28. Discovering Clues to Impending Explosive Volcanic Eruptions Through Geochemical Microanalysis and Magmatic Process Modeling

The largest explosive volcanic eruptions devastate vast areas and affect global climate. Collapse calderas form during these infrequent events when roof rock subsides into a shallow magma chamber while gas-charged magma rushes to the surface. Research under this Opportunity will focus on identifying conditions that led to such an event in the recent past by determining how a long-lived volcanic center evolved toward a caldera-forming eruption. The properties of erupted magma are products of subsurface processes, such as separation of crystals or aqueous fluid, assimilation of wallrock, and mixing of different magmas in the mid- to upper crust, acting upon inputs of magma from greater depth. The largest explosive eruptions occur when magma has evolved toward a silica- and gas-rich composition over time. The trace element and isotopic compositions of crystals and quenched melt (glass) in volcanic rocks contain microscopic records of the magmatic processes that led to past eruptions. These records are accessible using microbeam instrumentation such as laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), secondary ion mass spectrometry (SIMS, ion microprobe), and electron-probe microanalysis (EPMA) to determine compositions of material with exposed areas in rock thin sections on the order of 0.01 mm diameter. The Mendenhall Fellow will use these approaches to investigate in situ the microscopic geochemistry of rock samples that represent the long-term growth of a volcano that erupted catastrophically in the recent past. The geochemical data will serve as input for state-of-the-art computational modeling of the evolution of magma toward a caldera-forming eruption using the Magma Chamber Simulator code developed by Wendy Bohrson and co-workers (Bohrson and others, 2006; Bohrson and others, 2007). Each simulation in a series framed by a volcano's eruptive history will yield a thermodynamically based description of the chemical, physical, and energetic state of a magma body and its surrounding wallrock as the magma evolves along a complex pressure (depth below surface)–temperature–composition path. The resulting model of a well-studied volcanic system will test the efficacy of this approach for forecasting hazardous magma accumulations at volcanoes where large explosive eruptions are unprecedented.

The goals of the research are to answer two fundamental questions: What combination of processes and key parameters leads to accumulation of a large volume of gas-charged magma in the upper crust, setting the stage for a cataclysmic eruption? How can we recognize that path in the geologic record of a volcano in order to accurately assess hazards and forecast future behavior? Magma compositional variation commonly does not progress smoothly during the history of a volcano but instead may repeatedly revert to more primitive compositions between eruptions of evolved magma. It may not be clear from simply examining the eruptive history of a volcano that an exceptionally large or explosive eruption may be likely. The rocks themselves contain information on a microscopic scale that can be revealed by microbeam geochemical analyses and used in modeling subsurface processes with recently developed, energy constrained magma evolution software. The Mendenhall Fellow will design the project, make the geochemical measurements, perform the computational modeling, and likely discover patterns in the magma evolution history that provide clues to the style and size of later eruptions. The modeling will incorporate constraints imposed by eruptive histories of several hundred thousand years that are available for only a few volcanoes. A subset of these volcanoes have ejected samples of otherwise hidden, crystallized intrusive magma as accidental fragments in the deposits of explosive eruptions. As the non-erupted intrusive volume may be substantially greater than that of erupted magma, these fragments provide an important additional perspective on subsurface magmatic processes (e.g., Bacon and Lowenstern, 2005; Bacon and others, 2007).

Potential sites with suitable collections, geochronology, and geologic maps well known to the Research Advisors include Mount Mazama (Crater Lake, Oregon; Bacon and Lanphere, 2006) and Mount Veniaminof (Alaska; Bacon and others, 2003), but a different well-characterized volcano may be identified. The Mendenhall Fellow will utilize instruments available at U.S. Geological Survey (USGS) locations, such as the USGS–Stanford SHRIMP-RG (SIMS; Stanford University, CA), laser ablation ICP-MS (Denver, CO), and electron microprobe/SEM laboratories (Menlo Park, CA, and Denver).

References

Bacon, C.R., and Lanphere, M.A., 2006, Eruptive history and geochronology of Mount Mazama and the Crater Lake region, Oregon: Geological Society of America Bulletin, v. 118, p. 1331–1359.

Bacon, C.R., and Lowenstern, J.B., 2005, Late Pleistocene granodiorite source for recycled zircon and phenocrysts in rhyodacite lava at Crater Lake, Oregon: Earth and Planetary Science Letters, v. 233/3–4, p. 277–293.

Bacon, C.R., Calvert, A.T., Nye, C.J., and Sisson, T.W., 2003, History and eruptive style of Mount Veniaminof, a huge Alaskan basalt-to-dacite volcano with Pleistocene and Holocene caldera-forming eruptions: EOS Transactions of the American Geophysical Union, v. 84, no. 46, abstract V32D-1048.

Bacon, C.R., Sisson, T.W., and Mazdab, F.K., 2007, Young cumulate complex beneath Veniaminof Caldera, Aleutian arc, dated by zircon in erupted plutonic blocks: Geology, v. 35, p. 491–494.

Bohrson, W.A., Spera, F.J., Fowler, S.J., Belkin, H.E., DeVivo, B., and Rolandi, G., 2006, Petrogenesis of the Campanian Ignimbrite: Implications for crystal-melt separation and open-system processes from major and trace elements and Th isotope data, in DeVivo, B., ed., Volcanism in the Campania Plain, Vesuvius, Campi Flegrei and ignimbrites: Developments in Volcanology 9: New York, Elsevier, p. 249–288.

Bohrson, W.A., Spera, F.J., and Ghiorso, M.S., 2007, The magma chamber simulator: An interactive computer program for modeling the chemical and physical evolution of complex magmatic systems: EOS Transactions of the American Geophysical Union, v. 88, no. 52, Fall Meeting Supplement, Abstract V43A-1120.

Proposed Duty Station: Menlo Park, CA

Areas of Ph.D.: Geology, geochemistry, volcanology

Qualifications: Applicants must meet one of the following qualifications: Research Geologist, Research Chemist

(This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.)

Research Advisor(s): Charles Bacon, (650) 329-5246, cbacon@usgs.gov; Alan Koenig, (303) 236-2475, akoenig@usgs.gov; Frank Mazdab, (650) 725-6536, fmazdab@usgs.gov; Wendy Bohrson (Central Washington University), (509) 963-2835, bohrson@geology.cwu.edu

Human Resources Office contact: Candace Azevedo, (916) 278-9393, caazevedo@usgs.gov

Summary of Opportunities