite to magnetite with varying compositions (Fig. 6b), and the composition is unrelated to the presence or absence of carbonate minerals (Figs. 6b–d) (Table 2). The spinel-group mineral in the harzburgite sample (RY14-02) is ferritchromite, and it exhibits slightly lower Y Fe values than the meta-harzburgite and OPX-rich rocks (Table 2). ZnO was rarely detected because of its low concentration (typically <0.5 wt.%). No difference in composition was observed between the inclusions and discrete grains of spinel (Table 2). The OPX in all the analyzed samples exhibited extremely low Al 2 O 3 , Cr 2 O 3 , and CaO concentrations (<0.15, <0.17, and <0.18 wt.%, respectively) and a high Mg# value (0.91–0.92) (Figs. 6e, f) (Table 2). In the OPX-rich rocks, the Al 2 O 3 , Cr 2 O 3 , and CaO concentrations are generally below the detection limit or slightly lower than those in the harzburgites (Figs. 6e, f; Table 2). The OPX composition does not vary with the grain morphology (i.e., prismatic elongated vs. deformed vs. rounded; data not shown). The chlorite in the OPX-rich rocks and meta-harzburgite exhibit a high Cr 2 O 3 (<2.9 wt.%) concentration and high Mg# (~0.94) value, and it is classified as clinochlore (Hey, 1954). Most of the chlorites in the harzburgite (RY14-02) have similar compositions to those in the OPX-rich rocks and meta-harzburgite (Cr 2 O 3 = 2.2–2.9 wt.%; Mg# = ~0.94). However, the green pleochroic chlorite exhibits lower and more variable Mg# values (0.76–0.88) and Al 2 O 3 (7.0–8.3 wt.%) and Cr 2 O 3 (0.12–1.36 wt.%) concentrations (Table 2). These chlorites low in Al 2 O 3 have a composition that is intermediate between those of typical chlorite and serpentine, and they might feature a mixture of the two minerals. The amphibole in the OPX-rich rocks and meta-harzburgites is classified as tremolite (Mg# = ~0.96; Leake et al. 1997), and it exhibits low contents of TiO 2 (<0.61 wt.%), Al 2 O 3 (<0.61 wt.%), and Na 2 O (0.22–0.59 wt.%). The amphibole in the harzburgite sample (RY14-02) exhibits narrow concentrations of Al 2 O 3 (2.9–4.6 wt.%) and Na 2 O (2.0–2.4 wt.%), as well as low Mg# values (0.93–0.95). In terms of composition, it lies along the boundary between edenite, magnesio-hornblende, and tremolite. The talc in the OPX-rich rocks features high Mg# values (~0.98) and exhibits low NiO concentrations (<0.3 wt.%). The serpentine in the OPX-rich rocks and meta-harzburgites exhibit low concentrations of NiO (<0.50 wt.%) and MnO (<0.35 wt.%), and a narrow range of high Mg# values (0.95–0.96). The serpentine in the harzburgite also exhibits a low NiO concentration (<0.40 wt.%) and a lower Mg# value (0.93) than that in the OPX-rich rocks. The carbonate in the OPX-rich rock (RY26-04) and meta-harzburgite (RY26-05) is mainly classified as magnesite; however, the proportion of siderite varies (~3% or ~2 wt.% of FeO* in the former, and 5%–7% or 3.6–5.3 wt.% of FeO* in the latter) (Table 2). Dolomite also occurs in both samples, along tremolite contacts and as small inclusions in magnesite grains. It exhibits a narrow range of Ca/(Ca + Mg) atomic ratios (0.52–0.54) (Table 2). The garnet in the garnet amphibolite sample is classified as grossular–almandine, and it exhibits low contents of TiO 2 (~0.3 wt.%), MnO (<3.1 wt.%), and MgO (<4.2 wt.%). The MnO content in garnet decreases from the core to the rim (from ~4.0 to ~2.0 wt.%), whereas the MgO content increases (from ~2.5 to ~3.2 wt.%). The plagioclase in the garnet amphibolite sample (RY26-08) exhibits a narrow range of Ca/(Na + Ca) atomic ratios (0.12–0.17), except for one grain that exhibits a ratio of 0.04. The amphibole in the garnet amphibolite sample exhibits a Mg# value of ~0.50 and is classified as pargasite–ferropargasite (Al 2 O 3 = ~13 wt.%; TiO 2 = 0.84–1.16 wt.%). The epidote mineral in the garnet amphibolite sample features low contents of TiO 2 (<0.17 wt.%), and its calculated Al 2 O 3 and Fe 2 O 3 concentrations are ~26.4 wt.% and ~10.0 wt.%, respectively. Discussion Nature of the OPX-rich rocks and metaperidotite in the Ray-Iz massif Petrographic and geochemical data indicate that the Ray-Iz harzburgite and lherzolite represent the residues of high-degree partial melting associated with slab influx in a suprasubduction zone setting (Shmelev 2011; Shmelev et al. 2014, 2018). The Ray-Iz peridotite massif has experienced variable stages of metamorphism under greenschist-facies conditions (Shmelev, 2011; Yang et al. 2015; Meng et al. 2018). Our observations of abundant chlorite and tremolite in the samples from the Ray-Iz ultramafic rocks, including a harzburgite sample from outside of the metaperidotite zone (Fig. 1b), are consistent with the conclusions of the previous study above. Furthermore, we observe minute inclusions of ferritchromite and magnetite in the olivine and OPX grains in the samples from the metaperidotite zone and the surrounding harzburgite, which is typical for deserpentinized peridotitic rocks (Trommsdorff and Evans 1972; Arai 1975; Trommsdorf et al. 1998). The serpentine and magnetite could have formed through the hydration of peridotite via the serpentinization reaction: olivine + H 2 O = Mg-rich serpentine + magnetite + H 2 O (Frost 1985). The large ferritchromite grains in our samples are locally associated with chlorite, interpreted as a metamorphic phase that replaced primary Al- and Cr-rich chromian spinel (Cr/(Cr + Al) atomic ratio = 0.2–0.6) (Shmelev et al. 2014). The high Cr 2 O 3 concentrations in chlorite in the OPX-rich rock samples (<3.4 wt.%; Table 2) indicate that Cr, in addition to Al, was sourced from the primary mantle chromian spinel. There are two possible formation processes for Cr-rich chlorite: the retrogressive hydration of the peridotitic assemblage (olivine, OPX, and spinel) and the interaction between chromite and antigorite during a progressive dehydration process under oxidizing conditions (Merlini et al. 2009). In both processes, spinel is required as a source of Cr and Al for the formation of Cr-rich chlorite; however, there is no close association between our Cr-rich chlorite and spinel-group minerals (ferritchromite and/or magnetite), as described above. This may reflect recrystallization and a complete change of the side-by-side mineral association. Whether the recrystallization is retrogressive or progressive, the reaction of chromian spinel with ferritchromite occurs effectively during metamorphism under amphibolite facies conditions (Barnes 2000; Gervilla et al. 2012). Furthermore, the moderate Cr/(Cr + Fe 3+ ) atomic ratio (0.28–0.72) measured in ferritchromite in our samples (Table 1; Figs. 6b–d) aligns with the equilibrium temperatures estimated via thermodynamic calculations (~600°C) (Sack and Ghiorso 1991). The presence of an almost homogeneous Cr/(Cr + Fe 3+ ) atomic ratio within a grain reflects the severe modification of primary chromian spinel to ferritchromite under amphibolite facies condition for extended periods. The inclusions of magnetite/ferritchromite in olivine and OPX were likely formed through the dehydration (prograde metamorphism) of hydrated peridotites, a process known as deserpentinization (Arai 1975). Generally, olivine produced through deserpentinization exhibits higher Mg# values than primary olivine (Arai 1975). However, irrespective of the presence/absence of magnetite and ferritchromite as inclusions in the olivine grains, olivine exhibits almost constant compositions within the sample. The observed small variation in the olivine Mg# value indicates that only a small proportion of magnetite was formed during serpentinization, as supported by the scarcity of magnetite and ferritchromite in most samples (Fig. 3). In contrast, the carbonate-bearing OPX-rich sample (RY26-04) exhibits a high Mg# value and contains abundant magnetite and ferritchromite (Fig. 3), indicating that it was formed from a relatively magnetite-rich serpentinite. In addition to the compositions of spinel, the extremely low concentrations of Al, Cr, and Ca in OPX (Figs. 3e, f) are characteristic of secondary OPX formed via deserpentinization. Such OPX minerals occasionally show a radially aggregated texture (Arai 1974, 1975; Arai and Kida 2000; Padrón-Navarta et al. 2011). We emphasize that the features of deserpentinization (i.e., magnetite inclusions in olivine and OPX, and the low diopside and tschermakite contents in OPX) are also observed in the harzburgite obtained outside the metaperidotite zone (Fig. 1b). However, the OPX in most Ray-Iz peridotites (lherzolite and harzburgite) exhibit typical mantle peridotite compositions (i.e., moderate Ca, Al, and Cr concentrations; Figs. 6e, f). Therefore, we infer that the entire Ray-Iz peridotite massif experienced retrograde recrystallization (from the greenschist facies to the amphibolite facies) and prograde recrystallization (or metamorphism). However, the degree of serpentinization varies, possibly depending on the availability of fluid (water) and the position (e.g., depth) within the lithosphere. The variations in the composition of tremolites in the OPX-rich rock and in other parts of the Ray-Iz peridotite indicate that the equilibrium temperature for the former was lower than that for the latter. The Al 2 O 3 content in the former is almost negligible (<0.50 wt.%); however, those in the latter occasionally exceeds 4.0 wt.%, and the material is classified as edenite (Leake et al. 1997). The sections of the massif that were unaffected by hydration (serpentinization) do not record the dehydration (deserpentinization) event. A chlorite-bearing harzburgite from Cerro del Almirez, Spain has been determined to be a product of the high-pressure dehydration of antigorite serpentinite (Padón-Navarta et al. 2011). The abundant OPX in our OPX-rich rocks indicates that the protolith was a silica-enriched rock before dehydration. The silica enrichment likely occurred during serpentinization (Padrón-Navarta et al. 2011) or during an earlier mantle event (metasomatism or magmatism) (Kelemen et al. 1998; Ishimaru et al. 2007). In fact, some of our samples, RY26-01 (OPX-rich rock) and RY26-05 (meta-harzburgite with carbonate), exhibit slightly different whole-rock compositions: the former and the latter show a higher Al 2 O 3 /SiO 2 ratio and a lower MgO/SiO 2 ratio relative to reported Ray-Iz peridotitic rocks (Shmelev 2011), respectively (Fig. 5a). Sample RY26-01, exhibiting a high Al 2 O 3 /SiO 2 ratio and low Mg# value, is rich in SiO 2 (46.4 wt.