Weaker drainage of the limited continental surface of Eo-Paleoarchean continents was equivalent to a reduced capacity to neutralize the large volume of fluids emitted by the high-temperature vents of mid-ocean ridges. Without this continental runoff and the absence of detrital deposits, the Paleoarchean Ocean had very specific characteristics (e.g. high proton and Cl- contents, low alkalinity and low Mg content). Thermodynamic models also show that high-temperature hydrothermal fluids are supersaturated with Si, Ge and K.
In the lack of an efficient biosphere, the oceanic Si oversaturation is therefore equilibrated through the low-temperature (80-150°C) off ridge hydrothermal activity. The percolation of seawater into the seafloor basalts beneath an impermeable layer of cherts gave rise to deep-seated alteration of the oceanic crust, structured in three major layers: silicification at the surface, followed at depth by intense carbonatization and then a classic association of greenschist facies. This alteration manifests itself in a massive transfer of heavy Si (δ30Si>>0 in the seafloor) and its depletion in Ge relative to Si (Ge/Si<1).
Similar heavy Si isotopic and low Ge/Si ratio anomalies are systematically observed in all Eo-Paleoarchean crustal granitoids, either as batholithic intrusions or transformed into grey gneisses. This demonstrates that the growth of the very first continents originated from the deep interaction between the seafloor and the silica-saturated primitive ocean. The consequences of these results in terms of intracrustal and mantle recycling will be briefly discussed.