HR: 15:10h
AN: V53E-07    [Abstracts]
TI: Chlorine Isotope Geochemistry as a Monitor of Fluid-Rock Interaction in Volcanic Systems
AU: * Sharp, Z D
EM: zsharp@unm.edu
AF: Department of Earth and Planetary Sciences, University of New Mexico, 200 Yale Blvd, Albuquerque, NM 87131, United States
AU: Barnes, J D
EM: jdbarnes@unm.edu
AF: Department of Earth and Planetary Sciences, University of New Mexico, 200 Yale Blvd, Albuquerque, NM 87131, United States
AU: Fischer, T
EM: fischer@unm.edu
AF: Department of Earth and Planetary Sciences, University of New Mexico, 200 Yale Blvd, Albuquerque, NM 87131, United States
AB: Near-surface interaction of aqueous fluid with ascending magma may cause phreatomagmatic eruptions, enhanced stoping and fracturing leading to rapid ascent of magma and, in some cases, the formation of epithermal mineral deposits. Many of the conservative tracers commonly used in hydrology, such as stable isotope and chlorine or bromine chemistry, fall apart in the higher temperature systems, where phase separation associated with boiling modifies a fluid's chemistry. Chlorine isotope geochemistry provides a novel technique for tracing fluid-rock interaction in volcanic systems. The value of fumaroles from the Central American arc cover a range of over 16 ‰ (Barnes et al., this meeting). The remarkable spread is probably caused by partitioning of HCl gas from an acidic solution. Measured equilibrium fractionation between HCl(g) and acidic chloride solutions are 1.57 ‰ (at 70°C), in excellent agreement with theoretical estimates (Schauble et al., 2003). However, when a kinetic 'flow-through' experiment is conducted, fractionations as high as 8 ‰ occur, related to a continuous distillation process, in which ³⁵Cl-enriched HCl preferentially dissolves in the aqueous condensate along the flow path. The similarly high δ³⁷Cl values found in a number of Central American volcanic fumaroles are explained by continual vaporization and condensation in the near-surface plumbing system. The low δ³⁷Cl values (down to -5‰) found in other volcanoes are more difficult to explain. One possibility is that the gases represent the residue of extensive volatilization, whereby heavy Cl has left the system, lowering the δ³⁷Cl value of the remaining fluid. In this case, the system must be essentially closed, so that no reintroduction of chloride-rich waters occurs. In contrast, all high δ³⁷Cl value fumaroles are associated with large volcanic lakes, where a more-or-less infinite reservoir of Cl exists. The temporally constant high values indicate a high degree of interconnectivity in these systems. The very different delta values constrain the plumbing systems of each volcano.
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8430 Volcanic gases
DE: 8434 Magma migration and fragmentation
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting