Abstract
Hourglass inclusions are bodies of bubble-bearing glass in volcanic phenocrysts that extend to the crystal rim through a narrow neck. Compared to enclosed inclusions, hourglass inclusions are less devitrified, contain more gas, but contain less dissolved H2O, CO2, and Cl. Hourglass formation plausibly involves the unsuccessful capture of a gas bubble. Their evolution probably involves loss of some melt through the neck as a result of decompression in surrounding magma. The gas fraction attained within hourglass inclusions is governed by their size and shape as well as by both the amount and duration of external decompression. A quantitative model of rhyolitic hourglass emptying is developed and applied to Bishop Tuff hourglass inclusions. Those in plinian pumice suggest rapid ascant at 10 m/s consistent with theoretical eruption models. Hourglass inclusions from the Mono ash-flow lobe of the Bishop Tuff suggest (1) initial crystallization of quartz, formation of some enclosed and some hourglass inclusions at approximately 2400 bars; (2) magma decompression to approximately 1100 bars for at least a week (duration of eruption for the Bishop Tuff?) while hourglass inclusions further evolved and bubbles of gas attained a 50 μm diameter; (3) magma ascent from 1100 to approximately 700 bars at approximately 1 m/s, consistent with theory for ash-flow-producing (collapsing) eruption columns; (4) entrainment of some crystals that had decompressed to a pressure of 400 bars for several weeks; (5) thermal quenching of hourglass evolution as magmatic foam disrupted into fast-moving spray, erupted, and entrained cold air. Pressure at quenching is greater than predicted magma disruption pressures but possibly consistent with preservation of long vesicles. Uncertainties are large but can be reduced by future studies of postdepositional cooling, hourglass volatile compositions, temperature, and viscosity to obtain estimates of eruptive and preeruptive magma movement and crystallization rate.
