TEMPERATURES GRADIENTS IN THE MARINE ARCHEAN EXOGENIC CYCLE DERIVED FROM THE ALTERATION OF SEAFLOOR BASALTS, BARBERTON GREENSTONE BELT, SOUTH AFRICA

The Archean exogenic cycle is evident in coupled metabasalt-chert sequences of the Mesoarchean Onverwacht Suite of the Barberton Greenstone Belt. The pillowed basalts were erupted under >2 km of water and record intense water/rock interactions and greenschist metamorphism. Traces of life have been reported in both the cherts and altered glass of pillow margins of the metabasalts. The challenge is to see through the metamorphism and elucidate the paleo-environment of these microbial communities. Steep δ ¹⁸O gradients within the pillow sequences, δ ¹⁸O contrasts between pillow rims and cores, as well as a banded chert xenolith amongst pillows with hydrothermal quartz that has retained its high δ¹⁸O, show that metamorphism has not erased the signatures of seafloor alteration. Temperature gradient can be inferred from the δ¹⁸O of the cherts and basalts. The highest δ¹⁸O (near +20‰) are observed in regional bedded cherts, whereas cherts with hydrothermal field associations range between 9 to 15‰. The basalts range from +6 to +16‰. In multiple profiles, the δ¹⁸O of basalt is positively correlated with the degree of their silicification. Both the δ ¹⁸O and SiO₂ content of the basalts grade to the composition of the cherts at their contacts. The basalts that underlie the high δ¹⁸O regional cherts are proportionately higher in δ¹⁸O than those near the lower δ¹⁸O hydrothermal cherts. These observations can be explained by at least two differing mechanisms for silicification of the basalts. The first may follow, passively, the formation of the bedded chert bodies at the seawater/basalt pile interface. The second is more aggressive; occurring within the basalt pile in response to the passage of focused and diffuse hydrothermal fluids. In the first instance the basalts altered at a lower temperature than in the second, but the fluid would be the same (chemically modified) seawater. The data is best explained by 0‰ seawater forming the regional cherts and silicifying the basalts at the top of the sections around 80°C. Simultaneously, seawater derived hydrothermal systems alter some of the buried basalts at 180 to 250°C.