% on anhydrous basis) in addition to Al 2 O 3 (2.7 wt.% on anhydrous basis). This is interpreted to be due to the addition of Al 2 O 3 and SiO 2 during a secondary event, as described above, which explains why it does not exhibit the primary fertile character of the protolith due to the low-CaO content (<0.1 wt.% on anhydrous basis). The patchy distribution of carbonate minerals might reveal a variable (and locally high) CO 2 /(H 2 O + CO 2 ) ratio within the metaperidotite zone formed during the last serpentinization (= exhumation stage), based on the presence of magnesite at the expense of the radial aggregate of OPX and tremolite (Fig. 4f). The close association of talc with carbonate and OPX and/or the tremolite assemblages indicates that the interaction between the CO 2 -rich fluid and OPX and/or tremolite and the release of SiO 2 occurred after the formation of the secondary OPX. The olivine in the carbonate-bearing meta-harzburgite (RY26-05) exhibits remarkably high Ni concentrations (Fig. 6a), which cannot be ascribed to the formation of low-Ni metamorphic minerals (OPX and carbonate) because these minerals occur in much lower proportions than olivine (Fig. 3c). The carbonate-bearing OPX-rich rock (RY26-04) contains abundant Ni-rich magnetite (>1 wt.%; Figs 3b and 4c), which represents a major Ni sink in the sample. The olivines in these two samples are enriched with Ni relative to typical mantle peridotites (olivine NiO concentration = 0.3–0.4 wt.%) (Takahashi 1986; Herzberg et al. 2013). However, the whole-rock Ni contents in these two samples are moderate (2100–2300 ppm; Table 1). Therefore, the olivine in these two samples might have been enriched in Ni, possibly through the formation of OPX-rich rock via metasomatic interactions between Si-rich fluids/melts and peridotite under a suprasubduction zone (stage Ⅱ of Shmelev et al. 2019). This is consistent with the high amount of calculated normative OPX (42%) (Table 1; Fig. 3). P–T history of the Ray-Iz peridotite massif Previous researchers concluded that the residual peridotite of the Ray-Iz peridotite massif was formed by multistage partial melting under different tectonic settings (beneath a mid-ocean ridge and a suprasubduction zone) (Shmelev 2011; Shmelev et al. 2014, 2018). It is considered that the protolith of OPX-rich rocks in this study was the metasomatized OPX-rich peridotite with/without hornblende, and was formed under a suprasubduction zone setting (stage Ⅱ of Shmelev et al. 2019). As discussed above, the entire Ray-Iz peridotite massif underwent retrogressive recrystallization with/without hydration and prograde recrystallization (deserpentinization). The hydration and recrystallization occurred within the suprasubduction zone (Shmelev et al. 2019). The decomposition of serpentine requires increased temperature (i.e., contact metamorphism) or pressure (i.e., regional metamorphism via subduction). The Sob gabbro-diorite complex to the south of the study area might represent a heat source; however, the Sob complex and Ray-Iz peridotite massif are separated by a tectonic contact (Fig. 1b) (Shmelev and Meng 2013; Shmelev et al. 2014). Further, the lack of a notable mineral-reaction isograd parallel to the boundary of the Sob gabbro-diorite (Arai 1975) is inconsistent with the idea that the Sob complex represents the heat source for deserpentinization. Therefore, we infer that the compression of the forearc mantle during subduction with the downgoing slab into the deep mantle wedge was responsible for the deserpentinization of the Ray-Iz peridotites. Serpentinization stage After the formation of the Ray-Iz residual peridotites and OPX-rich metasomatized peridotite, the massif was settled at a low temperature (~600°C) part within a mantle wedge, after which it was partly serpentinized. During this stage, the primary chromian spinel reacted with an H 2 O-rich fluid together with olivine and OPX to form Al- and Cr-rich chlorites via the following reaction: olivine + OPX + spinel + fluid (H 2 O) = chlorite + ferritchromite. Chlorite is not always observed around coarse ferritchromite/magnetite, and its formation is possibly due to later dehydration (deserpentinization) and recrystallization. Clinopyroxene, where present, appears to be converted into tremolite; however, this is inconsistent with the lack of clinopyroxene and/or its pseudomorphs within the OPX-rich rocks and meta-harzburgite. The pressure condition during serpentinization is unclear considering the later low-temperature modification; however, the presence of abundant magnetite/ferritchromite suggests the occurrence of metamorphism in the amphibolite-eclogite facies (1–2 GPa) (Gervilla et al. 2012). The chemical homogeneity of ferritchromite grains (i.e., the lack of core–rim zoning) indicates a long residence time at a low temperature (~600°C). Deserpentinization stage The deserpentinization of the Ray-Iz peridotites required an increase in pressure via the subduction of the serpentinized mantle with the slab into the deep mantle. The multiphase inclusions of antigorite and talc in the OPX grains indicate that the serpentinized Ray-Iz massif was subducted at a relatively high pressure (>1.6 GPa) and a low temperature (650°C; Padrón-Navarta et al. 2010). The composition of ferritchromite and the lack of aluminous green spinel constrain the deserpentinization temperature to below the chlorite decomposition value in the curve in Fig. 7 (~700°C). The chlorite in our samples is possibly stable at considerably high temperatures (~900°C) because of its Cr-rich nature (up to 3.4 wt.% of Cr 2 O 3 ) (Fumagalli et al. 2014). However, the almost negligible contents of Al 2 O 3 , Cr 2 O 3 and CaO(< 0.2 wt.%) in the coexisting (recrystallized) OPX mineral prevent the realization of such a high temperature. This is consistent with the presence of abundant euhedral to radially aggregated tremolite grains in our samples (Figs. 4e, 7). Further, no geobarometer was available to constrain the maximum pressure during serpentine decomposition, although the Ray-Iz peridotite massif might have undergone subduction at ultrahigh pressures (deeper than the graphite–diamond transition; Fig. 7), as indicated by the abundant diamond in the Ray-Iz chromitite (Yang et al. 2015). Exhumation stage Following the formation of deserpentinized secondary peridotites, the Ray-Iz massif was serpentinized again during exhumation to produce the present mineral assemblage (olivine, OPX, chlorite, tremolite, talc, serpentine, and magnetite/ferritchromite). Chlorite and magnetite/ferritchromite appear to have been stable throughout, from the first serpentinization stage to the exhumation stage (points 2 and 3 in Fig. 7). Our conclusion is consistent with the previous estimation, which is that near-isochemical serpentinization and deserpentinization events (Shmelev 2011) occurred, preceded by metasomatism. The mineral chemistry and assemblage of the garnet amphibolite (N-MORB composition) (Shmelev et al. 2014) indicate the pressure–temperature conditions of ~1.5 GPa and ~700°C (Ernst and Liu 1998). Therefore, we infer that the garnet amphibolite was entrained in the Ray-Iz peridotite during exhumation. Conclusion The Ray-Iz rocks and meta-harzburgite contain olivine and OPX with abundant inclusions of opaque minerals (magnetite and ferritchromite), unrelated to hydrous minerals (e.g., serpentine, chlorite, tremolite, and talc). However, hydrous minerals do occur ubiquitously as discrete grains within the rock. The relationship between magnetite/ferritchromite and olivine or OPX indicates a deserpentinization origin for the peridotites. Serpentinization began at the tip of the mantle wedge and proceeded during subduction. High-pressure antigorite serpentinite was dehydrated to afford chlorite- and amphibole-bearing harzburgite during further subduction, and Si-enriched serpentinite was converted into the OPX-rich peridotite (Fig. 7). The peridotite might have entered the diamond-stability field during the deepest point of subduction, immediately before exhumation (Fig. 7). Diamonds can form within chromitites at ultrahigh pressures under the peak condition of this subduction zone setting. Declarations Acknowledgments We thank D. Dyuragina and D. Kuznetsov for their help during fieldwork. We greatly appreciate the assistance of T. Nishiyama and H. Isobe at Kumamoto University during microprobe analyses and Raman spectroscopy. We thank M. Obata and K. Naemura for their constructive reviews of an earlier version of the manuscript. This research was supported by the MONKASHO SPECIAL BUDGET “Decoding ocean-floor dynamics from ophiolites” to SA and JSPS KAKENHI (grant numbers JP24540518, JP16K1783400, JP22K03761) to SI. Ethical approval Not applicable. Funding This research was supported by the MONKASHO SPECIAL BUDGET “Decoding ocean-floor dynamics from ophiolites” to SA and JSPS KAKENHI (grant numbers JP24540518, JP16K1783400, JP22K03761) to SI. Availability of data and materials All data presented in the text of this article are fully available without restriction from the author upon request. References Arai S (1974) ‘Non-calciferous’ orthopyroxene and its bearing on the petrogenesis of ultramafic rocks in Sangun and Joetsu zones. J Japan Assoc Min Petr Econ Geol 69:343-353 Arai S (1975) Contact metamorphosed dunite-harzburgite complex in the Chugoku District, western Japan. 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Institute of the Lithosphere, Russian Academy of Sciences, Moscow, 101 p Brown D, Spadea P, Puchkov V, Alvarez-Marron J, Herrington R, Willner AP, Hetzel R, Gorozhanina Y, Juhlin C (2006) Arc-continent collision in the southern Urals. Earth Sci Rev 79:261-287 Evans BW, Trommsdorff V (1974) Stability of enstatite + talc, and CO2-metasmatism of metaperidotite, Val d’Efra, Lepontine Alps. Am J Sci 274:274-296 Frost BR (1975) Contact metamorphism of serpentine, chloritic Blackwall and rodingite at paddy-go-easy Pass, central Cascades, Washington. J Petrol 16:272-313 Frost BR (1985) On the stability of sulfide, oxides, and native metals in serpentinite. J Petrol 26:31-63 Gervilla F, Padrón-Navarta JA, Kerestedjian T, Sergeeva I, González-Jiménez JM, Fanlo I (2012) Formation of ferrian chromite in podiform chromitites from the Golyamo Kamenyane serpentinite, Eastern Rhodopes, SE Bulgaria: a two-stage process. 