HYDRATION AND SULFUR METASOMATISM OF FOREARC MANTLE

Conical and Torishima Seamounts in the Mariana forearc are serpentinite protrusions onto the seafloor that result from large scale hydration of the forearc mantle wedge by dewatering of the underlying subducting slab. We have analyzed the stable isotope compositions and sulfur contents of serpentinized peridotite sampled by ODP drilling of these seamounts to investigate the transport of sulfur from the subducting slab to the mantle wedge. Low-chlorinity fluids actively venting from the summit of Conical Seamount are derived from dehydration reactions in the subducting slab, 30 km below. Twenty three variably serpentinized harzburgites (to 295 m below seafloor) contain 1-105 ppm sulfide-S having dS values of -6.7 to +9.8‰, and 20-540 ppm sulfate-S having dS values of -1.1 to +21.8‰. Four highly serpentinized peridotites from inactive Torishima Seamount have sulfide-S contents generally too low for isotope analysis (<20 ppm), but one sample has dS_sulfide=4.7‰. Sulfate-S contents are variable (15-3590 ppm) and dS_sulfate is low (-20 to +6.3‰). Six serpentinites from Conical Seamount have dD values of -29.5 to -84.0‰. Coarser grained samples have the highest values and probably preserve metamorphic values, whereas fine grained samples may have exchanged hydrogen with seawater at low temperatures. Oxygen isotope analyses are underway in order to further constrain temperatures and fluid compositions. The serpentinized peridotites record a complex history of fluids and serpentinization at depth and during exposure at the seafloor. Elevated dS values of sulfide in serpentinite (up to 9.8‰) compared to mantle sulfur (0‰) are the result of input of S-enriched sulfur from sediments in the subducting slab, consistent with results for arc lavas. Locally low dS values of sulfide (-1.1 to -5.5‰) in serpentinized peridotite probably result from microbial sulfate reduction within the seamount edifices, consistent with results for interstitial waters in serpentinite muds on Torishima seamount.  The dS of sulfate in the serpentinized peridotites ranges from seawater values near the surface of Conical Seamount (21.8‰) to low values.predominatly reflecting oxidation of sulfide on the seafloor and during sample storage and chemical extractions.