Thermodynamic constraints on fayalite formation on parent bodies of chondrites

Thermochemical equilibria are calculated in the multicomponent gas-solution-rock system in order to evaluate the formation conditions of fayalite, (Fe_0.88-1.0Mg_0.12-0)₂ SiO₄, Fa_88-100, in unequilibrated chondrites. Effects of temperature, pressure, water/rock ratio, rock composition, and progress of alteration are evaluated. The modeling shows that fayalite can form as a minor secondary and transient phase with and without aqueous solution. Fayalite can form at temperatures below ∼350 °C, but only in a narrow range of water/rock ratios that designates a transition between aqueous and metamorphic conditions. Pure fayalite forms at lower temperatures, higher water/rock ratios, and elevated pressures that correspond to higher H₂/H₂O ratios. Lower pressure and water/rock ratios and higher temperatures favor higher Mg content in olivine. In equilibrium assemblages, fayalite usually coexists with troilite, kamacite, magnetite, chromite, Ca-Fe pyroxene, and phyllosilicates. Formation of fayalite can be driven by changes in temperature, pressure, H₂/H₂O, and water/rock ratios. However, in fayalite-bearing ordinary and CV3 carbonaceous chondrites, the mineral could have formed during the aqueous-to-metamorphic transition. Dissolution of amorphous silicates in matrices and/or silica grains, as well as low activities of Mg solutes, favored aqueous precipitation of fayalite. During subsequent metamorphism, fayalite could have formed through the reduction of magnetite and/or dehydration of ferrous serpentine. Further metamorphism should have caused reductive transformation of fayalite to Ca-Fe pyroxene and secondary metal, which is consistent with observations in metamorphosed chondrites. Although bulk compositions of matrices /chondrites have only a minor effect on fayalite stability, specific alteration paths led to different occurrences, quantities, and compositions of fayalite in chondrites.