HR: 0800h
AN: T41C-0706    [Abstracts]
TI: A Summary of Chlorine Stable Isotopes as a Volatile Tracer in the Central American and Izu- Bonin-Mariana Volcanic Arcs
AU: * Barnes, J D
EM: jdbarnes@unm.edu
AF: University of New Mexico, Department of Earth and Planetary Sciences MSCO3-2040 1 University of New Mexico, Albuquerque, NM 87131, United States
AU: Sharp, Z
EM: zsharp@unm.edu
AF: University of New Mexico, Department of Earth and Planetary Sciences MSCO3-2040 1 University of New Mexico, Albuquerque, NM 87131, United States
AU: Fischer, T
EM: fischer@unm.edu
AF: University of New Mexico, Department of Earth and Planetary Sciences MSCO3-2040 1 University of New Mexico, Albuquerque, NM 87131, United States
AB: The Cl isotope composition of volcanic gases (gas fumarole and gas condensate samples), hydrothermal waters, and lava and ash samples from volcanic centers along the Central American (CA) and Izu-Bonin-Mariana (IBM) arcs have δ³⁷Cl values ranging from -5 to +12‰. Gas and hydrothermal waters from CA span almost this entire range, whereas lava and ash samples span -2.6 to +3.0‰. In contrast, gas and hydrothermal waters from IBM only range from -5.4 to -0.1‰; overlapping with ash and lava samples ranging from -2.6 to +0.4‰. The high δ³⁷Cl values of volcanic gases are found only at volcanic centers in CA with large associated hydrothermal fields and/or crater lakes, such as Poás, Momotombo, and Santa Ana. It is thought that these enriched ³⁷Cl enriched gases are due to liquid-vapor fractionation in acidic systems and record information regarding the plumbing system of a volcano (see Sharp et al., this meeting). The chlorine isotope composition of the ash and lava samples are probably more representative of the ascending magma, which may serve as a tracer of chlorine sources. δ³⁷Cl values of ash and lava samples in Costa Rica are positive (+0.4 to +3.0‰), with the exception of one Arenal sample. In S. Nicaragua samples are negative and become slightly positive in N. Nicaragua. Samples from at then northern end of the arc are near zero (El Salvador: +0.1 to +0.9‰; Guatemala: -0.5‰). The slightly positive values in Costa Rica may indicate a mantle component with some contribution from isotopically heavy serpentinites. The negative values in Nicaragua are consistent with subducting sediments (structurally bound Cl (SBC) in offshore sediments average -0.5 ± 0.8‰; n = 11), sedimentary pore fluids, and/or isotopically light serpentinites associated with plate bending on the outer rise. Samples from the end of the arc in El Salvador and Guatemala indicate more of a mantle-like signature. In the IBM, there is no variation in δ³⁷Cl values along the length of the arc in either gas or ash samples. In contrast, there are distinct variations across the arc, from the forearc seamounts, frontal arc gases and ashes, and the cross chain, implying variations in the fluid source at different depths within the subduction zone. δ³⁷Cl values for SBC of serpentinite clasts and serpentine clays from the Conical, S. Chamorro, and Torishima seamounts are slightly positive (average = +0.3 ± 0.4‰; n = 19), identical values to seafloor serpentinites, suggesting a serpentinite (crysotile to antigorite transition) source. Volcanic gases and ashes are negative, consistent with a sediment-breakdown source. Subducting sediments have isotopically negative SBC δ³⁷Cl values, ranging from -0.6 to -2.5‰ (n = 10). Basalts from the Guguan cross- chain range from 0.0 to +0.5‰ (n = 3), again indicating a serpentinite source, such as fluids derived from antigorite breakdown.
DE: 1030 Geochemical cycles (0330)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting