ABSTRACT

The highly siderophile elements (HSE) include the economically critical platinum group elements (PGE; Os, Ir, Ru, Rh, Pt, Pd, Au and Re), gold and rhenium. The HSE are redox sensitive in mantle and seafloor environments and have a strong affinity to iron and sulphur, therefore their distribution within the subducted mantle lithosphere record changes to oxidation state and sulphidation. The mobility of the HSE during subduction has important implications for Re–Os isotopic signatures in the mantle, and the formation of Cu-Au arc-related ore deposits.In this study, subducted rock samples from Alpine Corsica are used to track the HSE in serpentinites and hybrid ultramafic/mafic rocks through the subduction cycle. A comparison of bulk rock HSE concentrations to those in pre-subduction analogues provides insights into the transfer of the HSE throughout the subduction cycle. Serpentinites subducted to blueschist-eclogite facies conditions have similar HSE concentrations to primitive upper mantle (PUM) concentrations, therefore it is concluded that the HSE are not mobilised from serpentinites on the scale of the whole rock or greater. Therefore, as suggested in previous studies, crustal lithologies may be more important contributors of the HSE to the sub-arc mantle, particularly Pt, Pd and Re. In contrast, HSE concentrations in hybrid rocks (talc schist and chlorite schist) deviate from protolith concentrations. Rhenium is higher in the talc schist, and Ir and Ru are lower in the chlorite schist than in the primitive upper mantle (PUM), or possible mafic protoliths.Mineral parageneses place temporal constraints on the growth of hosts to the HSE (sulphides, oxides and metal alloys), and record changes to the activities of oxygen and sulphur (aO₂–aS₂), and hence redox conditions, from pre-subduction to exhumation. Laser ablation-inductively couple plasma mass spectrometry (LA-ICP-MS) was used to determine the HSE concentrations in sulphides and oxides, and the detection of small (∼2–25 µm²) platinum group minerals utilised high resolution SEM mapping techniques. The prograde and retrograde sulphides have lower HSE concentrations compared to sulphides from pre-subduction settings. Therefore, the redistribution of the HSE on a mineral scale from sulphides to alloys and/or other sulphides has occurred within the serpentinites, which may reflect more reducing conditions during serpentinisation or subduction, consistent with the results of thermodynamic modelling. In contrast, the mineral assemblages in the hybrid rocks imply an increase in the extent of sulphidation and oxidation, and higher fluid:rock ratios during exhumation, coincident with Re enrichment in the talc schist, and a decrease in the concentrations of Ir and Ru in the chlorite schist, at length scales greater than those of the rock samples. Therefore, hybridisation of lithologies at the slab-mantle interface may enhance the transfer of the HSE to the sub-arc mantle. If Re transfer from the slab to the sub-arc mantle is possible, this questions the robustness of Re-Os isotope signatures as tracers of crustal recycling.

中文翻译：

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通过俯冲来追踪高度嗜铁亲铁元素：来自高山可西嘉岛的高压蛇纹岩和“混合”岩石的见解

摘要

高度亲铁性元素（HSE）包括经济上至关重要的铂族元素（PGE； Os，Ir，Ru，Rh，Pt，Pd，Au和Re），金和rh。HSE在地幔和海底环境中对氧化还原敏感，并且对铁和硫具有很强的亲和力，因此它们在俯冲的地幔岩石圈中的分布记录了氧化态和硫化的变化。俯冲过程中HSE的流动性对地幔中Re-Os同位素特征以及与Cu-Au弧有关的矿床的形成具有重要意义。在这项研究中，使用了来自高山可西嘉岛的俯冲岩石样本来追踪HSE。俯冲循环形成蛇纹岩和超镁铁质/镁铁质杂岩。将散装HSE浓度与俯冲前类似物中的HSE浓度进行比较，可以深入了解整个俯冲周期中HSE的转移。俯冲到蓝片岩-榴辉岩相条件下的蛇纹岩的HSE浓度与原始上地幔（PUM）浓度相似，因此得出的结论是，HSE并非从蛇纹岩中迁移出整个岩石或更大。因此，正如先前的研究所表明的那样，地壳岩性可能是HSE对弧下地幔尤其是Pt，Pd和Re的重要贡献。相反，混合岩石（滑石片岩和亚氯酸盐片岩）中的HSE浓度与原石浓度不同。滑石片岩中的is含量较高，绿泥石片岩中的Ir和Ru含量低于原始上地幔（PUM），₂ -aS ₂），以及因此的氧化还原条件，从预俯冲到掘尸。激光烧蚀-电感耦合等离子体质谱法（LA-ICP-MS）用于确定硫化物和氧化物中的HSE浓度，并检测少量（〜2–25 µm ²）铂族矿物利用高分辨率SEM映射技术。与来自俯冲前环境的硫化物相比，正向和逆向硫化物的HSE浓度较低。因此，在蛇纹岩中发生了HSE在矿物规模上从硫化物到合金和/或其他硫化物的重新分布，这可能反映了蛇纹石化或俯冲过程中更多的还原条件，这与热力学建模的结果一致。相反，杂化岩石中的矿物组合意味着硫化和氧化程度的增加，掘出过程中流体与岩石的比率更高，与滑石片岩中Re的富集相吻合，而Ir和Ru的浓度降低。亚氯酸盐片岩，其长度尺度大于岩石样品的尺度。因此，平板-地幔界面的岩性混合可以增强HSE向弧下地幔的转移。如果将Re从板块转移到弧下地幔是可能的，这会质疑Re--同位素同位素特征作为地壳再循环示踪剂的鲁棒性。