HR: 0830h
AN: V51E-0322    [PDF]
TI: Devolatilization Induced Crystallization Experiments of Hydrous Basaltic Magma, Implications for Lava Flow Emplacement
AU: * Teasdale, R
EM: teasdale@asu.edu
AF: University of Bristol, Department of Earth Sciences Wills Memorial Bldg Queens Rd, Bristol, BS8 1RJ United Kingdom
AU: Richard, B
EM: r.a.brooker@bristol.ac.uk
AF: University of Bristol, Department of Earth Sciences Wills Memorial Bldg Queens Rd, Bristol, BS8 1RJ United Kingdom
AU: Steve, S
EM: steve.sparks@bristol.ac.uk
AF: University of Bristol, Department of Earth Sciences Wills Memorial Bldg Queens Rd, Bristol, BS8 1RJ United Kingdom
AB: Lava flow solidification occurs by degassing induced crystallization and with external cooling; as a result both are potential controls for groundmass crystallization during ascent through the conduit and extrusion of lavas. Decompression controlled crystallization rates have been determined experimentally for more evolved magma compositions but are relatively unknown for basaltic magmas. Isothermal decompression TZM experiments have been used to constrain the general style and rates of devolatilization induced crystallization in a basaltic magma. Initial experiments determined plagioclase liquidus temperatures for a range of water contents from dry to supersaturated. The observed liquidus phase is aluminus spinel, followed by clinopyroxene, then plagioclase. Plagioclase-in temperatures vary from 1175 C at 1atm (dry) to 1050 C at 200 MPa (P H2O = P total). The effects of devolatilization are modeled by decompressing to 1 atm from 200 MPa, 100 MPa, and 50 MPa at a rate of 100 MPa/hr. These pressures are consistent with depths at which basaltic magmas are likely to begin ascent, based on petrologic evidence (1-2) and decompression rates are consistent with reported basaltic dike propagation of 1 m/s (3). Initial results show that crystal morphology and abundance are not significantly different for experiments decompressed from 200 MPa and 100 MPa suggesting that most devolatilization occurs in the final stages of decompression. Experiments produce up to 20 % groundmass plagioclase crystals which range from 1-20 microns in length. Results of devolatilization-induced crystallization experiments, in combination with external cooling models for groundmass crystallization will produce a more complete understanding of the solidification of hydrous basaltic lavas. This in turn will improve the characterization of basaltic magma transfer prior to eruption, and in particular at shallow depths where degassing becomes important. These results are part of ongoing efforts to describe the relationship between groundmass crystallinities and development of a'a and pahoehoe flow morphologies. (1) Geist et al 1998; (2) Bureau et al 1998; (3) Rubin 1993
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 8429 Lava rheology and morphology
DE: 8434 Magma migration
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting