Picritic glasses found in turbidite sands near the submarine part of Kilauea's East Rift Zone contain up to 15 wt.% MgO and are the most magnesian Hawaiian volcanic glasses reported to date (Clague et al., 1991, 1995). They have olivine phenocrysts as magnesian as Fo_90.7, and when their compositions are plotted together on normative diagrams, they form a distinct olivine fractionation trend. Melt geothermometers indicate that the eruption temperatures of the picrite magmas were as high as about 1320¾C. On the assumption that these glasses represent primary melt compositions that coexisted with a lherzolite phase assemblage, the CMASNF geothermometer (Gudfinnsson & Presnall, 2001) yields a maximum temperature of generation of about 1480¾C, which corresponds roughly to a pressure of 2.5 GPa. This assumes that the melts were essentially free of H₂O and CO₂. However, both of these volatile components have the potential to lower significantly the solidus temperatures of mantle peridotite and alter the chemistry of primary melts. The approximately 0.4 wt.% H₂O measured in the Hawaiian picrite glasses is probably below the saturation limit for H₂O, and can be assumed to be close to the original H₂O content of the picrite melts. The measured amount of CO₂ in the glasses is low as most CO₂ was probably lost by degassing at the time of eruption. The CO₂ content of primary magmas at Kilauea has been determined as 0.7 wt.% (Gerlach & Graeber, 1985; Gerlach et al., 2001). Whereas the addition of CO₂ tends to shift melts derived from peridotite toward greater alkalinity, the addition of H₂O tends to move liquids toward the quartz normative side of the basalt tetrahedron. Results of CO₂- and H₂O-bearing melting experiments indicate that with the low amounts of H₂O and CO₂ expected in the primary melts of Kilauea their effect on the position of phase boundaries will be small. From phase relations involving melt in equilibrium with a garnet lherzolite phase assemblage in the system CaO-MgO-Al₂O₃-SiO₂-CO₂, we estimate that the solidus temperature of mantle lherzolite is lowered by about 10¾C for each 1 wt.% increase in the amount of CO₂ in the melt. The effect of H₂O is to lower the solidus temperature about 40¾C for each 1 wt.% increase in this component. This yields a minimum potential temperature (T_P) for Hawaii of about 1420¾C, which is consistent with data indicating at most only a very slight increase of heat flow at Hawaii relative to Pacific crust of the same age (Stein & Stein, 1993). This T_P is 140-160¾C higher than petrological estimates of the average T_P of the MORB source (McKenzie & Bickle, 1988; Presnall et al., 2002). Our data do not constrain the upper limit of T_P at Hawaii.