CONSTRAINTS ON ARCHEAN (2.7 GA) SEAWATER SALINITY FROM FLUID INCLUSION DATA, BEN NEVIS AREA, ABITIBI GREENSTONE BELT, ONTARIO, CANADA

The Abitibi Greenstone Belt has long been recognized as one of the world’s best preserved greenstone belts. The Ben Nevis area, located just west of the Ontario-Quebec border, has undergone prehnite-pumpellyite grade seafloor metamorphism; this low grade is a favorable precondition for finding well preserved fluid inclusions related to Archean seawater-rock interaction. Structurally, the 2.7 Ga rocks of the Blake River Group form an easterly plunging syncline. Two north-south sample traverses give good spatial control of the data. Sampled material includes quartz-filled vesicles, quartz-, and quartz-epidote veins, quartz-calcite veins, and interstitial quartz from pillowed flows. Six main different fluid inclusion types have been recognized but do not show results distinctly different from each other. Careful petrographic investigations have revealed very good indications that the analyzed material is of true primary nature: locally, euhedral to subhedral quartz shows zoning due to variations in fluid inclusion density, geopetal features occur in undeformed vesicles, and no triple junctions occur between quartz grains. Eutectic temperatures between –30ºC and –45ºC show the abundance of divalent cations. Salinities (n=535) were modeled using the CaCl2 – H2O system; the bulk of the data range between 6 and 22wt% CaCl2 equivalent. Homogenization temperatures range from ~100 – 240ºC. These determinations are consistent with data from the ~3.2 – 3.5 Ga Barberton Greenstone Belt, South Africa, and are among the first to characterize Archean seawater from the Canadian Shield. The information suggests that Archean seawater may have been characterized by variable and high salinities for ≥ ~600 Ma.