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

本项目拟通过对硫化物矿物的原位微量元素和δ 34 S组成的研究，结合新的和以前发表的大量岩石微量元素和δ 34 S数据，以及岩石学观测，开发一种新的俯冲带硫源和硫循环过程的示踪剂。本研究的重点将是描述传入板块的化学特征，重点是沿着沿着快速和缓慢扩张的山脊产生的镁铁质和超镁铁质岩性，以及它们代表性的、从俯冲带（蓝片岩、榴辉岩、安辉岩-蛇纹岩）以及与脱水有关的矿脉中挖出的高压等价物。未变质洋底的取样地点包括特征良好的大洋钻探计划地点，其中包括沿着东太平洋海隆和大西洋中脊形成的岩石。此外，来自阿尔巴尼亚米尔迪塔蛇绿岩的橄榄岩保存了厚达10公里的侏罗纪海底，将提供对快速扩张的洋壳下的上地幔的微量元素和硫同位素特征的深入了解。这种样品选择将允许区分来自不同岩性的S通量（镁铁质与超镁铁质），不同的热液系统（高温与低温）和不同的构造环境与俯冲有关的样品的目标地区是厄瓜多尔的Raspas复合体，该复合体包括一个完整的俯冲大洋岩石圈部分，包括变质沉积物;在意大利的Voltri地块，重点将是挖出的安替戈石-蛇纹岩和部分脱水的含橄榄石蛇纹岩;以及新喀里多尼亚的Pouebo Ecloquillo Mélange，其中包括脱水-在板块俯冲过程中，内、外源流体共同作用形成了相关的脉体。大洋岩石圈俯冲前后原位与体相微量元素和δ 34 S特征对比和折返将提供深入了解S通量到地球内部，以及如何这些通量取决于构造环境，并可能随着时间的推移而改变。此外，与俯冲有关的样品将提供深入了解俯冲变质作用对俯冲带中硫和微量元素分馏以及硫循环过程的影响。利用热力学模型，我们进一步的目标是评估这些过程对地幔氧化还原收支的影响，并提供新的见解成矿过程中的弧设置。总的来说，这项研究可以揭示新的有效影响的俯冲蚀变海洋岩石圈对全球硫循环和地球的内部和外部硫水库的预算和同位素组成的演变。

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.