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Penrose Conference Icon
This conference was supported in part by National Science Foundation grant EAR 0926449.

Low δ18O rhyolites and crustal melting: Growth and redistribution of the continental crust

Twin Falls, Idaho, and Yellowstone National Park, Wyoming, USA
9–13 September 2009


Peter Larson
Washington State University, School of Earth and Environmental Sciences, Pullman, Washington 99164-2812, USA
Ilya Bindeman
University of Oregon, Dept. of the Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403, USA
John Wolff
Washington State University, School of Earth and Environmental Sciences, Pullman, Washington 99164-2812, USA

Snake River
Snake River, Twin Falls, Idaho, USA.
Photo by keasmus (click photo for larger image).

This Penrose Conference reflects a renewed interest in the formation of low-δ18O rhyolite magmas and in melting at intermediate crustal levels, stimulated by a combination of the application of new analytical techniques and the discovery of voluminous low-δ18O rhyolites in the Snake River Plain, Idaho, USA. Low-δ18O rhyolites are important because significant similar processes are required to dramatically affect the isotope ratio of the most major of elements in silicic magmas.

Forty-five scientists from around the world participated in the September 2009 Penrose Conference. Hugh Taylor, who conducted some of the original fundamental work on oxygen isotope applications to igneous petrogenesis, began the first day’s presentations with an historical overview of low-δ18O magmas and a review of genetic models for their origin. He concluded that low-δ18O meteoric hydrothermally altered wall rocks must be involved in their genesis. Karlis Muehlenbachs followed with a discussion of his early work and a review of newer data on the low-δ18O basalts of Iceland. Ilya Bindeman reviewed recent developments in low-δ18O magmatism and presented data for the eastern Snake River Plain and Yellowstone rhyolites. He proposed bulk shallow melting of altered rocks as the origin of the Snake River Plain/Yellowstone rhyolites based on in situ analyses of oxygen ratios in zircons. Shan de Silva discussed his research on large ignimbrite flare-ups in the central Andes and noted that the overall thermal evolution of the crust can control ignimbrite flare-ups because it controls melt production and storage. Peter Larson described the mineralogy and O-isotope characteristics of caldera hydrothermal systems, and suggested that propylitically altered rocks in the deeper parts of caldera hydrothermal systems are reasonable sources for low-δ18O rhyolites.

The day concluded with poster presentations by Tamara Carley (zircon in Icelandic rhyolite), Lily Claiborne (zircon record at Mount St. Helens), Chris Folkes (O ratios in central Andean silicic ignimbrites), Allison Phillips (hydrothermal alteration of the Tuff of Sulfur Creek at Yellowstone), and Laura Waters (water concentrations in western Mexican volcanic arc siliceous magmas).

The second day focused on Snake River Plain rhyolite magmatism. Barbara Nash provided an overview of the Bru-neau-Jarbidge eruptive center and found that rhyolites there erupted at relatively high temperatures. Mike McCurry pointed out that the Snake River Plain is fundamentally a basaltic magma system and noted that some of the rhyolite domes in the eastern Snake River Plain could be products of fractional crystallization. Eric Christiansen compared Snake River Plain and Great Basin rhyolites. He proposed that crustal partial melting and subsequent fractionation are reasonable explanations for their formation. Bill Leeman described a model whereby the Snake River Plain/Yellowstone rhyolites are produced by extension where subcrustal adiabatic basalt melts can rise to moderate crustal levels and produce the rhyolites via partial melting.

Scott Boroughs (central Snake River Plain low-δ18O rhyolites), Ben Ellis (phreatomagmatic rhyolites in the Snake River Plain), Kathryn Watts (the Kilgore Tuff, Heise center), and Chad Pritchard (Yellowstone post-collapse Upper Basin Member rhyolites) gave short presentations in preparation for the next day’s field trip. These were followed by posters: Matthew Brueseke (silicic magmatism in the Santa-Rosa Calico volcanic field, Nevada), Jeff Callicoat (spatial and temporal significance of mid-Miocene volcanism in northeast Nevada), Henny Cathey (O isotopes in zircons and geothermometry from the Bruneau-Jarbidge center, Snake River Plain), Matthew Cobble (Ti-in-quartz geothermometry for 15-Ma calderas, NW Nevada), Richard Gaschnig (timing of magmatism in the Atlanta Lobe of the Idaho batholith), Will Starkel (radiogenic isotope constraints on Snake River Plain rhyolite genesis), and John Wolff (Snake River Plain rhyolite and basalt genesis).

