HR: 1340h
AN: V33C-1523    [Abstracts]
TI: Experimental Determination of Olivine Reaction Rim Growth Rates in the Lassen Peak, CA Magma Chamber
AU: * Schwab, B E
EM: bes21@humboldt.edu
AF: Humboldt State University, Department of Geology 1 Harpst Street, Arcata, CA 95521, United States
AU: Castro, J M
EM: castroj@si.edu
AF: Smithsonian Institution, Department of Mineral Sciences National Museum of Natural History, Washington, DC 20013, United States
AB: A series of hydrothermal (high-pressure and –temperature) experiments have been performed on starting materials consisting of ~5 wt% of Fo85 olivine separated from a spinel lherzolite xenolith (UM-5) from Kilbourne Hole, NM combined with powdered natural dacite pumice from the 1915 eruption of Lassen Peak, CA. The dacite represents the silicic end member (67.8 wt% SiO₂) of mixed dacite and andesite magmas present in the 1915 Lassen Peak eruptive products. The olivine represents xenocrystic grains from the admixed andesite that combined with the dacite magma prior to the eruption. In the natural dacitic eruptive products, xenocrystic olivine grains display morphologically complex reaction rims of orthopyroxene. These rims are interpreted to have grown during magma mixing/mingling prior to the eruption. The growth rate of these rims may be used to constrain the length of time between magma recharge and eruption. Time series experiments were performed to reproduce olivine reaction rims within the dacite. Experiments were performed with run durations of 50, 100, 200, 400, and 600 hours at 50 MPa and 825°C and 875°C. Water-saturated run conditions at the Ni-NiO buffer represent plausible dacite equilibration conditions based on earlier phase equilibria experiments. Preliminary measurements of orthopyroxene rim widths measured on two olivine grains from experiments run at 50 MPa and 825°C for 50 and 600 hours show an increase in rim width from 2.2±1.8 μm to 3.3±2.5 μm, respectively (average and 1σ standard deviation of 490 and 651 linear measurements). In the 50-hour experiment, rim widths range from 0 to 13.4 μm, whereas in the 600 hour run, widths are 0.63-23.1 μm. The 600 hour run also has a larger percentage of width measurements above 5 μm (24.8% of measured widths, compared to 2.8% of measurements in the 50-hour experiment.) In the 50-hour experiment, 21.0% of the rim width measurements are 1 μm or less, whereas only 6.4% of measured olivine reaction rim widths are <1μm in the 600-hour run. Additional measurements on these and other experiments are under way using the linear measurement technique applied here, as well as other methods to estimate rim width and area. Experimentally determined rim growth rates will be compared with rims on the natural samples in order to constrain the timing and duration of magma mixing prior to the eruption.
DE: 3618 Magma chamber processes (1036)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 8412 Reactions and phase equilibria (1012, 3612)
DE: 8445 Experimental volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting