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 delta18O (6.5-8.5‰). Antigorite separates have deltaD values of -29.5‰ to -45.5‰that reflect serpentinization within the mantle wedge whereas chrysotile has low deltaD values (-51.8‰to -84.0‰) 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 °C. Two late cross-fiber chrysotile veins have higher delta18O values of 8.9‰ to 10.8‰ and formed at lower temperatures (as low as ~100 °C). Aqueous fluids in equilibrium with serpentine at 300-375 °C had delta18O=6.5-9‰ and deltaD=-4‰to -26‰, consistent with sediment dehydration reactions at temperatures <200 °C in the subducting slab rather than a basaltic slab source. Three aragonite veins in metabasalt and siltstone clasts within the serpentinite flows have delta18O=16.7-24.5‰, consistent with the serpentinizing fluids at temperatures <250 °C. delta13C values of 0.1-2.5‰ suggest a source in subducting carbonate sediments. The delta34S values of sulfide in serpentinites on Conical Seamount (-6.7‰ to 9.8‰) result from metasomatism through variable reduction of aqueous sulfate (delta34S=14‰) 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 34S (median delta34S (sulfide) = 4.5‰), consistent with a 34S-enriched mantle wedge as inferred from arc lavas.
