EFFECTS OF ERUPTION AND LAVA DRAINBACK ON THE H2O CONTENTS OF BASALTIC MAGMAS AT KILAUEA VOLCANO

Lava drainback has been observed during many eruptions at Kilauea Volc ano: magma erupts, degasses in lava fountains, collects in surface pon ds, and then drains back beneath the surface. Time series data for mel t inclusions from the 1959 Kilauea Iki picrite provide important evide nce concerning the effects of drainback on the H2O contents of basalti c magmas at Kilauea. Melt inclusions in olivine from the first eruptiv e episode, before any drainback occurred, have an average H2O content of 0.7 +/- 0.2 wt.%. In contrast, many inclusions from the later episo des, erupted after substantial amounts of surface degassed lava had dr ained back down the vent, have H2O contents that are much lower (great er than or equal to 0.24 wt.% H2O). Water contents in melt inclusions from magmas erupted at Pu'u 'O'o on the east rift zone vary from 0.39- 0.51 wt.% H2O in tephra from high fountains to 0.10-0.28 wt.% H2O in s patter from low fountains. The low H2O contents of many melt inclusion s from Pu'u 'O'o and post-drainback episodes of Kilauea Iki reveal tha t prior to crystallization of the enclosing olivine host, the melts mu st have exsolved H2O at pressures substantially less than those in Kil auea's summit magma reservoir. Such low-pressure H2O exsolution probab ly occurred as surface degassed magma was recycled by drainback and mi xing with less degassed magma at depth. Recognition of the effects of low-pressure degassing and drainback leads to an estimate of 0.7 wt.% H2O for differentiated tholeiitic magma in Kilauea's summit magma stor age reservoir. Data for MgO-rich submarine glasses (Clague et al. 1995 ) and melt inclusions from Kilauea Iki demonstrate that primary Kilaue an tholeiitic magma has an H2O/K2O mass ratio of similar to 1.3. At tr ansition zone and upper mantle depths in the Hawaiian plume source, H2 O probably resides partly in a small amount of hydrous silicate melt.