Origin of the Earth’s First Felsic Crust: A Hydrogen Perspective?

Abstract

The paper presents experimental data on melting model compositions of basaltic komatiite (BK) and enstatite chondrite (ECH) at a temperature of T = 1300°C and hydrogen pressure \({{P}_{{{{{\text{H}}}_{{\text{2}}}}}}}\) = 100 MPa. The experiments modeled interaction between the magma ocean and the early Earth’s hydrogen atmosphere. The experimental products consist of silicate glass (quenched melts), which is notably depleted in FeO but enriched in lithophile oxides and H₂O, and iron with minor Si and O admixtures. The equilibrium oxygen fugacity in the experimental runs was approximately two logarithmic units below the Fe−FeO buffer. Calculation of the fractional crystallization of the melts indicates that the complete crystallization products are granodiorite, which consists of two feldspars, clinopyroxene, and quartz with a minor amount of black mica (for the starting composition obtained in the run with BK), or quartz−two feldspars granite with minor amounts of biotite and muscovite (for the starting composition obtained in the run with ECH). Crystallization of zircon from the ECH melt might occur at T = 730−750°C. Our proposed model is the first that explains generation of melts enriched in SiO₂ and H₂O by internal processes of planetary evolution and does not invoke pre-hydrated upper crust for generating the Earth’s first felsic material.