CRYSTALLIZATION OF TITANIFEROUS CHROMITE, MAGNESIAN ILMENITE AND ARMALCOLITE IN THOLEIITIC SUITES IN THE KAROO IGNEOUS PROVINCE

Titaniferous chromite (up to 8 wt% TiO2) and magnesian ilmenite (up to 10 wt% MgO) coexist at the base of the differentiated tholeiitic Moun t Ayliff Intrusion in the Karoo Province of southern Africa, suggestin g that the original magma was TiO2-rich. Picritic lavas with 3% TiO2 f rom the Lebombo monocline of the Karoo Province also contain microphen ocrysts of magnesian ilmenite (up to 6 wt% MgO) and armalcolite (up to 7 wt% MgO). These oxide mineral associations and compositions are aty pical of tholeiitic magmas, in which chromite usually has less than 1 wt% TiO2, ilmenite less than 3 wt% MgO and armalcolite is rarely a pri mary mineral. Experiments have been conducted at one atmosphere pressu re on a range of compositions to determine the effect of TiO2 on the c rystallization and composition of chromite, ilmenite and armalcolite. The results indicate that increasing the TiO2 content of picritic magm as increases the TiO2 content of the spinel, mainly at the expense of Al2O3, whereas Cr2O3 is not affected. Spinel compositions in the Mount Ayliff Intrusion (with over 45 wt% Cr2O3, less than 10 wt% Al2O3 and 8 wt% TiO2) were duplicated in experiments on a picrite at temperature s of about 1,200-degrees-C at the Ni/NiO buffer. Increasing fO2 from f ayalite-magnetite-quartz to Ni/NiO buffer is shown to increase the cry stallization temperature of armalcolite and to decrease that of ilmeni te. The total FeO content of the liquid has little influence on the cr ystallization temperature of these phases. The TiO2 content of the liq uid, when either ilmenite or armalcolite crystallizes, varies inversel y with SiO2 content. The MgO content of the liquid at which ilmenite o r armalcolite crystallizes depends upon the TiO2 content of the starti ng composition, with naturally occurring and experimentally determined saturation being demonstrated for liquids with 5 wt% MgO and 5.5 wt% TiO2. The partition coefficient for MgO between armalcolite or ilmenit e and liquid is about 1.5. Observed magnesian armalcolite and ilmenite compositions in picrite lavas (both minerals) and in the Mount Ayliff Intrusion (ilmenite only) are consistent with crystallization from a TiO2-rich magma with approximately 5 wt% MgO. The Fe23+TiO5 component of armalcolite in the picrite lavas matches those formed experimentall y at temperatures of 1,150-1,110-degrees-C and fO2 Of the Ni/NiO to Ni /NiO + 1 log unit. Similarities also exist between the compositions of chromite, ilmenite and armalcolite and liquid fractionation trends of some Hawaiian high-TiO2 lavas and the experimental studies presented here.