Evolution of the Laacher See magma chamber: Evidence from SIMS and TIMS measurements of U-Th disequilibria in minerals and glasses

Details Evolution of the Laacher See magma chamber: Evidence from SIMS and TIMS measurements of U-Th disequilibria in minerals and glasses Evolution of the Laacher See magma chamber: Evidence from SIMS and TIMS measurements of U-Th disequilibria in minerals and glasses

Aug 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994e%26psl.126...75b&link_type=abstract

Earth and Planetary Science Letters (ISSN 0012-821X), vol. 126, no. 1-3, p. 75-90

Physics

Geophysics

15

Crystallization, Earth Crust, Geophysics, Phase Transformations, Radioactive Isotopes, Thorium Isotopes, Uranium Isotopes, Volcanoes, Crystals, Felsite, Magma, Mathematical Models, Planetary Evolution, Pumice, Secondary Ion Mass Spectrometry

Scientific paper

U-Th disequilibrium was measured by Secondary Ion Mass Spectrometry (SIMS) (Th) and Thermal Ionization Mass Spectrometry (TIMS) (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.

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