Modelling caldera collapse into a crystal mush, with application to the Bandelier Tuff, Valles caldera, New Mexico
Abstract
The Valles caldera, NM, was formed in two eruptions at 1.61 Ma and 1.25 Ma that produced the Bandelier Tuff. The older Otowi Member of the Bandelier is a compositionally zoned high-silica rhyolite (>75% SiO2) with a dense rock equivalent volume of ~450 km3, and up to 5-fold variation in the incompatible element content of pumice fragments. Volume-composition relations require that 3 - 4 times the erupted volume, i.e. 1350 - 1800 km3, of leucogranitic crystal residue must complement the erupted magma. Quartz-sanidine glomerocrysts in late-erupted pumices are interpreted as fragments of the residue, but have a cumulative volume << 100 km3. Therefore, most of the residue was not erupted. The younger Tshirege member of the Bandelier Tuff, erupted from the same site 360,000 years later, is a ~250 km3 zoned high-silica rhyolitic ignimbrite similar to the Otowi, but additionally contains magmatic components of low-silica rhyolite and dacite. Also, incompatible elements in the Tshirege exhibit a much greater degree of scatter on covariation diagrams than is the case for the Otowi Member. The dacite is not closely related to the rhyolites and was introduced during a late recharge event that may have triggered the eruption. The extent and style of recycling of Otowi crystal residue into the Tshirege magma body is a long-standing question. To begin to investigate this problem, we have initiated a series of laboratory experiments designed to simulate the settling of a caldera block into a crystal mush layer underlying supernatant, eruptible, crystal-poor magma during caldera-forming super-eruptions. The extent and character of mush disruption and eruption on collapse vary with the amounts of crystal-rich and crystal-poor magma, roof block weight and thickness, melt viscosity and heterogeneity and lead to a first-order prediction that rejuvenation of the disturbed crystal mush should result in greater chemical complexity during subsequent magmatic cycles. This is consistent with our observations of both members of the Bandelier Tuff.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V13C2377K
- Keywords:
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- 1036 GEOCHEMISTRY / Magma chamber processes;
- 1042 GEOCHEMISTRY / Mineral and crystal chemistry;
- 1065 GEOCHEMISTRY / Major and trace element geochemistry;
- 8440 VOLCANOLOGY / Calderas