HR: 08:45h
AN: V11A-04    [Abstracts]
TI: Sector and/or Concentric Zoned Augite Phenocryts in Mafic Arc Lavas: Evidence for Decompression Induced Crystallization With Implications for the Mechanism of Andesite Fractionation
AU: * Brophy, J G
EM: brophy@indiana.edu
AF: Department of Geological Sciences, Indiana University, Bloomington, IN 47405 United States
AB: The assumption that heat loss to the surrounding country rock is the primary driving force for crystallization and crystal-liquid fractionation underlies nearly every model of magmatic differentiation. In calc-alkaline magmas, which contain large amounts of dissolved H₂O, another driving force for crystallization exists, that being decompression and H₂O exsolution during magma ascent. Numerous mafic (basalt to basaltic andesite) lavas from volcanic arcs throughout the world contain large (2 to 10 mm diameter) augite phenocrysts that are commonly sector zoned and invariably concentrically zoned, where the concentric zoning consists of zones of optically clear augite separated by thin zones rich in mineral and melt inclusions. In many crystals the melt inclusions display a systematic core-to-rim decrease in H₂O concentration. Together these observations are evidence for rapid crystallization during decompression-induced H₂O exsolution. Starting with this assumption, detailed analysis of a single hydrous (2 wt. % H₂O) augite bearing Aleutian basalt yielded the following results: (1) crystallization commenced at a depth of around 1 km; (2) augite crystal growth rates are on the order of 10^-7 cm/s; (3) augite residence times was on the order of 40 days; which (4) yields an average magma ascent rate of 0.03 cm/s. This ascent rate is several orders of magnitude greater than calculated crystal settling rates implying that most such mafic magma bodies are likely to become congested and stall within the upper most crust. If and when this happens, extraction of the fractionation-generated interstitial andesitic liquid followed by cumulate entrainment and/or mixing with more mafic liquids could explain many of the chemical, mineralogic and textural features commonly observed in andesitic lavas throughout the world.
DE: 1036 Magma chamber processes (3618)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005