Fractionation of noble gases (He, Ar) during MORB mantle melting: a case study on the Southeast Indian Ridge

Abstract

New measurements of the He, Ar and CO 2 abundances trapped in basaltic glasses from the Southeast Indian Ridge (SEIR) show that volatile concentrations in the SEIR magmas were controlled by fractional degassing. Fractionation between volatile species is consistent with their solubilities in silicate melts. As a result, there are linear relationships between (for example) ln( 4He/ 40Ar*) vs. ln[ 40Ar*] and between ln( 4He/ 40Ar*) vs. ln( 40Ar*/CO 2) (where 40Ar* is the 40Ar corrected for atmospheric contributions). The slopes of these correlations permit the relative He/Ar and Ar/CO 2 solubilities to be estimated; these are generally consistent with experimentally determined noble gas solubilities in basaltic melts. However, there are systematic differences in the degassing trajectories. For example, in a plot of ln( 4He/ 40Ar*) vs. ln( 40Ar*/CO 2), samples from the deepest portions of the ridge consistently plot at lower 4He/ 40Ar* for a given 40Ar*/CO 2, compared to shallower sections of ridge. These variations in 4He/ 40Ar* likely reflect variations in He/Ar in the primary melt, i.e. their relative abundances prior to degassing. We estimated the variation in 4He/ 40Ar* in the initial melts (i.e. the 4He/ 40Ar* prior to degassing) by extrapolating the degassing trend to a constant mantle-like 40Ar*/CO 2 ratio and assuming that the relative He–Ar–CO 2 solubilities do not vary between samples. The 4He/ 40Ar* corrected for degassing in this manner varies by a factor ≈10 and correlates positively with the 3He/ 4He ratio. It is possible that the correlation between “degassing corrected” 4He/ 40Ar* ratios and the 3He/ 4He ratio results from preferential diffusion of 3He relative to 4He and of 4He relative to 40Ar from the solid mantle into primary melts during melting. However, modeling this diffusive process fails to reproduce the comparatively large variations in 3He/ 4He found in the basalts; therefore, it seems likely that mantle heterogeneities, in combination with diffusive fractionation, resulted in coupled He isotope and He/Ar variations.