HR: 1340h
AN: V33C-1522    [Abstracts]
TI: Dynamic Magma Chamber Processes Reflected in Trace-Element Data From the May 1915 Eruption of Lassen Peak, California
AU: * Kernen, R A
EM: kerner16@uwosh.edu
AF: University of Wisconsin Oshkosh, Geology Dept. 800 Algoma Blvd., Oshkosh, WI 54901, United States
AU: Wenner, J M
EM: wenner@uwosh.edu
AF: University of Wisconsin Oshkosh, Geology Dept. 800 Algoma Blvd., Oshkosh, WI 54901, United States
AU: Clynne, M A
EM: mclynne@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS910, Menlo Park, CA 94025, United States
AU: Mahoney, J B
EM: mahonej@uwec.edu
AF: University of Wisconsin - Eau Claire, Geology Dept., Eau Claire, WI 54702, United States
AU: Ferguson, J W
EM: fergusjw@uwec.edu
AF: University of Wisconsin - Eau Claire, Geology Dept., Eau Claire, WI 54702, United States
AB: We present new WDXRF trace-element data from products of the May 1915 eruption of Lassen Peak in Lassen Volcanic National Park (LVNP), California. Four main rock types are present in the 1915 deposits: light dacite, black dacite, dark andesite and andesite inclusions in light and black dacites. Trace-element data collected for this study confirm that rocks produced by the 1915 eruption at Lassen Peak are the result of complex magma mixing. Similar to the conclusions of Clynne (1999), we find that dacites present in the 1915 eruption have been variably modified by addition and disaggregation of andesite inclusions. Geochemical evidence shows that dark andesite was modified by mixing with variable amounts of dacitic magma. Andesite inclusions, although the most mafic of the rocks erupted in May 1915, display disequilibrium mineral textures and, thus, cannot represent the ultimate mafic end member. New trace element data also indicate that processes that generate the mafic end member may be more complicated than previously recognized. Although not incompatible with mixing, our trace element data record large variations in some incompatible trace element compositions of andesite inclusions, suggesting two possible mechanisms: 1) varying proportions of incompatible element-bearing phenocrysts or 2) interaction with an aqueous phase. Our conclusions mirror those based on published major-element data, supporting the idea of a multiple-stage mixing process and providing insight into magma chamber dynamics (Clynne, 1999). The 1915 eruption of Lassen Peak involves mixing of basaltic andesite with dacite to generate hybrid andesites that crystallized and vesiculated to form mafic foam. The vesiculated foam layer is unstable and breaks up into fragments (andesite inclusions), which are buoyant and are stirred by convection into the overlying dacite. Continued hybridization and heating of the reservoir dacite through partial disaggregation of andesite inclusions increases the temperature, modifies the composition of dacite and eventually cuts off formation of foam. Increased temperature and buoyancy of black dacite may have triggered the eruption on May 19, 1915, when three types of magma erupted: light dacite, black dacite and andesite inclusions. The final step in the mixing process involves the mingling of black dacite and dark andesite magmas to produce the banded pumice observed only in the May 22 eruption. Our trace element study strengthens conclusions supporting a complex mixing process to generate compositionally diverse products of the 1915 eruption and adds to current knowledge about dynamic magma chamber processes and the chemistry of magma mixing.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1036 Magma chamber processes (3618)
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting