Pre-eruptive conditions in the Wah Wah Springs Tuff: No Evidence of Thermal Rejuvenation
Abstract
The Wah Wah Springs Tuff (29.5 Ma) is one of four super volcanic eruptions (>1000 km3 of magma) of dacite that occurred near the peak of the ignimbrite flare-up in the Great Basin of western North America. It erupted from the Indian Peak caldera complex that straddles the border of Utah and Nevada and can be characterized as a "monotonous intermediate" ignimbrite because of its intermediate concentrations of silica (63 to 70 wt%), apparent uniform chemical and mineralogical characteristics, and crystal-rich nature (32 ± 10 percent phenocrysts). The major phase assemblage found throughout the deposit is similar to other monotonous intermediates (pl>hnbl>bio>qtz>cpx>opx>mt>ilm; sanidine is absent). Based on experiments on the monotonous intermediate Fish Canyon Tuff, the Wah Wah Springs magma equilibrated between 775°C and 800°C at water-undersaturated conditions and a pressure near 2 kb. The Holland and Blundy hornblende-plagioclase thermometer (with or without quartz) and the QUILF Fe-Ti oxide thermometer most consistently yield temperatures within this range and the oxides give an fO2 of 2 to 3 log units above the QFM oxygen buffer. Two-pyroxene thermometry gives temperatures that average 860°C, and we conclude the pyroxenes are relicts of a higher temperature phase of crystallization. The Johnson and Rutherford Al-in-hornblende geobarometer indicates pressures between 2.0 and 2.5 kb. Thus, the Wah Wah Springs Tuff is similar to the Fish Canyon Tuff but erupted at a temperature about 15° to 40°C higher. On the other hand important differences between these two super eruptions are revealed by detailed compositional profiles across hornblende and plagioclase grains that constrain how intensive parameters changed during the evolution of the Wah Wah magma shortly before eruption. Plagioclase in the Wah Wah Springs displays oscillatory zonation with overall normal zonation (An70 cores to An40 rims). Hornblende is also zoned normally especially in Al2O3 and TiO2 which decrease as much as 2.5 wt% and 1 wt%, respectively, from cores to rims. These zoning patterns are consistent with a decrease of temperature during crystallization of these phases. We find no low Al- hornblendes (Altotal =0.9 and 1.1 apfu) like those in the Fish Canyon, which were interpreted to be low temperature, near-solidus phases. Moreover, orthopyroxene is found only embedded in hornblende, apparently as a result of a down temperature reaction with melt. Resorption of quartz and plagioclase was probably the result of decompression of the water-undersaturated magma during eruption. Thus, we see no evidence that the Wah Wah magma was a near-solidus magma body that was "rejuvenated" or reheated immediately prior to eruption as proposed by Bachman et al., (2002) for the Fish Canyon Tuff. Pre-eruptive melting does not seem to be a necessary prerequisite for caldera collapse and eruption of very large bodies of magma.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.V12A..07W
- Keywords:
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- 8404 Volcanoclastic deposits;
- 8411 Thermodynamics (0766;
- 1011;
- 3611);
- 8428 Explosive volcanism;
- 8439 Physics and chemistry of magma bodies;
- 8440 Calderas