Title: The Bishop Tuff: Evidence for the origin of compositional zonation in silicic magma chambers
Abstract:The ash-fall and outflow sheets of the 0.7-m.y.-old Bishop Tuff represent >170 km3 of compositionally zoned rhyolitic magma emplaced during collapse of the Long Valley caldera, California. Field, mine...The ash-fall and outflow sheets of the 0.7-m.y.-old Bishop Tuff represent >170 km3 of compositionally zoned rhyolitic magma emplaced during collapse of the Long Valley caldera, California. Field, mineralogic, and chemical evidence agree that tapping of the thermally and chemically zoned chamber was continuous, without interruptions sufficient to permit mixing or phase re-equilibration. Fe-Ti oxide temperatures for 68 glassy samples increase systematically with eruptive progress from 720 to 790 °C; this increase corresponds well with the stratigraphic sequence, but the temperatures in no way correspond to the degree of welding. Ubiquitous quartz, sanidine, oligoclase, biotite, ilmenite, titanomagnetite, zircon, and apatite change composition progressively with temperature. The uniformity within every sample of each mineral species (irrespective of size and whether discrete or as inclusions) is not compatible with protracted crystal settling. Euhedral allanite (ρ > 4, La + Ce > 16% by weight) occurs in all early-erupted samples (720 to 763 °C) but in none erupted later. Despite this, whole-rock La + Ce values increased threefold during the eruption. Pyrrhotite, hypersthene, and augite appear abruptly at 737 °C and occur in all later samples. These sharp isothermal interfaces indicate lack of any extensive history of crystal settling.Read More
Publication Year: 1979
Publication Date: 1979-01-01
Language: en
Type: book-chapter
Indexed In: ['crossref']
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Cited By Count: 693
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