Argon diffusivities and solubilities in single, gem-quality crystals of forsterite,

enstatite and quartz were deduced from experiments conducted under 0.5 to

~6000bar Ar pressures at 425 to 1200C. Surface-sensitive analyses were used to

confirm that the near-surface regions of the crystals were crystalline, unreacted and

free from structural damage. Polished single-crystal slabs and specimens with

natural facets were placed in open containers and exposed to an argon atmosphere

either in a pressure vessel or in a gas-flow tube furnace at near-atmospheric

pressure. Argon atoms from the pressure medium diffused into the crystals to

produce near-surface concentration gradients, which were directly profiled using

Rutherford back-scattering spectrometry. The following Arrhenius relationships

were obtained:

forsterite: D (m2/s) = 7.2 x 10-20exp[-42(kJ/mol)/RT]

enstatite: D (m2/s) = 1.3 x 10-20exp[-32(kJ/mol)/RT]

quartz: D (m2/s) = 3.1 x 10-19exp[-43(kJ/mol)/RT]

These activation energies were suspected to be apparent ones that were a

combination of the effects of the lattice diffusion of Ar atoms and trapping in

point-defect vacancies. There were no discernible differences in Ar diffusion in

different crystallographic directions, and the diffusivities did not vary as a function

of the intrinsic oxygen fugacity of the experimental vessels. The uptake gradients

also yielded Ar lattice solubilities, as represented by the concentrations at the

mineral surfaces. There were small differences between the solubilities in forsterite

and quartz, but all of the solubilities were high: ~2000ppm[wt]. The Ar solubilities

for iron-bearing enstatite and San Carlos olivine were much higher than for the

other minerals. The mean value for enstatite was 30 to 60% higher than for ironfree

minerals. This was attributed to the incorporation of Ar in point defects that

were more abundant in enstatite due to the oxidation of minor FeO contents. The

Ar solubilities were independent of pressure above ~1bar, but were slightly lower

at 0.5 to 1bar. This suggested that all of the sites capable of accommodating Ar

atoms were filled at pressures of ~1bar. In all cases, Ar appeared to be compatible;

although the degree of compatibility was expected to depend upon the minimum Ar

fugacity required to fully populate the available vacancies.

Lattice Diffusion and Solubility of Argon in Forsterite, Enstatite, Quartz and

Corundum. J.B.Thomas, D.J.Cherniak, E.B.Watson: Chemical Geology, 2008,

253[1-2], 1-22