DIRECT MEASUREMENT OF HYDROTHERMAL FLUIDS FROM ANCIENT SEAFLOOR VENTS: IMPLICATIONS FOR THE COMPOSITION OF ARCHEAN 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. A minimum water depth for the pods of ~980 m has been calculated from fluid inclusion homogenization temperatures and gas data.

Ironstone Pod hydrothermal fluid end-member 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 fluid also contains up to 1.07 mol.% CO2, 0.03 mol.% N2, 0.02 mol.% CH4, 262 ppm COS, and minor amounts of C2-C4 hydrocarbons. Hydrothermal end-member 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, end-member 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 d18OH2O values for the pod hydrothermal end-member fluid define a narrow range from 0.9 to 1.6 per mil, very similar to that of modern vent fluid values (0.4 to 2.1 per mil).

A best estimate for the Ironstone Pod seawater end-member 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. Barberton ambient seawater is considered to have been an evaporative brine of NaCl-CaCl2 composition during the time of pod deposition.

Fluid inclusions contained within quartz-filled pillow lava withdrawal (drainage) cavities have also been analyzed from; (i) ~3.5 Ga Marble Bar lavas from the Pilbara greenstone belt, Australia, (ii) ~3.4 Ga Komati Formation lavas from Barberton, and (iii) ~2.7 Ga Reliance and Zeedersburg Formation lavas from the Belingwe greenstone belt, Zimbabwe. Results for all three areas are similar and show mixing between a fluid similar to the Ironstone Pod seawater composition and a much more saline brine.