Chromite in Komatiites, 1. Magmatic Controls on Crystallization and Composition

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Chromite is a widespread accessory mineral in komatiites, ranging Zhou & Kerrich, 1992). This is the first in a series of papers which addresses the occurrence of chromite in from skeletal through euhedral and equant to lobate and rarely poikilitic habits depending on cooling regime and the composition komatiites and its chemical variability. This paper looks at the factors controlling the crystallization of chromite of the coexisting olivine. Chromite is least abundant in highly magnesian chrome-undersaturated lavas, and most abundant in from komatiites, and at the textural and compositional variation within chromites which have apparently restrongly differentiated layered cumulate bodies. Abundances are typically lower than the expected cotectic proportions. Chromite tained some or all of their igneous characteristics. Textural and compositional features of chromites are compositions reflect variations in lava composition and oxygen fugacity; reaction with trapped intercumulus liquid; and sub-solidus interrelated in complex and interesting ways, and show systematic relationships to host rock chemistry and crysFe–Mg exchange with olivine. Primary chromites in thick dunitic channels and sheet flows have very low Fe contents, whereas tallization regime. Future papers in the series will deal with superimposed metamorphic effects. comparable thin flow chromites contain higher Fe. This is attributed to an initial reduced state of the magma, thin flow lavas A large body of new data on chromite compositions, including high-precision wavelength-dispersive analyses being subsequently oxidized because of post-eruption processes. Extensive reaction of chromite with trapped liquid causes decreasing of minor and trace elements Ti, V, Mn, Zn, Ni and Co, is presented from the Norseman–Wiluna Greenstone Belt Mg/(Mg+ Fe), and enrichment in Fe, TiO2, V2O5, MnO, CoO and ZnO. Nickel trends depend on sulphide content. These in Western Australia, and synthesized with a compilation of literature data taken mainly from Roeder (1994) and trends are best developed in rocks combining presence of trapped intercumulus liquid with prolonged cooling histories. The deficiency various unpublished sources. Whole-rock geochemical data are similarly compiled from the literature and some of chromite in channelized environments is partly a consequence of more primitive lavas being found in these environments, but is unpublished sources, and include a large body of data collected over the last 15 years at CSIRO. Details of mainly due to the difficulty in nucleating chromite crystals under low degrees of supercooling. data sources are listed in Appendix A, and details of electron microprobe analytical techniques in Appendix B.