CYCLING OF SULFUR IN SUBDUCTION ZONES - THE GEOCHEMISTRY OF SULFUR INTHE MARIANA-ISLAND ARC AND BACK-ARC TROUGH

The sulfur contents and sulfur isotopic compositions of 24 glassy subm arine volcanics from the Mariana Island Arc and back-arc Mariana Troug h were determined in order to investigate the hypothesis that subducte d seawater sulfur (deltaS-34 = 21 parts per thousand) is recycled thro ugh arc volcanism. Our results for sulfur are similar to those for sub aerial arc volcanics: Mariana Arc glasses are enriched in S-34 (deltaS -34 = up to 10.3 parts per thousand, mean = 3.8 parts per thousand) an d depleted in S (20-290 ppm, mean = 100 ppm) relative to MORB (850 ppm S, deltaS-34 = 0.1 +/- 0.5 parts per thousand). The back-arc trough b asalts contain 200-930 ppm S and have deltaS-34 values of 1.1 +/- 0.5 parts per thousand, which overlap those for the arc and MORB. The low sulfur contents of the arc and some of the trough glasses are attribut ed to (1) early loss of small amounts of sulfur through separation of immiscible sulfide and (2) later vapor-melt equilibrium control of sul fur contents and loss of sulfur in a vapor phase from sulfide-undersat urated melts near the minimum in sulfur solubility at fO2 almost-equal -to NNO (nickel-nickel oxide). Although these processes removed sulfur from the melts their effects on the sulfur isotopic compositions of t he melts were minimal. Positive trends of deltaS-34 with Sr-87/Sr-86, LILE and LREE contents of the arc volcanics are consistent with a meta somatic seawater sulfur component in the depleted sub-arc mantle sourc e. The lack of a S-34-rich slab signature in the trough lavas may be a ttributed to equilibration of metasomatic fluid with mantle material a long the longer pathway from the slab to the source of the trough volc anics. Sulfur is likely to have been transported into the mantle wedge by metasomatic fluid derived from subducted sediments and pore fluids . Gases extracted from vesicles in arc and back-arc samples are predom inantly H2O, With minor CO2 and traces of H2S and SO2. CO2 in the arc and back-arc rocks has deltaC-13 values of -2.1 to -13.1 parts per tho usand, similar to MORB. These data suggest that degassing of CO2 could explain the slightly lower deltaC-13 values for some Mariana Trough v olcanic glasses, and that incorporation of subduction-derived organic carbon into the Mariana Trough mantle source may not be necessary. Mor e analyses are required to resolve this question, however.