In situ Re–Os isotopic analysis of platinumgroup

Authors: Claudio Marchesi José María GonzálezJiménez Fernando Gervilla Carlos J Garrido William L Griffin Suzanne Y O’Reilly Joaquín A Proenza Norman J Pearson

Publish Date: 2010/09/05

Volume: 161, Issue: 6, Pages: 977-990

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Abstract

Chromitite pods in the MayaríCristal ophiolitic massif eastern Cuba were formed in the Late Cretaceous when island arc tholeiites and MORBlike backarc basin basalts reacted with residual mantle peridotites and generated chromiterich bodies enclosed in dunite envelopes Platinumgroup minerals PGM in the podiform chromitites exhibit important Osisotope heterogeneities at the kilometric hand sample and thin section scales 187Os/188Os calculated at the time of chromitite crystallization ~90 Ma ranges between 01185 and 01295 γOs = −71 to +16 relative to enstatite chondrite and all but one PGM have subchondritic 187Os/188Os Grains in a single hand sample have initial 187Os/188Os that spans from 01185 to 01274 and in one thin section it varies between 01185 and 01232 in two PGM included in chromite which are only several millimeters apart As the Os budget of a single micrometric grain derives from a mantle region that was at least several m3 in size the variable Os isotopic composition of PGM in the MayaríCristal chromitites probably reflects the heterogeneity of their mantle sources on the 10–100 m scale Our results show that this heterogeneity was not erased by pooling and mingling of individual melt batches during chromitite crystallization but was transferred to the ore deposits on mineral scale The distribution of the Os model ages calculated for PGM shows four main peaks at ~100 500 750 and 1000 Ma These variable Os model ages reflect the presence of different depleted domains in the oceanic Pacificrelated upper mantle of the Greater Antilles paleosubduction zone The concordance between the age of crystallization of the MayaríCristal chromitites and the most recent peak of the Os model age distribution in PGM supports that Os in several grains was derived from fertile domains of the upper mantle whose bulk Os isotopic composition is best approximated by that of enstatite chondrites on the other hand most PGM are crystallized by melts that tapped highly refractory mantle sourcesWe thank Anders Meibom and two anonymous reviewers for their constructive remarks on the submitted version of the manuscript We are grateful to O Alard for his useful comments on a preliminary version of the manuscript The analytical data were obtained using instrumentation funded by ARC LIEF and DEST Systemic Infrastructure Grants industry partners and Macquarie University This study was financially assisted by the “International Lithospheric Project” ILP task force CC4MEDYNA by the Spanish “Ministerio de Ciencia e Innovación” MICINN research grants CGL201014848/BTE CGL200761205/BTE AIHF2008073 and FPI BES20058328 by the “Generalitat de Catalunya” grant 2009 SGR 444 and by the “Junta de Andalucía” research groups and grant RNM131 RNM145 and “Proyecto de Excelencia2009RNM4495” CM’s research has been supported by a Marie Curie Intra European Fellowship within the 7th European Community Framework Programme and by a postdoctoral fellowship from the Universidad de Granada Spain This is contribution 676 from the Australian Research Council National Key Centre for the Geochemical Evolution and Metallogeny of Continents http//wwwgemocmqeduau

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