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.
Volcanic super-eruptions have occurred over geologic history and can be devasting to humans, animals, and plants worldwide. The last super-eruption occurred about 74,000 years ago and is thought to have had effects that changed climate dramatically and severely reduced the human population. Yellowstone caldera in Wyoming is known for multiple, volcanic super-eruptions (1000 to 3000 times the size of Mt St. Helens) and could be the site of another in the geologic future. Although not a super-eruption, Changbaishan volcano located along the China/N. Korea border also generated a large eruption (30 times the size of Mt. St. Helens) one thousand years ago that likely also had worldwide impacts and was the site of a seismic emergency in 2006. Valles caldera, located in north-central New Mexico, was similar to both Yellowstone and Changbaishan in that it generated both large volume eruptions and super-eruptions. This study attempts to identify how long liquid rock resides after a volcanic super-eruption occurs, how long mineral components are present in subsequent eruptions, and how long it takes until magmas associated with the next eruption appears. This study also determines that highly evolved, highly explosive magmas (rhyolites) that erupted at Valles caldera come from deeper crustal levels rather than them originating from less evolved magmas (basalts) that reach shallow levels and that fractionate into highly explosive rhyolitic magmas. In addition, we try to determine what processes are occurring and changing magmas while they reside at shallow levels prior to eruption and whether any of these processes act as a catalyst causing the actual eruptions. We have found that crustal assimilation, a process in which crust surrounding a magma is melted and incorporated into the magma, is a significant process that is partially dependent on variable temperature magmas being injected from deeper levels. These insights regarding volcanism at an older volcano such as Valles can then be used to better understand the processes that are likely occurring under the surface where magma currently resides at Yellowstone and Changbaishan volcanoes. In addition to understanding large climate changing eruptions, this research supported four students. Two undergraduate students did research projects undertaking crystal dissolution work in a cleanroom and analyzing crystals for Sr and Pb isotopes at New Mexico State University. These undergraduate students gained hands-on experience doing cutting edge scientific research and built an understanding of how volcanoes work and how scientific analyses are undertaken. Both students are now in graduate programs and working on their masters degrees. Two graduate students also worked on the project. The first obtained his masters degree and now works at Chesapeake Energy and the second is finishing a masters degree at New Mexico State University.
