Coexisting garnets and ilmenites synthesized at high pressures from pyrolite and olivine basanite and their significance for kimberlitic assemblages |
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| Authors: | D H Green N V Sobolev |
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| Institution: | 1. Research School of Earth Sciences, Australian National University, Canberra
2. Department of Geology, Australian National University, Canberra
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| Abstract: | Coexisting garnets and ilmenites have been synthesized at high pressure (21–40 kb) within the temperature range between 900 and 1100 °C from pyrolite-less-40% olivine and olivine basanite with various water contents. The two compositions yield phases with a range in the 100 Mg/Mg+Fe ratio for both garnet (41–76) and ilmenite (15–47). The distribution coefficient for iron and magnesiaum (K D(Fe, Mg) ilm-ga = 4.0±0.5) for coexisting phases does not appear to vary with change in the bulk composition or temperature of synthesis. The synthesized ilmenites are of similar composition to those of kimberlites in 100 Mg/Mg+Fe ratio and Al2O3 and Cr2O3 solid solution. Cr2O3 content in ilmenite is dependent on Cr2O3 in the bulk composition and also on Fe2O3 content of ilmenite. Fe2O3 content of ilmenite is very sensitive to f O2 and natural ilmenites from peridotites have formed under low f O2. Al2O3 solid solution in ilmenite as well as TiO2 in coexisting garnet tend to be higher with higher temperature. All the variety of compositions of ilmenites from kimberlites may be obtained from rocks rather close in composition to those used in experiments, within the same range of pressure and temperature but at variable oxygen fugacities. |
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