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Mineral Mag J Mineral Soc 30:277-292 Ishimaru S, Arai S, Ishida Y, Shirasaka M, Okrugin VM (2007) Melting and multi-stage metasomatism in the mantle wedge beneath a frontal arc inferred from highly depleted peridotite xenoliths from the Avacha volcano, southern Kamchatka. J Petrol 48:395-433 Ishimaru S, Arai S, Miura M, Shmelev VR, Pushkarev E (2015) Ruby–bearing feldspathic dike in peridotite from Ray–Iz ophiolite, the Polar Urals: implications for mantle metasomatism and origin of ruby. J Mineral Petrol Sci 110:76-81 Kelemen PB, Hart SR, Bernstein S (1998) Silica enrichment in the continental upper mantle via melt/rock reaction. Earth Planet Sci Lett 164:387-406 Leake BE, Woolley AR, Arps CES, Birch WD, Gilbert MC, Grice JD, Hawthorne FC, Kato A, Kisch HJ, Krivovichev VG, Linthout K, Laird J, Mandarino J, Maresch WV, Nickel EH, Rock NMS, Schumacher JC, Smith DC, Stephenson NCN, Ungaretti L, Whittaker EJW, Youzhi G (1997) Nomenclature of amphiboles report of the subcommittee on amphiboles of the international mineralogical association commission on new minerals and mineral names. Eur J Mineral 9:623-651 McDonough WF, Sun S-S (1995) The composition of the Earth. Chem Geol 120:223-253 Meng F, Shmelev VR, Kulikova KV, Ren Y (2018) A red-corundum-bearing vein in the Rai-Iz ultramafic rocks, Polar Urals, Russia: the product of fluid activity in a subduction zone. Lithos 320-321:302-314 Merlini A, Grieco G, Diella V (2009) Ferritchromite and chromian-chlorite formation in melange-hosted Kalkan chromitite (Southern Urals, Russia). Am Mineral 94:1459-1467 Moody JB (1976) Serpentinization: a review. 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J Petrol 52:2047-2078 Savelieva GN, Nesbitt RW (1996) A synthesis of the stratigraphic and tectonic setting of the Uralian ophiolites. J Geol Soc 153:525-537 Sack RO, Ghiorso MS (1991) Chromian spinels as petrogenetic indicators: thermodynamics and petrological applications. Am Mineral 76:827-847 Schmidt MW, Poli S (1998) Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation. Earth Planet Sci Lett 163:361-379 Sharma M, Wasserburg GJ, Papanastassiou DA, Quick JE, Sharkov EV, Laz’ko EE (1995) High 143Nd/144Nd in extremely depleted mantle rocks. Earth Planet Sci Lett 135:101-114 Shmelev VR (2011) Mantle ultrabasites of ophiolite complexes in the Polar Urals: petrogenesis and geodynamic environments. Petrology 19:618-640 Shmelev VR, Meng F-C (2013) The nature and age of basic rocks of the Rai-Iz Ophiolite Massif (Polar Urals). Dokl Earth Sci 451:758-761 Shmelev VR, Perevozchikov BV, Moloshag VP (2014) The Rai-Iz ophiolite massif in the Polar Urals: geology and chromite deposits. Field trip guidebook. 12th International Platinum Symposium, Yekaterinburg, IGG UB RAS, 44. Shmelev VR, Arai S, Tamura A (2018) The nature of mantle rocks un ophiolites of the Polar Urals. Dokl Earth Sci 479:472-476 Shmelev VR, Arai S, Tamura A (2019) Heterogeneity of mantle peridotites from the Polar Urals (Russa): evidence from the new LA-ICP-MS data. J Earth Sci 30:431-450 Straub SM, LaGatta AB, Martin-Del Pozzo AL, Langmuir CH (2008) Evidence from high-Ni olivines for a hybridized peridotite/pyroxenite source for orogenic andesites from the central Mexican volcanic Belt. Geochem Geophys Geosyst 9:Q03007, doi:10.1029/2007GC001583 Takahashi E (1986) Origin of basaltic magmas: implications from peridotite melting experiments and an olivine fractionation model. Bull Vol Soc Jpn 30:S17-S40 (in Japanese with English abstract). Trommsdorff V, Evans BW (1972) Progressive metamorphism of antigorite schist in the Bergell tonalite aureole (Italy). Am J Sci 272:423-437 Trommsdorff V, López Sánchez-Vizcaíno V, Gómez-Pugnaire MT, Müntener O (1998) High pressure breakdown of antigorite to spinifex-textured olivine and orthopyroxene, SE Spain. Contrib Mineral Petrol 132:139-148 Yang J, Meng F, Xu X, Robinson PT, Dilek Y, Makeyev AB, Wirth R, Wiedenbeck M, Cliff J (2015) Diamonds, native elements and metal alloys from chromitites of the Ray-Iz ophiolite of the Polar Urals. Gondwana Res 27:459-485 Tables Table 1 and 2 are available in the Supplementary Files section. Additional Declarations No competing interests reported. Supplementary Files Table1.xlsx Table 1. Whole-rock chemical compositions of OPX-rich rocks and meta-harzburgite in the Ray-Iz massif. Cc.* column indicates the presence or absence of carbonate in the sample. Footnote: Mg# = Mg/(Mg + Fe 2+ ) . Table2.xlsx Table 2. Major-element compositions of representative minerals in the Ray-Iz samples. The Cc.* column indicates the presence or absence of carbonate in the sample. Footnote: FeO* is total FeO for silicate minerals and calculated FeO for spinel. Mg# is Mg/(Mg + Fe) for silicate minerals and Mg/(Mg + Fe 2+ ) for spinel. Cr# is Cr/(Cr + Al). Y Al , Y Cr , and Y Fe indicate Al/(Al + Cr + Fe 3+ ), Al/(Al + Cr + Fe 3+ ), and Al/(Al + Cr + Fe 3+ ), respectively. opx, OPX; amph, amphibole; chl, chlorite; carb, carbonate; Fe-Chr, ferritchromite; mgt, magnetite. Cite Share Download PDF Status: Published Journal Publication published 11 Apr, 2026 Read the published version in Mineralogy and Petrology → Version 1 posted Editorial decision: Revision requested 08 Apr, 2024 Editor assigned by journal 08 Apr, 2024 Submission checks completed at journal 05 Apr, 2024 First submitted to journal 03 Apr, 2024 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-4210973","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":288814960,"identity":"c1e7d679-7c52-40c0-b3bb-dee8b17ae9f3","order_by":0,"name":"Satoko Ishimaru","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABEklEQVRIiWNgGAWjYFACxgZmBgMQg/kwutQBQlrYkiECbAS1AI2HUDzG6FqwA/kG5ubPBQXb5M1n5Hw25s2xi+aXbz7AzMNgJ8/AeBarNQYHGNukZxjcNpxzI3dzMu+25NyZbWwJQC3Jhg0M5xKwamFgbGPmMbjNOEMid/Nh3m3MuRuO8Zj/5mFgBio/Y4DdYYzNn4Fa7GdI5DwGaqkHaTEA2lKPUwvDAcYGaaCWRKAWZqDDDsO0HMapBewXoJbkGTzPjA3nbjsO9EtaAuMcg+OGbTj8It/A/vgzz5/btjPYkx9LvN1WndvPfPgAw5uKanl+CewhxiD/AKsw0ElsEmewSuED/D0kaxkFo2AUjIJhCQBt6FiuH8XLaAAAAABJRU5ErkJggg==","orcid":"","institution":"Kumamoto University","correspondingAuthor":true,"prefix":"","firstName":"Satoko","middleName":"","lastName":"Ishimaru","suffix":""},{"id":288814961,"identity":"ff4fb53d-372b-4c32-9366-e64aa03494bb","order_by":1,"name":"Rie Sonoda","email":"","orcid":"","institution":"Kumamoto University","correspondingAuthor":false,"prefix":"","firstName":"Rie","middleName":"","lastName":"Sonoda","suffix":""},{"id":288814962,"identity":"cebc38ac-c77f-4e6b-bd33-4cbef8b53b57","order_by":2,"name":"Makoto Miura","email":"","orcid":"","institution":"Gemological Institute of America, Tokyo","correspondingAuthor":false,"prefix":"","firstName":"Makoto","middleName":"","lastName":"Miura","suffix":""},{"id":288814963,"identity":"33bcc2d1-0616-4d31-a478-0947340fee15","order_by":3,"name":"Vladimir R. Shmelev","email":"","orcid":"","institution":"Russian Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Vladimir","middleName":"R.","lastName":"Shmelev","suffix":""},{"id":288814964,"identity":"2523c5a9-a666-4e8d-90b1-278d3331d6aa","order_by":4,"name":"Shoji Arai","email":"","orcid":"","institution":"Kanazawa University","correspondingAuthor":false,"prefix":"","firstName":"Shoji","middleName":"","lastName":"Arai","suffix":""}],"badges":[],"createdAt":"2024-04-03 08:05:14","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4210973/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4210973/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00710-025-00962-w","type":"published","date":"2026-04-11T15:58:13+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":54432139,"identity":"ea33c6b0-82bc-473d-ae55-6eba2305c661","added_by":"auto","created_at":"2024-04-10 11:42:27","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":2151138,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Simplified geological map of the Ray-Iz peridotite massif (modified after Shmelev et al. 2014). The two circles indicate the locations of samples collected for U–Pb dating (Shmelev and Meng 2013; Meng et al. 2018). The star indicates the location of OPX-rich samples. The central chromite deposit (CCD) is the largest chromite deposit in the massif. Abbreviation are as follows: Dn, dunite; Cpxite, clinopyroxenite; Gb, gabbro; Harz, harzburgite; Lher, lherzolite; Hb, hornblende; Dio, diorite; pl-Grd, plagio-granodiorite; Moz-Dio, monzodiorite; L-M Paleozoic, lower–middle Paleozoic. (b) Sampling points (stars with sample numbers) overlain on a Google Earth satellite image. Samples RY26-01, RY26-03, and RY26-04 are OPX-rich rocks; RY26-02, RY26-05, and RY26-07 are meta-harzburgites; and RY26-08 is a garnet-amphibolite enclosed in meta-harzburgite.\u003c/p\u003e","description":"","filename":"Fig.1.png","url":"https://assets-eu.researchsquare.com/files/rs-4210973/v1/0728394b8905dbebac2d5334.png"},{"id":54432136,"identity":"05e8a5c1-c15d-4388-9d98-3a1da958823d","added_by":"auto","created_at":"2024-04-10 11:42:27","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":3117867,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Spherulitic OPX aggregates in a meta-peridotite. (b) Large OPX aggregates in a meta-peridotite (boulder). (e) Magnetite inclusions in olivine and OPX in a meta-harzburgite (RY26-02). Cross-polarized light.\u003c/p\u003e","description":"","filename":"Fig.2.png","url":"https://assets-eu.researchsquare.com/files/rs-4210973/v1/3b5eb89460d5ba10b8a18ed6.png"},{"id":54432133,"identity":"e97a1f71-7cdd-4948-9734-fdfe229885f9","added_by":"auto","created_at":"2024-04-10 11:42:26","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":4386660,"visible":true,"origin":"","legend":"\u003cp\u003eSelected scanned images of thin sections of studied samples. (a) Ray-Iz OPX-rich rock (RY26-01). (b) Ray-Iz OPX-rich rock including carbonate (RY26-04). (c) Ray-Iz meta-harzburgite (RY26-05). Brightness and contrast are slightly modified. Left and right columns are plane-polarized light and crossed-polarized light images, respectively.