Melt/mantle mixing produces podiform chromite deposits in ophiolites: implications of Re-Os systematics in the Dongqiao Neo-tethyan ophiolite, northern Tibet

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.