Day four focused on Yellowstone and other rhyolite provinces. Chris Harris began with a discussion of the oxygen and radiogenic isotopic characteristics of Archean to Mesozoic magmatic rocks in South Africa. Tom Vogel described the origin of silicic volcanic rocks in caldera settings from central America and emphasized subtle δ18O decreases due to assimilation of rocks hydrothermally altered by tropical waters. Sarajit Sensarma provided an interesting review of the geochemistry of the Dongargarch bimodal igneous province, India, which, at 2.5 Ga, contains one of the oldest known ignimbrites on Earth. Jorge Vasquez described the thermochemical evolution of the Pitchstone Plateau rhyolite, the youngest Yellowstone rhyolite. These were followed by two presentations on rhyolites of the High Lava Plains in eastern Oregon by Martin Streck and Anita Grunder. They reviewed the distribution, timing, and geochemistry of these rhyolites and compared them with the Snake River Plain/Yellowstone rhyolites. Poster presentations included Mark Ford (High Lava Plains rhyolite δ18O values), Guillaume Girard (trace elements in post-collapse Yellowstone rhyolites), and Terry Spell (extra-caldera Yellowstone rhyolites).

The last day began with a double-headed talk by George Bergantz and Jim Beard, who described thermal aspects of assimilation-fractional crystallization (AFC) controls on crustal magma evolution. Calvin Miller discussed the applications of sphene and zircon as monitors of magmatic processes. Rebecca Lange talked about the origin of low-Sr Mexican western volcanic arc rhyolites and proposed that they are the product of multiple episodes of partial melting. Finally, Craig Lundstrum presented an alternative model for forming compositionally zoned silicic magma chambers, whereby thermal diffusion plays a major role in differentiation. Poster presentations followed: Catherine Curtis (South African Koegel Fontein low-δ18O rhyolites), Jim Beard (O-isotope behavior during magmatic processes), Leif Karlstrom (shallow crustal magma chamber growth and longevity), and Dorsey Wanless (mid-ocean ridge crustal assimilation models).

The attendees split into six discussion groups in the concluding session, listed here with the topics and discussion leaders. (1) How should an 18O-depleted magma be defined, and is there a correlation between rhyolite magma temperature and degree of 18O depletion? (Grunder and Wolff); (2) Is the 18O-depleted signal produced by a unique or general process of rhyolite genesis? (Miller); (3) What chemical fingerprints correlate with 18O/16O and D/H? (Beard); (4) How are large-volume 18O-depleted magmas produced? (Watts and Phillips); (5) What plausible models can be generated that include structural, thermal, and chemical constraints? (Branney); and (6) How can the mantle contribution to silicic magmas be determined, and what are the mechanisms for its involvement? (Leeman). Further discussion of these points will certainly stimulate additional ideas and research on rhyolite formation.

Jim Beard, Museum of Natural History
George Bergantz, Univ. of Washington
Ilya Bindeman, Univ. of Oregon
Scott Boroughs, Washington State Univ.
Mike Branney, Univ. of Leicester
Matthew Brueseke, Kansas State Univ.
Jeff Callicoat, Kansas State Univ.
Tamara Carley, Vanderbilt Univ.
Henny Cathey, Univ. of Utah
Eric Christiansen, Brigham Young Univ.
Lily Claiborne, Vanderbilt Univ.
Matthew Coble, Stanford Univ.
Catherine Curtis, Univ. of Cape Town
Shan De Silva, Oregon State Univ.
Ben Ellis, Univ. of Leicester
Chris Folkes, Monash Univ.
Mark Ford, Oregon State Univ.
Richard Gaschnig, Washington State Univ.
Guillaume Girard, McGill Univ.
Anita Grunder, Oregon State Univ.
Chris Harris, Univ. of Cape Town
Leif Karlstom, UC-Berkeley
Rebecca Lange, Univ. of Michigan
Peter Larson, Washington State Univ.
Bill Leeman, National Science Foundation
Craig Lundstrum, Univ of Illlinois–Urbana Champaign
Mike McCurry, Idaho State Univ.
Calvin Miller, Vanderbilt Univ.
Karlis Muehlenbachs, Univ. of Alberta
Barbara Nash, Univ. of Utah
Mike Perfit, Univ. of Florida
Allison Phillips, Washington State Univ.
Chad Pritchard, Washington State Univ.
Sarajit Sensarma, Univ. of Lucknow
Terry Spell, Univ. of Nevada–Las Vegas
Willliam Starkel, Washington State Univ.
George Stone, Milwaukee Area Technical College
Martin Streck, Portland State Univ.
Kevin Tarbert, Washington State Univ.
Hugh Taylor, California Institute of Technology
Jorge Vazquez, California State Univ. Northridge
Tom Vogel, Michigan State Univ.
V. Dorsey Wanless, Univ. of Florida
Laura Waters, Univ. of Michigan
Kathryn Watts, Univ. of Oregon
John Wolff, Washington State Univ.