Abstract
Ru-Os-Ir alloys from podiform chromitites in the Luobusa and Dongqiao ophiolites (see figure) were analysed for PGEs and Os/Os (in situ). Most grains are osmiridium or iridosmine (<5% Ru; IMA nomenclature). Re/Os is 60.001; individual grains are isotopically homogeneous (Os/Os within 0.1%). In the Luobusa ophiolite, Os/Os 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, Os/Os in alloys from the Dongqiao ophiolitic chromitite form two groups, mirroring whole-rock Os data for the chromitites. Group I has Os/Os 0.12616 ± 5-0.12664 ± 3 (1r) and TRD from 208 to 276 Ma. Group II Os/Os 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 Os/Os 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.
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