Isotope geochemistry and petrology of African carbonatites

Abstract

The carbonatite bodies examined are the Palabora, Spitskop and Premier Mine carbonatites of Precambrian age, the Mbeya carbonatite of Mesozoic age, Tertiary Homa Mountain carbonatite and the Recent eruptions of the Oldoinyo Lengai carbonatite. The field of oxygen and carbon isotopic ratios of the Palabora carbonatite, especially of the older carbonatite, coincides with that of primary igneous carbonatites. Spitskop carbonatite shows a slight deviation in oxygen and carbon isotopic ratios from primary igneous carbonatites. The Spitskop carbonatite forms a pipe-like body whose composition was modified after emplacement in several stages by circulating fluids which showed progressive enrichment in magnesium and iron. In the Premier Mine, the main carbonatite dyke mass intruded the centre of the black kimberlite. The oxygen isotopic compositions show a significant enrichment in their 18O compared to the “sub-volcanic” type carbonatites. Carbon and especially oxygen isotopic compositions show a considerable range in the Mbeya carbonatite. The fractionation factors of oxygen and carbon isotopes between the coexisting calcites and dolomites in the primary Mbeya carbonatites suggest that the carbonates crystallized at temperatures from 800°C to 380°C and > 700°C to 380°C, respectively. Carbon and oxygen isotopic compositions of the Homa Mountain carbonatite show a considerable spread and this is probably due to interaction with atmospheric oxygen and meteoric water during eruption. Sodium carbonate lava from Oldoinyo Lengai is deliquescent and readily alters in air. The lower value of δ 13C in the sodium carbonate lava is thought to be due to the partitioning of heavy carbon into a CO 2 gas phase during vulcanicity. It appears to suggest that no genetical relationship exists between sodium carbonatite lava and evaporite trona.