Compositional Convection Caused by Olivine Crystallization in a Synthetic Basalt Melt

Jon K. Seedhouse* and Colin H. Donaldson
Department of Geology, St Andrews University, Fife KY16 9ST
1Current address; British Gas Research and Technology, Gas Research Centre, Ashby Road, Loughborough, LE11 3QU.

Abstract: Compositional convection in magma chambers is thought to be an important process in the fractionation of liquid from crystals during the differentiation of magmas. It has been tested for in this study by undertaking isothermal crystal growth experiments in a silicate melt at atmospheric pressure in air. The melt used is a synthetic basalt in which iron is replaced by cobalt to minimise redox problems. Co-Mg olivine rims were overgrown on forsteritic olivine seeds cemented to the floor of a 2.4 cm deep alumina crucible. Following quenching and sectioning, glasses were examined optically for colour variations and by EPMA for compositional variations. It had been expected that the colour intensity of the blue glass would diminish in the Co-depleted zone that develops around crystal overgrowths, whereas in fact little difference is normally found, except for a slight fading of colour in glass above the apex of a seed in a few experiments. By contrast EPMA revealed zones up to 50 μm wide around seeds that are depleted in Co and Mg by up to 25 % at the crystal-glass interface and in patches above some crystals. Contour maps of X-ray count-rate data obtained in grids of analytical points show Co- and Mg-depleted glass around the overgrowths and in patches above the highest point of each seed, demonstrating that convection in the melt does occur during growth of individual crystals. As the experiments were carried out in a stable temperature gradient and the crystal seeds had no contact with the melt meniscus, thermal and surface-tensional convection are both eliminated, and the convection is inferred to be caused by a density difference resulting from compositional variation across the chemical boundary layer around a growing crystal. The density difference between the inside and outside of a boundary layer is calculated to be approximately −1%.

Keywords: magma chambers • convection • olivine • basalt melt

Mineralogical Magazine; February 1996 v. 60; no. 398; p. 115-130; DOI: 10.1180/minmag.1996.060.398.08
© 1996, The Mineralogical Society
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