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

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

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.