CARBON-ISOTOPE VARIATIONS OF CO2 IN CENTRAL LAU BASIN BASALTS AND FERROBASALTS

CO2 abundance and carbon isotope data are reported for basalt glasses from three spreading centres in the Central Lau Basin. Abundances of CO2 dissolved in glasses vary from 9 ppm to 217 ppm with deltaC-13 ranging from -16.4 parts per thousand to -8.8 parts per thousand. Evolved ferrobasalts from the propagating rift tip of the Central Lau Spreading Centre have both the lowest CO2 concentrations and delatC-13 values, while more magnesian basalts from the Intermediate and Eastern Lau Spreading Centres have higher CO2 abundances with higher and less variable deltaC-13 values. Variations in abundance and carbon isotope ratios of dissolved CO2 can be modelled by degassing during eruption of melts originally containing 410-440 ppm CO2, with deltaC-13 of -7.7 parts per thousand, using a vapour-melt isotope fractionation of 2.3 parts per thousand. The type of degassing experienced by melts is related to their style of eruption, which is, in turn, governed by their eruption site. Central Lau Basin glasses, along with glasses from other marginal basins, display carbon isotope ratios which are lower than MORB. While the variations within the suite are adequately described by various degrees of degassing, carbon isotope heterogeneity between sources of oceanic basalts can not be discounted.