Magmatic Longevity of Laacher See Volcano (Eifel, Germany) Indicated by U-Th Dating of Intrusive Carbonatites

Abstract

Uranium-series dating of carbonatitic ejecta clasts constrains the crystallization and differentiation timescales of the Laacher See volcano, which erupted 6 center dot 3 km(3) of magma (dense rock equivalent) during one of the largest Late Quaternary eruptions in Central Europe. Carbonatites form a distinct population among plutonic ejecta that are present in the middle and late erupted Laacher See tephra. Characteristic trace element patterns of the carbonatites, including negative Eu anomalies, and mantle-like oxygen isotopic compositions preserved in zircon indicate that the Laacher See carbonatites are cogenetic with their phonolite host. Carbonatite U-Th zircon isochron ages range from 32 center dot 6 +/- 4 center dot 1 ka (2 Sigma; MSWD = 1 center dot 7; n = 24) to near-eruption age (12 center dot 9 ka). Uranium-series carbonatite ages qualitatively agree with alkali feldspar compositions that lack prominent magmatic zonation, but show evidence for perthitic unmixing during subsolidus residence at elevated temperatures (< 700 degrees C) in an intrusive carapace surrounding the liquid-dominated interior of the magma system (> 720 degrees C). Model differentiation ages and crystallization ages for the carbonatites overlap within a few thousand years as resolved by U-Th dating and indicate rapid crystallization following carbonatite segregation from its parental phonolite. Model differentiation and zircon isochron ages peak at similar to 17 ka, suggesting a major phase of differentiation of the Laacher See magma system at this time, although the onset of phonolite differentiation dates back to at least similar to 10-20 kyr prior to eruption. Phenocrysts in the middle and late erupted phonolite magma crystallized shortly before eruption, and the lack of older crystals implies crystal removal through settling or resorption. Crystal ages from both crystal-rich and liquid-dominated parts of a magma system are thus complementary, and reveal different aspects of magma differentiation and residence timescales.