HR: 14:25h
AN: V43C-04 INVITED     [Abstracts]
TI: Temperature Effects on Phase Relations in Ultramafic-Hosted Hydrothermal Systems
AU: * Seyfried, W E
EM: wes@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, Minneapolis, MN 55455 United States
AU: Foustoukos, D I
EM: Fous0009@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, Minneapolis, MN 55455 United States
AU: Fu, Q
EM: fuxx0033@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, Minneapolis, MN 55455 United States
AB: The effect of temperature on alteration processes in ultramafic hosted hydrothermal systems is significant and manifest by complex changes in secondary mineralization and the composition of coexisting fluids, as suggested by recent experimental and theoretical data. At relatively high temperatures (400C) olivine recrystallization reactions are sluggish, generally limiting mass transfer. In SiO2 bearing fluids, such as the case for the Rainbow hydrothermal system (36N, MAR), evidence indicates olivine recrystallization to a more fayalite-rich phase and talc, enhancing olivine stability at reaction zone conditions. These phases plus tremolite play a key role in maintaining fluid acidity, accounting for the unusually high levels of dissolved metals that characterize the high temperature Rainbow vent fluids. In contrast, hydrothermal alteration of ultramafic rocks by seawater at temperatures below 300C generally results in high pH fluids, serpentinization of olivine and coexisting pyroxene, and Ca for Mg exchange in the fluid. Data also indicate potentially high dissolved H2 and low dissolved Fe and total dissolved sulfide species. Analogous processes likely characterize subseafloor reaction zones at the Lost City Hydrothermal Field (LCHF), which lies on the Atlantis Massif at 30N, 15 km west of the Mid-Atlantic Ridge. Indeed, geochemical modeling of the Lost City vent fluid chemistry suggests subseafloor temperatures of approximately 200C, which are considerably greater than the measured vent fluid temperatures (40 to 90C), suggesting conductive cooling and seawater mixing effects; processes consistent with the reported mineralization of chimney structures. Available data also suggest moderately high fluid/rock mass ratios, which in combination with reaction zone temperature estimates make it unlikely that hydrothermal circulation can be a direct result of the exothermic nature of the conversion of olivine to serpentine. Accordingly, alternative heat sources need to be considered for the LCHF, perhaps involving tectonically driven circulation and a closer linkage between on-axis and off-axis hydrothermal processes. The high methane concentrations reported for Lost City vent fluids are consistent with this scenario, assuming methane formation by FTT synthesis.
DE: 1012 Reactions and phase equilibria (3612, 8412)
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005