Volatiles in Basaltic Glasses from the Easter-Salas y Gomez Seamount Chain and Easter Microplate: Implications for Geochemical Cycling of Volatile Elements

We present H₂O, CO₂ and Cl concentrations in 66 basaltic glasses from the Easter Microplate (EMP) and Easter-Salas y Gomez Seamount Chain (ESC) system in the Southeastern Pacific. The EMP-ESC system is characterized by binary mixing between a depleted MORB source and an incompatible element and radiogenic isotope enriched plume source, with channeling of plume material toward the ridge crest centered at ~27° S on the east rift of the microplate. Water concentrations on the EMP are highest on the east rift at ~27° S and become progressively lower to the north and south, following the spatial pattern of other geochemical tracers. EMP basalts have not lost H₂O to degassing, but have assimilated variable quantities of a Cl-rich hydrothermal component. In contrast, some seamount basalts have lost water by shallow degassing, but very few have gained Cl, indicating little assimilation of Cl-rich materials. Several seamount glasses from the ESC may have assimilated a hydrous component, e.g., serpentinized harzburgite, during magma ascent through the lithosphere. Based on samples unaffected by shallow processes, the main plume component has H₂O/Ce of ~210 +/- 20 and is neither preferentially enriched nor depleted in H₂O relative to other similarly incompatible elements. The depleted MORB source has a H₂O/Ce of ~150 +/- 10 Estimated mantle volatile concentrations for the Salas y Gomez plume are 590 +/- 40 ppm for H₂O and 26 +/- 2 ppm for Cl. The most depleted source region for the Easter Microplate basalts has 64 +/- 20 ppm H₂O and 1.8 +/- 0.6 ppm Cl. The coupled behavior of H₂O and Cl with similarly incompatible elements suggests that the volatiles are juvenile, representative of a component common to mantle plumes, with minor contribution from recycled lithospheric components.