CHROMITE IN KOMATIITES, 1 - MAGMATIC CONTROLS ON CRYSTALLIZATION AND COMPOSITION

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 coexist ing olivine. Chromite is least abundant in highly magnesian chrome-und ersaturated lavas, and most abundant in strongly differentiated layere d cumulate bodies. Abundances are typically lower than the expected co tectic proportions. Chromite compositions reflect variations in lava c omposition and oxygen fugacity; reaction with trapped intercumulus liq uid; and sub-solidus Fe-Mg exchange with olivine. Primary chromites in thick dunitic channels and sheet flows have very low Fe3+ contents, w hereas comparable thin flow chromites contain higher Fe3+. This is att ributed to an initial reduced state of the magma, thin flow lavas bein g subsequently oxidized because of post-eruption processes. Extensive reaction of chromite with trapped liquid causes decreasing Mg/(Mg + Fe 2+), and enrichment in Fe3+, TiO2, V2O5, MnO, CoO and ZnO. Nickel tren ds depend on sulphide content. These trends are best developed in rock s combining presence of trapped intercumulus liquid with Prolonged coo ling histories. The deficiency of chromite in channelised environments is partly a consequence of more primitive lavas being found in these environments but is mainly due to the difficulty in nucleating chromit e crystals under low degrees of supercooling.