A Study of the Fe–Ti Oxides in the Whitestone Anorthosite, Dunchurch, Ontario

Abstract

The Fe–Ti oxides hemo-ilmenite, magnetite, and minor ferrian ilmenite occur predominantly in the foliated oxide-rich, garnet-bearing border facies of the Whitestone anorthosite. The rhombohedral phases comprises 80–90 modal % of the oxides, and shows variation in bulk composition from Ilm90Hem10 to Ilm50Hem50 (mol %), which we interpret to be a result of local variation in [Formula: see text]. Hemo-ilmenites with bulk compositions more ilmenite rich than approximately Ilm85Hem15 exsolve predominantly by a continuous exsolution mechanism, whereas more hematite rich phases exsolve discontinuously. Along the hematite limb of the solvus, the exsolution mechanism changes at a bulk composition of approximately Hem60Ilm40. The compositions of hematite lamellae and host phase ilmenite in hemo-ilmenite are constant at Hem72Ilm28 and Ilm95Hem5 respectively, as indicated by electron-probe analyses. This suggests that the position of the hematite–ilmenite solvus is at more ilmenite rich compositions than was determined by Carmichael (1961).Magnetite contains less than 3.5 mol % Usp, and the average unit cell dimension of 14 samples is 8.396 ± 0.004 Å. In hemo-ilmenite grains adjacent to magnetite, hematite lamellae thin and disappear leaving a zone up to 50 μ wide of homogeneous ferrian ilmenite (Ilm95Hem5). This ilmenite was added during cooling, as the result of oxidation of ülvospinel in magnetite. We envisage that oxides of the Whitestone anorthosite cooled slowly, from an initially high temperature stage in a tectonically quiet environment.