HR: 1330h
AN: B12A-0777    [PDF]
TI: Serpentinization and Heat Generation at Slow Spreading Ridges
AU: * Allen, D E
EM: alle0167@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, Minneapolis, MN 55455 United States
AU: Seyfried, W E
EM: wes@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, Minneapolis, MN 55455 United States
AB: The discovery of ultramafic hosted hydrothermal systems at Rainbow on the Mid Atlantic Ridge and Lost City, a vent site approximately 15km west of the MAR, provides unique perspectives on chemical and heat generating processes associated with serpentinization at a range of chemical and physical conditions. Vent fluid chemistry associated with alteration of ultramafic rocks at an axial and an off axis site on the MAR provides important constraints on heat and mass transport in subseafloor reaction zones. Low pH, high temperature fluids issuing from vents at the Rainbow site indicate slow rates of olivine hydrolysis and the near proximity of an intermittent magmatic heat source, which is needed to account for the high temperatures and flow rates observed at this site. Based on geochemical constraints, the Lost City vent fluids suggest higher temperatures than actually measured at vents. Therefore, the relatively low measured temperature of the Lost City vent fluid, likely reflects the effects of conductive cooling during transit from the site of alteration to the seafloor. Heat balance models suggest that heat production during serpentinization is ineffective, especially at relatively low and high temperatures due to kinetic constraints. Even at intermediate temperatures, however, where the olivine to serpentine conversion rate is more rapid, relatively low fluid to rock mass ratios are needed for exothermic heat generation to have even a minor influence on alteration temperature. Vent fluid chemistry reported for Lost City, however, suggests relatively high fluid to rock mass ratios as indicated by near seawater concentrations of dissolved Cl, K and Na. Although it can be argued that Cl is taken up during alteration in amounts that exactly balance hydration effects, the same would have to be true of K and Na, which is an unlikely scenario. The combination of vent fluid chemistry and heat balance models indicate that Lost City vent fluids may be associated with tectonically driven circulation where long lived faults allow fluid to access deep crustal sections at relatively high temperatures. Alternatively, vent fluids at Lost City may be produced by off axis gabbroic intrusions or near axis locations where magmatic heat sources are available and then transported to the current off axis location.
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 3035 Midocean ridge processes
DE: 8130 Heat generation and transport
DE: 8135 Hydrothermal systems (8424)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting