HR: 1330h
AN: V12A-0555    [PDF]
TI: Magma Ascent Rates Determined Using Multiple Petrographic Methods: A Comparison of Decompression-induced Amphibole and Plagioclase Reactions and Crystal Size Distribution
AU: * McCanta, M
EM: molly_mccanta@brown.edu
AF: Brown University, Department of Geological Sciences Box 1846, Providence, RI 02912 United States
AU: Rutherford, M
EM: malcolm_rutherford@brown.edu
AF: Brown University, Department of Geological Sciences Box 1846, Providence, RI 02912 United States
AB: Petrographic analysis of crystal-melt reactions driven by decompression induced magma degassing can potentially yield valuable information regarding magma ascent rates. The degassing process causes destabilization of OH-bearing phenocrysts, such as amphibole and biotite, and results in the crystallization of interstitial melts. Magma ascent rates for extrusions of H2O-rich magma can be quantified petrographically utilizing measurements of (1) decompression-driven amphibole breakdown rims [Rutherford and Hill, 1993], (2) compositional changes in plagioclase phenocryst rims and (3) crystal size distribution (CSD) of groundmass plagioclase microlites that are known to have grown during ascent and cooling [i.e., Marsh, 1998]. Measurements of these parameters in both natural and experimental samples allows for an investigation into the relative rates and correlations between amphibole breakdown, plagioclase growth (of both phenocrysts and microlites) and magma ascent. The dacitic domes of Black Butte, California, provide a good opportunity to study the effects of ascent rate as determined using the above methods as the pre-eruption magma represents one injection, not repeated cycles, based on the presence of completely unzoned amphibole and plagioclase phenocrysts. The Black Butte phase assemblage consists of phenocrystic amphibole surrounded by rims of breakdown material and phenocrystic plagioclase with compositionally distinct rims in a completely crystalline, high silica groundmass, composed primarily of microlites of plagioclase, pyroxene, and oxides. Measurements of the plagioclase and amphibole rims and the plagioclase microlites can be made for the natural rocks and compared to experimental results in order to determine magma ascent rates. Isothermal, constant rate and one-step decompression (P$_{1}$=200 MPa, P$_{2}$=2 MPa, dP/dt=0.002m/s, 0.01m/s, or 1-step) experiments were run to determine the conditions necessary to produce the rims and microlite characteristics measured in the natural samples. The starting material was a partially crushed Black Butte rock which was brought to conditions determined to have existed in the magma storage region prior to ascent (870$\deg$C, 200 MPa) and held for 48 hours to remelt and homogenize the holocrystalline groundmass. Plagioclase compositions and rim widths (natural = 20$\pm$10$\mu$m, experimental = 30$\mu$m) were best replicated in 30-day decompression experiments, which also produced comparable amphibole rim widths (natural = 30-45$\mu$m, experimental = 25$\mu$m). In addition, plagioclase microlites in the 30-day experiments exhibited a close compositional match with both the natural plagioclase phenocryst rim and microlite compositions (An52 vs. An59 and 50, respectively). Crystal size distribution calculations reveal that the characteristic crystal length in the experimental samples (26$\mu$m) is less than that of the natural samples (56$\mu$m). Preliminary experiments indicate this to be the result of experimental decompressions starting at slightly lower pressures (shallower depths) than the natural rocks. Ascent rates calculated from both the amphibole and plagioclase methods indicate similar ascent rates (0.002m/s and 0.001m/s, respectively).
DE: 8400 VOLCANOLOGY
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting