HR: 09:30h
AN: V31H-07    [Abstracts]
TI: Glass Inclusions and Volatile Contents of Lamprophyres, Basanites, and Basalts from the Colima Rift and Mascota Volcanic Field (Western Mexican Volcanic Belt)
AU: * Maria, A H
EM: ahmaria@usi.edu
AF: Department of Geology, University of Southern Indiana, 8600 University Blvd., Evansville, IN 47712, United States
AU: Luhr, J F
EM: luhrj@si.edu
AF: Department of Mineral Sciences, Smithsonian Institution, P.O. Box 37012 NHB-119, Washington, DC 20013-7012, United States
AB: We present geochemical data for coeval basaltic to lamprophyric tephra erupted during Quaternary time in the Colima and Mascota volcanic fields at the western end of the Mexican Volcanic Belt. Focusing on glass inclusions (124 analyzed) trapped within olivine, our goal was to evaluate a vein/wall-rock melting relationship between the lamprophyres (vein-dominated melts) and the mafic calc-alkaline rocks (diluted by partial melting of peridotite wall rock after exhaustion of phlogopite and other vein minerals). We measured major-element compositions of glass inclusions and host olivines by electron microprobe. Volatile contents in glass inclusions were measured by electron microprobe (S, %S6+, Cl), Fourier-transform infrared spectroscopy (H and C species), and secondary-ion mass spectrometry (H, C, F, S, Cl). Estimates of magmatic fO2 from measured bulk-rock Fe3+/Fe2+ and glass-inclusion %S6+ reveal that all studied magmas were highly oxidized, up to several log units above the Ni-NiO buffer. The highest concentrations of water and most other volatile elements (7% H2O, 1,460 ppm CO2, ~2% SO3Total, 2,400 ppm Cl, and ~1% F) were recorded for a glass inclusion from a Colima minette (48.2 wt.% SiO2, 6.0% K2O, and 1.2% P2O5, normalized anhydrous), and are consistent with a depth of entrapment of 24.1 km (calculated pressure of 6,660 bars). We consider these chemical properties of the melt to be little changed since it formed at a depth of 90-100 km in the mantle wedge from nearly pure partial melting of phlogopite-garnet-pyroxenite veins. The mafic calc-alkaline bulk-rock and glass-inclusion compositions are consistent with formation through approximately 7-fold dilution of the lamprophyre-type vein melts with partial melts from the surrounding depleted peridotite wall-rock. Among the calc-alkaline glass inclusions, the Mascota basaltic andesite has the highest concentrations of water and most other volatile elements with 49.6 wt.% SiO2, 1.0% K2O, 0.3% P2O5 (normalized anhydrous), 2.8% H2O, 296 ppm CO2 (1,425 bars pressure and 5.2 km depth of entrapment), 0.8% SO3Total, 870 ppm Cl, and 720 ppm F. Such mafic calc-alkaline melts are envisioned as parental to the volumetrically dominant andesites of western Mexico.
DE: 1043 Fluid and melt inclusion geochemistry
DE: 3619 Magma genesis and partial melting (1037)
DE: 3640 Igneous petrology
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting