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A nanoscale investigation of the Re-Os molybdenite geochronometer
Conference proceeding

A nanoscale investigation of the Re-Os molybdenite geochronometer

Fernando Barra, Artur Deditius, Martin Reich, Matt R. Kilburn, Paul Guagliardo and Malcolm Roberts
Actas - Congreso Geologico Chileno, Vol.15, p.886
XV Congreso Geológico Chileno (University of Concepción, Concepcion, Chile, 18/11/2018–23/11/2018)
2018
url
https://congresogeologicochileno.cl/wp-content/uploads/2018/12/Libro-de-Actas-XVCongresoGeologicoChileno2018-2.pdfView
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Abstract

crystallization dates deformation diffusivity emission spectroscopy Geochronology Isotope geochemistry isotope ratios isotopes laser-induced breakdown spectroscopy metals mineralization molybdenite osmium platinum group radioactive isotopes Re-187/Os-188 Re/Os rhenium spectroscopy stable isotopes sulfides
Rhenium-Os dating of molybdenite (MoS2), the most abundant molybdenum ore mineral, is commonly used to determine the age of mineralization by measurement of its 187Re and 187Os isotope concentrations. Unlike most sulfides found on Earth and in planetary materials, molybdenite is unique because it contains high Re (in the ppm range) and Os (at ppb levels), but almost no initial or common 187Os, hence all 187Os in molybdenite is of radiogenic origin; i.e., produced from the decay of 187Re. The potential of molybdenite as a single-mineral geochronometer was recognized years ago, although some researchers have suggested that 187Re and 187Os isotopes are not spatially linked at the micro-scale in molybdenite precluding the use of microbeam methods for Re-Os dating. It has been argued that this isotopic decoupling of Re and Os is caused by radiogenic 187Os diffusion which may accumulate in crystal deformation sites. Hence, to obtain accurate and reliable ages whole molybdenite crystals should be analyzed in order to overcome the inferred decoupling. In order to determine the crystallographic controls on the distribution of these elements within the molybdenite structure, elemental and isotope mapping of molybdenite grains using electron microprobe and nano-SIMS were performed. Our results show that Re is heterogeneously distributed in molybdenite and that 187Re and 187Os isotopes are not decoupled as previously thought. Both Re and Os are present in solid-solution and as nanoparticles in molybdenite, indicating a complex formation history that can compromise the accurate determination of mineralization ages.

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