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

通过硫化物中的原位微量元素和 δ34S 特征跟踪俯冲带中的硫循环

• 批准号: 438104271
• 负责人: Professorin Dr. Esther Schwarzenbach
• 金额: --
• 依托单位: Institut für Geologische Wissenschaften
• 依托单位国家: 德国
• 项目类别: Research Grants
• 资助国家: 德国
• 项目状态: 未结题
• 来源: https://gepris.dfg.de/gepris/projekt/438104271?language=en
• 关键词: Tracking; sulphur; cycling; subduction; zones

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.

在这个项目中，我们建议结合新发表的块状岩石痕量元素和δ34S数据以及岩石学观察，研究硫化物矿物的原位微量元素和δ34S组成，以开发一种新的俯冲带S源和S旋回过程的示踪剂。这项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海脊产生的镁铁质和超镁铁质岩性，以及它们从俯冲带(蓝片岩、榴辉岩、反榴辉岩-蛇纹岩)和与脱水有关的矿脉中剥离出来的具有代表性的高压对应物。未变质海底的样本位置包括特征良好的大洋钻探计划地点，以及沿着东太平洋隆起和中大西洋海脊形成的岩石。此外，阿尔巴尼亚Mirdita蛇绿岩中的橄榄岩保存了一段厚达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洞察。这种样品选择将允许通过俯冲海洋岩石圈来区分来自不同岩性(镁铁质与超镁铁质)、不同热液系统(高T与低T)和不同构造环境(快速与缓慢扩张的洋壳)的S通量。与俯冲相关的样品目标区是厄瓜多尔的Raspas杂岩，它包括一整段俯冲的大洋岩石圈，包括变质岩；意大利的Voltri地块，重点将放在折返的反榴辉岩-蛇纹岩和部分脱水的含橄榄石的蛇纹岩上；和新喀里多尼亚的Pouébo榴辉岩混杂岩，包括板块俯冲过程中由内外流体形成的脱水相关脉体。对比俯冲和折返前后大洋岩石圈的原地和块体岩石微量元素和δ34S特征，将有助于深入了解S流入地球内部的通量，以及这些通量如何依赖于构造环境，并可能随着时间的变化而变化。此外，与俯冲有关的样品还将深入了解俯冲变质作用对S的影响以及俯冲带中的微量元素分馏和S旋回过程。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预算的影响，并对弧形背景下的成矿过程提供新的见解。总体而言，这项研究可以为研究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部和外部硫磺储量的预算和同位素组成的演变提供新的线索。