Hourglass inclusions: Theory and application to the Bishop rhyolitic tuff

Journal Article · · American Mineralogist; (United States)

OSTI ID:5784988

Anderson, Jr, A T ^[1]

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1. Univ. of Chicago, IL (United States)

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 H{sub 2}O, CO{sub 2}, and Cl. A quantitative model of rhyolitic hourglass emptying is developed and applied to Bishop Tuff hourglass inclusions. Those in plinian pumice suggest rapid ascent 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 2,400 bars; (2) magma decompression to approximately 1,100 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{mu}m diameter; (3) magma ascent from 1,100 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. 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.

• OSTI ID: 5784988
• Journal Information: American Mineralogist; (United States), Vol. 76:3-4; ISSN 0003-004X
• Country of Publication: United States
• Language: English

Related Subjects

15 GEOTHERMAL ENERGY
58 GEOSCIENCES
INCLUSIONS
ORIGIN
TUFF
MICROSTRUCTURE
BUBBLES
CRYSTALLIZATION
GEOLOGIC MODELS
MAGMA
PRESSURIZATION
QUARTZ
QUENCHING
RESERVOIR PRESSURE
SHAPE
VOLCANIC REGIONS
VOLCANISM
CRYSTAL STRUCTURE
MINERALS
OXIDE MINERALS
PHASE TRANSFORMATIONS
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