\u003c/p\u003e","description":"","filename":"Fig.3.png","url":"https://assets-eu.researchsquare.com/files/rs-4210973/v1/100df68e74d919666f4ff36a.png"},{"id":54432132,"identity":"d1918cb7-7750-4d6a-b7ef-6a27581e4ef8","added_by":"auto","created_at":"2024-04-10 11:42:26","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":5789922,"visible":true,"origin":"","legend":"\u003cp\u003ePhotomicrographs (a) Serpentinized and relic OPX in OPX-rich rock (RY26-01). Crossed-polarized light image. (b) Plane-polarized image of (a). (c) Abundant opaque inclusions (Cr-mgt/Fe-Chr) in olivine (ol) and OPX in OPX-rich rock (RY26-04). Crossed-polarized light image. (d) Abundant opaque inclusions (Cr-mgt/Fe-Chr) in OPX and olivine grains in an OPX-rich rock (RY26-03). Cross-polarized light. (c) Tremolite in the Ray-Iz OPX-rich rock (RY26-04). Magnesite (mgs) is locally associated with amphibole. Crossed-polarized light image. (d) OPX-replacing magnesite (mgs) in sample RY26-05. Crossed-polarized light image.\u003c/p\u003e","description":"","filename":"Fig.4.png","url":"https://assets-eu.researchsquare.com/files/rs-4210973/v1/45b4d89fa80778f4cb5e630d.png"},{"id":54432134,"identity":"5aae9fdc-1fd5-4c6b-ac22-34436feb25a0","added_by":"auto","created_at":"2024-04-10 11:42:26","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":455092,"visible":true,"origin":"","legend":"\u003cp\u003eWhole-rock chemical compositions of studied samples. Data from Shmelev et al. (2019) are shown for comparison. (a) Relationships between Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/SiO\u003csub\u003e2\u003c/sub\u003e and MgO/SiO\u003csub\u003e2\u003c/sub\u003e on anhydrous basis. Reference rock values (DM, depleted mantle; PM; primitive mantle) and mineral compositions (Lz, lizardite; Atg, antigorite; Tlc, talc; En, enstatite; Brc, brucite) and trends are from Deschamps et al. (2013). Compositional variation of abyssal peridotites is from Niu (2004). (b) REE pattern, normalized to chondrite value (McDonough and Sun, 1995).\u003c/p\u003e","description":"","filename":"Fig.5.png","url":"https://assets-eu.researchsquare.com/files/rs-4210973/v1/8c8a4e21649c5dced163ff3d.png"},{"id":54432142,"identity":"3c0da4ef-e6dc-460b-99fe-a14048316b53","added_by":"auto","created_at":"2024-04-10 11:42:28","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":772443,"visible":true,"origin":"","legend":"\u003cp\u003eMajor-element compositions of representative minerals from the Ray-Iz OPX-rich rocks and related samples. Data from the Ray-Iz metaperidotite (MP) and Ray-Iz lherzolite/harzburgite (L/H) are shown for comparison (Shmelev et al. 2014). A legend for the symbols is shown in (d). (a) Relationship between olivine Mg# and NiO concentration. MOR = mantle olivine array (Takahashi 1986). (b) Trivalent cation ratio of spinel. The regional metamorphic trend (R. meta trend) is after Müntener et al. (2000). The compositions of abyssal peridotites are shown for comparison (Arai et al. 2006). (c) Relationship between spinel Y\u003csub\u003eFe\u003c/sub\u003e and MnO concentration. (d) Relationship between spinel Y\u003csub\u003eFe\u003c/sub\u003e and NiO concentration. (e) Relationship between OPX Mg# and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e concentration. (f) Relationship between OPX Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e concentrations.\u003c/p\u003e","description":"","filename":"Fig.6.png","url":"https://assets-eu.researchsquare.com/files/rs-4210973/v1/e0a52a58d9f62c039dc49d53.png"},{"id":54432141,"identity":"85f7b378-2502-48f5-af0a-7fd033575bdb","added_by":"auto","created_at":"2024-04-10 11:42:28","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":484583,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic P–T path for the Ray-Iz ophiolite. The phase diagram for H\u003csub\u003e2\u003c/sub\u003eO-saturated peridotite is modified after Schmidt and Poli (1998). The up-temperature reaction from antigorite and talc to OPX and H\u003csub\u003e2\u003c/sub\u003eO is from Padrón-Navarta et al. (2010) and was calculated for a Si-rich bulk-rock composition. Grey arrow indicates deserpentinization paths via thermal metamorphism. The P–T path of the Ray-Iz peridotite massif is indicated by the black arrow. Points [1], [2], and [3] indicate hydration, compression, and exhumation stages, respectively. Dashed lines with arrowheads indicate parts of the P–T path that are speculative. No geobarometer was available to constrain the maximum pressure during serpentine decomposition, although it is possible that the Ray-Iz peridotite massif reached ultra-high pressures.\u003c/p\u003e","description":"","filename":"Fig.7.png","url":"https://assets-eu.researchsquare.com/files/rs-4210973/v1/f35aacaaab010b43b392acf3.png"},{"id":106809184,"identity":"35b9f23d-1498-40e4-8469-8c6321137c15","added_by":"auto","created_at":"2026-04-13 16:07:50","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":25410421,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4210973/v1/32210928-d063-4258-9f49-546e6e4a22a7.pdf"},{"id":54432130,"identity":"c3da1e99-169e-49ca-80d1-0a326de4866f","added_by":"auto","created_at":"2024-04-10 11:42:26","extension":"xlsx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":13021,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTable 1.\u003c/strong\u003e Whole-rock chemical compositions of OPX-rich rocks and meta-harzburgite in the Ray-Iz massif. Cc.* column indicates the presence or absence of carbonate in the sample.\u003c/p\u003e\n\u003cp\u003eFootnote: Mg# = Mg/(Mg + Fe\u003csup\u003e2+\u003c/sup\u003e) .\u003c/p\u003e","description":"","filename":"Table1.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-4210973/v1/8c53a4a37058a95fc1f57009.xlsx"},{"id":54432135,"identity":"55d138ff-dfde-4da9-addc-081b744a61f9","added_by":"auto","created_at":"2024-04-10 11:42:26","extension":"xlsx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":18949,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTable 2.\u003c/strong\u003e Major-element compositions of representative minerals in the Ray-Iz samples. The Cc.* column indicates the presence or absence of carbonate in the sample.\u003c/p\u003e\n\u003cp\u003eFootnote: FeO* is total FeO for silicate minerals and calculated FeO for spinel. Mg# is Mg/(Mg + Fe) for silicate minerals and Mg/(Mg + Fe\u003csup\u003e2+\u003c/sup\u003e) for spinel. Cr# is Cr/(Cr + Al). Y\u003csub\u003eAl\u003c/sub\u003e, Y\u003csub\u003eCr\u003c/sub\u003e, and Y\u003csub\u003eFe\u003c/sub\u003e indicate Al/(Al + Cr + Fe\u003csup\u003e3+\u003c/sup\u003e), Al/(Al + Cr + Fe\u003csup\u003e3+\u003c/sup\u003e), and Al/(Al + Cr + Fe\u003csup\u003e3+\u003c/sup\u003e), respectively. opx, OPX; amph, amphibole; chl, chlorite; carb, carbonate; Fe-Chr, ferritchromite; mgt, magnetite.\u003c/p\u003e","description":"","filename":"Table2.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-4210973/v1/223f3cfdec7b68134fd657a0.xlsx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Spherulitic orthopyroxene–bearing metaperidotite in Ray-Iz ophiolites (Polar Urals): high-pressure deserpentinization in a mantle wedge","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe Ray-Iz (or Rai-Iz) ophiolite comprises mainly peridotites, which is considered a residual mantle formed by multistage partial melting under different tectonic settings (beneath a mid-ocean ridge and a suprasubduction zone), based on the mineral chemistry of the residual peridotites (Shmelev 2011; Shmelev et al. 2018, 2019). The Ray-Iz peridotite massif underwent metasomatism (via an interaction with an arc-type melt or a hydrous fluid) (Glodny et al. 2003; Ishimaru et al. 2015; Meng et al. 2018) and retrograde metamorphism during exhumation (Shmelev 2011).\u0026nbsp;The Ray-Iz ophiolite has received significant attention, attributed to the occurrence of diamond and other unusual minerals in the podiform chromitites residing in peridotites (Yang et al. 2015).\u003c/p\u003e\n\u003cp\u003eThe peridotites in the Ray-Iz ophiolite locally contain abundant metamorphic minerals, such as tremolite, talc, and chlorite, as well as minute inclusions of opaque minerals (ferritchromite, Cr-rich magnetite, and various sulfides) in olivine and orthopyroxene (OPX) grains. The presence of these inclusions indicates that the peridotites were formed through serpentinization and deserpentinization of primary peridotites (Arai 1975; Moody 1976). As shown in\u0026nbsp;Fig. 1a,\u0026nbsp;spherulitic OPX-bearing harzburgite\u0026nbsp;occurs in the \u0026ldquo;metaperidotite zone\u0026rdquo; (within the center of the Ray-Iz peridotite massif; Shmelev et al. 2014). Notably, the occurrence of spherulitic OPX in metaperidotites has been reported elsewhere (Arai, 1974, 1975; Evans and Trommsdorff 1974; Frost 1975). The Ray-Iz spherulitic OPX-bearing metaperidotite has been described as a sagvandite (a magnesite-bearing orthopyroxenite), which implies that a CO\u003csub\u003e2\u003c/sub\u003e-rich fluid was involved in its formation (Shmelev et al. 2014). However, we did not observe any enrichment in carbonate minerals or the presence of a heat source close to the spherulitic OPX-bearing harzburgite block (Fig. 1a).\u003c/p\u003e\n\u003cp\u003eWe present detailed petrological descriptions of several peridotite samples, which are employed to discern the P-T history of the Ray-Iz mantle slice. The OPX-bearing harzburgite block records various metamorphic events related to the subduction/obduction of the Ray-Iz peridotite massif.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eGeological background\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe Ural Mountains represent a linear mid-Paleozoic orogenic zone that was formed during the closure of an ocean basin\u0026ndash;island arc system between the European Plate to the west and the Siberian Plate to the east (Savelieva and Nesbitt 1996; Brown et al. 2006). U\u0026ndash;Pb dating of zircon in an eclogite from the Marun\u0026ndash;Keu complex in the Polar Urals indicates that the collision occurred at 360\u0026ndash;355 Ma (Glodny et al. 2004). The Ray-Iz ultramafic massif is located in a long ophiolitic belt in the Polar Urals and is associated with the Voikar and Syum-keu ultramafic massifs. Together, these three massifs are interpreted to represent a single large-scale ophiolite (Bogatsky et al. 1996) that was thrust over the Paleozoic sedimentary rocks of the East European Platform (Shmelev et al. 2014). Sm\u0026ndash;Nd dating indicates that partial melting and the formation of residual mantle peridotite occurred at 387 \u0026plusmn; 34 Ma (Sharma et al. 1995). The ophiolite massifs are accompanied by high-pressure metamorphic rocks, including eclogites, blueschists, amphibolites, and albite\u0026ndash;lawsonite-facies rocks (Yang et al. 2015).