DEGASSING AND REHYDRATION OF EXPLOSIVE PRODUCTS OF THE 900 BP GLASS MOUNTAIN RHYOLITIC ERUPTION OF MEDICINE LAKE VOLCANO, CALIFORNIA

Effusive eruptions of silicic magma are usually preceded by an intense explosive phase (e.g., Mono Craters; Newberry; Medicine Lake Volcano; Chaitén; Cordón Caulle). The deposits of the explosive phase are pumices with minor amounts of obsidian pyroclasts. The latter are often interpreted to be fragments of vanguard magma produced by permeable foam collapse before the explosive phase but recent studies have suggested that they can also be produced syn-eruptively by ash sintering above the fragmentation level. Pumices and pyroclastic obsidians therefore record different degassing histories, which, if correctly interpreted, can provide information concerning magma ascent and fragmentation. With this aim in mind, a series of 37 pumices and pyroclastic obsidians from the Plinian phase of the 900 BP Glass Mountain eruption of Medicine Lake Volcano, California, were analyzed for their water content and δD values. Water contents in the pyroclastic obsidians are 0.3-0.9 wt% with δD values of -106 to -77‰, consistent with the degassing of magma in open-system and indicative of a quench at low pressures. Water content in the porous pyroclasts varies more widely, from 0.3 to 1.4 wt%, and is positively correlated with the clast porosity (3-85 vol.%), while δD is much lower than in the dense obsidian pyroclasts, with values of -120 to -102‰, inconsistent with a degassing trend. Numerical modeling of water diffusion in rhyolitic glass shows that the values measured in the pumices are consistent with a rehydration of an initially water-poor (~0.3 wt%) glass at atmospheric conditions for about 900 years.