Experimental Determination of Oxygen-isotope Fractionations Between CO_2 Vapour and Soda-melilite Melt

Oxygen-isotope measurements of igneous rocks and minerals can constrain their origin and evolution, but they are interpretable only in light of known ^(18)O/^(16)O fractionations among co-existing silicate melt, minerals, and magmatic volatiles. There have been many experimental studies of mineral-mineral and mineral-fluid fractionations but few involving silicate melt, particularly basic and ultrabasic melts relevant to study of mantle geochemistry and basaltic volcanism (Muehlenbachs & Kushiro, 1974). Consequently, fractionations involving such melts are generally estimated based on the systematics of differences in δ^(18)O between phenocrysts and co-existing glass or groundmass. Estimates of this type, although useful, are insufficient for interpreting the usually subtle oxygen isotope variations observed in many basaltic suites. Therefore, we have undertaken a series of experiments examining oxygen-isotope fractionations involving basic silicate melts. We report here measurements of fractionations between the CO_2 and soda-melilite melt. This composition has been used previously as an analog for basic silicate melts.