Experimental evidence for a hydrous transition zone in the early Earth's mantle

Partition coefficients of H₂O between β and γ phases of olivine stoichiometry and coexisting ultra mafic melt have been estimated to be > 0.1 ± 0.04 (1σ) and 0.04, respectively; based on experiments at 15-16.5 GPa, 1300-1500°C in a hydrous KLB-1 peridotite system. The high H₂O contents of β (1.5-3 wt%) and γ phases (0.7 wt%) would form a reservoir for H₂O after cooling and crystallization of a hydrous magma ocean. Subsequent upwelling of this hydrous mantle would release H₂O at the β phase-olivine boundary near 400 km depth, inducing partial melting of the peridotite to produce hydrous ultramafic magma. Most subducting hydrous minerals dehydrate at pressures shallower than 6.5 GPa if the down-dragged hydrous peridotite follows a P-T path hotter than 900°C at 8 GPa and cannot re-hydrate the transition zone. Therefore, the above proposed partial melting would gradually deplete the H₂O reservoir, which is consistent with the decrease in the activity of ultramafic magmatism and the apparent degree of melting of komatiites from the Archean to the Mesozoic.