The Effect of Alumina on Phase Transformations at the 660-Kilometer Discontinuity from Fe-Mg Partitioning Experiments

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Science  13 Sep 1996:
Vol. 273, Issue 5281, pp. 1522-1524
DOI: 10.1126/science.273.5281.1522


Experimental data on the partitioning of Fe and Mg between coexisting silicate perovskite, magnesiowüstite, and γ-(Mg,Fe)2SiO4 demonstrate that Fe substitution in perovskite is strongly coupled to Al2O3 concentration. In Al2O3-free compositions, perovskite has a low Fe/(Fe+Mg) ratio: for a bulk Fe/(Fe+Mg) of 0.11, perovskite has a value close to 0.04, whereas magnesiowüstite has a ratio near 0.17. In peridotitic mantle, however, where the perovskite should contain 4 to 5 weight percent Al2O3, it has essentially the same Fe/(Fe+Mg) ratio as coexisting magnesiowüstite. Under lower mantle conditions, therefore, perovskite and magnesiowüstite should, in peridotite, each have an Fe/(Fe+Mg) ratio close to 0.11.

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