Carbon and oxygen isotope investigations in carbonatites and related rocks from the Kaiserstuhl, Germany

Abstract

A systematic carbon and oxygen isotope study was carried out on the carbonate phase of different carbonatites and related volcanic rocks from the Miocene Kaiserstuhl alkaline complex. Carbonatites are grouped into sövitic intrusions, small alvikitic dykes of varying textures, and extrusive carbonatites in the form of tear-drop lapilli and carbonate ashes. Each group exhibits distinct isotopic compositions. The sövites show the most primitive carbon and oxygen isotope ratios with an average of −5.5‰ δ 13C vs. PDB and +7.8‰ δ 18O vs. SMOW, respectively. The extrusive carbonatite lapilli have lower 13C contents, possibly due to an exchange with increasing amounts of CO 2 that accumulated in the magma prior to eruption. The elevated 18O content of up to + 16‰ δ 18O vs. SMOW for different samples and different eruptions is assumed to characterize the crystallization of these volcanic carbonatites in equilibrium with an aqueous fluid phase. The alvikitic dykes exhibit a wide variation in isotopic compositions, in particular for oxygen. Isotope ratios vary from the unfractionated primary compositions of intrusive sövites to the fractionated ratios of the extrusive lapilli. These large variations are ascribed to isotope-exchange processes between a carbonate melt and an aqueous gas phase at decreasing temperatures. The variations in oxygen isotopic composition reflect differences in the equilibration conditions between melt and gas phase for respectively deep-seated subvolcaniv vs. near-surface and eruptive crystallization levels. Additional factors for the large isotopic variation are probably non-equilibrium exchange processes due to the escape of fluids prior or during the emplacement of the alvikites. Recrystallization in the fine-grained calcitic matrix of dykes and lapilli in equilibrium with low-temperature meteoric pore fluids may further contribute to the fractionated isotopic pattern. Primary igneous isotope ratios were also found for the carbonate phase of diatreme breccias genetically related to the sövite intrusions. Among the alkalic silicate rocks, some bergalites (melitite-hauyne-nepheline-calcite rocks) show primary carbon and oxygen isotope ratios emphasizing their geochemical transition character between silicate magmatism and carbonatites.