Melt/mantle mixing produces podiform chromite deposits in ophiolites

implications of Re-Os systematics in the Dongqiao Neo-tethyan ophiolite, northern Tibet

Rendeng Shi*, William L. Griffin, Suzanne Y. O'Reilly, Qishuai Huang, Xiaoran Zhang, Deliang Liu, Xiachen Zhi, Qiongxia Xia, Lin Ding

*Corresponding author for this work

Research output: Contribution to journalArticle

91 Citations (Scopus)

Abstract

Podiform chromite deposits occur in the mantle sequences of many ophiolites that were formed in supra-subduction zone (SSZ) settings. We have measured the Re-Os isotopic compositions of the major chromite deposits and associated mantle peridotites of the Dongqiao Ophiolite in the Bangong-Nujiang suture, Tibet, to investigate the petrogenesis of these rocks and their genetic relationships.The 187Os/188Os ratios of the chromite separates define a narrow range from 0.12318 to 0.12354, less variable than those of the associated peridotites. Previously-reported 187Os/188Os ratios of the Os-rich alloys enclosed in the chromitites define two clusters: 0.12645±0.00004 (2s; n=145) and 0.12003 to 0.12194. The ultra-depleted dunites have much lower 187Os/188Os (0.11754, 0.11815), and the harzburgites show a wider range from 0.12107 to 0.12612. The average isotopic composition of the chromitites (187Os/188Os: 0.12337±0.00001) is low compared with the carbonaceous chondrite value (187Os/188Os: 0.1260±0.0013) and lower than the average value measured for podiform chromitites worldwide (0.12809±0.00085). In contrast, the basalts have higher 187Os/188Os, ranging from 0.20414 to 0.38067, while the plagioclase-bearing harzburgite and cumulates show intermediate values of 187Os/188Os (0.12979~0.14206). Correspondingly, the basalts have the highest 187Re/188Os ratios, up to 45.4±3.2, and the chromites have the lowest 187Re/188Os ratios, down to 0.00113±0.00008. We suggest that melts/fluids, derived from the subducting slab, triggered partial melting in the overlying mantle wedge and added significant amounts of radiogenic Os to the peridotites. Mass-balance calculations indicate that a melt/mantle ratio of approximately 15:1 (melt: 187Re/188Os: 45.4, 187Os/188Os: 0.34484; mantle peridotite: 187Re/188Os: 0.0029, 187Os/188Os: 0.11754) is necessary to increase the Os isotopic composition of the chromitite deposits to its observed average value. This value implies a surprisingly low average melt/mantle ratio during the formation of the chromitite deposits. The percolating melts probably were of variable isotopic composition. However, in the chromitite pods the Os from many melts was pooled and homogenized, which is why the chromitite deposits show such a small variation in their Os isotopic composition. The results of this study suggest that the 187Os/188Os ratios of chromitites may not be representative of the DMM, but only reflect an upper limit. Importantly, the Os-isotope compositions of chromitites strongly suggest that such deposits can be formed by melt/mantle mixing processes.

Original languageEnglish
Pages (from-to)194-206
Number of pages13
JournalGondwana Research
Volume21
Issue number1
DOIs
Publication statusPublished - Jan 2012

Keywords

  • chromite deposit
  • melt/mantle mixing
  • geochemistry
  • Os isotope
  • dongqiao ophiolite
  • Tibet

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