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Compositional convection caused by olivine crystallization in a synthetic basalt melt

Published online by Cambridge University Press:  05 July 2018

Jon K. Seedhouse  and
Colin H. Donaldson
Jon K. Seedhouse
Affiliation:
Department of Geology, St Andrews University, Fife KY16 9ST
Colin H. Donaldson
Affiliation:
Department of Geology, St Andrews University, Fife KY16 9ST
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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 chambersconvectionolivinebasalt melt
Type
The 1995 Hallimond Lecture
Information
Mineralogical Magazine , Volume 60 , Issue 398 , February 1996 , pp. 115 - 130
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1996

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Footnotes

*

Current address; British Gas Research and Technology, Gas Research Centre, Ashby Road, Loughborough, LE11 3QU.

References

Bottinga, Y. and Weill, D.H. (1970) Densities of liquid silicate systems from partial molar volumes of oxide components. Amer. Sd., 269 169—82.Google Scholar
Brearley, M. and Scarfe, C.M. (1986) Dissolution rates of upper mantle minerals in an alkali basalt melt at high pressure: an experimental study and implications for ultramafic xenolith survival. J. Petrol., 27, 1157–82.CrossRefGoogle Scholar
Coons, W.E., Holloway, J.R. and Navrotsky, A.(1976) CO2+ as a chemical analogue for Fe2+ in high temperature experiments in basaltic systems. Earth Planet. Sci. Lett., 30, 303–8.CrossRefGoogle Scholar
Coons, W.E. and Holloway, J.R. (1979) Cobaltous oxide as a chemical analogue for ferrous iron in experimental petrology: An alternative solution to the iron-loss problem. Amer. Mineral., 64, 1097–106.Google Scholar
Donaldson, C.H. (1975) Calculated diffusion coefficients and the growth rate of olivine in a basaltic magma. Lithos, 8, 163–74.CrossRefGoogle Scholar
Donaldson, C.H. (1976) An experimental investigation of olivine morphology. Contrib. Mineral. Petrol, 57, 187–213.CrossRefGoogle Scholar
Donaldson, C.H. (1993) Convective fractionation during magnetite and hematite dissolution in silicate melts. Mineral, Mag., 57, 469–88.CrossRefGoogle Scholar
Donaldson, C.H. and Hamilton, D.L. (1987) Compositional convection and layering in a rock melt. Nature, 327, 413–5.CrossRefGoogle Scholar
Hill, R.E.T. (1969) The crystallization of basaltic melts as a function of oxygen fugacity. Unpublished Ph.D. thesis. Queen's University, Belfast.Google Scholar
Hofmann, A.W. (1980) Diffusion in silicate melts: a critical review. In: Physics of Magmatic Processes. (Hargraves, R.B., ed.) Princeton University Press, 585 pp.Google Scholar
Kuo, L-C. and Kirkpatrick, R.J. (1985a) Kinetics of crystal dissolution in the system diopside-forsterite- siiica. Amer. J. Sci., 285, 51–91.CrossRefGoogle Scholar
Kuo, L-C. and Kirkpatrick, R.J. (1985/j) Dissolution of mafic minerals and its implication for the ascent velocities of peridotite-bearing basaltic magmas. J. GeoL, 93, 691–700.CrossRefGoogle Scholar
Martin, D., Griffiths, R.W. and Campbell, I.H. (1987) Compositional and thermal convection in magma chambers. Contrib. Mineral Petrol., 96, 465–75.CrossRefGoogle Scholar
Seedhouse, J.K. (1994) Testing for compositional convection in silicate melts; crystal growth experiments and a petrographic study of a differentiated ring dyke. PhD. Thesis, Univ. of St Andrews.Google Scholar
Shaw, H.R. (1972) Viscosities of magmatic silicate liquids: An empirical method of prediction. Amer. J. ScL, 272, 870–93.CrossRefGoogle Scholar
Sparks, R.S.J., Huppert, H.E. and Turner, J.S. (1984) The fluid dynamics of evolving magma chambers. Phil. Trans. Roy. Soc. London. A310, 511—34.Google Scholar
Tait, S.R., Huppert, H.E. and Sparks, R.S.J. (1984) The role of compositional convection in the formation of adcumulate rocks, Lithos, 17 139—46.CrossRefGoogle Scholar
Tait, S.R. and Jaupart, C. (1992) Compositional convection in a reactive crystalline mush and melt differentiation. J. Geoph. Res., 97, 6735–56.CrossRefGoogle Scholar
Turner, J.S. (1980) A fluid dynamical model of differentiation and layering in magma chambers. Nature, 285, 213–5.CrossRefGoogle Scholar
Turner, J.S. and Campbell, I.H. (1986) Convection and mixing in magma chambers. Earth Sci. Rev., 23, 255–352.CrossRefGoogle Scholar
Wager, L.R., Brown, G.M. and Wadsworth, W.J. (1960) Types of igneous cumulates. J. Petrol., 1, 73–85.CrossRefGoogle Scholar
Zhang, Y., Walker, D. and Lesher, C.E. (1989) Diffusive crystal dissolution. Contrib. Mineral. Petrol., 102, 492–513.CrossRefGoogle Scholar