Variations of oxygen and carbon isotopes in carbonatites: A study of Brazilian alkaline complexes

Abstract

We have studied the oxygen and carbon isotopic compositions of carbonatite complexes from South America (Jacupiranga, Araxá, Catalão, Tapira, and Mato Preto) and interpreted the results in terms of magmatism, contamination by country rocks, and hydrothermal processes. These complexes range in age from 130 to 65 Ma and were emplaced during the reactivation of the South American Platform during the Mesozoic Era. Except for the samples from Mato Preto ( δ 13C = −6.9 to + 0.8%), which have been contaminated by limestone from the country rock, there are no large carbon isotope differences among the samples of Jacupiranga (−7.3 to −6.6%), Araxá (−7.5 to −4.8%), Catalão (−7.1 to −5.3%), and Tapira (−6.8 to −4.8%). In contrast, the carbonatites have a wide range in oxygen isotopic composition, which seems to be related to their degree of hydrothermal alteration and their emplacement level. For instance, while the samples from Jacupiranga have a narrow range of δ 18O (6.6 to 7.3%) and have not been extensively affected by fertilization, the carbonatites from the complexes of Araxa (8.7 to 16.3%o), Catalão (7.3 to 19.3%), and Tapira (9.7 to 15.4%) have a wide range in δ 18O and are accompanied by pervasive potash-fenitization of their host rock. The potash-fenitization is marked by the replacement of carbonatite host rock (mainly pyroxenite) by carbonate-phlogopite-magnetite-rich rocks. We conclude that fractional crystallization and liquid immiscibility may not significantly affect the oxygen and carbon isotopic composition of carbonatites. Large isotopic variations may be produced when these rocks intrude limestone country rocks (i.e., Mato Preto) and/or have been subjected to postcrystallization alteration processes. Variations of δ 18O and δ 13C in the carbonatites may be explained by isotopic exchange between these rocks and H 20CO 2-fluids at different temperatures and with different H 2O/CO 2 ratios. The isotope exchange model implies that the isotopic variations in carbonatites take place under low-temperature conditions (below 300°C) and involve fluids with high H 2O/CO 2 ratios.