The role of O isotope exchange between amorphous silicate and CO gas in the isotopic evolution of SS

Abstract

A series of oxygen isotope exchange experiments between amorphous silicates with forsterite composition and low-pressure 18O-enriched CO gas was carried out to interpret the mass-independent oxygen isotope fractionation observed in primitive Solar System materials. The time evolutions of the degree of oxygen isotope exchange give kinetic parameters of the reaction whose rates are controlled by the supply of CO molecules to the grain surface and diffusion of oxygen-bearing species in the amorphous structure. The difference in reactivity between CO and H2O molecules at the grain surface for amorphous forsterite is comparable to that for Ca-, Al-rich inclusion melt. The obtained kinetic data in this study show that the oxygen isotope exchange between amorphous silicates with CO and H2O gases precedes the gaseous isotope exchange between CO and H2O. This suggests that isotope equilibrium between CO and H2O gases, which has been considered to be kinetically inhibited at low temperatures, may be accomplished through the efficient isotope exchange of amorphous silicate dust with both CO and H2O gases.