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Laacher See revisited: High-spatial-resolution zircon dating indicates rapid formation of a zoned magma chamber

¹ Department of Earth and Space Sciences, University of California–Los Angeles, Los Angeles, California 90095-1567, USA

High-spatial-resolution (²³⁰Th)/(²³⁸U) disequilibrium dating reveals rapid zircon crystallization prior to the 12,900 yr B.P. Laacher See (Germany) phonolite eruption. Zircons from Lower Laacher See tephra (LLST) and syenitic subvolcanic nodules share REE and low U/Th characteristics and define an isochron age of 17.1 ± 1.3 ka (1). Zircons also show higher initial (²³⁰Th)/(²³²Th)₀ and oxygen isotopic disequilibrium compared to their host phonolite. Thus, LLST zircons shortly predate the eruption and yet are not strictly part of the phonolite crystallization sequence. Based on their similarity to zircons in subvolcanic nodules, they are instead interpreted to be scavenged from a precursor syenite intrusion, which implies short (a few k.y.) filling and differentiation timescales for the Laacher See magma chamber. Thorium and oxygen isotopes in zircon as well as evidence for abundant crustal xenocrysts further indicate extensive crustal contamination, which allows for considerably faster basanite-phonolite differentiation than the 100 k.y. time span previously estimated from bulk U-series data.

Key Words: U-238/Th-230 • zircon • phonolite • magma chambers • Allerød

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