Origin of chromite compositional variation in the Panton Sill,Western Australia |
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| Authors: | Paul R Hamlyn Reid R Keays |
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| Institution: | (1) Department of Geology, University of Melbourne, 3052 Parkville, Australia;(2) Present address: Lamont-Doherty Geological Observatory of Columbia University, 10964 Palisades, New York, USA |
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| Abstract: | The compositional variation of chromite and associated olivine in chromite-rich and chromitepoor cumulus layers of the Panton Sill is described and a diffusion-controlled crystallization mechanism is proposed to explain this variation. By this mechanism, chromite initially precipitates with a fairly uniform composition, irrespective of the relative proportions of coprecipitating olivine and chromite, and is modified by continued growth during the postcumulus stage. The effect of postcumulus overgrowth of chromite, K
d
=(Mg/Fe2+)liquid/(Mg/Fe2+) chromite 6, is to deplete the surrounding magma in chromium and decrease Fe2+ relative to Mg such that a chemical gradient exists between the overlying magma, through which the cumulus grains settled, and the magma in contact with settled chromite grains near the magma/crystal pile interface. Postcumulus equilibration of olivine and chromite with the surrounding magma results in higher Mg/(Mg + Fe2+) ratios of both olivine and chromite and higher Al content of chromite. The extent of this postcumulus modification is directly related to the proportion of chromite to olivine in a particular layer. This model can be extended to stratiform intrusions elsewhere in which chromite coprecipitates with olivine, orthopyroxene or plagioclase and displays similar compositional trends. |
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