D/H ratios in basalt glasses from the Salas y Gomez mantle plume interacting with the East Pacific Rise: Water from old D‐rich recycled crust or primordial water from the lower mantle?

Abstract

Deuterium/Hydrogen isotope ratios were measured in 22 fresh basalt glasses dredged from young seamounts along the Easter‐Salas y Gomez seamount chain (ESC) and from the spreading centers of the Easter Microplate (EMP). The δD values decrease regularly from −36‰ near Salas y Gomez to −63‰, 800 km west on the East Pacific Rise (EPR) and the west rift of the EMP. Similar gradients are observed in H2O, Pb and Sr radiogenic isotope ratios, trace element concentrations, and in the ratios of incompatible to moderately incompatible trace elements. Fractionation of the D/H ratios by recent low‐temperature near surface processes during the emplacement of these basalts is discounted on the basis of (1) degassing and hydrothermal alteration modeling, (2) the observed correlation of H2O with refractory incompatible elements such as Ce and Th, and (3) the young age and freshness of the basalt glasses studied. The δD variation shows a strong geographical coherence with the long‐lived Pb and Sr isotope ratios, suggesting long‐lived upper mantle source heterogeneities in D/H and H2O as well. Binary mixing of the δD and H2O‐rich Salas y Gomez mantle plume with the depleted upper mantle low in δD and H2O is clearly evident in the covariation of Pb radiogenic‐bearing isotope ratios, δD, and H2O along the ESC‐EMP. The positive correlations of δD with Pb isotope ratios indicate that the Salas y Gomez mantle plume with HIMU‐affinity is rich in deuterium and in H2O. A mantle plume model with two possible variations, not mutually exclusive, namely recycling of old hydrothermally altered and dehydrated crust, with or without entrainment of primordial water from the lower mantle is proposed and evaluated.