HR: 16:45h
AN: V44A-04    [Abstracts]
TI: Contrasting sulfur isotope compositions of sulfide minerals between on-ridge and off-ridge hydrothermal fields in the southern Mariana back-arc region
AU: * Kakegawa, T
EM: kakegawa@mail.tains.tohoku.ac.jp
AF: Tohoku University, Graduate School of Science, Aramaki Aza Aoba, Sendai, Miy 980-8578 Japan
AB: New submarine hydrothermal fields were discovered in the southern Mariana back-arc spreading region during the Yokosuka-Shinkai 6500 cruise (October, 2003). One is located on the ridge of spreading center and the other is located on the off-rige site: A low-temperature hydrothermal activity and 10 m-high sulfide chimneys were found on the ridge site and the black-smoker activity with various sulfide chimneys was found on the top of the off-ridge seamount. Both hydrothermal fields were directly drilled by the benthic-multiple coring system during the Hakurei 2 cruise (February, 2004), in order to examine the subsurface hydrothermal processes. Elemental maps of drilled core samples and surface chimneys were constructed using X-ray scanning microscope, and an alteration pattern and types of sulfide minerals were examined. Forming steps of Fe-rich clays near the seafloor were traced and vesicle-filling process by clays and sulfides were found in the examined samples. Sulfur isotope analyses were performed using EA-IRMS on the separated sulfide and sulfate minerals. Sulfur isotope compositions of sulfate minerals are the identical to the seawater sulfate value. Sulfur isotope compositions of sulfides range from +2.9 to +8.7?n at the on-ridge site and _|3.2 to +3.6 ?n at the off-ridge site, respectively. Such regional difference in sulfur isotope compositions of sulfides is probably reflecting the difference in crustal processes: either (1) involvement of sulfate reduction near the discharge zone or (2) isotope exchange among several sulfide and sulfate phases in the deep reaction zone. Chronological change of sulfur isotope compositions was also found in each region: lighter sulfur isotope compositions were found in the younger generation of sulfides. This suggests the style of sulfur cycle in the basaltic crusts (e.g., depth and temperature change for sulfate reduction, potential biological process, etc.) are varying through the development and/or decay of each hydrothermal field.
UR: http://www.ganko.tohoku.ac.jp
DE: 8424 Hydrothermal systems (8135)
DE: 1040 Isotopic composition/chemistry
DE: 0330 Geochemical cycles
DE: 0370 Volcanic effects (8409)
DE: 0400 Biogeosciences
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting