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

Chromites in olivine adcumulates, mesocumulates and orthocumulatesfrom drill core of the Jimberlana intrusion have been analysedand related to the cumulate type and to the nature of the surroundingsilicate mineral. Chromites in adcumulates and mesocumulatesshow a restricted range of composition and are high in Mg, Aland Cr. The orthocumulate chromites vary in composition fromthat found in adcumulates to chromites which are much higherin Fe and Ti and with a higher Fe^3?: Fe^2? ratio. The chromitesin orthocumulates vary in composition depending upon the natureof the enclosing silicate mineral. This is believed to reflectthe ability of the enclosing mineral to protect the originalcumulus chromite from reaction with the intercumulus liquid.Thus chromite within early bronzite oikocrysts was protectedfrom reaction whereas that in plagioclase and phlogopite wasprotected at a much later stage and has a higher Fe and Ti contentChromite within olivine appears to have been able to equilibratewith intercumulus liquid until late in the magmatic historyexcept where the olivine enclosing chromite has itself beensurrounded by bronzite. It is suggested that chromite can exchangeelements with intercumulus liquid through the olivine. Thereare two possibilities; either elements such as Cr, Al, Ti andFe^{3 ?} were able to diffuse through the olivine structure orthe apparently enclosed chromite crystals were able to maintaindirect contact with the melt along fine fractures produced bythe differential thermal contraction of olivine and chromite. The average diameters of chromite crystals within orthocumulatebronzite and olivine are 28 and 20 microns respectively whereaschromites in plagioclase and phlogopite have average diametersof 48 and 56 microns. There is no obvious correlation betweenthe size of the chromite and their composition for grains foundwithin a particular silicate. Chromites of every size have beenable to equilibrate with the liquid unless they were protectedfrom reaction. Nucleation of reaction products played an important role indetermining the final composition of any particular chromitecrystal. The significance of reaction and nucleation on a localscale of millimetres is considered with respect to the majorsilicates and to the location of the last liquid. It is suggestedthat the last liquid tended to concentrate in pockets of reactantcrystals, where product crystals failed to nucleate until latein the magmatic history. It is estimated that in rocks withan orthocumulate texture, the intercumulus liquid crystallizedover a temperature range as large as 300 ?C and that it becamesignificantly more oxidizing near the solidus temperature.