The Rates of Dissolution of Olivine, Plagioclase, and Quartz in a Basalt Melt

Colin H. Donaldson
Department of Geology, University of St Andrews, St Andrews, Fife, KY16 9ST, Scotland

Abstract: The dissolution rates of spheres of two magnesian olivines, two plagioclases, and quartz in tholeiitic basalt have been determined at three super-liquidus temperatures and one-atmosphere pressure. There are considerable differences in the rates among the minerals, e.g. at 1210°, 12° above the liquidus temperature of the basalt, labradorite dissolves at 86 µm/h. and the magnesian olivines at 9 and 14 µm/h. The rates are not time dependent and this, coupled with the existence of concentration gradients in the composition of quenched melt adjacent to partially dissolved crystals, indicates that the dissolution rates are dictated by a combination of diffusion and convection of components to and from the crystal-liquid interface. Values for the activation enthalpy of dissolution are small for quartz and plagioclase (40–50 kcal mol−1) but large for olivine 73–118 kcal mol−1). Dissolution of plagioclase in rock melts seems to be a much more rapid process than crystal growth, whereas olivines apparently dissolve and grow at similar rates. Crystal dissolution is sufficiently slow that ascending, crystal-bearing magma may become superheated and yet fail to dissolve the crystal fraction before quenching; this may be the reason that olivine phenocrysts are often rounded.

Keywords: olivine • plagioclase • quartz • tholeiite • basalt • dissolution

Mineralogical Magazine; December 1985 v. 49; no. 354; p. 683-693; DOI: 10.1180/minmag.1985.049.354.07
© 1985, The Mineralogical Society
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