Multiple events in oceanic upper mantle: Ru-Os-Ir alloys in Tibetan ophiolites

R. D. Shi, X. C. Zhi, S. Y. O'Reilly, W. L. Griffin, N. J. Pearson, W. J. Bai, Q. S. Fang, O. Alard, M. Zhang

Research output: Contribution to journalMeeting Abstractpeer-review

Abstract

Ru-Os-Ir alloys from podiform chromitites in the Luobusa and Dongqiao ophiolites (see figure) were analysed for PGEs and 187Os/188Os (in situ). Most grains are osmiridium or iridosmine (<5% Ru; IMA nomenclature). 187Re/188Os is 60.001; individual grains are isotopically homogeneous (187Os/188Os within 0.1%). In the Luobusa ophiolite, 187Os/188Os ratios range from 0.12620 ± 4(1r) to 0.12672 ± 6(1r); the average for all grains (n = 145) is 0.12645 ± 2(1r). Re-depleted model ages (TRD) (Enstatite Chondritic Reservoir) range from 197-270 Ma, consistent with the opening of the Neo-Tethyan Ocean. In contrast, 187Os/188Os in alloys from the Dongqiao ophiolitic chromitite form two groups, mirroring whole-rock Os data for the chromitites. Group I has 187Os/188Os 0.12616 ± 5-0.12664 ± 3 (1r) and TRD from 208 to 276 Ma. Group II 187Os/188Os ranges from 0.12003 ± 5(1r) to 0.12194 ± 3(1r) and the TRD ranges from 871 to 1139 Ma. We suggest: (1) the ophiolitic podiform chromitites originated as mantle-melting residues in the Permian to early Triassic time; (2) the Yarlung-Zangbo and Bangong-Nujiang Neo-Tethyan Oceans opened nearly simultaneously; (3) the 187Os/188Os of the Mesozoic upper mantle ranges from 0.12639 ± 4(1r) to 0.12645 ± 2(1r); (4) the Dongqiao ophiolite contains older material, perhaps relict Rodianian subcontinental lithospheric mantle.
Original languageEnglish
Pages (from-to)A583-A583
Number of pages1
JournalGeochimica et Cosmochimica Acta
Volume70
Issue number18
DOIs
Publication statusPublished - 2006
Event16th Annual V M Goldschmidt Conference - Melbourne, Australia
Duration: 1 Aug 20061 Sept 2006

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