Convective Fractionation During Magnetite and Hematite Dissolution in Silicate Melts

C. H. Donaldson
School of Geography and Geology, University of St. Andrews, Fife KY16 9ST, Scotland

Abstract: Single crystals of magnetite and of hematite have been dissolved at atmospheric pressure in superheated melts in the systems CaO-MgO-Al2O3-SiO2 and CaO-Al2O3-SiO2, and in a basalt. The crystals were suspended in alumina crucibles containing ca 3.5 cm3 of melt. Quenched run products were examined optically and by electron probe analysis to establish the distribution of Fe in the glassy charges. There is usually a concentration of Fe at the base of a run product, consistent with flow of dissolved matter from the crystal to the floor. One or more columns of brown, Fe-rich glass may extend from the underside of a relic crystal towards the floor. In CMAS run products, such columns typically extend this entire distance, whereas in the CAS and basalt run products, the columns are either detached from the crystal or do not reach the floor. In the CMAS melt (viscosity ∼1 poise) there is apparently continuous release of Fe-bearing melt from around a dissolving crystal, whereas in the CAS and basalt melts (viscosities 7000 and 300 poise, respectively) release is intermittent. In CMAS run products the Fe content is usually greatest, in glass, at the base, and declines gradually upwards; in CAS and basalt runs the bottom of a crucible is occupied by discrete, sharply bounded pillows of Fe-rich glass, with only slight, or no, gradation in composition. Rising gas bubbles can elevate small blobs of the denser, Fe-bearing melt from around a dissolving crystal, and trains of bubbles in the CAS melt may guide this Fe-bearing melt, against gravity, to the surface of the charge. When the bubbles burst at the surface, this dense melt is left in an unstable location and releases diapirs which descend to the bottom of the crucible. In spite of the evidence that convective fractionation occurs in these haplomagmas and in the basalt, it remains to be demonstrated that it will occur during sidewall crystallization or during the growth of minerals in a cumulus mush to cause magmatic differentiation.

Keywords: fractionation • magnetite • hematite • silicate melts • magmatic differentiation

Mineralogical Magazine; September 1993 v. 57; no. 388; p. 469-488; DOI: 10.1180/minmag.1993.057.388.09
© 1993, The Mineralogical Society
Mineralogical Society (