The bubble-by-bubble volatile evolution of two mid-ocean ridge basalts

Analyses of He, Ar and CO2 in individual vesicles from two basaltic glasses from the mid-Atlantic Ridge show that volatile compositions in basaltic glass vary from vesicle to vesicle. This volatile fractionation is consistent with Rayleigh-type fractional degassing from the magma, producing a well-defined evolution in the magmatic volatile compositions. Volatile fractionation during vesicle formation may be superimposed on the overall Rayleigh degassing trend of one of the samples. Estimates of the relative He/CO2 and Ar/CO2 fractionation factors (5 and 0.5 respectively) from these degassing trends suggest that noble gases are more soluble and/or CO2 less soluble during some mid-ocean ridge magmatic processes than predicted by experimental studies (relative He/CO2 and Ar/CO2 solubilities predicted to be about 2 and 0.2 respectively). Assuming the magmatic Ar/He ratio was in the range 0.3–0.6, the initial He/CO2 ratio of the magma was between 9.7 and 12.4×10−5. The calculated pressures of volatiles in most vesicles are within error of the eruption pressure (≈0.3 kbar). However, some vesicles appear to preserve internal pressures up to 1 kbar. If true, this suggests magma ascent through the crust must have occurred at high pressure with instantaneous quenching of the magma.