\u003c/p\u003e\n\u003cp\u003eThe Ray-Iz ultramafic massif comprises mainly lherzolite\u0026ndash;harzburgite, dunite\u0026ndash;harzburgite, and metaperidotites (Fig. 1a), and the residual features indicate that they were formed by at least two partial melting events: the former occurring in a mid-ocean ridge setting and the latter in a suprasubduction zone setting (Shmelev 2011; Shmelev et al. 2014, 2019). The Moho transition zone is located at the southern end of the Ray-Iz massif, and the peridotites are in tectonic contact with a hornblende gabbro-diorite\u0026ndash;plagiogranodiorite complex (Sob complex) to the south (Fig. 1a) (Shmelev and Meng 2013; Shmelev et al. 2014). U\u0026ndash;Pb dating of zircon obtained from the gabbroic rocks yielded a crystallization age of 418 \u0026plusmn; 2 Ma (Shmelev and Meng 2013). The Ray-Iz peridotite massif underwent polyphase metamorphism and deformation, and the peridotite shows signs of undergoing variable serpentinization (Meng et al. 2018). Corundum-bearing felsic veins, formed through interactions between slab-derived fluid/melt and peridotite (Glodny et al. 2003; Ishimaru et al. 2015), occur in the central dunite zone (Fig. 1a). The \u003csup\u003e40\u003c/sup\u003eAr/\u003csup\u003e39\u003c/sup\u003eAr dating of phlogopite and the U\u0026ndash;Pb dating of zircon indicate that these veins were formed at ~380 Ma (Meng et al. 2018), which is similar to a Rb/Sr isochron age (373.1 \u0026plusmn; 5.4 Ma) (Glodny et al. 2003).\u003c/p\u003e\n\u003cp\u003eThe metaperidotite zone at the center of the Ray-Iz ophiolite (Fig. 1a) comprises mainly harzburgitic metaperidotites that commonly contain large proportions of chlorite and amphibole (Figs. 2a, b). Within the zone lies a block of spherulitic OPX-bearing harzburgite (Figs. 2a, b) that was interpreted to have been produced during the infiltration of a CO\u003csub\u003e2\u003c/sub\u003e-rich fluid (Shmelev et al. 2014). We observed large spherulitic OPX aggregates (diameter: \u0026lt;10 cm) in a small block (20 \u0026times; 20 \u0026times; 10 m) of OPX-rich harzburgite located at N66\u0026deg;53.547\u0026cent;, E65\u0026deg;11.821\u0026cent;. A small amphibolite block was enclosed in the metaperidotite at the northern end of our sampling points (RY26-08 in Fig. 1b).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSample descriptions\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe collected eight samples from the metaperidotite zone: three from the OPX-rich harzburgite (RY26-01, RY26-03, and RY26-04), four from the surrounding metaperidotite (chlorite- and talc-bearing harzburgite/lherzolite; RY26-02, RY26-05, RY26-06, RY26-07, RY26-10), and one from an associated garnet amphibolite block (RY26-08) (Fig. 1b). Hereinafter, we refer to the metaperidotite containing spherulitic OPX and the harzburgite from the metaperidotite zone as OPX-rich rock and meta-harzburgite, respectively (cf. Figs. 2 and 3). In addition, we collected samples of harzburgite and dunite from the main body of the Ray-Iz peridotite massif for comparison and conducted a major-element mineral analysis of one harzburgite (RY14-02).\u003c/p\u003e\n\u003cp\u003eThe OPX-rich rocks comprise typical peridotite minerals (olivine, OPX, and spinel) and hydrous minerals (chlorite and talc \u0026plusmn; amphibole). Only one sample (RY26-04) contains carbonate (\u0026le;5%) in addition. Although the modal amount of serpentine in the OPX-rich samples is typically small, one OPX-rich sample (RY26-01) contains a large amount of serpentine as veinlets that cross-cut other minerals, as well as locally serpentinized spherulitic OPX aggregates (Figs. 3a, 4a,b). We observed two OPX morphologies: elongated prisms and rounded grains displaying wavy extinction and kink bands (Fig. 3b). Abundant opaque grains occur as inclusions in the relict olivine and OPX grains (Figs. 4c,d). Amphibole is heterogeneously distributed in the OPX-rich rocks, except for sample RY26-01. The amphibole grains show a prismatic or euhedral shape and were locally replaced by carbonate and talc (Fig. 4e). Carbonate is associated with chlorite and amphibole, and it locally replaces OPX porphyroblasts (Fig. 4e). However, no clear replacement relationships with other minerals were observed. Chlorite forms lath and tabular grains and occurs either as inclusions in olivine and OPX or as discrete aggregates (Figs. 4c,e). There is no obvious association with spinels (ferritchromite and Cr-bearing magnetite) (Fig. 4c), except for sample RY26-01. The chlorite grains in sample RY26-01 are abundant and always enclose magnetite in a serpentine matrix. Talc is closely associated with OPX and olivine. The minute inclusions in olivine and OPX were identified as talc and minor low-temperature serpentine. No fluid phase was observed as inclusions in minerals.\u003c/p\u003e\n\u003cp\u003eThe surrounding meta-harzburgite displays the same mineral assemblage as the OPX-rich rock (Fig. 3). One sample (RY26-05) additionally contains carbonate (\u0026lt;5 vol.%), which does not exhibit a noticeable replacement texture. However, some olivine, OPX, and amphibole grains were locally replaced by carbonate (Fig. 4f). Serpentine minerals are rare in meta-harzburgites and occur only as veinlets or along olivine grain boundaries with minute concentrations of magnetite grains. Minute opaque inclusions occur in olivine and OPX grains. The petrographic characteristics of the meta-harzburgite (e.g., replacement textures) are similar to those of OPX-rich rocks.\u003c/p\u003e\n\u003cp\u003eThe garnet amphibolite associated with the OPX-rich rock is composed of garnet, amphibole, quartz, and plagioclase, with minor titanite, ilmenite, rutile, zircon, epidote, and apatite. Garnet grains appear pale pink under plane-polarized light and contain abundant inclusions of ilmenite, rutile, and titanite. Amphibole displays strong green\u0026ndash;yellowish brown pleochroism.\u003c/p\u003e\n\u003cp\u003eThe chlorite- and amphibole-bearing harzburgites are the products of polyphase metamorphism (Meng et al. 2018) and are common within and outside of the metaperidotite zone (e.g., close to the Central Chromite Deposit (CCD), where diamond has been observed; Fig. 1b) (Shmelev et al. 2014; Yang et al. 2015). The olivine and OPX grains in the OPX-rich rocks and metaperidotites contain inclusions of opaque minerals (Figs. 4c,d). The harzburgite samples collected from areas close to the CCD are moderately to highly serpentinized and contain abundant chlorite and minor amphibole and talc. Chlorite occurs as dark-green grains displaying weak-to-strong pleochroism and non-pleochroic grains associated with chromian spinel. Chromian spinel exhibits anhedral to subhedral features, and it contains inclusions of chlorite laths. Amphibole grains are elongated, and talc is typically observed surrounding OPX grains.\u0026nbsp;\u003c/p\u003e"},{"header":"Analytical methods","content":"\u003cp\u003eThe whole-rock chemical compositions were determined by inductively coupled plasma\u0026ndash;optical emission spectrometry (ICP\u0026ndash;OES) and inductively coupled\u0026ndash;mass spectrometry (ICP\u0026ndash;MS) at Activation Laboratories Ltd., (Actlabs), Ancaster, Canada. The samples were also prepared (crushing and pulverization) at Actlabs. The data quality was determined using the analytical results of international standard samples (NIST 694, DNC-1, BIR-1a), and the relative standard deviations were mostly \u0026lt;5% and \u0026lt;10% for major and trace elements, respectively.\u003c/p\u003e\n\u003cp\u003eThe major-element mineral compositions were measured in harzburgite, meta-harzburgite, and OPX-rich rock samples using a field emission-scanning electron microscopy (FE-SEM, JEOL JSM-7001F) system equipped with an energy-dispersive X-ray spectrometry (EDS) system (INCA Energy or AZtecEnergy, Oxford Instruments) at Kumamoto University, Kumamoto, Japan. We employed an accelerating voltage of 15 kV, a probe current of 1 nA, and a probe diameter of \u0026lt;1 \u0026micro;m. The analytical precision during the EDS measurement of the NiO concentration in olivine was generally less than \u0026plusmn;0.1 wt.% (Fig. 3a). We define Mg# as the atomic ratios of Mg/(Mg + total Fe) and Mg/(Mg + Fe\u003csup\u003e2+\u003c/sup\u003e) for silicate minerals and spinels, respectively. The concentrations of ferric and ferrous iron in the spinel minerals within peridotitic rocks and in the garnet and epidote minerals within garnet amphibolite were calculated based on their stoichiometry. Parameter Y\u003csub\u003eFe\u003c/sub\u003e represents the ratio of Fe\u003csup\u003e3+\u003c/sup\u003e relative to the trivalent cations (Cr, Al, and Fe\u003csup\u003e3+\u003c/sup\u003e) in the spinels.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e\u003cstrong\u003eWhole-rock compositions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe obtained whole-rock chemical compositions for six samples: two OPX-rich rocks, two meta-harzburgite samples, one meta-lherzolite, and one garnet amphibolite. All peridotitic samples are plotted in the distribution of abyssal peridotite (Fig. 5a), and there is no noticeable relationship between the whole-rock Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/SiO\u003csub\u003e2\u003c/sub\u003e ratio and their petrographical features, i.e., the presence of carbonate, abundance of OPX and/or amphibole, and calculated normative OPX amount (Table 1; Fig. 5a). The slightly low whole-rock MgO/SiO\u003csub\u003e2\u003c/sub\u003e ratio in one carbonate-bearing sample, RY26-05, is consistent with the calculated high normative OPX amount (42%) (Table 1; Figs. 3, 5a). The whole-rock rare earth element (REE) contents of the studied samples are within the 0.1\u0026ndash;1.0 range, relative to the chondrite values (McDonough and Sun 1995), and the shape of the REE patterns is concave upward, with depletion at Eu (Fig. 5b). The pattern is almost the same as that reported in Shmelev et al. (2019) (Fig. 5b).