Of 50 analyzed glass inclusions in olivine phenocrysts from the 1959 Kilauea Iki eruption, 41 formed at pressures < 1 kbar, seven between 1 and 2 kbar, and two at pressures > 2 kbar. The surprisingly low formation pressures suggest that most 1959 olivines, including most of those with preeruptive equilibration temperatures above 1200 degrees C, crystallized in an upper part of Kilauea) s summit magma storage reservoir. The implication that the parental magma was buoyant relative to stored magma is consistent with an expected preeruptive bulk CO2 content near 0.2 wt% and published evidence for mixing between hot, newly arrived parental and preexisting magma. That the 1959 magma was rich not only in crystals but also in gas, as evidenced by its high lava fountains, suggests that the storage time in a shallow reservoir was too short for either crystals or gas to be lost. Therefore, the 1959 Kilauean magma probably is a near-parental magma that rose and formed a gas- and crystal-rich cap near the top of a shallow body of stored magma beneath Kilauea) s summit region. Whether newly arriving parental magma is buoyant relative to stored magma depends mainly on pressure and magma gas content. Consequently, it seems likely that the eruptive and degassing behavior of Kilauea is regulated in part by an interplay between the CO, content of parental magma and the pressure at which new magma intrudes stored, degassed magma.

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