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Pre-eruption recharge of the Bishop magma system

¹ Department of Earth & Environmental Sciences, Rensselaer Polytechnic Institute, Troy, New York 12180, USA
² U.S. Geological Survey, MS-910, Menlo Park, California 94025, USA
³ Department of Earth & Environmental Sciences, Rensselaer Polytechnic Institute, Troy, New York 12180, USA

The 650 km³ rhyolitic Bishop Tuff (eastern California, USA), which is stratigraphically zoned with respect to temperatures of mineral equilibration, reflects a corresponding thermal gradient in the source magma chamber. Consistent with previous work, application of the new TitaniQ (Ti-in-quartz) thermometer to quartz phenocryst rims documents an 100 °C temperature increase with chamber depth at the time of eruption. Application of TitaniQ to quartz phenocryst cores, however, reveals lower temperatures and an earlier gradient that was less steep, with temperature increasing with depth by only 30 °C. In many late-erupted crystals, sharp boundaries that separate low-temperature cores from high-temperature rims cut internal cathodoluminescent growth zoning, indicating partial phenocryst dissolution prior to crystallization of the high-temperature rims. Rimward jumps in Ti concentration across these boundaries are too abrupt (e.g., 40 ppm across a distance of <10 µm) to have survived magmatic temperatures for more than 100 yr. We interpret these observations to indicate heating-induced partial dissolution of quartz, followed by growth of high-temperature rims (made possible by lowering of water activity due to addition of CO₂) within 100 yr of the climactic 760 ka eruption. Hot mafic melts injected into deeper parts of the magma system were the likely source of heat and CO₂, raising the possibility that eruption and caldera collapse owe their origin to a recharge event.

Key Words: Long Valley caldera • Bishop Tuff • mafic recharge • thermal rejuvenation • rhyolite

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