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

Partition coefficients of H2O between beta and gamma phases of olivine stoichiometry and coexisting ultra mafic melt have been estimated to be > 0.1 +/- 0.04 (1 sigma) and 0.04, respectively; based on experiments at 15-16.5 GPa, 1300-1500 degrees C in a hydrous KLB-1 peridotite system. The high H2O contents of beta (1.5-3 wt%) and gamma 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 beta 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 degrees 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.