Fluid chemistry of Archean seafloor 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 temperature data, when combined with gas chromatographic data, provide a minimum calculated water depth for the pods of 982 m. Ironstone Pod hydrothermal fluid endmember concentrations (Mg = 0) of various dissolved components derived from bulk fluid inclusion crush-leach experiments, 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 contains 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, commonly 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 contributions from sediments and/or organic matter. Calculated delta(18)O(H2O) values for the pod hydrothermal endmember fluid define a narrow range 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 the 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 exception of significantly higher I, NH4, Ca, and Sr in the inclusions. Sulfate concentrations are minimum estimates for Barberton seawater. Fluid inclusion samples containing the greatest amount of seawater component have higher N-2 (up to 0.1 mol%) and low CO2, when compared to samples dominated by the hydrothermal endmember fluid. Barberton ambient seawater is considered to have been an evaporative brine of NaCl-CaCl2 composition during the time of pod deposition. Ironstone Pod fluid inclusion seawater endmember Br/Cl and I/Cl values of 2.45 x 10(-3) and 40.2 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 Elsevier Science Ltd.