“Null result” diffusion experiments with diopside: implications for pyroxene equilibria

doi:10.1016/0012-821X(82)90201-1

Earth and Planetary Science Letters

Volume 58, Issue 2, April 1982, Pages 285-292

https://doi.org/10.1016/0012-821X(82)90201-1 Get rights and content

Abstract

Data for the diffusion of cations in pyroxenes are relevant to a variety of sub-solidus processes including order-disorder and exsolution. Similar data must also be available if the reliability of geobarometers and geothermometers involving pyroxenes is to be assessed. Two types of diffusion experiment have been performed to determine cation diffusion rates in pyroxenes: (1) interdiffusion between single crystals of diopside and polycrystalline sinters enriched in Al and Fe, and (2) interdiffusion between single crystals of diopside and a glass of the same composition which was isotopically enriched in²⁶Mg and⁴³Ca. Following high-temperature annealing for periods up to several hundred hours, analysis of the diffusion couples, using an electron microprobe and an ion microprobe respectively, failed to show any measurable diffusion profiles. From these “null result” experiments the diffusion coefficients (D) for Al and Fe in diopside are estimated to be less than4×10⁻¹⁴cm²s⁻¹ at 1200°C, and values ofD for Ca and Mg in diopside are estimated to be less than7 × 10⁻¹⁴cm²s⁻¹ at 1250°C. These rates are significantly slower than published tracer-type diffusion data for Ca and Al.

A review of studies of order-disorder, microstructural coarsening, and diffusion in pyroxenes suggest that activation energies for cation exchange are typically in excess of 60 kcal mol⁻¹. Transport rates will be assisted, and activation energies lowered by sample non-stoichiometry, inhomogeneities, high dislocation densities and the presence of water.

The collective data for Al, Mg and Ca diffusion in diopside indicate diffusion coefficients⋍ 10⁻¹⁵cm²s⁻¹ at 1200°C. A comparison with data for diffusion in garnet, olivine and spinel suggests that pyroxenes may have the highest blocking temperatures.

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“Null result” diffusion experiments with diopside: implications for pyroxene equilibria

Abstract

Chem. Geol.

Subsolidus phenomena in pyroxenes

The coarsening kinetics associated with exsolution in an iron-free clinopyroxene

Contrib. Mineral. Petrol.

Experimental coarsening of antiphase domains in a silicate mineral

Science

Evaluation of a crust model of eucrites from the width of exsolved pyroxene

Geochem. J.

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Am. Mineral.

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Contrib. Mineral. Petrol.

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