Carbon and oxygen isotope variations in southern African carbonatites

Abstract

The C and O isotope composition of 54 carbonate samples from 18 carbonatite localities throughout southern Africa was determined. Sample frequency distributions indicate a dependence of the δ 13C and δ 18O values of southern African carbonatites on their carbonate mineralogy. The calcite carbonatite samples fall into the range −8% to −4% for δ 13C and 6% to + 10% for δ 18O, which is defined as the primary igneous carbonatite field (Taylor et al., 1967). Those samples with higher δ 13C and δ 18O values are either dolomite/ankerite carbonatites or contain dolomite/ankerite, possibly indicating the involvement of different magmatic or hydrothermal phases in the carbonatite formation. Emplacement level, type of country rock intruded and secondary alteration by e.g. hydrothermal fluids, seem to be the most important parameters which affected the C and O isotope patterns found in southern African carbonatite complexes. Magmatic processes like fractional crystallisation and liquid immiscibility are of minor importance in controlling C and O isotope ratios. An alteration model of at least two stages is proposed for the evolution of the C/O isotope patterns observed in southern African carbonatites. The first stage involved magmatic-metasomatic processes during or shortly after the formation of carbonatites. These processes affected both the C and O isotope composition of carbonatites by exchange with a H 2O/CO 2-rich fluid and led to higher δ 13C and δ 18O values when compared to primary igneous values. The elevated 18O content of carbonatite carbonates was introduced by a second stage (or more stages) of post-magmatic, hydrothermal alteration and/or interaction with ground water at low (<200–250°C) temperatures. These alterations seem to be accompanied by a growing mineralogical diversity, which reflects, to some extent, the degree of alteration affecting the carbonatites. Information on stable isotope heterogeneities of the igneous sources of southern African carbonatites is obscured. Small scale C isotope heterogeneities within the igneous sources of carbonatites may be found, provided that the effects of a first stage, magmatic-metasomatic alteration can be eliminated. Changes in δ 18O values of carbonatites by sub-solidus second stage alteration processes within the crust further complicate an estimation of stable isotope variations of igneous carbonatite sources.