A Critical Review of Molybdenite 187Re Parent‐187Os Daughter Intra‐Crystalline Decoupling in Light of Recent In SituMicro‐Scale Observations

Abstract

Two recent advances in instrumentation convey rhenium and osmium elemental and isotopic distribution in molybdenite at the micro‐scale: (1) NanoSIMS imaging which reveals heterogeneities that could preclude micro‐scale geochronology (spot dating), and (2) reaction‐cell LA‐ICP‐MS/MS which partially resolves overlapping masses of isobaric 187Re and 187Os which otherwise confound micro‐scale analyses. The results from each of these measurement principles led respective researchers to conclude that no apparent parent‐daughter decoupling occurs, at least in their chosen samples. This contradicts 20 years of prior macro‐ to micro‐scale Re‐Os molybdenite data. Given these contradictions, we compare their results to our macro‐scale isotope dilution (ID) N‐TIMS measurement results of NIST Reference Material 8599, Henderson molybdenite. We document the homogeneity of the NIST molybdenite, including model age variability, precision of ID N‐TIMS measurement results and statistical treatment. We demonstrate that parent‐daughter decoupling cannot be precluded by visual inspection of NanoSIMS isotopic maps. In addition, we prove mathematically that quantifying reaction‐cell reacted 187Os by LA‐ICP‐MS/MS is far too imprecise to preclude parent‐daughter decoupling at an extent that hinders high‐precision geochronology, although full 2‐D ablation of a molybdenite crystal surface may yield an accurate Re‐Os age. Nevertheless, LA‐ICP‐MS/MS is useful for documenting the spatial extent of heterogeneous parent‐daughter distribution within individual crystals and supports whole crystal ID N‐TIMS.