THE CONCENTRATION AND ISOTOPIC COMPOSITION OF CARBON IN BASALTIC GLASSES FROM THE JUAN-DE-FUCA RIDGE, PACIFIC-OCEAN

The abundance and C-13/C-12 ratios of carbon were analyzed in basaltic glass from twenty locations along the Juan de Fuca Ridge using a 3-step combustion /extraction technique. Carbon released during the first two combustion steps at 400-500-degrees-C and 600-650-degrees-C is interpreted to be secondary, and only the carbon recovered during a final combustion step at almost-equal-to 1200-degrees-C is thought to be indigenous to the samples. For carbon released at almost-equal-to 1200-degrees-C, glasses analyzed as 1-2 mm chips contained 23-146 ppm C with deltaC-13 values of -4.8 to -9.3 parts per thousand, whereas samples crushed to 38-63 mum or 63-90 mum yielded 56-103 ppm C with deltaC-13 values of -6.1 to -9.2 parts per thousand. The concentrations and isotopic compositions of the primary carbon dissolved in the glasses and present in the vesicles are similar to those previously reported for other ocean-ridge basalts. The Juan de Fuca basaltic magmas were not in equilibrium with respect to carbon when they erupted and quenched on the sea floor. Evidence of disequilibrium includes (1) a large range of carbon contents among glasses collected at similar depths, (2) a highly variable calculated carbon isotopic fractionation between melt and vapor determined by comparing crushed and uncrushed splits of the same sample, and (3) a lack of correlation between vesicle abundance, carbon concentration, and depth of eruption. Variations in carbon concentration and deltaC-13 ratios along the ridge do not correlate with major element chemistry. The observed relationship between carbon concentrations and deltaC-13 values may be explained by late-stage, variable degrees of open-system (Rayleigh-like) degassing.