Tracking sulphur cycling in subduction zones by in situ trace element and δ34S signatures in sulphides

本项目拟结合新的和已发表的大块岩石微量元素和δ34S数据，结合岩石学观测，对硫化物矿物的原位微量元素和δ34S组成进行研究，为俯冲带S源和S循环过程开发一种新的示踪剂。本研究的重点将是描述进入板块的化学特征，重点是沿着快速和缓慢扩张的山脊产生的基性和超基性岩性，以及它们的代表，从俯冲带（蓝片岩、榴辉岩、反长花岗岩-蛇纹岩）以及与脱水有关的脉中挖掘出的高压岩性。未变质海底的样本位置包括具有良好特征的ODP（海洋钻探计划）地点，其中包括沿东太平洋隆起和大西洋中脊形成的岩石。此外，来自阿尔巴尼亚Mirdita蛇绿岩的橄榄岩，保存了长达10公里厚的侏罗纪海底剖面，将为快速扩张的海洋地壳下的上地幔的微量元素和硫同位素特征提供深入了解。这种样品选择将允许通过蚀变海洋岩石圈的俯冲作用区分来自不同岩性（基性与超基性）、不同热液系统（高t与低t）和不同构造环境（快速扩张与缓慢扩张的海洋地壳）的S通量。与俯冲有关的样品的目标区域是厄瓜多尔的拉斯帕斯杂岩，该杂岩包括一个完整的俯冲海洋岩石圈剖面，包括变质沉积层；意大利的Voltri地块，重点将是发掘出的反长花岗岩-蛇纹岩和部分脱水的含橄榄石的蛇纹岩；新喀里多尼亚的poumacubo榴辉岩，包括在板块俯冲过程中由内源和外源流体形成的与脱水有关的矿脉。对比俯冲和挖掘前后海洋岩石圈原位微量元素和大体积岩石的δ34S特征，将有助于了解地球内部的S通量，以及这些通量如何依赖于构造环境和可能随时间而变化。此外，与俯冲有关的样品将有助于深入了解俯冲变质作用对俯冲带S和微量元素分馏和S循环过程的影响。利用热力学模型，我们进一步旨在评估这些过程对地幔氧化还原收支的影响，并为弧环境中的成矿过程提供新的见解。总体而言，该研究有助于揭示蚀变海洋岩石圈俯冲对全球硫循环的有效影响以及地球内外硫储层收支和同位素组成的演化。

In this project we propose to study the in situ trace element and δ34S compositions of sulphide minerals in combination with new and previously published bulk rock trace element and δ34S data, and petrological observations to develop a new tracer for S sources and S cycling processes in subduction zones. The focus of this study will be to characterize the chemical signature of the incoming plate with emphasis on both mafic and ultramafic lithologies produced along fast- and slow-spreading ridges, and their representative, exhumed high-pressure equivalents from subduction zones (blueschists, eclogites, antigorite-serpentinites) as well as dehydration-related veins. Sample locations for unmetamorphosed ocean floor include well-characterized ODP (Ocean Drilling Program) sites, amongst others, rocks that formed along the East Pacific Rise and the Mid-Atlantic Ridge. In addition, peridotites from the Mirdita ophiolite in Albania, which preserves an up to 10 km thick section of Jurassic seafloor, will provide insight into the trace element and sulphur isotopic signatures of the upper mantle underlying fast-spreading oceanic crust. This sample selection will allow discrimination of S fluxes from different lithologies (mafic versus ultramafic), different hydrothermal systems (high-T versus low-T) and different tectonic settings (fast- versus slow-spreading ocean crust) by subduction of altered oceanic lithosphere.Target areas for subduction-related samples are the Raspas complex in Ecuador that comprises a complete section of subducted oceanic lithosphere including metasediments; the Voltri Massif in Italy where the focus will be on exhumed antigorite-serpentinites and partly dehydrated olivine-bearing serpentinites; and the Pouébo Eclogite Mélange in New Caledonia that includes dehydration-related veins formed by internally derived and externally derived fluids while the slab was subducting.Comparison between in situ and bulk rock trace element and δ34S signatures of oceanic lithosphere before and after subduction and exhumation will provide insight into the S fluxes into Earth’s interior and how these fluxes depend on tectonic setting and may have changed over time. In addition, subduction-related samples will provide insight into the effects of subduction metamorphism on S and trace element fractionation and S cycling processes in subduction zones. Using thermodynamic modelling we further aim to evaluate the impact of these processes on the redox budget of the mantle and provide new insights into ore forming processes within arc settings. Overall, this study can shed new light into the effective impact of subduction of altered oceanic lithosphere on the global sulphur cycle and the evolution of the budget and isotopic composition of the Earth’s internal and external sulphur reservoirs.