Ancient mantle domains with unradiogenic Os isotopes have been well documented in the Yarlung-Tsangpo ophiolites (YTO), southern Tibet. Nonetheless, previous Os isotope studies have yielded relatively young Re-depletion model (TRD) ages of <1.0 Ga for mantle peridotites of the Zedong ophiolite in the eastern segment of the YTO. In this study, a comprehensive petrological and geochemical study has been conducted on a suite of mantle samples including harzburgites, lherzolites and one pyroxenite. Our results indicate that the Zedong harzburgites have been subjected to low to moderate degrees (∼6–20 %) of partial melting and late-stage melt-rock interaction with basaltic melts. In contrast, both lherzolites and pyroxenite were generated by melt refertilization. The Zedong lherzolites display flat patterns of highly siderophile elements (HSE; Os, Ir, Ru, Pt, Pd, Re), whereas the harzburgites show depletion in Pt, Pd and Re relative to Os, Ir and Ru. The Re-Os isotopes of the Zedong lherzolites have been modified by melt refertilization and thus cannot provide reliable age information. Three Zedong harzburgites with the lowest PdN/IrN and ReN/IrN ratios have the least radiogenic 187Os/188Os of 0.11497–0.11537, giving the old TRD ages of 1.89–1.98 Ga. These ages are considerably older than previously reported TRD ages for the Zedong peridotites (< 1.0 Ga), but comparable to the oldest TRD age for the Luobusa peridotites. This supports the occurrence of similarly old mantle domains in both Zedong and Luobusa ophiolites in the eastern part of the YTO. The Zedong pyroxenite has a radiogenic 187Os/188Os ratio of 0.158 that is similar to the 187Os/188Os ratios (0.156–0.180) of previously reported for the YTO pyroxenites, which can be explained by selective assimilation of interstitial sulfides during the percolation of migrating melts through the mantle lithosphere. Grain boundary sulfides may play a significant role in the high 187Os/188Os of the YTO pyroxenites.

中文翻译：

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Re-Os 同位素证据证明受精的新特提斯海底幔域中古代熔体耗竭


具有非放射性 Os 同位素的古老地幔结构域已在藏南雅鲁-藏布蛇绿岩 （YTO） 中得到充分记录。尽管如此，以前的 Os 同位素研究已经为 YTO 东部的泽东蛇绿岩的地幔橄榄岩产生了相对年轻的再耗尽模型 （TRD） 年龄，为 <1.0 Ga。在这项研究中，对一组地幔样品进行了全面的岩石学和地球化学研究，包括哈茨堡石、针铁矿和一种辉石石。我们的结果表明，泽东哈茨堡岩经历了低到中度 （∼6-20%） 的部分熔融和晚期熔岩与玄武岩熔体的相互作用。相比之下，钙辉石和辉石都是通过熔融施肥生成的。Zedong 针状钙石显示出高度嗜铁元素 （HSE;Os、Ir、Ru、Pt、Pd、Re），而哈茨堡石相对于 Os、Ir 和 Ru 显示 Pt、Pd 和 Re 的消耗。Zedong lherzolites 的 Re-Os 同位素已被熔融受精修饰，因此无法提供可靠的年龄信息。三个 PdN/IrN 和 ReN/IrN 比值最低的泽东哈茨堡岩的放射性 187Os/188Os 最低，为 0.11497-0.11537，老 TRD 年龄为 1.89-1.98 Ga。这些年龄比以前报道的泽东橄榄岩的 TRD 年龄（x3C 1.0 Ga）要古老得多，但与罗布萨橄榄岩最古老的 TRD 年龄相当。这支持了 YTO 东部的 Zedong 和 Luobusa 蛇绿岩中也存在类似的古老地幔域。泽东辉石的放射性 187Os/188Os 比值为 0.158，与 187Os/188Os 比值 （0.156–0.180） 之前报道的 YTO 辉石岩，这可以通过迁移熔体通过地幔岩石圈的渗流过程中间隙硫化物的选择性同化来解释。晶界硫化物可能在 YTO 辉石矿的高 187Os/188Os 中起重要作用。