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMineral compositions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eOlivine generally displays similar compositions of harzburgite (Fo = 90.7\u0026ndash;91.3; NiO = 0.25\u0026ndash;0.53 wt.%) and meta-harzburgite (Fo = 90.8\u0026ndash;91.4; NiO = 0.27\u0026ndash;0.62 wt.%) (Fig. 6a). The only exception is sample RY26-05, which contains carbonate minerals (\u0026lt;5 vol.%) and yields higher NiO concentrations (0.50\u0026ndash;0.90 wt.%) and similar Fo values (90.6\u0026ndash;91.6) (Table 2; Fig. 6a) compared with the values for the harzburgite. The olivine in the OPX-rich rocks exhibits variable NiO concentrations (0.28\u0026ndash;0.62 wt.%) and Fo values (90.3\u0026ndash;91.8; Fig. 6a; Table 2). Sample RY26-04, which contains carbonate and abundant magnetite (Fig. 4c), exhibits high Fo values (95.3\u0026ndash;95.9) and NiO concentrations of 0.35\u0026ndash;0.64 wt.% (Fig. 6a; Table 2). The measured NiO concentrations in olivine in our OPX-rich samples and the carbonate-rich meta-harzburgite sample (RY26-05) are higher than those reported previously for metaperidotites (Shmelev 2011).\u003c/p\u003e\n\u003cp\u003eThe spinel in the OPX-rich rocks has unusual compositions, characterized by low Mg# values (0\u0026ndash;0.26) and very low Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e concentrations (\u0026lt;2.5 wt.%), and the constituent minerals are classified as chromite, magnetite, and ferritchromite (Fig. 6b) (Table 2). Most grains lack compositional zoning; however, some contain chromite or ferritchromite cores surrounded by magnetite rims, i.e., the Cr concentration decreases outward from the grain cores. No clear relationship is observed between the degree of chemical homogeneity and the spinel grain size. Spinel-group minerals contain up to 2 wt.% TiO\u003csub\u003e2\u003c/sub\u003e, with Y\u003csub\u003eFe\u003c/sub\u003e values of \u0026lt;0.7. Their MnO concentrations are typically \u0026lt;0.8 wt.% and are weakly negatively correlated with the Y\u003csub\u003eFe\u003c/sub\u003e values (Fig. 6c). The only spinel-group mineral in the carbonate-bearing OPX-rich sample (RY26-04) is magnetite, which contains minor Cr (Figs. 6c\u0026ndash;d) and abundant NiO (typically \u0026gt;1.0 wt.%) (Fig. 3d) (Table 2). The spinel-group minerals in the meta-harzburgite range from chromite to magnetite with varying compositions (Fig. 6b), and the composition is unrelated to the presence or absence of carbonate minerals (Figs. 6b\u0026ndash;d) (Table 2). The spinel-group mineral in the harzburgite sample (RY14-02) is ferritchromite, and it exhibits slightly lower Y\u003csub\u003eFe\u003c/sub\u003e values than the meta-harzburgite and OPX-rich rocks (Table 2). ZnO was rarely detected because of its low concentration (typically \u0026lt;0.5 wt.%). No difference in composition was observed between the inclusions and discrete grains of spinel (Table 2).\u003c/p\u003e\n\u003cp\u003eThe OPX in all the analyzed samples exhibited extremely low Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, and CaO concentrations (\u0026lt;0.15, \u0026lt;0.17, and \u0026lt;0.18 wt.%, respectively) and a high Mg# value (0.91\u0026ndash;0.92) (Figs. 6e, f) (Table 2). In the OPX-rich rocks, the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, and CaO concentrations are generally below the detection limit or slightly lower than those in the harzburgites (Figs. 6e, f; Table 2). The OPX composition does not vary with the grain morphology (i.e., prismatic elongated vs. deformed vs. rounded; data not shown).\u003c/p\u003e\n\u003cp\u003eThe chlorite in the OPX-rich rocks and meta-harzburgite exhibit a high Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (\u0026lt;2.9 wt.%) concentration and high Mg# (~0.94) value, and it is classified as clinochlore (Hey, 1954). Most of the chlorites in the harzburgite (RY14-02) have similar compositions to those in the OPX-rich rocks and meta-harzburgite (Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e = 2.2\u0026ndash;2.9 wt.%; Mg# = ~0.94). However, the green pleochroic chlorite exhibits lower and more variable Mg# values (0.76\u0026ndash;0.88) and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (7.0\u0026ndash;8.3 wt.%) and Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (0.12\u0026ndash;1.36 wt.%) concentrations (Table 2). These chlorites low in Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e have a composition that is intermediate between those of typical chlorite and serpentine, and they might feature a mixture of the two minerals.\u003c/p\u003e\n\u003cp\u003eThe amphibole in the OPX-rich rocks and meta-harzburgites is classified as tremolite (Mg# = ~0.96; Leake et al. 1997), and it exhibits low contents of TiO\u003csub\u003e2\u003c/sub\u003e (\u0026lt;0.61 wt.%), Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (\u0026lt;0.61 wt.%), and Na\u003csub\u003e2\u003c/sub\u003eO (0.22\u0026ndash;0.59 wt.%). The amphibole in the harzburgite sample (RY14-02) exhibits narrow concentrations of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (2.9\u0026ndash;4.6 wt.%) and Na\u003csub\u003e2\u003c/sub\u003eO (2.0\u0026ndash;2.4 wt.%), as well as low Mg# values (0.93\u0026ndash;0.95). In terms of composition, it lies along the boundary between edenite, magnesio-hornblende, and tremolite.\u003c/p\u003e\n\u003cp\u003eThe talc in the OPX-rich rocks features high Mg# values (~0.98) and exhibits low NiO concentrations (\u0026lt;0.3 wt.%). The serpentine in the OPX-rich rocks and meta-harzburgites exhibit low concentrations of NiO (\u0026lt;0.50 wt.%) and MnO (\u0026lt;0.35 wt.%), and a narrow range of high Mg# values (0.95\u0026ndash;0.96). The serpentine in the harzburgite also exhibits a low NiO concentration (\u0026lt;0.40 wt.%) and a lower Mg# value (0.93) than that in the OPX-rich rocks.\u003c/p\u003e\n\u003cp\u003eThe carbonate in the OPX-rich rock (RY26-04) and meta-harzburgite (RY26-05) is mainly classified as magnesite; however, the proportion of siderite varies (~3% or ~2 wt.% of FeO* in the former, and 5%\u0026ndash;7% or 3.6\u0026ndash;5.3 wt.% of FeO* in the latter) (Table 2). Dolomite also occurs in both samples, along tremolite contacts and as small inclusions in magnesite grains. It exhibits a narrow range of Ca/(Ca + Mg) atomic ratios (0.52\u0026ndash;0.54) (Table 2).\u003c/p\u003e\n\u003cp\u003eThe garnet in the garnet amphibolite sample is classified as grossular\u0026ndash;almandine, and it exhibits low contents of TiO\u003csub\u003e2\u003c/sub\u003e (~0.3 wt.%), MnO (\u0026lt;3.1 wt.%), and MgO (\u0026lt;4.2 wt.%). The MnO content in garnet decreases from the core to the rim (from ~4.0 to ~2.0 wt.%), whereas the MgO content increases (from ~2.5 to ~3.2 wt.%). The plagioclase in the garnet amphibolite sample (RY26-08) exhibits a narrow range of Ca/(Na + Ca) atomic ratios (0.12\u0026ndash;0.17), except for one grain that exhibits a ratio of 0.04. The amphibole in the garnet amphibolite sample exhibits a Mg# value of ~0.50 and is classified as pargasite\u0026ndash;ferropargasite (Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e = ~13 wt.%; TiO\u003csub\u003e2\u003c/sub\u003e = 0.84\u0026ndash;1.16 wt.%). The epidote mineral in the garnet amphibolite sample features low contents of TiO\u003csub\u003e2\u003c/sub\u003e (\u0026lt;0.17 wt.%), and its calculated Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and\u0026nbsp;\u003cbr\u003eFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e concentrations are ~26.4 wt.% and ~10.0 wt.%, respectively.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003e\u003cstrong\u003eNature of the OPX-rich rocks and metaperidotite in the Ray-Iz massif\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePetrographic and geochemical data indicate that the Ray-Iz harzburgite and lherzolite represent the residues of high-degree partial melting associated with slab influx in a suprasubduction zone setting (Shmelev 2011; Shmelev et al. 2014, 2018). The Ray-Iz peridotite massif has experienced variable stages of metamorphism under greenschist-facies conditions (Shmelev, 2011; Yang et al. 2015; Meng et al. 2018). Our observations of abundant chlorite and tremolite in the samples from the Ray-Iz ultramafic rocks, including a harzburgite sample from outside of the metaperidotite zone (Fig. 1b), are consistent with the conclusions of the previous study above.\u003c/p\u003e\n\u003cp\u003eFurthermore, we observe minute inclusions of ferritchromite and magnetite in the olivine and OPX grains in the samples from the metaperidotite zone and the surrounding harzburgite, which is typical for deserpentinized peridotitic rocks (Trommsdorff and Evans 1972; Arai 1975; Trommsdorf et al. 1998). The serpentine and magnetite could have formed through \u003cs\u003ethe hydration of peridotite via\u003c/s\u003e the serpentinization reaction: olivine + H\u003csub\u003e2\u003c/sub\u003eO = Mg-rich serpentine + magnetite + H\u003csub\u003e2\u003c/sub\u003eO (Frost 1985). The large ferritchromite grains in our samples are locally associated with chlorite, interpreted as a metamorphic phase that replaced primary Al- and Cr-rich chromian spinel (Cr/(Cr + Al) atomic ratio = 0.2\u0026ndash;0.6) (Shmelev et al. 2014). The high Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e concentrations in chlorite in the OPX-rich rock samples (\u0026lt;3.4 wt.