Chemistry of Ilmenites Crystallized within the Anhydrous Melting Range of a Tholeiitic Andesite at Pressures Between 5 and 26 kb

R. N. Thompson
Dept. of Geology, Imperial College of Science and Technology, London SW7 2AZ

Summary: Microprobe analyses of eight ilmenites, a titaniferous magnetite, and a rutile are presented. They were synthesized at pressures from 5 to 26 kb and temperatures between 1075 and 1225 °C in the anhydrous melting interval of a tholeiitic andesite from the Snake River Plain, Idaho, U.S.A. Both the Fe/Ti distribution between coexisting ilmenite and titanomagnetite at 11 kb/1075 °C and the low calculated Fe3+ contents of the other ilmenites confirm previous suggestions that the fo2 in these experiments was buffered by graphite capsules to values near those generated by the wüstitemagnetite assemblage. Mn in the ilmenites (0·30–0·51% MnO) shows moderate negative correlations with both the pressures and temperatures of their formation, whilst AI (0·41–1·06 % Al2O3) shows poor positive correlations with these parameters. In contrast, the only significant trend shown by Mg in the ilmenites (2·09–5·26 % MgO) is with the position of each experimental run in the melting interval of the lava. Mg/(Mg + Fe2+) of the ilmenite decreases during equilibrium crystallization at a given pressure and appears to be controlled solely by Mg/Fe2+ distribution amongst the coexisting ferromagnesian minerals and interstitial liquid (glass).

Mineralogical Magazine; December 1976 v. 40; no. 316; p. 857-862; DOI: 10.1180/minmag.1976.040.316.06
© 1976, The Mineralogical Society
Mineralogical Society (