Crystallization of titaniferous chromite, magnesian ilmenite and armalcolite in tholeiitic suites in the Karoo Igneous Province

Abstract

Titaniferous chromite (up to 8 wt% TiO2) and magnesian ilmenite (up to 10 wt% MgO) coexist at the base of the differentiated tholeiitic Mount Ayliff Intrusion in the Karoo Province of southern Africa, suggesting that the original magma was TiO2-rich. Picritic lavas with 3% TiO2 from the Lebombo monocline of the Karoo Province also contain microphenocrysts of magnesian ilmenite (up to 6 wt% mgO) and armalcolite (up to 7 wt% MgO). These oxide mineral associations and compositions are atypical of tholeiitic magmas, in which chromite usually has less than 1 wt% TiO2, ilmenite less than 3 wt% MgO and armalcolite is rarely a primary mineral. Experiments have been conducted at one atmosphere pressure on a range of compositions to determine the effect of TiO2 on the crystallization and composition of chromite, ilmenite and armalcolite. The results indicate that increasing the TiO2 content of picritic magmas 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 temperatures of about 1,200°C at the Ni/NiO buffer. Increasing fO2 from fayalite-magnetite-quartz to Ni/NiO buffer is shown to increase the crystallization temperature of armalcolite and to decrease that of ilmenite. The total FeO content of the liquid has little influence on the crystallization temperature of these phases. The TiO2 content of the liquid, when either ilmenite or armalcolite crystallizes, varies inversely with SiO2 content. The MgO content of the liquid at which ilmenite or armalcolite crystallizes depends upon the TiO2 content of the starting composition, with naturally occurring and experimetally determined saturation being demonstrated for liquids with 5 wt% MgO and 5.5 wt% TiO2. The partition coefficent for MgO between armalcolite or ilmenite 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 experimentally at temperatures of 1,150–1,110°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 fraction-ation trends of some Hawaiian high-TiO2 lavas and the experimental studies presented here.