Petrogenesis and volatile stratigraphy of the Bishop Tuff - evidence from melt inclusion analysis
- New Mexico Inst. of Mining and Technology, Socorro (United States) Arizona State Univ., Tempe (United States)
The preeruptive volatile gradient of the Bishop Tuff (BT) is determined on the basis of detailed analyses of the H2O, F, and Cl contents of melt inclusions in phenocrysts from different stratigraphic levels of the deposit. The magma which produced BT was found to be strongly zoned with respect to H2O, and slightly zoned with respect to F and B. The bulk of the volatile zonation within the BT magma was in the 120 cu km of magma which produced the Plinian tephra, where the H2O content decreases from a mean of 6 wt pct to a mean of 3.3 wt pct to between 2 and 3 wt pct for the magma which formed the ignimbrite. The strong H2O gradient suggests that only the magma from the very upper portion of the magma chamber could have been saturated with respect to an H2O-dominated vapor phase, because of the decrease in H2O contents with decreasing pressure. The volatile enrichment in the magma which formed the BT Plinian tephra appears to have occurred independently of strong trace element enrichment and may have been generated by rapid diffusion from a sidewall crystallization zone. 60 refs.
- OSTI ID:
- 6994610
- Journal Information:
- Journal of Geophysical Research; (United States), Vol. 97; ISSN 0148-0227
- Country of Publication:
- United States
- Language:
- English
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