Diffusion and solubility of argon in forsterite, enstatite, quartz, corundum and MgO

The geochemistry of argon has proven important in geochronology and for the development of models for terrestrial accretion, mantle structure, outgassing and atmosphere formation. In order to develop models of geochemical processes using noble gases, their diffusivities and solubilities must be known. Many studies during the last several decades have increased our understanding of noble gas diffusion and solubility in glasses and melts. Fewer studies have measured these properties in minerals, and existing noble gas solubility measurements in minerals are highly variable and inconclusive. We determined argon diffusivities and solubilities in forsterite, enstatite, quartz, corundum and MgO from experiments conducted at 1230 to 1550 bars Ar pressure and temperatures from 400°C to 1020°C. Single, gem-quality crystals of each mineral were cut into ~3 mm2 wafers and polished prior to loading them into unsealed ceramic containers, which were then inserted into cold-seal reaction vessels for pressurization with Ar. Rutherford backscattering spectroscopy was used to measure diffusive uptake concentration gradients that developed in the crystals, from which solubilities and diffusivities were extracted. The temperature dependence of argon diffusion is described by Arrhenius relationships. The D0 values range from 1.45x10^{-20} to 8.05x10^{-20} for forsterite, enstatite, corundum and quartz. Activation energies (Ea) range from 30 to 47 kJ/mol for all the minerals except MgO, which preliminary results suggest has a significantly higher D0 and Ea for diffusion of argon. The results for quartz are in agreement with a previously determined Arrhenius relation (Watson and Cherniak, 2003). Our results indicate that argon is retained in these minerals (except perhaps MgO) through protracted, high temperature thermal events. Fractional loss calculations show that a forsterite crystal with a 1 mm radius would retain ~20% of its argon for >106 years at 1300°C. The solubility of argon in forsterite increases with increasing temperature indicating a positive enthalpy of solution (ΔH) whereas the argon solubility in enstatite decreases with increasing temperature indicating a negative ΔH. Argon solubility in the other minerals is independent of temperature within the present uncertainty of the data. Extrapolating the solubility data for forsterite and enstatite to 1300°C (1 bar) and dividing by published argon solubilities for basaltic composition melts yields mineral/melt partition coefficients greater than unity, consistent with the results of Broadhurst et al. (1992). The new data may explain observations of excess argon in mafic xenocrysts of volcanic rocks, but they also complicate mantle degassing scenarios. Watson E.B. and Cherniak D.J. (2003) Lattice diffusion of Ar in quartz, with constraints on Ar solubility and evidence of nanopores. Geochimica et Cosmochimica Acta 67, 2043-2062. Broadhurst C.L., Drake M.J., Hagee B.E., Bernatowicz T.J. (199) Solubility and partitioning of Ne, Ar, and Xe in minerals and synthetic basaltic melts. Geochimica et Cosmochimica Acta 56, 709-723.