Recycling of water, carbon, and sulfur during subduction of serpentinites: A stable isotope study of Cerro del Almirez, Spain

We use the concentrations and isotope compositions of water, carbon, and sulfur in serpentinites and their dehydration products to trace the cycling of volatiles during subduction. Antigorite serpentinites from the Cerro del Almirez complex, Spain, contain 9-12 wt.% H2O and 910 +/- 730 ppm sulfur, and have bulk delta O-18 values of 8.6 +/- 0.4 parts per thousand, delta D = -54 +/- 5 parts per thousand, and delta S-34 = 5.0 parts per thousand., consistent with serpentinization at temperatures of similar to 200 degrees C by seawater hydrothermal fluids in a seafloor setting. The serpentinites were dehydrated to chlorite-harzburgite (olivine + orthopyroxene + chlorite) at 700 degrees C and 1.6-1.9 GPa during subduction metamorphism, resulting in loss of water, and sulfur. The chlorite-harzburgites contain 5.7 +/- 1.9 wt.% H2O, and have bulk delta O-18 = 8.0 +/- 0.9 parts per thousand., and delta D = 77 +/- 11%. The rocks contain 650 +/- 620 ppm sulfur having delta S-34 = 1.2 parts per thousand. Dehydration of serpentinite resulted in loss of 5 wt.% H2O having delta O-18 = 8-10 parts per thousand and delta D = -27 to -65%. and loss of 260 ppm sulfur as sulfate, having delta S-34 =14.5 parts per thousand,. The contents and delta C-13 of total carbon in the two rock types overlap, with a broad trend of decreasing carbon contents and delta C-13 from similar to 1300 to 200 ppm and -9.6 to -20.2 parts per thousand. This reflects mixing between reduced carbon in the rocks (210 ppm, delta C-13 approximate to - 26 parts per thousand) and seawater-derived carbonate (delta C-13 approximate to -1 parts per thousand). Our results indicate: 1) Serpentinized oceanic peridotites carry significant amounts of isotopically fractionated water, carbon and sulfur into subduction zones; 2) Subduction of serpentinites to high P and T results in loss of water, and sulfur, which can induce melting and contribute to O-18, D. and S-34 enrichments and oxidation of the sub-arc mantle wedge; and 3) Isotopically fractionated water, carbon, and sulfur in serpentinite dehydration products are recycled deeper into the mantle where they can contribute to isotope heterogeneities and may be significant for volatile budgets of the deep Earth. (C) 2012 Elsevier B.V. All rights reserved.