Self diffusion of network formers (silicon and oxygen) in naturally occurring basaltic liquid

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°C. Simple diffusion couples were composed of isotopically normal basaltic glass synthesized from chemical reagents mated to chemically identical glass enriched in ¹⁸O and ³⁰Si. Concentrations of ¹⁸O and ³⁰Si across the interfacial region of the couples were analyzed by ion microprobe. At 1 and 2 GPa, D*_o is consistently larger than D*_Si for a given diffusion couple, but only at the highest temperature (1600°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*_(Si,O) = (-12.5 ± 0.2) - (170000 ± 2000)/RT, where T is temperature in K, R is the gas constant in J K^-1 mole^-1, and D* is expressed in m² s^-1. Self diffusion coefficients at 2 GPa are a factor of 1.5 greater and at 1400°C the activation volume (V_a) is -6.7 cm³ mol^-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 η-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. These features of network former diffusion are consistent with the formation of high-coordinated Si as a transition complex in melts populated by Q², Q³, and Q⁴ species. In light of inferred changes in melt structure with increasing silica content, we further speculate that the dominant mode of network former diffusion changes from a cooperative process in basaltic liquid, perhaps involving SiO₅ transition complexes, to an ionic process (Si⁴⁺ and O^2-) in liquids approaching full polymerization.