Addressing Molybdenite187 Re Parent- 187 Os Daughter Intra-Crystalline Decoupling In Light of Recent In-Situ Micro-Scale Observations

Every so often, an analytical advancement or challenging geological occurrence necessitates re-evaluation of well-established geochronological methods. Rhenium-osmium (Re-Os) dating of sulfide minerals, especially molybdenite, by isotope dilution-negative thermal ionization mass spectrometry (ID-NTIMS) yields demonstrably accurate, robust ages. Difficulty in determining an age is caused by either 1) geological complexity including intra-crystalline heterogeneity or 2) inadequate analytical capabilities. Established, systematic methods overcome most of these difficulties, with exceptions. With respect to molybdenite, geological complexity is encountered as macro-scale polyphase overgrowths. Micro-scale complexity is observed as primary Re oscillatory zoning and secondary intra-granular 187 Re parent- 187 Os daughter decoupling. Macro- and micro-scale geologic heterogeneities are overcome by systematic, targeted sampling protocols built upon careful hand sample and microscopic observations linking molybdenite crystallization to the host rock's geologic history.Advances in instrumentation permit more accurate and precise determination of elemental and isotopic compositions at the micro-scale, which ultimately reveals geological complexity and heterogeneity. Yet, newly produced high-resolution data must be carefully scrutinized and interpreted correctly. Two new micro-scale analytical techniques have been proposed: (1) to use NanoSIMS imaging to reveal heterogeneities that could preclude micro-scale geochronology (spot dating) methods, and (2) enable micro-scale geochronology using LA-ICP-MS/MS to distinguish the interfering masses of parent Re and daughter Os isotopes. We turn first to NIST Reference Material 8599, Henderson molybdenite, to compare its Re-Os characteristics and ID-NTIMS results with the newly published Re-Os molybdenite data. We identify the "best possible results" by quantifying the homogeneity of the NIST molybdenite, including model age variability, precision of ID-NTIMS analyses, and statistical treatment. Subsequently, we examine the limitations of the two newly proposed methods. We demonstrate that parent-daughter decoupling cannot be precluded by visual inspection of NanoSIMS isotopic maps. In addition, we prove mathematically that the quantifying the contribution of 187 Os to the total measured mass 187 by LA-ICP-MS/MS is far too imprecise to achieve high-precision geochronology. Statistical data treatment and reporting can also result in misguided conclusions and applications.