HR: 08:00h
AN: V31H-01 INVITED    [Abstracts]
TI: Ascent, Degassing, and Crystallization of Water-Rich Mafic Magmas in the Trans-Mexican Volcanic Belt: A Melt Inclusion Perspective
AU: * Wallace, P
EM: pwallace@uoregon.edu
AF: University of Oregon, Dept. of Geological Sciences, Eugene, OR 97403,
AU: Johnson, E
EM: ejohns10@uoregon.edu
AF: University of Oregon, Dept. of Geological Sciences, Eugene, OR 97403,
AU: Vigouroux, N
EM: nathalie.vigouroux@gmail.com
AF: Simon Fraser University, Dept. of Earth Sciences, Burnaby, BC V5A 1S6, Canada
AU: Delgado Granados, H
EM: hugo@geofisica.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Instituto de Geofisica Ciudad Universitaria, Coyoacan, D.F 04510, Mexico
AB: Mafic cinder cones are common in subduction-related volcanic arcs. Such cones exhibit a wide range of eruptive styles and are often violently explosive, but the mechanisms driving explosive cinder cone eruptions are still poorly understood, as is the nature of their underlying plumbing systems. To investigate magmatic processes beneath cinder cones, we have analyzed major elements and volatiles in olivine-hosted melt inclusions from 14 cones in the central and western Trans-Mexican Volcanic Belt (TMVB). Our samples include the historic eruptions of Paricutin and Jorullo and the highly potassic minette and basanite cinder cones in the Colima Graben. Melt inclusions from each cone have variable H₂O (typically <1 wt% to maximum values of 4 to 5.5 wt%) and CO₂ (<50 to 5000 ppm), corresponding to crystallization pressures of <100 bars to ~5 kb. This indicates that olivine crystallized from variably degassed melts over a range of depths extending from the mid- crust to near-surface, probably within a network of dikes (±sills). The highest CO₂ contents (≤5000 ppm) are found in the potassic minettes and basanites from the western TMVB, whereas values ≤1800 ppm are typical of calc-alkaline basalts and basaltic andesites. Modeling of major element variations and comparison to phase equilibria demonstrate that polybaric olivine crystallization beneath these cones was driven by H₂O loss from melts during ascent. A puzzling feature of the inclusion data is that CO₂ does not decrease as rapidly with degassing as predicted by degassing models. A likely explanation involves fluxing of magma in the conduit system with CO₂-rich gas released by degassing magma at greater depths. Decreases in both olivine crystallization depths and the vigor of gas fluxing over time in longer lived cinder cone eruptions reflect evolution of the conduit system and deeper storage reservoir.
DE: 1036 Magma chamber processes (3618)
DE: 3640 Igneous petrology
DE: 8400 VOLCANOLOGY
DE: 8412 Reactions and phase equilibria (1012, 3612)
DE: 8430 Volcanic gases
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting