DIFFUSION OF NOBLE-GASES IN MELTS OF THE SYSTEM SIO2-NAALSI2O6

Noble gas sorption experiments were performed in an internally heated pressure vessel at pressures of 1000 and 2500 bar and temperatures of 800 degrees C on vitreous NaAlSi3O8 (albite) and NaAlSi2O6 (jadeite), and at 1200 degrees C on silica. Desorption was studied by Knudsen cel l mass spectrometry. The amount of desorbed gas decreases from silica to albite and jadeite composition. At constant heating rate the maximu m 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-squar es fit to an Arrhenius-type equation D = D(0)exp (-E(a)/RT), where D-0 in cm(2) s(-1) and E(a) in J mol(-1), yielding D-Ar = 2.4 . 10(-6)exp (-96000/RT), D-Kr = 2.2 . 10(-5)exp (-154000/RT) and D-Xe = 6.3 . 10( -2)exp (-293000/RT) for silica; D-Xe = 2.9 . 10(4)exp (-357000/RT) for albite; and D-Ar = 8.3 . 10(-4)exp (-142000/RT), D-Kr = 2.8 . 10(-2)e xp (-179000/RT) and DXe = 2.7 . 10(4)exp (-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 ot her with log D = -7.9 +/- 0.1 (D* in cm(2) s(-1)) at 1225 +/- 25 degr ees C for both albite and jadeite melt.