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.
The aim of this project was to understand how peridotite, from the Earth’s mantle, is hydrated to serpentinite when exposed at mid-ocean ridges, and what happens to serpentinite when it is metamorphosed as it is returned to the mantle in subduction zones. Specifically, we looked at cycling of carbon, sulfur, and water, as these are good tracers of biological activity, can be indicators of temperatures of reaction, and are important for the volatile budgets of the Earth. Seafloor serpentinites from northern Italy document two general processes. First, peridotites associated with gabbro intrusions and hydrothermal deposits are serpentinized by high-temperature hydrothermal circulation (>250?C) driven by the gabbro intrusions. Second, peridotites close to the seafloor are serpentinized by seawater at low temperatures (down to near 0?C), and microbial activity is important. Both processes lead to enrichments of water, carbon and sulfur in the serpentinites, but are distinguished by the isotope compositions of sulfur and oxygen in the rocks. We calculate chemical budgets for serpentinization of peridotite, and show that serpentinization results in sinks for carbon and sulfur that are comparable to those in typical altered basaltic ocean crust. Serpentinization is thus an important sink for carbon and sulfur in oceanic basement. Peridotites from the southernmost Alps were serpentinized, then recrystallized and partly dehydrated during metamorphism at temperatures of 600?C during subduction. Our analyses of these rocks indicate serpentinization on the seafloor, with no change in sulfur, carbon, or water during early metamorphism in subduction zones. Peridotites from Cerro del Almirez, Spain, were serpentinized, recrystallized at high pressures like those from the southern Alps, then metamorphosed at higher pressure and temperature (700?C) and dehydrated. Our analyses of these rocks indicate that serpentinization occurred on the seafloor, and that later metamorphic dehydration resulted in loss of significant water and sulfur. Our results indicate: 1) Peridotites serpentinized on the seafloor carry significant amounts of isotopically fractionated water, carbon, and sulfur into subduction zones; 2) Recycling of serpentinites to high pressure and temperature results in loss of water, carbon, and sulfur, which can induce melting, oxidize the mantle, and contribute to isotope enrichments of these elements in island arc volcanoes; and 3) Isotopically fractionated sulfur, water and carbon in serpentinite dehydration products are recycled deeper into the mantle where they are significant for volatile budgets of the deep Earth. This work provided support for a graduate student, maintenance of the fluorination line in the Stable Isotope Laboratory, and the PI's sulfur extraction facilities. The PI gave talks at Forsythe middle school about careers in marine geology and oceanography.
