HR: 11:35h
AN: V12A-06    [Abstracts]
TI: Old phenocrysts gather on the shores of the Aegean
AU: * Bachmann, O
EM: olivier.bachmann@terre.unige.ch
AF: Section des Sciences de la Terre, University of Geneva, 13, rue des maraichers, Geneve, 1205 Switzerland
AU: Charlier, B L
EM: b.l.a.charlier@open.ac.uk
AF: Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA United Kingdom
AU: Lowenstern, J B
EM: jlwnstrn@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: Information derived from zircon crystals has proven to be extremely useful in deciphering magmatic processes. In particular, their crystallization age can be deduced from U-Th-Pb dating due to the slow diffusion of U, Th and Pb in the zircon lattice, leading to a closure temperature above the magmatic temperature for most silicic magmas. This characteristic of zircon U-Th-Pb dating establishes it as one of the best tools to assess the duration of assembly and longevity of silicic magma bodies above their solidi. It also provides a means of recognizing xenocrystic material in the form of cores inherited from assimilated lithologies. The ~150 km³ rhyolitic Kos Plateau Tuff eruption occurred ~160 ka ago (⁴⁰Ar/³⁹Ar sanidine age) in the eastern part of the Aegean Arc, leaving a >20 km diam. caldera off shore of the island of Kos (Greece). SHRIMP U-Th-Pb dating of zircons from multiple samples at different stratigraphic levels records an age distribution from ~160 ka (indistinguishable from eruption age by ⁴⁰Ar/³⁹Ar) to ~500 ka and a peak of zircon crystallization from 200 to 250 ka. These results highlight two important observations: (1) large silicic magma bodies can form and reside above their solidi for >300 ka, (2) the absence of U-Pb ages older than 500 ka, together with the whole-rock isotopic ratios of this rhyolite (ε_Nd = 0.2-0.9 and ⁸⁷Sr/⁸⁶Sr = 0.7040-0.7044) in an area where Palaeozoic crust is exposed at the surface, implies minimal crustal contamination. The low basalt flux, hence low thermal flux, of the dying Aegean subduction zone may explain the growth of this voluminous rhyolitic body without significant mass contribution from the surrounding pre-existing crust.
DE: 1036 Magma chamber processes (3618)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1105 Quaternary geochronology
DE: 1120 Isotopic disequilibrium dating
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005