Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XVIII, No.3


Experimental Study on the Partitioning of Fe and Mg Between Garnet and Olivine and its Applications to Kimberlites

Oka, Yastami

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The temperature dependence of the Fe-Mg partition coefficient is studied experimentally on the garnet-olivine system. The experiments are carried out in the temperature and pressure range of 1000℃ ― 1400℃, 25 ― 30 kb, respectively. The composition of the starting materials are (Fe17Mg72Ca11) for garnet and (Fe12Mg88) for olivine. It is shown that the temperature-pressure relation of this reaction is represented as T = (ΔH** + PΔV**)/(ΔS** ― RlnK'Fe-Mg^Gt-O1) with the thermodynamic parameters as ΔH**=-3.06 kcal, ΔS**=-1.33 cal/deg, and ΔV**=-10.5 cal/kb where ΔV** is estimated from the unit cell volume-composition relations. The calculated temperature and pressure agree with the experimental data within an error of ±50℃. The partition coefficient of the granular lherzolites in the kimberlites from Thaba Putsoa pipe, Lesotho, varies from 2.4 ― 2.8. It is shown that this variation is almost due to the effect of Cr in garnet. The effect of Cr in garnet is also demonstrated on the partitioning of the Mg-Tschermak's molecule between orthopyroxene and garnet, and the Cr-Al exchange reaction between garnet and chromite. In the present treatment, the effect of Cr in garnet is corrected by assuming a nearly isophysical condition for this granular lherzolite nodules. The temperature and pressure of these nodules are estimated to be 975℃ and 45 kb from the application of the present experiments with the combination of the garnet-orthopyroxene geobarometry. The applications to some other granular lherzolite nodules are also presented.