DIFFUSION OF NOBLE GASES IN MELTS OF THE SYSTEM SIO2NAALSI2O6

Abstract

Noble gas sorption experiments were performed in an internally heated pressure vessel at pressures of 1000 and 2500 bar and temperatures of 800°C on vitreous NaAlSi3O8 (albite) and NaAlSi2O6 (jadeite), and at 1200°C on silica. Desorption was studied by Knudsen cell mass spectrometry. The amount of desorbed gas decreases from silica to albite and jadeite composition. At constant heating rate the maximum of gas release shifts to higher temperatures with increasing atomic diameter of the noble gas. Analysis of rate heating experiments yields estimates of the temperature dependence of diffusivity D. Least-squares fit to an Arrhenius-type equation D=D0exp(−EaRT), where D0 in cm2 s−1 and Ea in J mol−1, yielding DAr=2.4·10−6exp(−96000/RT), DKr=2.2·10−5exp(−154000/RT) and DXe=6.3·10−2exp(−293000/RT) for silica; DXe=2.9·104exp(−357000/RT) for albite; and DAr=8.3·10−4exp(−142000/RT), DKr=2.8·10−2exp(−179000/RT) and DXe=2.7·104exp(−352000/RT) for jadeite. The activation energy of noble gas diffusion depends non-linearly on the radius of the diffusing atom. Systematic trends suggest an increase of activation energy of diffusion with substitution of Si by Al + Na. At magmatic temperatures diffusivities of Ar, Kr and Xe approach each other with log D∗=−7.9±0.1 (D∗ in cm2s−1) at 1225±25°C for both albite and jadeite melt.