Calcium self-diffusion in diopside at high temperature: implications for transport properties |
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Authors: | A Dimanov O Jaoul |
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Institution: | (1) GeoForschungsZentrum Potsdam, Telegrafenberg, HD, E4, D-14473 Potsdam, Germany Fax: +49-331-2881328; e-mail: dima@gfz-potsdam.de, DE;(2) Laboratoire des Mécanismes et Transferts en Géologie, Université Paul Sabatier, 39 allées Jules Guesde, F-31000 Toulouse, France, FR |
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Abstract: | We have investigated 44Ca self-diffusion in natural diopside single crystals (containing ∼2 atomic % Fe) at temperatures up to 1320 °C (i.e. 30 °C
below the nominal melting point). Oxygen fugacity was controlled by gaseous mixtures. Diffusion profiles ranging from ∼50
to 500 nm were analysed by Rutherford Back-Scattering Spectrometry (RBS). The present results are complementary to previous
studies, and show that in both synthetic (Fe-poor) and natural (Fe-rich) diopside, there are two different diffusion regimes
for Ca with a transition at ∼1230±15 °C. Below this temperature diffusion is characterised by an activation enthalpy of ∼284±10
kJ/mol, while at higher temperatures it increases up to ∼1006±75 kJ/mol. These regimes are proposed to be respectively extrinsic
and intrinsic. For the intrinsic regime Ca self-diffusion may involve Ca-Frenkel point defects. These are pairs of a vacancy
on a M2 site and a calcium cation on an interstitial (normally unoccupied) site. The concentration of such point defects depends
only on temperature, and it is especially important at very high temperatures. The activation enthalpy for intrinsic diffusion
may represent the half defect formation enthalpy plus the migration enthalpy for movement through interstitial sites. For
the extrinsic regime we propose Ca self-diffusion to involve extrinsic interstitial point defects with concentration proportional
to ()–0.19±0.03. We suggest that for both regimes, Ca diffusion involves the well known M3 sites in the octahedral layers, as well as sites in the tetrahedral layers, that we call M4. These sites are especially convenient to explain the observed isotropic diffusion. Increasing concentration of Ca-Frenkel
point defects may be related to the onset of premelting, which affects the thermodynamic properties of Fe-“free” diopside
above 1250 °C. In the light of the present results, premelting is also expected to occur in natural Fe-bearing diopside and
it could strongly influence its thermodynamic and transport properties. Subsequently, in deep upper mantle conditions (T≈1250 °C–1300 °C)
where premelting could occur, diffusional cation exchanges with surrounding phases and diffusion controlled creep might be
facilitated. Finally, our diffusion data support a previous suggestion that electrical conductivity may be electronic rather
than ionic.
Received: 17 December 1997 / Revised, accepted: 17 April 1998 |
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