Stable isotope studies and processes of carbonate formation in Hungarian alkali basalts and lamprophyres: evolution of magmatic fluids and magma-sediment interactions

Abstract

Processes of carbonate formation have been related to C and O isotopic compositions in the Mesozoic alkali basalt (Mecsek Mts.) and lamprophyre (Transdanubian Range) suites of Hungary. In the studied magrnatic rocks, carbonates are present as ocelli, amygdales, xenoliths, veins and groundmass carbonate. C and O isotope studies of these types of carbonate have yielded information on the origin of the carbonates and indicated the following processes of formation that determined the δ 13C and δ 18O values of the carbonates:(1) Crystallization of magmatic carbonate. Textural characteristics and δ 13C values suggest formation of magmatic carbonate in alkali basalt and lamprophyre dikes, whereas the δ 18O compositions of these carbonates indicate low temperature oxygen isotope exchange with magmatic fluids.(2) Assimilation of sedimentary carbonate by silicate magmas. Even completely recrystallized amygdales and ocelli of basalts and lamprophyres have preserved their sedimentary δ 13C values. In contrast, variations in the extent of mobilization and isotope exchange with magmatic fluids are reflected in differences in the ranges of the δ 18O values of amygdales, ocelli and veins, and can be attributed to different amounts of fluids involved in the magmatic events.(3) Low temperature alteration of magmatic rocks caused only 18O-enrichment in the carbonate amygdales of basalts and the groundmass carbonates of lamprophyres, indicating that no externally-derived CO 2 was present in the alteration fluids.(4) Degassing of magma and magmatic fluid. Correlations between δ 13C and δ 18O data, magma crystallization depths and amygdale sizes in the alkali basalts suggest that CO 2 degassing has been responsible for the negative δ 13C and positive δ 18O shifts observed. A similar trend was found in the lamprophyres, but the extent of the δ 18O shift indicates that in these rocks H 2O degassing also played an important role.