Multidiffusion mechanisms for noble gases (He, Ne, Ar) in silicate glasses and melts in the transition temperature domain: Implications for glass polymerization

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• 作者：Julien Amalberti^a ; ¹ ; ^{jvamalbe@umich.edu" class="auth_mail" title="E-mail the corresponding author} ; Pete Burnard^a ; Didier Laporte^b ; Laurent Tissandier^a ; Daniel R. Neuville^c
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：1 January 2016
• 年：2016
• 卷：172
• 期：Complete
• 页码：107-126
• 全文大小：1676 K

文摘

Noble gases are ideal probes to study the structure of silicate glasses and melts as the modifications of the silicate network induced by the incorporation of noble gases are negligible. In addition, there are systematic variations in noble gas atomic radii and several noble gas isotopes with which the influence of the network itself on diffusion may be investigated. Noble gases are therefore ideally suited to constrain the time scales of magma degassing and cooling. In order to document noble gas diffusion behavior in silicate glass, we measured the diffusivities of three noble gases (⁴He, ²⁰Ne and ⁴⁰Ar) and the isotopic diffusivities of two Ar isotopes (³⁶Ar and ⁴⁰Ar) in two synthetic basaltic glasses (G1 and G2; ²⁰Ne and ³⁶Ar were only measured in sample G1). These new diffusion results are used to re-interpret time scales of the acquisition of fractionated atmospheric noble gas signatures in pumices.

The noble gas bearing glasses were synthesized by exposing the liquids to high noble gas partial pressures at high temperature and pressure (1750–1770 K and 1.2 GPa) in a piston-cylinder apparatus. Diffusivities were measured by step heating the glasses between 423 and 1198 K and measuring the fraction of gas released at each temperature step by noble gas mass spectrometry. In addition we measured the viscosity of G1 between 996 and 1072 K in order to determine the precise glass transition temperature and to estimate network relaxation time scales. The results indicate that, to a first order, that the smaller the size of the diffusing atom, the greater its diffusivity at a given temperature: D(He) > D(Ne) > D(Ar) at constant T. Significantly, the diffusivities of the noble gases in the glasses investigated do not display simple Arrhenian behavior: there are well-defined departures from Arrhenian behavior which occur at lower temperatures for He than for Ne or Ar. We propose that the non-Arrhenian behavior of noble gases can be explained by structural modifications of the silicate network itself as the glass transition temperature is approached: as the available free volume (available site for diffusive jumps) is modified, noble gas diffusion is no longer solely temperature-activated but also becomes sensitive to the kinetics of network rearrangements. The non-Arrhenian behavior of noble gas diffusion close to T_g is well described by a modified Vogel–Tammann–Fulcher (VTF) equation:

where D is the diffusion coefficient, a the diffusion domain size (taken to be the size of the sample), A₁ and C are respectively equivalent to the pre-exponential factor and to the activation energy (E_a in J mol⁻¹) of the classical Arrhenius equation, B₁ can be interpreted as a “pseudo-activation energy” that reflects the influence of the silicate network relaxation, T₂ is the temperature where the diffusion regime switches from Arrhenian to non-Arrhenian, and R is the gas constant (=8.314 J K⁻¹ mol⁻¹).

Finally, our step heating diffusion experiments suggest that at T close to T_g, noble gas isotopes may suffer kinetic fractionation at a degree larger than that predicted by Graham’s law. In the case of ⁴⁰Ar and ³⁶Ar, the traditional assumption based on Graham’s law is that the ratio D⁴⁰Ar/D³⁶Ar should be equal to 0.95 (the square root of the ratio of the mass of ³⁶Ar over the mass of ⁴⁰Ar). In our experiment with glass G1, D⁴⁰Ar/D³⁶Ar rapidly decreased with decreasing temperature, from near unity (0.98 ± 0.14) at T > 1040 K to 0.76 when close to T_g (T = 1003 K). Replicate experiments are needed to confirm the strong kinetic fractionation of heavy noble gases close to the transition temperature.