Uranium and thorium diffusion in diopside

Details Uranium and thorium diffusion in diopside Uranium and thorium diffusion in diopside

Aug 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998e%26psl.160..505v&link_type=abstract

Earth and Planetary Science Letters, vol. 160, Issue 3-4, pp.505-519

Computer Science

13

Diffusion, Diopside, Pyroxene Group, Uranium Disequilibrium, U-238/Th-230, Igneous Processes

Scientific paper

This paper presents new experimental data on the tracer diffusion rates of U and Th in diopside at 1 atm and 1150-1300°C. Diffusion couples were prepared by depositing a thin layer of U-Th oxide onto the polished surface of a natural diopside single crystal, and diffusion profiles were measured by ion microprobe depth profiling. For diffusion parallel to [001] the following Arrhenius relations were obtained: log 10 D U =(-5.75±0.98)-(418±28 kJ/mol)/2.303 RT log 10 D Th =(-7.77±0.92)-(356±26 kJ/mol)/2.303 RT . The diffusion data are used to assess the extent to which equilibrium is obtained during near fractional melting of a high-Ca pyroxene bearing mantle peridotite. We find that the diffusion rates for both elements are slow and that disequilibrium between solid and melt will occur under certain melting conditions. For near-fractional adiabatic decompression melting at ascent rates >3 cm/yr, high-Ca pyroxene will exhibit disequilibrium effects. High-Ca pyroxene will become zoned in U and Th and the melts extracted will be depleted in these incompatible elements relative to melts produced by equilibrium fractional melting. U and Th diffusivities in high-Ca pyroxene are similar, and diffusive fractionation of these elements will be limited. Numerical solutions to a dynamic melting model with diffusion-controlled chemical equilibration indicate that the activity ratio [ 230 Th/ 238 U] in a partial melt of spinel peridotite will be slightly less than 1 for a broad range of melting parameters. This result reinforces the already widely accepted conclusion that melting of spinel peridotite cannot account for 230 Th excesses in mid-ocean ridge and ocean island basalts, and that garnet must therefore be present over part of the melting column.

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