HR: 1330h
AN: T32A-0909    [PDF]
TI: Chemistry of Springs Across the Mariana Forearc Shows Progressive Devolatilization of the Subducting Pacific Plate
AU: * Mottl, M J
EM: mmottl@soest.hawaii.edu
AF: SOEST, University of Hawaii, Honolulu, HI 96822 United States
AU: Wheat, C G
EM: wheat@mbari.org
AF: GURU, University of Alaska, Fairbanks, AK 99775 United States
AU: Fryer, P
EM: pfryer@soest.hawaii.edu
AF: SOEST, University of Hawaii, Honolulu, HI 96822 United States
AU: Gharib, J
EM: jgharib@soest.hawaii.edu
AF: SOEST, University of Hawaii, Honolulu, HI 96822 United States
AU: Hulme, S
EM: shulme@mlml.calstate.edu
AF: Moss Landing Marine Labs, Moss, Landing, CA 95039 United States
AB: Upwelling pore water fresher than seawater has been sampled from serpentinite mud volcanoes on six cruises to the Mariana forearc in 1987-2003: ODP Legs 125 and 195, Alvin and Shinkai-6500 diving cruises in 1987 and 1996, and two cruises in 1997 and 2003 using piston and gravity coring and the ROV Jason. Low-chlorinity water has been sampled at ten sites on nine serpentinite mud volcanoes from 13o47'N to 19o33'N, ranging from 50-90 km from the trench axis, corresponding to depths to the top of the subducting slab of ~15 to 29 km below the seafloor. Another six seamounts failed to yield freshened water, probably because we failed to hit the right spots. We achieved this success rate using high-resolution bathymetric and sonar maps generated on the same cruises. The freshened waters show systematic variations in composition across the forearc. Near the trench, at 50-56 km distance, springs have high Ca (47-75 mmol/kg) and Sr (310-920 umol/kg) and low alkalinity (3 meq/kg). Farther from the trench, at 70-90 km, springs have high carbonate alkalinity (35-56 meq/kg) and near-zero Ca and Sr, apparently because carbonate dissolution has joined dehydration as a major process at the top of the subducting plate. Sulfate, Na/Cl, K, Rb, Cs, and B all increase regularly with distance from the trench, from values less than in seawater to greater, in response to increasing temperature at depth. All of the waters have high pH and are heavily depleted in Mg, Si, and F relative to seawater. Springs with high carbonate alkalinity also have high CH4, which feeds microbial communities in the shallow subsurface that reduce sulfate to bisulfide at pH 12.5. Chlorinity ranges from 235-538 mmol/kg but does not vary systematically with distance from the trench, because of either variable supply at the source or variable uptake of H2O by serpentinization along the flow path. Volatile elements and compounds including H2O and CO2 thus are progressively leached from the sediment and altered oceanic crust at the top of the subducting Pacific Plate, probably in response to increasing temperature over a range of 100-250oC. Based on leaching of K, the 150oC isotherm is crossed approximately beneath Big Blue Seamount at a depth of ~22 km below the seafloor, 70 km behind the trench. The systematic and sharp chemical gradients across the forearc imply that the aqueous solutions carrying these volatiles rise vertically from their source region at the top of the subducting plate rather than traveling long distances along the decollement before ascending.
DE: 1030 Geochemical cycles (0330)
DE: 1045 Low-temperature geochemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 8150 Plate boundary--general (3040)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting