FLUID CHEMISTRY OF ARCHEAN SEA-FLOOR HYDROTHERMAL VENTS - IMPLICATIONS FOR THE COMPOSITION OF CIRCA 3.2 GA SEAWATER

Seafloor hydrothermal vents of mid-Archean age (ca. 3230 Ma) have been identified and mapped in the Barberton greenstone belt, South Africa and are known as the Ironstone Pods. Fluid inclusion homogenization te mperature data, when combined with gas chromatographic data, provide a minimum calculated water depth for the pods of 982 m. Ironstone Pod h ydrothermal fluid endmember concentrations (Mg = 0) of various dissolv ed components derived from bulk fluid inclusion crush-leach experiment s, include: Cl (730 mmol/L), Br (2.59), I (0.058), Na (822), NH4 (11.4 ), K (21.5), Ca (42.6), and Sr (0.15). This hydrothermal fluid also co ntains up to 1.07 mol% CO2, 0.03 mol% N-2, 0.02 mol% CH4, 262 ppm COS, and minor amounts of C-2-C-4 hydrocarbons. Hydrothermal endmember Ca, Sr, and NH4, in particular, and to a lesser degree K, I, and CO2, com monly plot on, or very close to, modern vent fluid trends. By contrast , endmember Na and Br concentrations are distinct (higher) from modern vent fluids. High I and NH4 concentrations are consistent with contri butions from sediments and/or organic matter. Calculated delta(18)O(H2 O) values for the pod hydrothermal endmember fluid define a narrow ran ge from 0.9 to 1.6 parts per thousand, very similar to that of modern vent fluid values (0.4-2.1 parts per thousand). A best estimate for th e Ironstone Pod seawater endmember composition is Cl (920 mmol/L), Br (2.25), SO4 (2.3), I (0.037), Na (789), NH4 (5.1), K (18.9), Mg (50.9) , Ca (232), and Sr (4.52). Barberton seawater components are commonly within an order of magnitude of modern seawater values, with the excep tion of significantly higher I, NH4, Ca, and Sr in the inclusions. Sul fate concentrations are minimum estimates for Barberton seawater. Flui d inclusion samples containing the greatest amount of seawater compone nt have higher N-2 (up to 0.1 mol%) and low CO2, when compared to samp les dominated by the hydrothermal endmember fluid. Barberton ambient s eawater is considered to have been an evaporative brine of NaCl-CaCl2 composition during the time of pod deposition. Ironstone Pod fluid inc lusion seawater endmember Br/Cl and I/Cl values of 2.45 x 10(-3) and 4 0.2 x 10(-6), respectively, are within error of bulk Earth (2.38 x 10( -3) and 190 x 10(-6)) and are consistent with the chemistry of 3.23 Ga Barberton seawater being buffered by the mantle. Copyright (C) 1997 E lsevier Science Ltd.