EVOLUTION OF THE LAACHER-SEE MAGMA CHAMBER - EVIDENCE FROM SIMS AND TIMS MEASUREMENTS OF U-TH DISEQUILIBRIA IN MINERALS AND GLASSES

U-Th disequilibrium was measured by Secondary Ion Mass Spectrometry (Th) and Thermal Ionization Mass Spectrometry (U) in mineral separates from pumices and cumulate nodules from the Laacher See Volcano (12.5 +/- 0.5 kyr, corrected C-14 age), in East Eifel, Germany. Phonolitic glasses all plot to the left of the equiline, with higher U/Th observed in the most felsic glasses. Measured partitioning of U and Th in the mineral phases is consistent with the observed increase in U/Th with progressive fractional crystallization. Apparent ages of crystallization inferred from internal isochrons in the pumices from the main part of the magma chamber are about 13 +/- 3 kyr, indistinguishable from the eruption age. This implies that the residence time of these phenocrysts did not exceed 1-2 kyr. Older, less precise, ages for the upper part of the magma chamber (approximately 30 kyr) suggest mixing of older crystals from cumulates and/or longer residence times. Mineral isochrons for cumulate nodules are not well defined, with 'ages' ranging from 10 to 30 kyr, suggesting in-situ cumulate formation 10-20 kyr prior to the eruption. The time required to explain the Th-230/Th-232 difference between parental basanite and Laacher See phonolite is inferred to be about 100 kyr. Together, these observations favor a cooling model where differentiation of parental basanite takes place in a deep magma body over about 100 kyr, followed by emplacement of the resulting mafic phonolite into an upper crustal chamber and continued differentiation toward evolved phonolite. The time-scale for the development of zonation in the Laacher See magma chamber is estimated to be 10-20 kyr at most.