HR: 10:25h
AN: V22A-01 INVITED     [Abstracts]
TI: Geochemical and Chronometric Data of Carbonate Veins Provide Insights Into Seawater-Ultramafic Rock Interactions
AU: * Bach, W
EM: wbach@whoi.edu
AF: WHOI, Department of Marine Chemistry and Geochemistry, MS#8,, Woods Hole, MA 02543 United States
AU: Rosner, M
EM: rosner@gfz-potsdam.de
AF: GFZ-Potsdam, Telegrafenberg Haus B, Potsdam, 14473 Germany
AU: Paulick, H
EM: Holger.Paulick@uni-bonn.de
AF: Universitaet Bonn, Mineralogisch-Petrologisches Institut Poppelsdorfer Schloss, Bonn, 53115 Germany
AB: Ocean Drilling Program Leg 209 recovered drill cores of serpentinized peridotite and hydrothermally altered troctolitic and gabbroic rocks from various locations on rift valley walls along magma-starved ridge segments adjacent to the 15$^o$20N Fracture Zone. We have examined carbonate veins to gain insights into chemical evolution paths of seawater circulating within the lithospheric mantle as it is exhumed and uplifted. Three types of carbonate veins were distinguished in cores from Sites 1270-1275 on the basis of their chemical and isotopic compositions: (1) aragonite veins in serpentinite (T=2-15$^o$C, Sr/Ca=6.6-14.1, $\delta^{13}$C=-2 to +3$\permil$), (2) calcite veins in troctolitic rocks (T=5-120$^o$C, Sr/Ca=0.12-0.79, $\delta^{13}$C=-4 to +3$\permil$, and (3) calcite veins in schistous metaultramafic rock (T=90-220$^o$C, Sr/Ca=0.07-0.14, $\delta^{13}$C=0 to +9 $\permil$). The high-T calcite veins formed within detachment faults, while the aragonite veins formed after exhumation and are related to cracking during uplift. These veins may hence provide insights into two different hydrological systems associated with exposure of ultramafic basement in inside corners of fracture zones. Calcite veins from talcous schists have chemical (low U/Ca, Mg/Ca, high K/Ca) and isotopic (low $^{87}$Sr/$^{86}$Sr and $\delta^{7}$Li, see Rosner et al., this session) signatures of high-temperature fluid-rock interactions. Their C and O isotope compositions preserve a record of thermogenic methane production at high temperatures ($>$300$^o$C), followed by cooling and possibly partial oxidation of methane as fluids migrated up along detachment faults. Aragonite veins are young (2 to 12 kyrs at Site 1274 and 38 to $>$55 kyrs at Site 1271) and reveal interesting co-variations between subseafloor depth and age, temperature, as well as chemical indicators of fluid maturity (e.g., Mg/Ca). These data can be used to estimate that (1) fluid flow rates are on the order of 1 cm/yr, (2) the modern thermal gradient in upper basement is about 100-150$^o$C/km, and (3) and 90-95$%$ of Mg are lost as the temperature of circulating seawater increases by 10-15$^o$C. Our results indicate that subseafloor low-temperature seawater-peridotite interactions constitute a Mg sink that can possibly counteract the Mg loss associated with brucite dissolution and olivine weathering at the seafloor. Our estimates suggest an annual loss of about 40 mol Mg per Watt convective heat loss in these settings.
DE: 8135 Hydrothermal systems (8424)
DE: 4825 Geochemistry
DE: 4832 Hydrothermal systems
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 3035 Midocean ridge processes
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting