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 mid‐ocean ridge basalt (MORB) mantle source (DMM) and an incompatible element and radiogenic isotope enriched source, the Salas y Gomez mantle plume (SyG). Plume material is channeled 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 ESC seamount glasses may have assimilated a hydrous component, for example, serpentinized harzburgite, during magma ascent through the lithosphere. On the basis of 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 H2O/Ce of ∼150 ± 10. Estimated mantle volatile concentrations are 750 ± 210 ppm H2O and 40 ± 11 ppm Cl for the SyG source, 120 ± 27 ppm H2O and 4.5 ± 1.4 ppm Cl for an average EPR source, and 54 ± 12 ppm H2O and 1.7 ± 0.4 ppm Cl for the DMM source. The coupled behavior of H2O and Cl with similarly incompatible elements, coupled with elevated 3He/4He ratios, suggests that the volatiles are dominantly juvenile, representative of a component common to mantle plumes, with minor contribution from recycled lithospheric components.