%; Table 2) indicate that Cr, in addition to Al, was sourced from the primary mantle chromian spinel. There are two possible formation processes for Cr-rich chlorite: the retrogressive hydration of the peridotitic assemblage (olivine, OPX, and spinel) and the interaction between chromite and antigorite during a progressive dehydration process under oxidizing conditions (Merlini et al. 2009). In both processes, spinel is required as a source of Cr and Al for the formation of Cr-rich chlorite; however, there is no close association between our Cr-rich chlorite and spinel-group minerals (ferritchromite and/or magnetite), as described above. This may reflect recrystallization and a complete change of the side-by-side mineral association. Whether the recrystallization is retrogressive or progressive, the reaction of chromian spinel with ferritchromite occurs effectively during metamorphism under amphibolite facies conditions (Barnes 2000; Gervilla et al. 2012). Furthermore, the moderate Cr/(Cr + Fe\u003csup\u003e3+\u003c/sup\u003e) atomic ratio (0.28\u0026ndash;0.72) measured in ferritchromite in our samples (Table 1; Figs. 6b\u0026ndash;d) aligns with the equilibrium temperatures estimated via thermodynamic calculations (~600\u0026deg;C) (Sack and Ghiorso 1991). The presence of an almost homogeneous Cr/(Cr + Fe\u003csup\u003e3+\u003c/sup\u003e) atomic ratio within a grain reflects the severe modification of primary chromian spinel to ferritchromite under amphibolite facies condition for extended periods. The inclusions of magnetite/ferritchromite in olivine and OPX were likely formed through the dehydration (prograde metamorphism) of hydrated peridotites, a process known as deserpentinization (Arai 1975).\u003c/p\u003e\n\u003cp\u003eGenerally, olivine produced through deserpentinization exhibits higher Mg# values than primary olivine (Arai 1975). However, irrespective of the presence/absence of magnetite and ferritchromite as inclusions in the olivine grains, olivine exhibits almost constant compositions within the sample. The observed small variation in the olivine Mg# value indicates that only a small proportion of magnetite was formed during serpentinization, as supported by the scarcity of magnetite and ferritchromite in most samples (Fig. 3). In contrast, the carbonate-bearing OPX-rich sample (RY26-04) exhibits a high Mg# value and contains abundant magnetite and ferritchromite (Fig. 3), indicating that it was formed from a relatively magnetite-rich serpentinite.\u003c/p\u003e\n\u003cp\u003eIn addition to the compositions of spinel, the extremely low concentrations of Al, Cr, and Ca in OPX (Figs. 3e, f) are characteristic of secondary OPX formed via deserpentinization. Such OPX minerals occasionally show a radially aggregated texture (Arai 1974, 1975; Arai and Kida 2000; Padr\u0026oacute;n-Navarta et al. 2011). We emphasize that the features of deserpentinization (i.e., magnetite inclusions in olivine and OPX, and the low diopside and tschermakite contents in OPX) are also observed in the harzburgite obtained outside the metaperidotite zone (Fig. 1b). However, the OPX in most Ray-Iz peridotites (lherzolite and harzburgite) exhibit typical mantle peridotite compositions (i.e., moderate Ca, Al, and Cr concentrations; Figs. 6e, f). Therefore, we infer that the entire Ray-Iz peridotite massif experienced retrograde recrystallization (from the greenschist facies to the amphibolite facies) and prograde recrystallization (or metamorphism). However, the degree of serpentinization varies, possibly depending on the availability of fluid (water) and the position (e.g., depth) within the lithosphere. The variations in the composition of tremolites in the OPX-rich rock and in other parts of the Ray-Iz peridotite indicate that the equilibrium temperature for the former was lower than that for the latter. The Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e content in the former is almost negligible (\u0026lt;0.50 wt.%); however, those in the latter occasionally exceeds 4.0 wt.%, and the material is classified as edenite (Leake et al. 1997). The sections of the massif that were unaffected by hydration (serpentinization) do not record the dehydration (deserpentinization) event. A chlorite-bearing harzburgite from Cerro del Almirez, Spain has been determined to be a product of the high-pressure dehydration of antigorite serpentinite (Pad\u0026oacute;n-Navarta et al. 2011).\u003c/p\u003e\n\u003cp\u003eThe abundant OPX in our OPX-rich rocks indicates that the protolith was a silica-enriched rock before dehydration. The silica enrichment likely occurred during serpentinization (Padr\u0026oacute;n-Navarta et al. 2011) or during an earlier mantle event (metasomatism or magmatism) (Kelemen et al. 1998; Ishimaru et al. 2007). In fact, some of our samples, RY26-01 (OPX-rich rock) and RY26-05 (meta-harzburgite with carbonate), exhibit slightly different whole-rock compositions: the former and the latter show a higher Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/SiO\u003csub\u003e2\u003c/sub\u003e ratio and a lower MgO/SiO\u003csub\u003e2\u003c/sub\u003e ratio relative to reported Ray-Iz peridotitic rocks (Shmelev 2011), respectively (Fig. 5a). Sample RY26-01, exhibiting a high Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/SiO\u003csub\u003e2\u003c/sub\u003e ratio and low Mg# value, is rich in SiO\u003csub\u003e2\u003c/sub\u003e (46.4 wt.% on anhydrous basis) in addition to Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (2.7 wt.% on anhydrous basis). This is interpreted to be due to the addition of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and SiO\u003csub\u003e2\u003c/sub\u003e during a secondary event, as described above, which explains why it does not exhibit the primary fertile character of the protolith due to the low-CaO content (\u0026lt;0.1 wt.% on anhydrous basis). The patchy distribution of carbonate minerals might reveal a variable (and locally high) CO\u003csub\u003e2\u003c/sub\u003e/(H\u003csub\u003e2\u003c/sub\u003eO + CO\u003csub\u003e2\u003c/sub\u003e) ratio within the metaperidotite zone formed during the last serpentinization (= exhumation stage), based on the presence of magnesite at the expense of the radial aggregate of OPX and tremolite (Fig. 4f). The close association of talc with carbonate and OPX and/or the tremolite assemblages indicates that the interaction between the CO\u003csub\u003e2\u003c/sub\u003e-rich fluid and OPX and/or tremolite and the release of SiO\u003csub\u003e2\u003c/sub\u003e occurred after the formation of the secondary OPX.\u003c/p\u003e\n\u003cp\u003eThe olivine in the carbonate-bearing meta-harzburgite (RY26-05) exhibits remarkably high Ni concentrations (Fig. 6a), which cannot be ascribed to the formation of low-Ni metamorphic minerals (OPX and carbonate) because these minerals occur in much lower proportions than olivine (Fig. 3c). The carbonate-bearing OPX-rich rock (RY26-04) contains abundant Ni-rich magnetite (\u0026gt;1 wt.%; Figs 3b and 4c), which represents a major Ni sink in the sample. The olivines in these two samples are enriched with Ni relative to typical mantle peridotites (olivine NiO concentration = 0.3\u0026ndash;0.4 wt.%) (Takahashi 1986; Herzberg et al. 2013). However, the whole-rock Ni contents in these two samples are moderate (2100\u0026ndash;2300 ppm; Table 1). Therefore, the olivine in these two samples might have been enriched in Ni, possibly through the formation of OPX-rich rock via metasomatic interactions between Si-rich fluids/melts and peridotite under a suprasubduction zone (stage Ⅱ of Shmelev et al. 2019). This is consistent with the high amount of calculated normative OPX (42%) (Table 1; Fig. 3).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eP\u0026ndash;T history of the Ray-Iz peridotite massif\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePrevious researchers concluded that the residual peridotite of the Ray-Iz peridotite massif was formed by multistage partial melting under different tectonic settings (beneath a mid-ocean ridge and a suprasubduction zone) (Shmelev 2011; Shmelev et al. 2014, 2018). It is considered that the protolith of OPX-rich rocks in this study was the metasomatized OPX-rich peridotite with/without hornblende, and was formed under a suprasubduction zone setting (stage Ⅱ of Shmelev et al. 2019). As discussed above, the entire Ray-Iz peridotite massif underwent retrogressive recrystallization with/without hydration and prograde recrystallization (deserpentinization). The hydration and recrystallization occurred within the suprasubduction zone (Shmelev et al. 2019). The decomposition of serpentine requires increased temperature (i.e., contact metamorphism) or pressure (i.e., regional metamorphism via subduction). The Sob gabbro-diorite complex to the south of the study area might represent a heat source; however, the Sob complex and Ray-Iz peridotite massif are separated by a tectonic contact (Fig. 1b) (Shmelev and Meng 2013; Shmelev et al. 2014). Further, the lack of a notable mineral-reaction isograd parallel to the boundary of the Sob gabbro-diorite (Arai 1975) is inconsistent with the idea that the Sob complex represents the heat source for deserpentinization. Therefore, we infer that the compression of the forearc mantle during subduction with the downgoing slab into the deep mantle wedge was responsible for the deserpentinization of the Ray-Iz peridotites.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSerpentinization stage\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAfter the formation of the Ray-Iz residual peridotites and OPX-rich metasomatized peridotite, the massif was settled at a low temperature (~600\u0026deg;C) part within a mantle wedge, after which it was partly serpentinized. During this stage, the primary chromian spinel reacted with an H\u003csub\u003e2\u003c/sub\u003eO-rich fluid together with olivine and OPX to form Al- and Cr-rich chlorites via the following reaction: olivine + OPX + spinel + fluid (H\u003csub\u003e2\u003c/sub\u003eO) = chlorite + ferritchromite. Chlorite is not always observed around coarse ferritchromite/magnetite, and its formation is possibly due to later dehydration (deserpentinization) and recrystallization. Clinopyroxene, where present, appears to be converted into tremolite; however, this is inconsistent with the lack of clinopyroxene and/or its pseudomorphs within the OPX-rich rocks and meta-harzburgite. The pressure condition during serpentinization is unclear considering the later low-temperature modification; however, the presence of abundant magnetite/ferritchromite suggests the occurrence of metamorphism in the amphibolite-eclogite facies (1\u0026ndash;2 GPa) (Gervilla et al. 2012). The chemical homogeneity of ferritchromite grains (i.e., the lack of core\u0026ndash;rim zoning) indicates a long residence time at a low temperature (~600\u0026deg;C).