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

蛇纹岩在硫和有机碳俯冲再循环中的作用

• 批准号: 0809000
• 负责人: Jeffrey Alt
• 金额: $ 32.82万
• 依托单位: Regents of the University of Michigan - Ann Arbor
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-07-01 至 2011-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0809000&HistoricalAwards=false
• 关键词: Role; Serpentinites; Subduction; Recycling; Sulfur

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.

蛇纹岩化的超镁铁质岩石约占洋壳缓慢扩张中心形成的10-20%，另外，随着岩石圈在俯冲带的弯曲，额外的海底地幔被蛇纹化，使蛇纹岩成为大洋岩石圈的重要组成部分。这些岩石暴露在海水中导致对H2O和Cl、B、S和C等元素的吸收。例如，与亏损的地幔橄榄岩相比，S的qne可以富集两个数量级(分别为~0.1wt%和~0.01wt%)，而S同位素从地幔d34S值显著分馏出来，分别为-43到+13permil和~0permil)。蛇纹岩还含有大量的有机碳，TOC(总有机碳)值高达2700ppm，d13C值低至-30permil。硫化物和有机碳在岩石中的添加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组分输送到弧下深处，在那里它们可以影响弧状岩浆活动和/或将这些组分返回地幔深处。这一过程还可能影响俯冲带岩浆的S和碳同位素组成，并有助于形成地幔中S和碳的同位素不均一性。由于对这些过程了解较少，建议对俯冲变质过程中经历了不同P-T条件的特提斯蛇绿岩超镁铁质岩石中的S和C的含量和同位素组成进行研究。总体目标是了解：(A)这些岩石中的海底蛇纹岩作用过程；(B)这些元素在俯冲和高压变质过程中的行为；(C)这些元素在多大程度上通过弧火山作用循环，而不是回到地幔深处。将评估以下假设：(1)海底低温蛇纹岩作用过程中海水硫酸盐的微生物还原可导致蛇纹岩中S和总有机碳含量升高，d34S和d13CTOC值降低(负)。(1B)与局部辉长岩侵入有关的蛇纹岩作用可能在较高的温度下发生，导致硫化物-S含量和d34S值升高。(2)蛇纹岩在高磷脱水反应中的硫损失导致同位素分馏很少。(2B)TOC在脱水反应中被保留。这些假说将通过对全岩含量和不同变质蛇纹岩中S和TOC的同位素组成的分析，以及对岩石中单独的不透明相(硫化物、氧化物和碳质物质)的岩相学和化学分析来验证。样本将来自北阿佩宁斯的特提斯蛇绿岩，利古里亚阿尔卑斯山的沃尔特里群，以及贝蒂克科迪勒拉的阿尔米雷兹地块。该项目将资助一名女研究生。它还将在国际和平研究所的监督下为稳定的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资助的研究项目使用。