Perovskite occurs as an accessory phase in late Quaternary alkali mafic rocks (e.g., nephelinites, melilitites, and leucitites) of the West Eifel Volcanic Field (WEVF) in western Germany. It is often present as euhedral to skeletal-arborescent crystals in clefts or vesicles of lava flows, which indicates late-stage formation during or briefly after eruptive emplacement. Selected perovskite crystals from four different volcanoes (Rother Kopf, Grauley, Loehley, and Emmelberg) were dated by a novel method of 238U/230Th perovskite geochronology using secondary ion mass spectrometry (SIMS). In addition, trace element abundances in perovskite were determined by SIMS to provide insight into the conditions and processes related to its formation. Perovskite crystals from the WEVF contain Fe, Nb, LREE, and in some cases Sr at concentrations of up to several wt%. Traces of Ta, U, and Th are present in concentrations of a few 1000 μg g−1. Comparatively high Pb with common isotopic composition renders U–Pb perovskite dating of such young crystals impossible. Perovskite crystals from the older, ca. 700–480 ka activity period of the WEVF (Rother Kopf, Grauley, and Loehley) are enriched in Th relative to U, with observed (238U)/(232Th) activity ratios ranging from 0.114 to 0.824. By contrast, Emmelberg perovskite from the younger, ca. <80 ka WEVF eruptive pulse is generally highly enriched in U with (238U)/(232Th) activity ratios up to 14. Perovskite from Rother Kopf, Grauley, and Loehley have reached secular equilibrium, consistent with known or inferred eruption ages of >480 ka from the literature. Emmelberg perovskite analyses, by contrast, plot along an isochron in the (230Th)/(232Th) versus (238U)/(232Th) diagram and define an isochron age of 48.4 ± 2.4 ka (2σ). The 238U/230Th perovskite age for Emmelberg volcano closely agrees with published 40Ar/39Ar leucite ages averaging 49 ± 4 ka, albeit with smaller uncertainties. This similarity underscores that 238U/230Th perovskite ages represent eruption ages and supports textural evidence that WEVF perovskite crystallized in a late-magmatic, pneumatolytic stage shortly after lava flow formation. The results demonstrate the potential of perovskite for 238U/230Th disequilibrium geochronology for late Pleistocene samples where analytical precision at the millennial timescale can be achieved. Perovskite present in deposits from late Pleistocene-Holocene alkaline volcanism such as in Southern Italy or the East African Rift, as well as in oceanic island settings (e.g., Hawaii), are other potential targets for this method.
238U/230Th disequilibrium dating and trace element geochemistry of perovskite from late Quaternary alkali mafic rocks of the West Eifel Volcanic Field (Germany)
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Yi Sun, Axel K. Schmitt; 238U/230Th disequilibrium dating and trace element geochemistry of perovskite from late Quaternary alkali mafic rocks of the West Eifel Volcanic Field (Germany). European Journal of Mineralogy doi: https://doi.org/10.1127/ejm/2019/0031-2883
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