Magma evolution of the East Eifel volcanic field from a perspective based on detrital zircon in modern river sediment
Palaeontology
Final Report Abstract
With the eruption of Laacher See volcano ca. 13,000 years ago, the Quaternary East Eifel volcanic field ranks among the youngest and most dynamic volcanically areas in Central Europe. Earthquakes, uplift, and gas emissions occur focused in this region, suggesting persistent and ongoing migration of magmatic fluids from an anomalously hot zone in the upper mantle. Geodetic and geophysical methods, however, can only image the current conditions in crust and mantle underneath the volcanic cover. To improve the understanding of the longterm processes and timescales of magma transfer from the mantle to the surface, investigations of past eruptions are therefore essential. Conventionally, samples of lava, pyroclastic fragments, or xenoliths from volcanic deposits are collected for such investigations. This punctuated sampling, however, is labor intensive and prone to bias. Instead, we investigated modern stream sediments from the East Eifel, from which zircon was extracted as a particularly resistant and datable index mineral with relative ease. Stream sediments of the Nette and Brohlbach catchments were sampled and zircon was enriched using in-situ hydraulic separation. Both stream systems drain an area that encompasses nearly all East Eifel volcanic centers. Investigation of detrital zircon from these sediments thus yielded a complete picture of all zircon-bearing sources among the regional volcanic deposits. More than 400 detrital zircon crystals were dated using uranium-thorium and uranium-lead geochronology, and their oxygen-hafnium isotopic compositions were determined. Based on these ages, the phonolitic-trachytic Wehr and Laacher See centers were identified as the source of Quaternary zircon, which dominates the detrital zircon population. Paleogene zircon comprises at most 15 % of the total population and is attributed to evolved (sub-)volcanic rocks from the volcanic Hocheifel. Devonian sandstone contributed zircon with ages between ca. 300 and 3200 Ma, and this population makes up 20–30% of the grains. These results indicate that the Laacher See magma system crystallized zircon in evolved melts already by ca. 63 ka, with most zircon crystals being derived from residual melts in the largely crystalline roof of the magma reservoir. A quasi-continuous zircon age spectrum between ca. 63 and 13 ka implies the lasting presence of partial melts in a near-surface environment, which requires frequent rejuvenation of the magma system by mafic recharge. The Laacher See magma system with its highly evolved and largely crystalline margins is thus more long-lived than previously known.
Publications
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Magma accumulation underneath Laacher See volcano from detrital zircon in modern streams. Journal of the Geological Society, 180(1).
Schmitt, Fabian H.; Schmitt, Axel K.; Gerdes, Axel & Harvey, Janet C.
