Pyroxenites, comprising only approximately 2 %–10 % by volume of the upper mantle, are among the most important lithological heterogeneity in the mantle. The formation of pyroxenite veins within peridotites is often attributed to migrating melts which provides an important mechanism for crust-mantle interaction. The Re–Os isotope system provides valuable insight into crust-mantle interaction, as there is a significant contrast in Os/Os between crustal and mantle rocks. Previous Os isotope studies have shown that the forearc mantle wedge mainly consists of highly heterogeneous ancient peridotites. However, limited Re–Os isotope data of forearc mantle pyroxenites exist. Here, two types of mantle pyroxenites from the New Caledonia forearc ophiolite were selected for a comparative study. Contrasting microtextures and geochemical compositions support that these pyroxenites were formed by different processes. The Massif du Sud pyroxenites, hosted in harzburgites, have a cumulate origin and crystallized from boninitic melts generated by remelting of depleted mantle during subduction initiation, whereas the Tiebaghi pyroxenites within the host lherzolites are the products of melt-peridotite reactions. Both types of pyroxenites display highly fractionated highly siderophile element (HSE) patterns. The Massif du Sud pyroxenites are strongly enriched in palladium group platinum-group elements (PPGE) relative to iridiumgroup platinum-group elements (IPGE) with remarkable positive Pt anomalies, which are similar to those of primitive arc lavas and mantle wedge pyroxenite xenoliths. In contrast, the Tiebaghi pyroxenites have flat IPGE but variable PPGE and Re patterns. Compared to the forearc mantle peridotites, the Tiebaghi pyroxenites have higher Os isotope ratios. Ancient basaltic melt percolation followed by long-term (>1 Ga) radiogenic ingrowth can successfully produce the observed correlations of Os/Os with Os or AlO contents. Besides, the oldest Re-depletion model age (T) of ∼1.35 Ga has been obtained for the host lherzolites. Therefore, the Tiebaghi pyroxenites and their host lherzolites probably represent a stranded old mantle relict that has undergone ancient partial melting and melt percolation events. Our study supports the presence of highly radiogenic mantle domains in the forearc and provides significant constraints on the role of ancient basaltic melt percolation for generating mantle heterogeneity.

中文翻译：

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弧前地幔辉石岩中的古代熔体渗流：来自高亲铁元素和锇同位素比的证据

辉石岩仅占上地幔体积的约 2%–10%，是地幔中最重要的岩性异质性之一。橄榄岩内辉石岩脉的形成通常归因于熔融物的迁移，这为壳幔相互作用提供了重要机制。 Re-Os 同位素系统为壳幔相互作用提供了宝贵的见解，因为地壳和地幔岩石之间的 Os/Os 存在显着差异。此前的Os同位素研究表明，弧前地幔楔主要由高度异质性的古代橄榄岩组成。然而，弧前地幔辉石岩的铼-锇同位素数据有限。本次选取新喀里多尼亚弧前蛇绿岩中的两种地幔辉石岩进行对比研究。对比显微结构和地球化学成分支持这些辉石岩是由不同过程形成的。藏于方辉橄榄岩中的 Massif du Sud 辉石岩具有累积起源，并由俯冲起始期间贫化地幔重熔产生的胶质熔体结晶而成，而主体二辉橄榄岩内的 Tiebaghi 辉石岩是熔体-橄榄岩反应的产物。两种类型的辉石岩都显示出高度分馏的高亲铁元素（HSE）模式。 Massif du Sud辉石岩相对于铱族铂族元素（IPGE）强烈富集钯族铂族元素（PPGE），并具有显着的正Pt异常，这与原始弧熔岩和地幔楔辉石岩捕虏体相似。相比之下，Tiebaghi 辉石岩具有平坦的 IPGE，但具有可变的 PPGE 和 Re 模式。与弧前地幔橄榄岩相比，Tiebaghi 辉石岩具有更高的 Os 同位素比。古代玄武岩熔体渗滤以及长期 (>1 Ga) 放射性向内生长可以成功地产生观察到的 Os/Os 与 Os 或 Al2O 含量的相关性。此外，还获得了宿主二辉橄榄岩最古老的Re耗尽模型年龄（T）~1.35 Ga。因此，铁巴吉辉石岩及其宿主二辉橄榄岩可能代表了经历了古代部分熔融和熔融渗滤事件的搁浅的古老地幔遗迹。我们的研究支持弧前存在高放射性地幔域，并为古代玄武岩熔体渗滤产生地幔异质性的作用提供了重要的限制。