Seafloor hydrothermal fluids, Ben Nevis area, Abitibi Greenstone Belt: Implications for Archean (∼2.7 Ga) seawater properties

A central part of the ∼2.7 Ga Abitibi Greenstone Belt, between the Destor-Porcupine and Cadillac-Larder Lake structures, has been subjected to sub-greenschist facies metamorphism (T < 300 °C). Quartz-filled vesicles and interstitial and drainage cavity quartz from pillowed flows are considered to be the best host for Archean seawater-related fluid inclusions. Petrology revealed primary depositional textures, indicating a lack of significant metamorphism or recrystallization. Hence, the material is considered to be of primary origin, containing primary, Archean trapped fluids. jority of observed fluid inclusions are two-phase and liquid-dominated with a vapor bubble volume of ∼10%. Microthermometry on 58 samples (∼1100 fluid inclusions) and eutectic temperatures between −45 and −30 °C indicate the presence of at least one significant divalent cation in the fluids; hence, salinity is calculated as wt.% CaCl2 equiv. The peak of the salinity distribution of inclusions from vesicles, and interstitial and drainage cavity quartz, falls between ∼4 and ∼10 wt.% CaCl2 equiv.; a small subset of inclusions yielded salinities between 18 and 22 wt.% CaCl2 equiv. Most homogenization temperatures fall in an interval between ∼110 and ∼160 °C. A geothermal gradient estimated from the data of ∼10–15 °C/km is reasonable and consistent with the interpretation of the area as a regional seafloor recharge zone. y porosity in the modern ocean floor does not show the rapid precipitation of quartz that is indicated for the Archean; however, the global occurrence of silica-rich banded iron formations and the lack of silica consuming marine organisms in the Precambrian indicates that marine geochemistry and SiO2-precipitation mechanisms may have been different during the Precambrian. The total salinity of a fluid is dependent primarily on its total chlorinity; hence water–rock interactions with ion exchange cannot be the only reason for the higher salinities encountered in this study. Indications for boiling or high temperature phase separation are not evident, suggesting that these options are not a reason for the observed high salinities. ta are in agreement with six other data sets—from the 3.75 Ga Isua Greenstone Belt (W. Greenland), the 3.24 Ga Pilbara region (W. Australia), the ∼3.2 Ga Barberton Greenstone Belt (South Africa; 2), the ∼2.7 Ga Matagami area (Abitibi Greenstone Belt, Canada), and the Proterozoic (∼2.2 Ga) Ongeluk Formation (South Africa). The consistency of fluid inclusion data suggests that Archean and early Proterozoic seawater may have been more saline, and potentially stratified, compared to modern seawater.