Major, minor, and dissolved volatile element concentrations were measured in tholeiitic glasses from the submarine portion (Puna Ridge) of the east rift zone of Kilauea Volcano, Hawaii. Dissolved H2O and S concentrations display a wide range relative to nonvolatile incompatible elements at all depths. This range cannot be readily explained by fractional crystallization, degassing of H2O and S during eruption on the seafloor, or source region heterogeneities. Dissolved CO2 concentrations, in contrast, show a positive correlation with eruption depth and typically agree within error with the solubility at that depth. We propose that most magmas along the Puna Ridge result from (1) mixing of a relatively volatile-rich, undegassed component with magmas that experienced low pressure (perhaps subaerial) degassing during which substantial H2O, S, and CO2 were lost, followed by (2) fractional crystallization of olivine, clinopyroxene, and plagioclase from this mixture to generate a residual liquid; and (3) further degassing, principally of CO2 for samples erupted deeper than 1000 m, during eruption on the seafloor. The degassed end member may form at upper levels of the summit magma chamber (assuming less than lithostatic pressure gradients), during residence at shallow levels in the crust, or during sustained summit eruptions. The final phase of degassing during eruption on the seafloor occurs slowly enough to achieve melt/vapor equilibrium during exsolution of the typically CO2-rich vapor phase. We predict that average Kilauean primary magmas with 16% MgO contain approximately 0.47 wt % H2O, approximately 900 ppm S, and have delta-D values of approximately -30 to -40%. Our model predicts that submarine lavas from wholly submarine volcanoes (i.e., Loihi), for which there is no opportunity to generate the degassed end member by low pressure degassing, will be enriched in volatiles relative to those from volcanoes whose summits have breached the sea surface (i.e., Kilauea and Mauna Loa).
