A PETROLOGIC PERSPECTIVE OF KILAUEA VOLCANO’S SUMMIT MAGMA RESERVOIR

Two hundred years of magmatic history are well documented by the lavas and tephra sampled from eruptions within or near Kilauea’s summit caldera. Detailed petrographic and geochemical data for a comprehensive suite of samples reveal the range of magmatic processes that operate within the volcano's summit magma reservoir and document two compositional trends that span nearly the entire known compositional range for the volcano. Although olivine fractionation is a dominant process in the summit reservoir, differentiation involving clinopyroxene and plagioclase is more extensive than previously recognized in Kilauea summit lavas. The effects of crystal fractionation are superimposed upon an evolving hybrid magma composition produced by mixing new, mantle-derived magmas with more fractionated reservoir magma. Prior to Kilauea’s 1924 summit crater collapse, a trend of increasing incompatible elements and CaO, and decreasing silica abundances (at a constant MgO) prevailed. Thereafter, the trend reversed direction and has persisted for the rest of the 20^th century, including during the current Puu O`o eruption. The rapid and systematic nature of these temporal geochemical variations indicates that the summit reservoir is a single, well mixed, relatively small body rather than a plexus of dikes and sills or a large chamber, as previously thought. Petrographic observations and olivine and whole-rock geochemical data suggest that the summit reservoir has a crown of aphyric, more evolved, low-density magma. Frequent eruptions of hybrid reservoir magma document the rapid variation in parental magma composition. These compositional variations correlate with large changes in magma supply rate and the inferred size of the reservoir. Both magma supply rate and the compositional variations are thought to be influenced by the degree of melting of small-scale source heterogeneities within the Hawaiian plume. Nonetheless, Kilauea’s source compositions and partial-melting processes have varied narrowly over the past 350 ka.