The ¹⁹⁰Pt-¹⁸⁶Os decay system applied to dating platinum-group element mineralization of the Bushveld Complex, South Africa

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• 作者：J.A. Coggon^a ; ^{jude@uni-bonn.de} ; G.M. Nowell^a ; D.G. Pearson^a ; T. Oberthü ; r^b ; J.-P. Lorand^c ; F. Melcher^b ; S.W. Parman^d
• 关键词：Pt&ndash ; Os ; Isotopes ; Dating ; Geochronology ; Bushveld ; PGM
• 刊名：Chemical Geology
• 出版年：2012
• 期刊代码：12_00092541
• 类别：ear
• 出版时间：2 April, 2012
• 卷：302-303
• 期：Complete
• 页码：48-60
• 文件大小：853 K

摘要

We evaluate the ¹⁹⁰Pt-¹⁸⁶Os decay system, measured by laser ablation MC-ICPMS, as a useful geochronometric tool for direct dating of platinum-group minerals (PGM) in large mafic/ultra-mafic intrusions by analysing mineralised rocks from the geochronologically well constrained Bushveld Complex, S. Africa. Mixed PGM (laurite, cooperite, laurite-platarsite, sperrylite and Pt-Fe alloy) from the Merensky Reef yield a Pt-Os isochron age of 1995 卤 50 Ma (2蟽, n = 48, MSWD = 1.16, initial ¹⁸⁶Os/¹⁸⁸Os = 0.119819 卤 0.000006). While this is 59 Myr younger than the U-Pb zircon age for the Merensky Reef (Scoates and Friedman, 2008) it is consistent with recent Re-Os ages for the Platreef. Considering the younger age recorded by both Re-Os and Pt-Os systems, we propose that the Pt-Os isochron age records a late, low temperature hydrothermal event affecting PGM and sulphides in the Platreef and Merensky Reef.

Pt-rich phases may also yield single grain ¹⁹⁰Pt-¹⁸⁶Os model ages since initial ¹⁸⁶Os/¹⁸⁸Os can be well constrained. A Pt-Os model age of 2024 Ma 卤 101 Ma was calculated for a Merensky cooperite; a Pt-Fe alloy from the Onverwacht pipe and a Tweefontein Hill sperrylite give Pt-Os model ages of 2113 卤 106 Ma and 2042 卤 102 Ma, respectively. A Pt-Os isochron derived from all Bushveld units gives an age of 2012 卤 47 Ma (2蟽, ¹⁸⁶Os/¹⁸⁸Os_i = 0.119818 卤 0.000006) with an MSWD of 1.19, hence PGE mineralisation in different stratigraphic horizons of the complex cannot be distinguished temporally using the Pt-Os chronometer. Nonetheless, the potential for the Pt-Os system in dating events in PGE mineralised systems is evident and the agreement of single grain model ages in high Pt/Os phases with the Pt-Os and Re-Os isochron ages indicates their usefulness in the geochronology of these systems.