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeserpentinization stage\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe deserpentinization of the Ray-Iz peridotites required an increase in pressure via the subduction of the serpentinized mantle with the slab into the deep mantle. The multiphase inclusions of antigorite and talc in the OPX grains indicate that the serpentinized Ray-Iz massif was subducted at a relatively high pressure (\u0026gt;1.6 GPa) and a low temperature (650\u0026deg;C; Padr\u0026oacute;n-Navarta et al. 2010). The composition of ferritchromite and the lack of aluminous green spinel constrain the deserpentinization temperature to below the chlorite decomposition value in the curve in Fig. 7 (~700\u0026deg;C). The chlorite in our samples is possibly stable at considerably high temperatures (~900\u0026deg;C) because of its Cr-rich nature (up to 3.4 wt.% of Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) (Fumagalli et al. 2014). However, the almost negligible contents of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and CaO(\u0026lt; 0.2 wt.%) in the coexisting (recrystallized) OPX mineral prevent the realization of such a high temperature. This is consistent with the presence of abundant euhedral to radially aggregated tremolite grains in our samples (Figs. 4e, 7). Further, no geobarometer was available to constrain the maximum pressure during serpentine decomposition, although the Ray-Iz peridotite massif might have undergone subduction at ultrahigh pressures (deeper than the graphite\u0026ndash;diamond transition; Fig. 7), as indicated by the abundant diamond in the Ray-Iz \u0026nbsp;\u003cbr\u003echromitite (Yang et al. 2015).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eExhumation stage\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFollowing the formation of deserpentinized secondary peridotites, the Ray-Iz massif was serpentinized again during exhumation to produce the present mineral assemblage (olivine, OPX, chlorite, tremolite, talc, serpentine, and magnetite/ferritchromite). Chlorite and magnetite/ferritchromite appear to have been stable throughout, from the first serpentinization stage to the exhumation stage (points 2 and 3 in Fig. 7). Our conclusion is consistent with the previous estimation, which is that near-isochemical serpentinization and deserpentinization events (Shmelev 2011) occurred, preceded by metasomatism.\u003c/p\u003e\n\u003cp\u003eThe mineral chemistry and assemblage of the garnet amphibolite (N-MORB composition) (Shmelev et al. 2014) indicate the pressure\u0026ndash;temperature conditions of ~1.5 GPa and ~700\u0026deg;C (Ernst and Liu 1998). Therefore, we infer that the garnet amphibolite was entrained in the Ray-Iz peridotite during exhumation.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003col\u003e\n \u003cli\u003eThe Ray-Iz rocks and meta-harzburgite contain olivine and OPX with abundant inclusions of opaque minerals (magnetite and ferritchromite), unrelated to hydrous minerals (e.g., serpentine, chlorite, tremolite, and talc). However, hydrous minerals do occur ubiquitously as discrete grains within the rock. The relationship between magnetite/ferritchromite and olivine or OPX indicates a deserpentinization origin for the peridotites.\u003c/li\u003e\n \u003cli\u003eSerpentinization began at the tip of the mantle wedge and proceeded during subduction. High-pressure antigorite serpentinite was dehydrated to afford chlorite- and amphibole-bearing harzburgite during further subduction, and Si-enriched serpentinite was converted into the OPX-rich peridotite (Fig. 7). The peridotite might have entered the diamond-stability field during the deepest point of subduction, immediately before exhumation (Fig. 7). Diamonds can form within chromitites at ultrahigh pressures under the peak condition of this subduction zone setting.\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank D. Dyuragina and D. Kuznetsov for their help during fieldwork. We greatly appreciate the assistance of T. Nishiyama and H. Isobe at Kumamoto University during microprobe analyses and Raman spectroscopy. We thank M. Obata and K. Naemura for their constructive reviews of an earlier version of the manuscript. This research was supported by the MONKASHO SPECIAL BUDGET \u0026ldquo;Decoding ocean-floor dynamics from ophiolites\u0026rdquo; to SA and JSPS KAKENHI (grant numbers JP24540518, JP16K1783400, JP22K03761) to SI.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical approval\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was supported by the MONKASHO SPECIAL BUDGET \u0026ldquo;Decoding ocean-floor dynamics from ophiolites\u0026rdquo; to SA and JSPS KAKENHI (grant numbers JP24540518, JP16K1783400, JP22K03761) to SI.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data presented in the text of this article are fully available without restriction from the author upon request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eArai S (1974) \u0026lsquo;Non-calciferous\u0026rsquo; orthopyroxene and its bearing on the petrogenesis of ultramafic rocks in Sangun and Joetsu zones. 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Geochem Geophys Geosyst 9:Q03007, doi:10.1029/2007GC001583\u003c/li\u003e\n \u003cli\u003eTakahashi E (1986) Origin of basaltic magmas: implications from peridotite melting experiments and an olivine fractionation model. Bull Vol Soc Jpn 30:S17-S40 (in Japanese with English abstract).\u003c/li\u003e\n \u003cli\u003eTrommsdorff V, Evans BW (1972) Progressive metamorphism of antigorite schist in the Bergell tonalite aureole (Italy). Am J Sci 272:423-437\u003c/li\u003e\n \u003cli\u003eTrommsdorff V, L\u0026oacute;pez S\u0026aacute;nchez-Vizca\u0026iacute;no V, G\u0026oacute;mez-Pugnaire MT, M\u0026uuml;ntener O (1998) High pressure breakdown of antigorite to spinifex-textured olivine and orthopyroxene, SE Spain. Contrib Mineral Petrol 132:139-148\u003c/li\u003e\n \u003cli\u003eYang J, Meng F, Xu X, Robinson PT, Dilek Y, Makeyev AB, Wirth R, Wiedenbeck M, Cliff J (2015) Diamonds, native elements and metal alloys from chromitites of the Ray-Iz ophiolite of the Polar Urals. Gondwana Res 27:459-485\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003eTable 1 and 2 are available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"mineralogy-and-petrology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"mipe","sideBox":"Learn more about [Mineralogy and Petrology](http://link.springer.com/journal/710)","snPcode":"710","submissionUrl":"https://submission.nature.com/new-submission/710/3","title":"Mineralogy and Petrology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"opaque inclusion, ferritchromite, deserpentinization, metaperidotite, Ray-Iz ophiolite","lastPublishedDoi":"10.21203/rs.3.rs-4210973/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4210973/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe Ray-Iz (or Rai-Iz) peridotite massif, the main part of the Paleozoic Ray-Iz ophiolite in the Polar Urals, comprises harzburgite, lherzolite, and dunite lenses, which contain abundant chromitite pods. The metaperidotite zone occurs as the ENE-WSW linear zone (width: ~4 km) at the center of the Ray-Iz peridotite massif. We sampled peculiar orthopyroxene-rich peridotites from the metaperidotite zone that contain spherulitic aggregates of orthopyroxene. This investigation provided insights into the key petrologic nature of the Ray-Iz peridotite massif, particularly its P-T history. The orthopyroxene-rich rocks and their surrounding metaperidotites contain various metamorphic minerals, such as tremolite, chlorite, serpentines (lizardite and chrysotile), and talc. Furthermore, the olivine and orthopyroxene grains in these lithologies contain minute inclusions of opaque spinels (ferritchromite and Cr-rich magnetite) and metamorphic minerals (tremolite, chlorite, serpentine minerals, etc.). The spinel occurring both as discrete grains and inclusions in the orthopyroxene-rich rocks exhibits low Al and high Fe\u003csup\u003e3+\u003c/sup\u003e concentrations, and the contents of Ca, Al, and Cr in orthopyroxene are extremely low. The presence of high-Fe\u003csup\u003e3+\u003c/sup\u003e spinel inclusions in olivine and orthopyroxene, combined with the presence of spherulitic aggregates and the extremely low Ca and Al concentrations in orthopyroxene, indicates deserpentinization as the origin of the metaperidotites, including the orthopyroxene-rich rocks. Contact metamorphism appears to be unbefitting as the cause of the deserpentinization because of the lack of a heat source close to the Ray-Iz massif. Instead, we propose that a hydration–dehydration path resulted from the subduction and exhumation of the forearc mantle wedge peridotite.\u003c/p\u003e","manuscriptTitle":"Spherulitic orthopyroxene–bearing metaperidotite in Ray-Iz ophiolites (Polar Urals): high-pressure deserpentinization in a mantle wedge","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-04-10 11:42:14","doi":"10.21203/rs.3.rs-4210973/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-04-08T12:05:00+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-04-08T11:44:53+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-04-05T13:07:55+00:00","index":"","fulltext":""},{"type":"submitted","content":"Mineralogy and Petrology","date":"2024-04-03T08:03:20+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"mineralogy-and-petrology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"mipe","sideBox":"Learn more about [Mineralogy and Petrology](http://link.springer.com/journal/710)","snPcode":"710","submissionUrl":"https://submission.nature.com/new-submission/710/3","title":"Mineralogy and Petrology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"a223acac-fe6e-42e1-8540-9ca675dcdcc8","owner":[],"postedDate":"April 10th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-04-13T16:03:52+00:00","versionOfRecord":{"articleIdentity":"rs-4210973","link":"https://doi.org/10.1007/s00710-025-00962-w","journal":{"identity":"mineralogy-and-petrology","isVorOnly":false,"title":"Mineralogy and Petrology"},"publishedOn":"2026-04-11 15:58:13","publishedOnDateReadable":"April 11th, 2026"},"versionCreatedAt":"2024-04-10 11:42:14","video":"","vorDoi":"10.1007/s00710-025-00962-w","vorDoiUrl":"https://doi.org/10.1007/s00710-025-00962-w","workflowStages":[]},"version":"v1","identity":"rs-4210973","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4210973","identity":"rs-4210973","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}