THE CL--BR--I- COMPOSITION OF SIMILAR-TO-3.23 GA MODIFIED SEAWATER - IMPLICATIONS FOR THE GEOLOGICAL EVOLUTION OF OCEAN HALIDE CHEMISTRY

Fluid inclusion leachates obtained from vug and vein quartz samples fr om an Archean (similar to 3.23 Ga) Fe-oxide hydrothermal deposit in th e west-central part of the Barberton greenstone belt, South Africa, we re analyzed by ion chromatography for chloride, bromide, and iodide. T he deposit, known as the ironstone pods, formed by seafloor hydrotherm al activity and fluid discharge. Quartz is dominated by type I liquid- vapor; aqueous inclusions with a bimodal salinity distribution (0-0.25 MCl- and 0.9-1.8 MCl-). Bulk analytical salinities range from 0.45 to 0.99 MCl- and represent averages of type I inclusions. Bulk fluid inc lusion bromide and iodide concentrations are 1.44-3.32 mM and 0.01-0.1 2 mM, respectively. For comparison, modem seawater has halogen content s of 590 mM chloride, 0.9 mM bromide, and 0.5 mu M total iodine. In th e fluids from the ironstone pods, bromide and iodide are enriched rela tive to chloride, when compared with modern seawater. Approximate Br-/ Cl- and I-/Cl- ratios of 3.2 Ga Barberton seawater are 2.5 x 10(-3) an d 40 x 10(-6), respectively. Dispersion to higher values was caused pr incipally by reaction with organic sediments whose trends are similar to those seen for modem vent fluids at unsedimented and sedimented rid ges, relative to modem seawater. These halide ratios are greater than those of modern seawater, suggesting a change in the halide ratios of seawater over geological time. The analytical data are consistent with a model in which marine organic sedimentation has fractionated bromin e and iodine out of seawater relative to chloride, thereby causing the halide ratios of seawater to decrease from high early and mid-Archean values towards their present day values. (C) 1997 Elsevier Science B. V.