HR: 1340h
AN: V53D-1588    [Abstracts]
TI: Complex P-T History of Amphiboles From the 2004-2005 Eruption of Mount St. Helens
AU: * Thornber, C R
EM: cthornber@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory, Vancouver, WA 98683 United States
AU: Pallister, J S
EM: jpallist@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory, Vancouver, WA 98683 United States
AU: Lowers, H
EM: hlowers@usgs.gov
AF: U.S. Geological Survey, MS973 Denver Federal Center, Denver, CO 80225-0046 United States
AU: Meeker, G P
EM: gmeeker@usgs.gov
AF: U.S. Geological Survey, MS973 Denver Federal Center, Denver, CO 80225-0046 United States
AB: Variable depth and temperatures within the Mount St. Helens (MSH) 2004-5 eruptive plumbing system are recorded by a diverse population of amphiboles. Three compositional arrays among amphiboles in dome dacite are most clearly distinguished by Al and Fe variations. Magnesiohornblende to edenite compositions (Leake classification) define a low-Al group ranging from 6 to 9.5 wt% Al₂O₃ with increasing FeO^T from 12 to 16.5 wt%. The predominant medium-Al group of tschermakite-magnesiohastingite-pargasite amphiboles ranges from 9.5 to 12 wt% Al₂O₃ with FeO^T decreasing from 16.5 to 10 wt%. The high-Al group of magnesiohastingsite-pargasite amphiboles has 12 to 14 wt% Al₂O₃ with FeO^T increasing from 10 to 16 wt%. (The FeO{T trends are consistent with Mg-number variations). Most discrete low-Al amphiboles are broken crystal fragments, often with resorbed edges and some with broad reaction rims. Low-Al compositions also form rims on mid-Al and high-Al amphiboles and are present as cores within some oscillatory- or sector-zoned crystals. Within the low-Al group, higher Al and Fe compositions overlap those in gabbro xenoliths from MSH 1980-86 dome rocks. The medium-Al group also includes phenocrysts with uniform cores, hopper-growth-like characteristics and negligible reaction rims in the most glass-rich 2004 samples. In typical 2004-05 dome dacite, medium- and high-Al groups have amphibole breakdown rims with uniform thickness of ~5 microns. A qualitative assessment of amphibole source depth is provided by the Anderson-Smith (1995) Al-in-hornblende barometer, with the caveats that this barometer requires independent knowledge of temperature and bulk composition. Because the 2004-05 amphibole assemblage is complex and oxide temperatures record only the latest magmatic equilibration, we cannot establish crystallization temperatures for individual amphibole crystals. Consequently, the pressures reported here should only be used in a relative sense and not as a measure of actual depths. The following examples are illustrative: at 850°C, high-and mid-Al amphiboles yield a pressure range of 430 to 270 MPa and 270 to 80 MPa, respectively, with lowest Al compositions yielding barometric results below the ~100 MPa limit of amphibole stability for MSH dacites, and suggesting that the low-Al group crystallized at considerably lower temperatures. At higher temperatures, progressively more of the medium-Al amphiboles yield untenable low pressures, e.g., at 900°C, only high-Al group amphiboles yield pressures >100 MPa. These barometry calculations and preservation of a range of medium- to high-Al amphiboles in each sample suggest: (1) entrainment of high-Al amphiboles formed at relatively low temperature and great depth followed by continued crystallization of medium-Al phenocrysts during nearly isothermal ascent, (2) crystallization during cooling in shallow crustal magma chambers, or a combination of (1) and (2). The inability of experiments to reproduce amphibole with Al₂O₃ >11 wt% at pressures up to 300 MPa in MSH dacite (Rutherford and Devine, this session) favors a high-pressure origin for the high-Al amphiboles, or crystallization from a more mafic bulk composition. Low-Al amphiboles (Al₂O₃<7.4 wt%) require <800°C temperatures to yield minimum pressures for amphibole stability (~100 MPa), and suggests remobilization and entrainment of older near-solidus dacite magma or gabbro in hotter 2004-05 magma.
DE: 3625 Petrography, microstructures, and textures
DE: 3640 Igneous petrology
DE: 3652 Pressure-temperature-time paths
DE: 7280 Volcano seismology (8419)
DE: 8412 Reactions and phase equilibria (1012, 3612)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005