HR: 0800h
AN: V41D-0814    [Abstracts]
TI: Hyperacid volcano-hydrothermal fluids from Copahue volcano, Argentina: Analogs for "subduction zone fluids"?
AU: * Varekamp, J C
EM: jvarekamp@wesleyan.edu
AF: Earth and Environmental Sciences, Wesleyan University, Middletown, CT 06459, United States
AB: Hyperacid concentrated Chlorine-Sulfate brines occur in many young arc volcanoes, with pH values <1, high concentrations of volcanogenic elements (S, Cl, F, As, B) and the main rock forming elements (Ca, Al, Mg, K, Na, P). Sulfur isotope data and Silica thermometry from such fluids sampled over a ten year period from the Copahue volcanic system (Argentina) suggest reservoir temperatures of 175-300 oC, whereas the surface fluids do not exceed local boiling temperatures. These fluids are generated at much lower P-T conditions than fluids associated with a dehydrating subducted sediment complex below arc volcanoes, but their fundamental chemical compositions may have similarities. Incompatible trace element, major element concentrations and Pb isotope compositions of the fluids were used to determine the most likely rock protoliths for these fluids. Mean rock- normalized trace element diagrams then indicate which elements are quantitatively extracted from the rocks and which are left behind or precipitated in secondary phases. Most LILE show flat rock-normalized patterns, indicating close to congruent dissolution, whereas Ta-Nb-Ti show strong depletions in the rock-normalized diagrams. These HFSE are either left behind in the altered rock protolith or were precipitated along the way up. The behavior of U and Th is almost identical, suggesting that in these low pH fluids with abundant ligands Th is just as easily transported as U, which is not the case in more dilute, neutral fluids. Most analyzed fluids have steeper LREE patterns than the rocks and have negative Eu anomalies similar to the rocks. Fluids that interacted with newly intruded magma e.g., during the 2000 eruption, have much less pronounced Eu anomalies, which was most likely caused by the preferential dissolution of plagioclase when newly intruded magma interacted with the acid fluids. The fluids show a strong positive correlation between Y and Cd (similar to MORB basalts, Yi et al., JGR, 2000), suggesting that Cd is mainly a rock-derived element that may not show chalcophilic behavior. The fluids are strongly enriched (relative to rock) in As, Zn and Pb, suggesting that these elements were carried with the volcanic gas phase into the system. In summary, if these fluids are broadly similar to fluids from dehydrating subducted sediments, they tend to transport preferently the LILE, LREE, U as well as Th, while the HFSE are left behind.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1043 Fluid and melt inclusion geochemistry
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting