HR: 1340h
AN: V33C-1521    [Abstracts]
TI: A New Explanation For Oscillatory Zoning In Plagioclase Phenocrysts
AU: * Ustunisik, G K
EM: ustunigk@email.uc.edu
AU: Kilinc, A
EM: attilakilinc@uc.edu
AB: Plagioclase phenocryts in volcanic rocks commonly record various differentiation processes in their magma chambers. A fundamental assumption is that the normal zoning followed by a reverse zoning or oscillatory zoning are either due to the changes in temperature (T) during mixing of two magmas or due to the fluctuations in water pressure (PH2O) and total pressure (PTotal) during decompressional crystallization. However, this assumption is not always true as demonstrated here for a parental basaltic composition from Small Hasandag Volcano, Central Anatolia, Turkey. Our hypothesis is that the changes recorded in the plagioclase zones of the Small Hasandag Volcano can be explained by the differentiation of magma under isochoric conditions. Once the magma fills the magma chamber, system is isochoric between two successive eruptions provided that wall rocks are rigid. Under these conditions, volume (V) and T are independent variables, whereas P is a dependent variable which changes with subtle differences in V of crystals and melt during crystallization. In order to test this hypothesis, a parental composition (Mg#=61.98 with 3% H2O) is allowed to differentiate under isochoric fractional crystallization conditions. First, a rapid decrease in P has been recorded due to the bulk compressibility 10-6bar-1. Later in the crystallization history of the magma, the water in the original magma, exsolves causing a P increase. As denser minerals crystallize in response to the decreasing T, P decreases significantly from 1500 bars to a minimum of 900 bars at about 7% crystallization and then increases to 1500 bars at about 75% crystallization and continues to increase to 1600 bars at 85% of crystallization and then decreases again to 1540 bars at 90% of crystallization. These fluctuations in P affect the anorthite content (XAn) resulting in normal, reverse and oscillatory zoning in plagioclase phenocrysts.
DE: 3610 Geochemical modeling (1009, 8410)
DE: 8428 Explosive volcanism
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting