The Effect of Postcumulus Reactions on Composition of Chrome-spinels from the Jimberlana Intrusion

Chromites in olivine adcumulates, mesocumulates and orthocumulates from drill core of the Jimberlana intrusion have been analysed and related to the cumulate type and to the nature of the surrounding silicate mineral. Chromites in adcumulates and mesocumulates show a restricted range of composition and are high in Mg, Al and Cr. The orthocumulate chromites vary in composition from that found in adcumulates to chromites which are much higher in Fe and Ti and with a higher Fe3³: Fe2³ ratio. The chromites in orthocumulates vary in composition depending upon the nature of the enclosing silicate mineral. This is believed to reflect the ability of the enclosing mineral to protect the original cumulus chromite from reaction with the intercumulus liquid. Thus chromite within early bronzite oikocrysts was protected from reaction whereas that in plagioclase and phlogopite was protected at a much later stage and has a higher Fe and Ti content Chromite within olivine appears to have been able to equilibrate with intercumulus liquid until late in the magmatic history except where the olivine enclosing chromite has itself been surrounded by bronzite. It is suggested that chromite can exchange elements with intercumulus liquid through the olivine. There are two possibilities; either elements such as Cr, Al, Ti and Fe3 ³ were able to diffuse through the olivine structure or the apparently enclosed chromite crystals were able to maintain direct contact with the melt along fine fractures produced by the differential thermal contraction of olivine and chromite. The average diameters of chromite crystals within orthocumulate bronzite and olivine are 28 and 20 microns respectively whereas chromites in plagioclase and phlogopite have average diameters of 48 and 56 microns. There is no obvious correlation between the size of the chromite and their composition for grains found within a particular silicate. Chromites of every size have been able to equilibrate with the liquid unless they were protected from reaction. Nucleation of reaction products played an important role in determining the final composition of any particular chromite crystal. The significance of reaction and nucleation on a local scale of millimetres is considered with respect to the major silicates and to the location of the last liquid. It is suggested that the last liquid tended to concentrate in pockets of reactant crystals, where product crystals failed to nucleate until late in the magmatic history. It is