Self diffusion of network formers (silicon and oxygen) in naturally occurring basaltic liquid
- Univ. of California, Davis, CA (United States)
- Arizona State Univ., Tempe, AZ (United States)
Self diffusion coefficients (D*) for silicon and oxygen in anhydrous basaltic liquid [O/(Si+Al) = 2.5] were measured at 1 and 2 GPa and temperatures between 1320 and 1600{degrees}C. Simple diffusion couples were composed of isotopically normal basaltic glass synthesized from chemical reagents mated to chemically identical glass enriched in {sup 18}O and {sup 30}Si. Concentrations of {sup 18}O and {sup 30}Si across the interfacial region of the couples were analyzed by ion microprobe. At 1 and 2 GPa, D{sub o}* is consistently larger than D{sub Si}* for a given diffusion couple, but only at the highest temperature (1600{degrees}C) is the difference outside the small uncertainties for the analytical measurements. At 1 GPa the self diffusivities for both Si and O are well-described by the Arrhenius relationship 1n D{sub Si,O}* = (-12.5 {+-} 0.2) - (170000 {+-} 2000)/RT, where T is temperature in K, R is the gas constant in J K{sup -1} mole{sup -1}, and D* is expressed in m{sup 2} s{sup -1}. Self diffusion coefficients at 2 GPa are a factor of 1.5 greater and at 1400{degrees}C the activation volume (V{sub a}) is -6.7 cm{sup 3} mol{sup -1}. The similarity in self diffusion coefficients, small activation energies (<50% of the Si-O bonding energy), and negative activation volumes for Si and O self-diffusion in basaltic liquid suggest that network former diffusion is a largely cooperative process involving local contraction of the anionic structure. An evaluation of the Eyring {eta}-D relationship implies a mean translation distance for network former diffusion that is 2-3 times the diameter of the oxygen ion and of the order of the Si-Si separation distance. 43 refs., 5 figs., 2 tabs.
- DOE Contract Number:
- FG03-92ER14240
- OSTI ID:
- 530428
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 60, Issue 3; Other Information: PBD: Feb 1996
- Country of Publication:
- United States
- Language:
- English
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