On the kinetics of textural equilibration in forsterite marbles |
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| Authors: | Marian B Holness M J Bickle Colin M Graham |
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| Institution: | (1) Department of Earth Sciences, University of Cambridge, Downing Street, CB2 3EQ Cambridge, UK;(2) Department of Geology and Geophysics, University of Edinburgh, EH9 3JW Edinburgh, UK;(3) Present address: Department of Geology and Geophysics, University of Edinburgh, EH9 3JW Edinburgh, UK |
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| Abstract: | In the aureole of the Beinn an Dubhaich granite, Skye, the minimum observed median forsterite-calcite-calcite dihedral angle
varies from 110° at the olivinein isograd to about 165° (the equilibrium value) at the granite-limestone contact. Laboratory
experiments were performed to investigate the kinetics of this textural change. It was found that the rate of change of the
forsterite-calcite-calcite dihedral angle followed approximately first-order kinetics with an activation energy of 48±4 kJ
mol-1 for fluid-present conditions, and 90 ±4 kJ mol-1 for fluid-absent conditions. Scanning ion imaging demonstrated that, at least in the early stages of textural change, solution-reprecipitation
of the calcite was the rate determining step in the fluid-present runs. The latter result and the value of the activation
energy are both consistent with the activation energy found by previous authors for (albeit zeroth order) silicate-aqueous
solution solution-reprecipitation reactions. The value of activation energy for the dry data does not correspond to those
for either grain boundary or volume diffusion of oxygen or magnesium in forsterite. The mechanism for textural equilibration
in the fluid-absent runs is uncertain. Application of the experimentally-derived rate equation to the Beinn an Dubhaich marbles
gave activation energies higher than those obtained experimentally. It is concluded from consideration of grain growth effects
that activation energies derived from the Beinn an Dubhaich marbles probably reflect textural equilibration under predominantly
fluid-absent conditions. |
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