Intellectual merit. Rhyolitic super-eruptions from calderas are rare events that lie outside recorded human experience, but it is quite certain that they will continue to occur. Insights into the magmatic processes that accompany the generation, storage, and eruption of large bodies of rhyolitic magma are provided by petrologic study of the eruptive products of past events. A consensus is now emerging among the scientific community that eruptions of crystal-poor rhyolite with volumes of the order of 100 - 1,000 km3 leave still greater quantities of complementary non-erupted crystal-rich residue beneath the surface. This residue may be capable of rejuvenation and production of subsequent eruptible bodies of crystal-poor magma. This proposal seeks funding for a detailed study of an example where two super-eruptions of crystal-poor rhyolite occurred on exactly the same site, separated in time by ~360,000 years. The Otowi (Lower) and Tshirege (Upper) Members of the Bandelier Tuff were erupted from the Valles caldera, New Mexico, in two episodes at 1.61 and 1.25 Ma respectively. Both exhibit strong chemical zonation. From prior work, the pre-caldera magmatic history is well understood, as are the development and pre-eruptive architecture of the Otowi magma body. The highly enriched, yet relatively simple, geochemical character of the Otowi implies that it parted from a large mass of crystals. The later Tshirege Member shares many petrologic and geochemical similarities with the Otowi, yet is considerably more complex. Preliminary data are consistent with a model in which the Tshirege is partly derived from Otowi crystal residue with added ancient crust. This mixture was not homogenized prior to production of the Tshirege magma. This hypothesis will be tested by analyzing pumices, glasses and crystals for major, trace elements and radiogenic isotopes to identify contributing components and the process by which they were assembled to form the Tshirege magma. Two additional key aspects are (1) associated analog experiments to constrain the extent of syn-eruptive disturbance of the crystal pile induced by the downdropping caldera block, and (2) high resolution imaging and analysis of Ti distributions in quartz phenocrysts, which will constrain the timing of recharge and heating event(s) that may trigger super-eruptions by thermal rejuvenation of the crystal pile.
Broader impacts. Given the potentially devastating global impact of rhyolitic super-eruptions, the need to forecast the next such event several decades beforehand is self-evident. Such premonitory indicators as may be detectable at the surface must necessarily be informed by an understanding of the nature and timing of magmatic processes that precede supereruptions. This in turn demands basic research into these processes, which is the subject of this research. The research outlined in this proposal will additionally serve to train undergraduates and a graduate student in modern methods associated with petrologic and geochemical investigations. Additionally, it will contribute to the development of a young research scientists and an early-career Hispanic scientist who has just taken a tenure-track position.
With the exception of asteroid strikes, large volcanic ‘super-eruptions’ are the most devastating of all geological phenomena. They are rare and outside human experience, but it is quite certain that they will continue to occur. The effects of a future such eruption on our increasingly populated planet would depend to some extent on its location, but have the potential to cause megadeaths, global disruption of climate, agriculture, transport and communications, and collapse of the global economy. Because none have been observed (or at least recorded) by humans, what we wish to know of them must be gained through study of the geologic record. This includes reconstructing the processes that cause the eruptions, in order that society may gain predictive capability for an event the like of which has never been witnessed. Two such events occurred at 1.60 and 1.24 Ma respectively on the site of the Valles caldera, New Mexico, to form the Otowi and Tshirege Members of the Bandelier Tuff. The two tuffs consist of ash and pumice in deposits up to hundreds of meters thick; the ash has been found as far away as the Gulf of Mexico. Between the two deposits are several minor ash layers, the products of much smaller eruptions that provide shapshots of the magma system at intervals during the 360,000 years between the two super-eruptions. In this research, we chemically analyzed studied glassy ash and pumice, mineral crystals of quartz, feldspar, pyroxene and zircon found in the ash, and small glass inclusions within the quartz and feldspar crystals. We determined abundances of the major chemical constituents of the ash and mineral grains as well as trace element concentrations and the ratios of isotopes of the elements strontium and lead. Using these chemical tracers, we were able to show that the big eruptions happened after a history of events involving recharge of the magma chamber from below. Each recharge event could have caused an eruption. The eruptions happened on the order of decades following the critical, triggering recharge event. It may not be possible to distinguish a critical from a non-critical (i.e. non-eruption-producing) event; however the critical events were accompanied by failure of the ‘ceiling’ of the magma chamber, which would be accompanied by measurable geophysical signals that would possible provide warning of an impending super-eruption.
