Viscosity of Peridotite Liquid up to 24 GPa: Predictions From Self-Diffusion Coefficients

Details Viscosity of Peridotite Liquid up to 24 GPa: Predictions From Self-Diffusion Coefficients Viscosity of Peridotite Liquid up to 24 GPa: Predictions From Self-Diffusion Coefficients

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmmr11a..06s&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #MR11A-06

Other

3621 Mantle Processes (1038), 3630 Experimental Mineralogy And Petrology, 3924 High-Pressure Behavior, 5460 Physical Properties Of Materials, 8147 Planetary Interiors (5430, 5724, 6024)

Scientific paper

Knowledge of the effect of pressure on the transport properties of peridotite liquid is required for understanding the dynamics of deep magma oceans and therefore the early differentiation of terrestrial planets. We previously measured the viscosity of peridotite liquid to 13 GPa by in-situ falling sphere viscometry and found a viscosity minimum at ~9 GPa. In order to investigate diffusional properties and extend the viscosity estimates to higher pressures (using the Eyring relation), we have determined self-diffusion coefficients of O, Si, Mg and Ca and chemical diffusivities of Co and Ni up to 25 GPa and 2653 K using multianvil apparatus. One half of each peridotite diffusion couple contained natural isotopic abundances and 1 wt% CoO. The other half was enriched in 18O, 30Si, 25Mg, 44Ca and contained 1 wt % NiO. Samples were heated to the desired temperature at 45 K/sec and run durations ranged from 5 to 30 seconds. Diffusion profiles were analysed by SIMS at the University of Edinburgh and by electron microprobe and were fitted to obtain diffusion coefficients. Both self-diffusivities and chemical diffusivities decrease with pressure to ~9 GPa, and then increase again to 16 GPa. The diffusivity minimum is thus consistent with the previously observed viscosity maximum. The diffusion coefficients determined in the range 20-25 GPa suggest a diffusivity maximum at 18-20 GPa above which diffusivities decease again with increasing pressure. The positive activation volumes determined at 0-9 GPa and >18 GPa are consistent with the behaviour of a simple liquid upon compression by which transport behaviour is inhibited by a decrease in free volume. The negative pressure dependence in the range 9-18 GPa is likely the result of increases in the coordination of network forming cations in the melt structure. The Eyring relation successfully relates diffusivity to viscosity up to 13 GPa and suggests an average diffusional jump distance of ~0.45 nm.

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