Contrasting behaviours of CO2, S, H2O and halogens (F, Cl, Br, and I) in enriched-mantle melts from Pitcairn and Society seamounts

Abstract

In order to improve characterisation of volatiles in the EM1 and EM2 mantle sources, which are interpreted to contain subducted sedimentary or lithospheric components, we report electron microprobe, FTIR and SIMS CO2, H2O, S, F and Cl concentrations of variably enriched glasses from Pitcairn and Society seamounts in Polynesia. The analyses complement previously published Cl, Br and I data for some of the same glasses and all the techniques show reasonable agreement of better than 5–9% for S, Cl and H2O. The concentrations of H2O and all the halogens increase as a function of melt evolution with the highest values of 1.6wt.% H2O, 2100ppm F, 1500ppm Cl, 3.7ppm Br and 80ppb I in melts with <2wt.% MgO. In contrast, CO2 and S are strongly influenced by degassing and it is the least evolved melts that preserve the highest CO2 concentrations, that indicate CO2 oversaturation of some lavas. Comparison of volatile and non-volatile elements of similar compatibility (e.g. H2O/Ce, Cl/K) as a function of equilibration depth and 87Sr/86Sr, suggests H2O and halogens were not significantly degassed from melts equilibrated at >100bars and most of the variation in H2O/Ce reflects source composition. Despite their overall enrichment in volatiles, both the Pitcairn and Society EM1 and EM2 sources are depleted in H2O as well as Cl, Br, and I relative to Ce and K. Slightly different behaviour is suggested for F and a correlation between F/Cl and K/Cl in Pitcairn melts, could be explained by preferential subduction of F relative to Cl into the EM sources. The relative abundances of H2O, halogens and lithophile elements in melts from different tectonic settings indicate subduction-related H2O and Cl loss efficiencies are similar at ~90–96%; F is suggested to have a lower loss efficiency; whereas Br and I are lost more efficiently. Nonetheless, dehydrated lithosphere containing 5–10% of its original volatile content is interpreted as the most likely source of volatile enrichment in the Pitcairn and Society mantle sources.