Longevity and Dynamics of Rhyolitic Magma Systems
June 712, 2001 Mammoth, California
- Kurt Knesel
- Department of Earth Sciences, University of Queensland, St. Lucia, Brisbane, Qld 4072, Australia, 61-7-33659779, fax 61-7-33651277
- George Bergantz
- Department of Geological Sciences, Office Box 351310, University of Washington, Seattle, WA 98195-1310, USA, (206) 685-4972, fax 206-543-3836
- Jon Davidson
- Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567, (310) 206-3042, fax 310-825-2779
Mammoth Mountain forms the southwest rim of the Long Valley caldera, one of three large Quaternary rhyolitic caldera centers in the United States. Long Valley, a site of recent volcanic unrest, lies at the heart of current debate over the mechanisms and time scales for the production, storage, and differentiation of rhyolite magma. Such information is critical to our understanding of fundamental geologic problems such as the formation and growth of Earth's continents and predicting volcanic hazards.
The conference aims to bring together petrologists, geochemists, volcanologists, and geophysicists actively studying the generation and evolution of silicic magmas. We hope to resolveor at least constraina number of very important and currently highly topical issues pertaining to the shallow-crustal evolution of large, typically caldera-forming, silicic magma bodies. Issues include:
- What is a magma chambera large, long-lived fractionating liquid body or a "sleepy" crystal mush that gets kicked to life every so often, remobilizing existing material? To what degree do plutons carry forward, in some integrated way, the expression of this?
- What do crystals really representphenocrysts vs. xenocrystsand what "memory" do they retain? Does crystal growth- and dissolution-zoning reflect protracted fractionation of a single magma body or remobilization and dispersal of crystal mush during injection of fresh magma into the subvolcanic system? How do crystals move in the magma system, or, are the crystals effectively static in a moving magma system?
- What is the efficacy of, and what are the driving forces for, convection and/or mixing in silicic magmas? Can crystal disequilibrium features, such as chemical and/or isotopic zoning and dissolution surfaces, serve to discriminate between thermal convection and magma mixing?
- What are the time scales needed to produce large, rhyolitic magma bodies? Recent work using 40Ar/39Ar, Rb/Sr, or U-series isotope data has led to the suggestion that rhyolite magmas in the Long Valley system are stored, following differentiation, for long (105106) time scales. This contention has been disputed principally on the basis that it would be difficult to keep a body of magma thermally viable for such long periods, even if >500 km3 volume. Alternative physical models have been proposed, such as remobilization of juvenile plutons or cumulate materials and ion microprobe work on zircons has variously upheld or contested the claims for long magma residence times. A key focus of the meeting will be to evaluate the different types of data available that bear on ages of magmatic events, and discuss their interpretations.
Keynote talks will outline the current state of knowledge concerning the generation and evolution of large rhyolitic magma systems and will set the foundation for evaluation of existing paradigms, development of new models, and discussion of future research directions. Most of the meeting will focus on poster sessions and group discussions. Mid-meeting field trips to selected Bishop Tuff and Sierran plutonic locations will serve to raise questions concerning limits and constraints on sampling and interpreting geochemical data from pyroclastic deposits based on our knowledge of how large silicic systems erupt, links between plutonic and volcanic environments, and the importance of recharge and mixing in magma evolution.
The conference is limited to approximately 50 participants to ensure a workshop atmosphere focussed on manageable discussions. We encourage participation of graduate students working on silicic magma systems; partial student subsidies will be available. The registration fee, which will include lodging, some meals, field trips, and all other conference costs except personal incidentals, is not expected to exceed $750. Information on travel to the conference will be provided in the letter of invitation.