Stable isotope compositions of serpentinite seamounts in the Mariana forearc: Serpentinization processes, fluid sources and sulfur metasomatism

The Mariana and Izu-Bonin arcs in the western Pacific are characterized by serpentinite seamounts in the forearc that provide unique windows into the mantle wedge. We present stable isotope (O, H, S, and C) data for serpentinites from Conical seamount in the Mariana forearc and S isotope data for Torishima seamount in the Izu-Bonin forearc in order to understand the compositions of fluids and temperatures of serpentinization in the mantle wedge, and to investigate the transport of sulfur from the slab to the mantle wedge. Six serpentine mineral separates have a restricted range of delta O-18 (6.5-8.5 parts per thousand). Antigorite separates have delta D values of -29.5 parts per thousand to -45.5 parts per thousand that reflect serpentinization within the mantle wedge whereas chrysotile has low delta D values (-51.8 parts per thousand to -84.0 parts per thousand) as the result of re-equilibration with fluids at low temperatures. Fractionation of oxygen isotopes between serpentine and magnetite indicate serpentinization temperatures of 300-375 degrees C. Two late cross-fiber chrysotile veins have higher delta O-18 values of 8.9 parts per thousand to 10.8 parts per thousand and formed at lower temperatures (as low as similar to 100 degrees C). Aqueous fluids in equilibrium with serpentine at 300-375 degrees C had delta O-18=6.5-9 parts per thousand and delta D = -4 parts per thousand to -26 parts per thousand, consistent with sediment dehydration reactions at temperatures < 200 degrees C in the subducting slab rather than a basaltic slab source. Three aragonite veins in metabasalt and siltstone clasts within the serpentinite flows have delta O-18 = 16.7-24.5 parts per thousand, consistent with the serpentinizing fluids at temperatures < 250 degrees C. delta C-13 values of 0.1-2.5 parts per thousand suggest a source in subducting carbonate sediments. The delta S-34 values of sulfide in serpentinites on Conical Seamount (-6.7 parts per thousand to 9.8 parts per thousand) result from metasomatism through variable reduction of aqueous sulfate (delta S-34 = 14 parts per thousand) derived from slab sediments. Despite sulfur metasomatism, serpentinites have low sulfur contents (generally < 164 ppm) that reflect the highly depleted nature of the mantle wedge. The serpentinites are mostly enriched in S-34 (median delta S-34(sulfide) = 4.5 parts per thousand), consistent with a S-34-enriched mantle wedge as inferred from arc lavas. (c) 2006 Elsevier B.V. All rights reserved.