Hydrogen-rich and water-rich fluids exert different control on dissolution mechanism of oxide and silicate minerals in the Earth’s interior. With Mg-silicate-H2 fluids, dissolution tends to be incongruent with the (Mg/Si)fluid < (Mg/Si)Mg-silicate with formation of SiOH and SiH4 complexes in the fluid (Shinozaki et al. 2013, 2014). In contrast, in Mg-silicate-H2O systems, Mg-silicate minerals in the mantle (pyroxene and forsterite) dissolve stoichiometrically (congruently) in aqueous fluids to at least 10 GPa pressure. Metasomatic alteration by H2-rich fluids enriches, therefore, the mantle in SiO2 compared with alteration by H2O fluid. This difference becomes increasingly important with mantle depth because the environment becomes more reducing, which results in an increase of H2/H2O fluids (Shinozaki et al. 2014, this issue). Chemical gradients with depth of the Earth could be affected by increased H2/H2O of mantle fluids whereby Mg/Si ratios, for example, will become variable. Silicate-H2 alteration processes likely also played major roles during the early, core-forming stages of the Earth. Such a process could be responsible for Mg/Si changes in the early silicate Earth.