ABSTRACT

Oceanic basalts are often altered by circulating seawater. During this process carbonates are formed frequently as veinlets and crystal aggregates. Additionally hydrothermal carbonate is found occasionally. The isotopic composition of secondary and possibly primary carbonates has been investigated for 75 samples from Hole 418 A, a 550m long basalt sequence drilled during DSDP Legs 52 and 53 to the south of the Bermuda Rise, in the Atlantic Ocean. Isotope ratios of secondary carbonates, carbonate rich basalts and some of the so called “fresh” basalts have values typical for marine carbonates, i.e., δ¹³C +2.8 to -2.3‰ vs. PDB. Isotope ratios of samples with small carbonate content fall in the range of primary magmatic carbon, i.e., δ¹³C -4 to -8‰. The carbon isotope composition of carbonates in oceanic basalts is therefore constituted by an overprinting of seawater-derived secondary carbonates on traces of primary carbonate carbon.