13C/12C and 34S/32S ratios in magmatic gases from ridge volcanism in Afar

Abstract

Most data on the stable isotope composition of deep-seated carbon and sulphur come from igneous rocks and low-temperature gases. Very little information comes from high-temperature gases which escape from magmas (T = 1,000±200 °C), even though these are the best specimens of primary igneous fluids. Moreover, the only available 13C/12C data on carbon in high-temperature basaltic gases2–4 are largely divergent: low ratios from −14 to −26‰ versus PDB were measured in gases from Hawaii2,3,5, which is consistent with the values for reduced carbon in basalts6, whereas higher ratios around −6.5‰,typical of carbon in most of carbonatites7,8, diamonds9 and kimberlites10, have been found in gases from northern Afar4 (P.A. and M. Javoy, in preparation). The high-temperature gas samples collected from the November 1978 Asal rift eruption, southern Afar1, have provided new isotope data on basaltic igneous volatiles, released from upper mantle magmatism11.