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

The Ben Nevis area, located just west of the Ontario-Quebec border, comprises part of the Abitibi Greenstone belt, which is widely recognized as one of the world’s best preserved greenstone belts. The rocks belong to the Blake River Group, a thick section of bimodal submarine volcanics that form an easterly plunging regional syncline. The metamorphic grade is prehnite-pumpellyite, which is a favorable precondition for finding well preserved fluid inclusions related to Archean seawater-rock interactions. Two N-S sample traverses give good spatial control of the data. Sample material includes: a breccia pipe, mineralized vein material, quartz-filled vesicles, interstitial quartz from pillowed flows, quartz fill from a flowtop breccia, quartz veins, and quartz-epidote veins. Fluid inclusions are two phase, mainly liquid dominated; a small number of inclusions are vapor rich. The sample material shows very good indications that it is of true primary nature: growth zoning in euhedral to subhedral quartz, visible due to variable fluid inclusion abundance, geopetal features in vesicles, and lack of recrystallization textures. Eutectic temperatures between –30 and –45ºC show the abundance of divalent cations; hence, salinities (n=~1100) were modeled using the CaCl2 – H2O system. The majority of salinities range between 6 and 14 CaCl2 wt% equiv. Homogenization temperatures vary by sample type; the majority ranges between 120 and 160ºC; vapor rich inclusions typically homogenize at slightly higher temperatures. Microthermometric results are similar to data from the ~3.2 – 3.5 Ga Barberton Greenstone Belt, South Africa and are among the first to characterize evolved Archean seawater from the Abitibi Greenstone Belt. These new data suggest that Archean seawater may have been more saline during the Archean compared to modern seawater.