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First in situ Re-Os dating of molybdenite by LA-ICP-MS/MS

Journal article
Authors Johan Hogmalm
Isabell Dahlgren
Irma Fridolfsson
Thomas Zack
Published in Mineralium Deposita
Volume 54
Issue 6
Pages 821-828
ISSN 0026-4598
Publication year 2019
Published at Department of Earth Sciences
Pages 821-828
Language en
Links dx.doi.org/10.1007/s00126-019-00889...
Keywords Molybdenite, In situ, Re-Os, ICP-MS, MS, CH4, chemical-separation, isotopic analysis, ablation, age, Geochemistry & Geophysics, Mineralogy
Subject categories Earth and Related Environmental Sciences

Abstract

Conventional dating of molybdenite (Re-187-Os-187) provides one of few options for direct dating of sulfide mineralization. Unfortunately, in situ dating of molybdenite is considered unreliable due to intra-granular decoupling of Re-187-Os-187. In this study, we developed a new analytical protocol for studies of micron- to grain scale Re-187-Os-187 systematics in molybdenite. Online chemical separation using ICP-MS/MS technology enables in situ dating by beta-decay systems (e.g., Rb-Sr and K-Ca in micas) using laser ablation. Here, the methodology is extended to the Re-187-Os-187 system, another beta-decay system that cannot be resolved by mass spectrometry. Several reaction gases were evaluated, and production of OsCH2 by reaction with CH4 was found to produce strong separation of Os from Re. However, in contrast to the e.g., Rb-87-Sr-87 system, 1-2% of the parent isotope Re also reacted to ReCH2, leaving a significant interference. A mathematical correction of this remaining interference is possible, and Re-187-Os-187 (mass-shifted) can be measured accurately even for fairly extreme ratios. For laser ablation, standards were developed by pressing particulate pellets of conventionally dated molybdenite (Moly Hill and Merlin), because there are no appropriate reference materials available. Six natural molybdenite samples from a range of geological settings, containing >10ppm Re, were analyzed by 70 mu m laser ablation spots, and ages were calibrated by analysis of molybdenite pellets. Contrary to our expectation, weighted average ages obtained were in agreement (within 1%) with conventional age determinations, with fairly good precision (from 1 to 5% 2 sigma depending on Re concentration), suggesting limited or essentially nonexistent decoupling within crystals. Two important implications of this result are that decoupling Re-Os is not universal, and that our new analytical protocol is useful both for dating and for studies of decoupling. The benefit of in situ dating compared to conventional dating is, apart from lower cost and time consumption, the possibility of targeting smaller molybdenite crystals (>= 100 mu m) in thin sections and epoxy mounts. The youngest sample in the study is 920Ma, but we see potential of dating significantly younger Re-rich molybdenite.

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