HR: 1340h
AN: V13B-0523    [Abstracts]
TI: Contrasting Ti Zoning Patterns in Quartz Phenocrysts from the Bishop and Fish Canyon Tuffs: An Expression of Volatile Composition in Recharge Melts
AU: * Wark, D A
EM: warkd@rpi.edu
AF: Dept of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180 United States
AU: Bachmann, O
EM: olivier.bachmann@terre.unige.ch
AF: Section des Sciences de la Terre, University of Geneva, 13 rue des Maraichers, Geneva, 1205 Switzerland
AB: Mafic recharge is increasingly recognized as playing an important role in the thermal and chemical evolution of silicic magma systems. Even in the absence of direct interaction between mafic and silicic magmas, volatiles released from recharge melts may be effective agents of heat exchange ("gas sparging" of Bachmann and Bergantz, 2003). These volatiles will also variably affect - depending on their abundance and composition - phase relations and physical properties of the overlying silicic magma. Addition of H₂O to rhyolite melt should decrease solidus temperatures and melt viscosity; addition of CO₂-rich bubbles (into which H₂O will migrate from the melt) will have the opposite effects. We interpret differences in quartz zoning patterns from the Bishop and Fish Canyon Tuffs - each of which has a near-eutectic, rhyolitic melt composition - to reflect differences in the composition of volatiles added by recharge melts. Late-erupted Bishop Tuff contains phenocrysts of quartz that are reversely zoned with respect to Ti content: low-Ti cores (~40-50 ppm) are separated from high-Ti rims (80-100 ppm) by a sharp boundary that appears to be a dissolution feature. This pattern is best explained by recharge involving CO₂-rich basalts typical of the region. Quartz that originally grew at 740°C partially dissolved due to heating and was later reprecipitated at the higher temperatures (780-800°C) of the CO₂-elevated solidus. In contrast, Fish Canyon quartz is strongly resorbed, with variable Ti contents (90-120 ppm) but no systematic zoning. Partial dissolution of quartz without growth of high-Ti rims can be explained by the addition of heat and H₂O-rich volatiles (which lowered the solidus temperature) released from recharge melts like those extruded after Fish Canyon eruption (the Huerto Andesite).
DE: 3618 Magma chamber processes (1036)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3640 Igneous petrology
DE: 8412 Reactions and phase equilibria (1012, 3612)
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005