PARTIAL PRESSURES OF CRYSTALLIZATION AND MAGMA PLUMBING SYSTEMS BENEATH HAWAIIAN VOLCANOES

To help interpret the plumbing systems beneath Hawaiian volcanoes, we have calculated the pressures of partial crystalliztion of basaltic magmas using the petrological method described by Kelly & Barton (2008). A total of 1576 major oxide analyses of glasses from four volcanoes (Kilauea and the Puna Ridge, Loihi, Mauna Loa, and Mauna Kea, on the Big Island) were compiled and used as input data. Glasses represent quenched liquid composition rather than mixtures of crystals and melts, and therefore glass analyses are preferable to whole-rock analyses for calculation of pressures of partial crystallization. The results were filtered to exclude samples that yielded unrealistic results based on high errors associated with the calculated pressure or negative value of pressure, and to exclude samples with non-basaltic compositions or that do not lie along the melt, olivine, plagioclase and clinopyroxene cotectic. Calculated pressures were converted to depths of partial crystallization. The majority (68.2%) of pressures of partial crystallization for the shield-stage subaerial volcanoes Kilauea, Mauna Loa, and Mauna Kea, fall in the range 0-140 MPa, corresponding to depths of 0-5 km. Glasses from the Puna Ridge yield pressures ranging from 18 to 126 MPa and are virtually identical to pressures determined from glasses from Kilauea (0 to 129 MPa), as expected because the Puna Ridge is an offshore extension of the East Rift zone. These results are consistent with the presence of magma reservoirs at depths of 0-5 km beneath the large shield volcanoes. The inferred depth of the magma reservoir beneath the summit of Kilauea (average = 1.8 km, maximum = 5 km) agrees extremely well with depths (~2-6 km) estimated from seismic studies (Dzurisin, 1984). The results for Kilauea and Mauna Kea indicate that significant partial crystallization also occurs beneath the summit reservoirs at depths up to 11 km. These results are also consistent with seismic evidence for the presence of a magma reservoir at 8-11 km beneath Kilauea (Lin et al 2014) at the base of the volcanic pile. The results for Loihi (100-400 MPa) indicate crystallization at higher average pressures and depths (3-14 km) than the large shield volcanoes, suggesting that the plumbing system is not yet fully developed, and that the Hawaiian volcanic plumbing systems evolve over time.