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

在缓慢扩张中心形成的海洋地壳中，蛇纹岩化的超基性岩石约占10-20%，再加上俯冲带岩石圈的弯曲，海底地幔也被蛇纹岩化，使蛇纹岩成为海洋岩石圈的重要组成部分。这些岩石暴露在海水中，吸收了H2O和Cl、B、S和c等元素。例如，与枯竭的地幔橄榄岩相比，S可以富集1到2个数量级（分别为~0.1- 1 wt%和~0.01 wt%），并且S同位素从地幔值（d34S = -43 ~ +13 permil vs ~0 permil）显著分异。蛇纹岩还含有大量的有机碳，TOC（总有机碳）值高达2700 ppm， d13C值低至-30 peril。岩石中硫化物和TOC的添加是在一定温度范围内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弧下深处，在那里它们可以影响弧岩浆活动和/或将这些成分返回深部地幔。这一过程也可能影响俯冲带岩浆的S和C同位素组成，并导致地幔中S和C的同位素非均质性。由于对这些过程的了解甚少，因此建议研究俯冲变质过程中经历变P-T条件的特提斯蛇绿岩超基性岩石中S和C的含量和同位素组成。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中的表现；(c)这些元素通过弧火山作用再循环的程度与返回到更深的地幔的程度。以下假设将被评估：（1a）海底低温蛇纹石化过程中海水硫酸盐的微生物还原可能导致蛇纹石中S和TOC含量升高，d34S和d13CTOC值降低（负）。（1b）与局部辉长岩侵入有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d34S值升高。（2a）蛇纹岩在高磷脱水反应中硫的损失导致同位素分馏较少。（2b） TOC在脱水反应中被保留。这些假设将通过分析整个岩石的含量和不同变质蛇纹岩中S和TOC的同位素组成，以及岩石中单个不透明相（硫化物、氧化物和碳质物质）的岩石学和化学分析来检验。样品将来自亚平宁北部的特提斯蛇绿岩，利古里亚阿尔卑斯山脉的Voltri群，以及Betic Cordillera的Almirez块体。更广泛的影响。该项目将资助一名女研究生。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和其他nsf支持的研究项目使用。

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.