Tracer diffusion of network formers and multicomponent diffusion in dacitic and rhyolitic melts

Journal Article · Sat Feb 01 04:00:00 EST 1992 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(92)90086-X· OSTI ID:5195492

Baker, D R ^[1]

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1. McGill Univ., Montreal, Quebec (Canada) Rensselaer Polytechnic Inst., Troy, NY (United States)

Tracer diffusivities of Si, B, and Ga were measured in anhydrous dacitic and rhyolitic melts at 1.0 GPa and 1,300 to 1,600C. In each melt silicon is the slowest diffusing cation followed by B and then Ga in the rhyolitic melt; in the dacitic melt B and Ga diffusivities are virtually identical. The low activation energy for Si in the rhyolitic melt (139 kJ/mol) is proposed to be due to reduction of the Si-O bond strength by clustering of alkalies. Differences between B and Ga diffusion in rhyolite are related to the large proportion of B in trigonal coordination in these melts whereas Ga is only in tetrahedral coordination. Activation energies for Ga diffusion are presumed similar to those necessary for aluminum tracer diffusion. The gallium activation energies are also similar to the activation energies for viscous transport in these two melts and the activation energy for Si-Al interdiffusion between these two melts. Tracer diffusivities of these network formers were used to model interdiffusion between dacitic and rhyolitic melts previously measured at similar conditions of pressure and temperature. Using the model of Lasaga (1979) with diffusivities measured in this study for network formers and diffusivities of network modifiers from other studies, diffusion profiles were calculated at 1,500, 1,400, and 1,300C. Additionally, the Oxide-Molecule' model of Oishi et al. (1982) was also used at 1,300C. Calculated and measured profiles agree well at 1,500C, but as temperature decreases the mismatch between experiment and model increases. Even the inclusion of nonideal activity terms into the 1,300C model does not resolve discrepancies between calculations and experiments. The Lasaga (1979) model provides a better fit to experiments than the Oishi et al. (1982) model at the conditions studied.

• OSTI ID: 5195492
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Journal Name: Geochimica et Cosmochimica Acta; (United States) Vol. 56:2; ISSN GCACA; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
ACTIVATION ENERGY
ARRHENIUS EQUATION
BORON
CALCIUM IONS
CHARGED PARTICLES
CHEMICAL COMPOSITION
CHEMISTRY
DIFFUSION
ELEMENTS
ENERGY
EQUATIONS
EQUILIBRIUM
GALLIUM
GEOCHEMISTRY
IGNEOUS ROCKS
IONS
MATHEMATICAL MODELS
METALS
PARTICLE MODELS
PARTITION
PETROCHEMISTRY
PETROGENESIS
PRESSURE EFFECTS
RHYOLITES
ROCKS
SEMIMETALS
SILICON
SILICON IONS
STATISTICAL MODELS
STRUCTURAL CHEMICAL ANALYSIS
TEMPERATURE EFFECTS
THERMODYNAMIC MODEL
VOLCANIC ROCKS