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Journal of Petrology, Volume 39, Issue 10: October 1 1998.
Chromite is a widespread accessory mineral in komatiites, ranging from skeletal through euhedral and equant to lobate and rarely poikilitic habits depending on cooling regime and the composition of the coexisting olivine. Chromite is least abundant in highly magnesian chrome-undersaturated lavas, and most abundant in strongly differentiated layered cumulate bodies. Abundances are typically lower than the expected cotectic proportions. Chromite compositions reflect variations in lava composition and oxygen fugacity; reaction with trapped intercumulus liquid; and sub-solidus Fe-Mg exchange with olivine. Primary chromites in thick dunitic channels and sheet flows have very low Fe3+ contents, whereas comparable thin flow chromites contain higher Fe3+. This is attributed to an initial reduced state of the magma, thin flow lavas being subsequently oxidized because of post-eruption processes. Extensive reaction of chromite with trapped liquid causes decreasing Mg/(Mg + Fe2+), and enrichment in Fe3+, TiO2, V2O5, MnO, CoO and ZnO. Nickel trends depend on sulphide content. These trends are best developed in rocks combining presence of trapped intercumulus liquid with prolonged cooling histories. The deficiency of chromite in channelized environments is partly a consequence of more primitive lavas being found in these environments, but is mainly due to the difficulty in nucleating chromite crystals under low degrees of supercooling.
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Pages 1689-1720