HR: 09:50h
AN: V51B-07    [Abstracts]
TI: Strontium Isotopic Evidence for Episodic Discharge of Slab Fluids to Mud Volcanos in the Marianas Forearc
AU: * Bickford, M
EM: mebickfo@syr.edu
AF: Department of Earth Sciences, Syracuse University, Syracuse, NY 13244-1070, United States
AU: Siegel, D I
EM: disiegel@syr.edu
AF: Department of Earth Sciences, Syracuse University, Syracuse, NY 13244-1070, United States
AU: Hill, B M
EM: bhill@swredev.com
AF: Department of Earth Sciences, Syracuse University, Syracuse, NY 13244-1070, United States
AU: Shosa, J
EM: jennifer.d.shosa@exxonmobil.com
AF: ExxonMobil Upstream Research, 3319 Mercer Street, Houston, TX 77027, United States
AB: Subduction of the Pacific plate beneath the Mariana forearc releases hydrous fluids which move upward along fractures and react with fault gouge and fractured harzburgitic mantle rocks to produce serpentinite "mud" that discharges with cold water as springs on the ocean floor. This mud accumulates as seamounts, tens of km wide and up to two km high. As part of Leg 195 of the Ocean Drilling Program, we measured the isotopic composition of Sr in water squeezed from intervals of the cores, the serpentine mud, leaches of the serpentine mud, and in entrained harzburgitic clasts. Sr87/Sr86 in the pore fluids is constant at ~ 0.7056 except for just below the seawater-mud interface where a ratio of 0.7091 and Sr concentration of ~80 ug/L indicate seawater is the major pore fluid. Strontium concentrations abruptly decrease below this interface to ~6 umol/kg, and 87Sr/86Sr decreases to ~0.7056 through the remainder of the penetrated section except where seawater -precipitated aragonite has equilibrated with the rising fluids, raising their isotopic composition to near seawater values, and at one horizon where isotopic composition and strontium concentrations briefly rise to near seawater values. Because the upward flow of fluids is on the order of cm/yr, Sr isotopic equilibration between rising fluids and the carbonate precipitates must have occurred within a maximum of a few hundred years. In contrast, the strontium isotopic compositions of leached serpentine mud, and of harzburgite clasts entrained in the mud, is always significantly greater that that of the pore fluids. In harzburgite clasts the ratio is as great as 0.7088, almost as high as that of seawater. Whereas strontium in aragonite clearly equilibrated isotopically within a few hundred years, strontium in harzburgite clasts has not re-equilibrated with adjacent pore fluids in the same time. The harzburgite clasts and associated serpentine mud must have been near the sea floor, unburied, for a yet undetermined but much longer period of time to have equilibrated from ~ 0.704 to 0.709 prior to subsequent burial and variable equilibration with poer fluids. These variations in the strontium isotopic composition of solids and pore waters suggest episodic rather than steady state expulsion of fluids from the subduction zone . It may be possible to characterize at least the periodicity of fluid release in the mud volcano setting by investigating zonation of strontium isotopic composition of hartzburgite clasts throughout the 60 meter deep composite cores.
DE: 3654 Ultra-high pressure metamorphism
DE: 3902 Creep and deformation
DE: 3924 High-pressure behavior
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
DE: 8162 Rheology: mantle (8033)
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly