HR: 17:45h
AN: V44C-08    [Abstracts]
TI: Zircon from Mount St. Helens Reveals Residence Times of Tens to Hundreds of Thousands of Years at Low Magmatic Temperatures Prior to Eruption
AU: * Claiborne, L L
EM: lily.e.lowery@vanderbilt.edu
AF: Earth and Environmental Sciences, Vanderbilt University, SC5717 Science and Engineering Building Stevenson Center Drive, Nashville, TN 37240, United States
AU: Miller, C F
EM: calvin.miller@vanderbilt.edu
AF: Earth and Environmental Sciences, Vanderbilt University, SC5717 Science and Engineering Building Stevenson Center Drive, Nashville, TN 37240, United States
AU: Clynne, M A
EM: mclynne@usgs.gov
AF: U.S. Geological Survey, Volcano Hazards Team, 345 Middlefield Road MS910, Menlo Park, CA 94025, United States
AU: Wooden, J L
EM: jwooden@usgs.gov
AF: U.S. Geological Survey, USGS-Stanford Micro-isotopic Analytical Center, Stanford University Green Building 367 Panama Street, Stanford, CA 94305, United States
AU: Pallister, J S
EM: jpallist@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory, 1300 SE Cardinal Court Suite 100, Vancouver, WA 98683, United States
AU: Lowenstern, J B
EM: Jlwnstrn@usgs.gov
AF: U.S. Geological Survey, Volcano Hazards Team, 345 Middlefield Road MS910, Menlo Park, CA 94025, United States
AU: Mazdab, F K
EM: frankm@pangea.stanford.edu
AF: U.S. Geological Survey, USGS-Stanford Micro-isotopic Analytical Center, Stanford University Green Building 367 Panama Street, Stanford, CA 94305, United States
AB: U-series and U-Pb geochronology of zircons from four samples that span the 300,000 year eruptive history of Mount St. Helens, combined with zircon trace element geochemistry and application of the Ti-in-zircon thermometer, provide critical constraints on the time-temperature-compositional history of the sub-volcanic magmatic system. Preliminary results indicate that prior to and throughout its eruptive history, one or more relatively cool, crystal-rich reservoirs have been accumulating beneath the volcano. SHRIMP-RG U-Pb ages of the oldest sample, a dacite erupted ~300 ka, reveal that zircons grew between ~320 and 520 ka, suggesting magmatic activity may have begun 200 ka before eruption. 238U-230Th age spectra in the three youngest samples indicate multiple ages of growth for each sample. The oldest of these three young samples (eruption constrained to ~35-50 ka) contains zircons ranging from ~50 to ~200 ka, with the main concentration of ages ~100 ka. Zircons from a 35 ka dacite range from ~65 to ~230 ka in age, with a dominant episode of growth ~130 ka. Dacite from the current eruption, sampled from the dome in 2005, contains zircons from ~40 to ~170 ka in age, with distinct populations at ~130 and ~170 ka. Taken together, these ages of tens to hundreds of thousands of years prior to eruption and the distinct episodes of growth suggest repeated injection and accumulation of one or more crystal-rich reservoirs beneath the volcanic edifice, a scenario that is further supported by Ti-in-zircon geothermometry (Watson et al. 2006). Ti concentrations indicate zircons grew at temperatures from ~840 to ~640 C, with 90% of analyzed spots recording temperatures between ~770 and ~670 C (T's carry uncertainties of tens of degrees, mostly from uncertainty in a(TiO2)). These temperatures are significantly lower than the eruption temperatures of their host magmas, which range from ~950-800 C. The rounded, resorbed surface morphology of many of the grains attests to the undersaturation of the phase in these hotter magmas. Calculated dissolution rates indicate that zircons would have completely dissolved in magmas such as these in tens to hundreds of years, suggesting that residence of the zircons at higher temperatures prior to eruption must have been brief. Gravity data (Williams et al., 1987) tentatively suggest that an intrusive complex lies beneath the volcano and may extend as far west as Goat Mountain, the earliest eruptive center of the St. Helens system. The accumulation of this complex may correspond with the ages represented by the zircons, although the conditions of magma storage in these intrusions are unknown. The zircons in these intrusions could be stored in sub- solidus rock, in discrete reservoirs of crystal mush, or in a larger crystal mush that continuously mixes magma batches and recycles zircon populations. Regardless of the conditions, it is apparent that eruptive events sample this storage zone and the zircons contained therein and carry them rapidly to eruption.
DE: 1115 Radioisotope geochronology
DE: 1120 Isotopic disequilibrium dating
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3640 Igneous petrology
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting