HR: 11:20h
AN: V12A-05    [Abstracts]
TI: Open-system degassing, gas fluxing, and the growth and entrainment of olivine during small volume mafic eruptions: A case study of Volcan Jorullo, central Mexico
AU: * Wallace, P J
EM: pwallace@uoregon.edu
AF: Department of Geological Sciences University of Oregon, 1272 University of Oregon, Eugene, OR 97403
AU: Johnson, E R
EM: ejohns10@uoregon.edu
AF: Department of Geological Sciences University of Oregon, 1272 University of Oregon, Eugene, OR 97403
AU: Delgado Granados, H
EM: hugo@tonatiuh.igeofcu.unam.mx
AF: Universidad Autonoma de Mexico, Departamento de Vulcanologia, Ciudad Universitaria, DF 04510 Mexico
AB: Olivine-hosted melt inclusions from mafic arc volcanoes commonly contain highly variable H₂O and CO₂ contents that are best explained by olivine crystallization over a wide range of depths during ascent and degassing. An excellent example of this is found in deposits from the 1759-1774 eruption of Jorullo volcano in the subduction-related Trans-Mexican Volcanic Belt. Jorullo's earliest lavas are primitive (9.3 wt% MgO, Fo_86-91 olivine), and lava compositions evolved over time as a result of crystal fractionation (Luhr and Carmichael, 1985). We analyzed melt inclusions in Mg-rich olivine from a 5-meter-thick proximal ash fall sequence. Melt inclusions from the basal tephra have variable H₂O (<1-5.2 wt%) and CO₂ (44-900 ppm), corresponding to crystallization pressures of <100 bars to 4.2 kb. This indicates that olivine crystallized over a range of depths extending from the mid-crust to very shallow levels beneath the volcano. Relations between H₂O and incompatible K, Ti, and P, and comparison to experimental phase equilibria demonstrates that olivine crystallization is driven primarily by the effects of H₂O loss from the melt during ascent. A puzzling feature of the H₂O and CO₂ data at Jorullo and other mafic arc volcanoes is that CO₂ concentrations do not decrease as rapidly with degassing as is predicted by degassing models. A likely explanation involves open-system degassing in which relatively CO₂-rich vapor fluxes upwards through the conduit. This vapor is probably released by crystallizing and degassing magma at greater depths within the system. Melt inclusions from the upper part of the tephra section at Jorullo have lower H₂O (0.2-1.4 wt%) than inclusions from the basal tephra and no detectable CO₂, suggesting shallow crystallization of degassed magma toward the end of the violent-Strombolian-style eruptions. The change in olivine crystallization depths over time probably results from evolution of the conduit system. During the early, pre-eruption stage, as magma fractures its way upward and creates a conduit system, olivine crystallizes over a wide range of depths, trapping variably degassed melts. These crystals are probably stored in the complex dike and sill network that is growing beneath the volcano. When magma finally breaks out at the surface, initiating the eruption, magma flux increases, and magma moving upward through the conduit system entrains olivine crystals stored in the dike and sill network. Over time, the conduit system becomes localized, so early-formed olivine from greater depths are no longer entrained, and crystallization occurs mainly at shallow depths beneath the volcano. Such a model may have general applicability to many mafic eruptions and helps to explain how olivine crystals formed at diverse depths and of sometimes diverse compositions become incorporated into a batch of erupting magma.
DE: 1043 Fluid and melt inclusion geochemistry
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3640 Igneous petrology
DE: 8428 Explosive volcanism
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005