The Role of Serpentinites in Subduction Recycling of Sulfur and Organic Carbon

蛇纹岩化超镁铁岩约占缓慢扩张中心洋壳的10-20%，加上岩石圈在俯冲带弯曲时，额外的海底地幔被蛇纹化，使蛇纹岩成为洋壳的重要组成部分。这些岩石暴露于海水导致吸收H2O和Cl、B、S和C等元素。例如，与亏损地幔橄榄岩相比，S可以被qne富集两个数量级（分别为~0.1- 1重量%和~0.01重量%），并且S同位素从地幔值d34 S中显著分馏（分别为-43至+13 permil和~0 permil）。蛇纹岩还含有大量的有机碳，TOC（总有机碳）值高达2700 ppm，d13 C值低至-30 permil。硫化物和TOC的加入是在一定温度范围内的热液和微生物作用的结果。俯冲带蛇纹岩的再循环可以将水和海水成分输送到亚弧深度，在那里它们可以影响弧岩浆作用和/或将这些成分返回到深部地幔。这一过程还可能影响俯冲带岩浆的S、C同位素组成，并导致地幔S、C同位素的不均匀性。由于这些过程知之甚少，建议检查的内容和同位素组成的特提斯蛇绿岩，经历了不同的P-T条件下俯冲变质的超镁铁岩中的S和C。总体目标是了解：（a）这些岩石中的海底蛇纹石化过程;（B）这些元素在俯冲和高压变质作用期间的行为;（c）这些元素通过弧火山作用再循环与返回更深地幔的程度。将对以下假设进行评估：（1a）海底低温蛇纹岩化过程中海水硫酸盐的微生物还原可导致蛇纹岩中S和TOC含量升高以及d34 S和d13 CTOC值降低（负值）。 (1b)与当地辉长岩侵入体相关的蛇纹石化可能发生在较高的温度下，导致硫化物-S含量和d34 S值升高。(2a)硫损失蛇纹岩在高磷脱水反应导致很少的同位素分馏。(2b)TOC在脱水反应过程中被保留下来。这些假设将通过全岩含量和硫同位素组成的分析以及变质蛇纹岩中TOC和S的同位素组成进行检验，并通过岩石中单个不透明相（硫化物、氧化物和含碳物质）的岩相学和化学分析进行检验。样本将来自北方山脉的特提斯蛇绿岩、利古里亚阿尔卑斯山脉的Voltri群和贝蒂奇科迪勒拉山脉的Almirez群。该项目将支持一名女研究生。它还将为PI监督下的稳定同位素实验室提供持续支持。这些设施通常可供学生使用和其他NSF支持的研究项目。

Serpentinized ultramafic rocks comprise ~10-20% of oceanic crust generated at slow spreading centers, plus additional suboceanic mantle is serpentinized as the lithosphere flexes at subduction zones, making serpentinites an important component of oceanic lithosphere. Exposure of these rocks to seawater results in uptake of H2O and elements such as Cl, B, S, and C. For example, S can be enriched by qne to two orders of magnitude compared to depleted mantle peridotites (~0.1- 1 wt% vs. ~0.01 wt%, respectively), and S isotopes are significantly fractionated from the mantle value d34S = from -43 to +13 permil vs ~0 permil, respectively). Serpentinites also contain significant amounts of organic carbon, with TOC (Total Organic Carbon) values up to 2700 ppm, having d13C values down to -30 permil. The addition of sulfide and TOC to the rocks results from hydrothermal and microbial processes over a range of temperatures.Recycling of serpentinites in subduction zones can transport water and seawater components to sub-arc depths, where they can influence arc magmatism and/or return these components to the deep mantle. This process may also influence S and C isotope compositions of subduction zone magmas, and contribute to isotopic heterogeneities of S and C in the mantle. Because these processes are poorly understood, it is proposed to examine the contents and isotope compositions of S and C in ultramafic rocks of Tethyan ophiolites that have experienced variable P-T conditions during subduction metamorphism. Overall goals are to understand: (a) seafloor serpentinization processes in these rocks; (b) the behavior of these elements during subduction and high-pressure metamorphism; and (c) the extent to which these elements are recycled through arc volcanism vs. returned to the deeper mantle. The following hypotheses will be evaluated: (1a) Microbial reduction of seawater sulfate during low-temperature serpentinization on the seafloor can result in elevated S and TOC contents and low (negative) d34S and d13CTOC values for serpentinites. (1b) Serpentinization associated with local gabbroic intrusions may have occurred at higher temperatures, leading to elevated sulfide-S contents and d34S values. (2a) Sulfur losses from serpentinites during high-P dehydration reactions result in little isotope fractionation. (2b) TOC is retained during dehydration reactions,.These hypotheses will be tested through analyses of whole rock contents and isotope compositions of S and TOC in variably metamorphosed serpentinites, and through petrographic and chemical analyses of individual opaque phases (sulfides, oxides, and carbonaceous material) in the rocks. Samples will be from Tethyan ophiolites in the Northern Appenines, the Voltri Group in the Ligurian Alps, and the Almirez massif in the Betic Cordillera.BROADER IMPACTS. This project will support a female graduate student. It will also provide ongoing support for a stable isotope lab under the PI's supervison. These facilities are generally available for student use and other NSF-supported reseach projects.