Abstract

Hydrothermal vents above serpentinite produce chemical potential gradients of aqueous and ionic hydrogen and thus provide a very attractive venue for the origin of life. This environment was most favourable before the Earth's massive CO2 atmosphere was subducted into the mantle in the late aftermath of the moon-forming impact. Thermophile to clement conditions persisted for several million years while atmospheric pCO2 dropped from ~25 bar to below 1 bar. The ocean was weakly acid pH ~6, and a large pH gradient existed for nascent life with pH 9 to 11 vent fluids. Total CO2 in water was significant so the vent environment was not carbon limited as in the modern ocean. Phosphorus and Fe(II) were somewhat soluble. This epoch occurred well before the earliest record of surface rocks ~3.8 billion years ago; by then photosynthetic life teemed on the Earth and the oceanic pH was the modern value of ~8. Banded iron formation cannot occur in pH ~8 seawater below a massive CO2 atmosphere, as Fe++ is insoluble, indicate that pCO2 was only modestly above modern levels. Serpentinite existed by 3.9 Ga, but older rocks that might retain evidence of its presence have not been found. The mantle of the Earth sequesters extensive evidence of Archaean and younger subducted biological material. There has been no systematic search of mantle-derived rocks for evidence of life or abiotic conditions in the Hadean