Plume–ridge interaction along the Galapagos Spreading Center: discerning between gas loss and source effects using neon isotopic compositions and 4He–40Ar∗–CO2 relative abundances

To document the plume–ridge interaction at the Galapagos Spreading Center (GSC), we present neon 3-isotope analyses performed on vesicles-trapped volatiles of MORB glasses dredged along the ridge axis between 86°W and 98°W. 4He–40Ar∗–CO2 relative abundances were also measured in order to study gas loss in this context and discern between source and degassing effects. Neon isotopic compositions are in the MORB range with 21Ne/22Ne ratios extrapolated to the 20Ne/22Ne mantle ratio of 12.5 varying between 0.053 and 0.072. Unradiogenic plume-like compositions were not measured. The 4He–40Ar∗–CO2 relative abundances are highly variable along the ridge, for example 4He/40Ar∗ ratio varies between 3 and 433, but these variations can be fully explained by a simple model of Rayleigh distillation, with a single volatile source composition for the entire GSC. Magma fractional crystallisation, which increases in the plume influenced zone, seems to be the main motor for degassing. As other geochemical and geophysical studies indicate a significant plume influence on the GSC, these results suggest the plume component feeding the ridge is either degassed or else different from the plume core.