The temporal evolution and volcanic plumbing system beneath the southeast Lammersdorf volcanic center, West Eifel Volcanic Field, Germany

Abstract

The West Eifel Volcanic Field, western Germany comprises ~240 volcanic edifices spread over an area of ~ 600 km2. Magma was intruded into Devonian and Triassic meta-sedimentary rocks over the period 940 ka BP to 11 ka BP with an increase in eruptive frequency in the last 100 ka. Low velocity anomalies indicate the presence of a thermal plume containing 1-2% melt in the asthenosphere below the field; indicating that it is still potentially active. Since there are several large towns in the region and the Eifel is on the flight path for many major airports any assessment of volcanic hazard must be based on the dynamics of magma emplacement. The Rockeskyllerkopf Volcanic Complex (RVC) first erupted the Southeast Lammersdorf volcanic center at ~474 ka BP with the final eruption of the Rockeskyllerkopf volcanic center, more than 100,000 years later at ca 360 ka BP. For each volcanic event the first stage was phreatomagmatic but progressed to strombolian as groundwater was reduced. The deposits of the first event contain mantle-derived and high pressure cumulate xenoliths that were entrained in the rising magma. Numerous studies have shown that the olivine in mantle xenoliths is in disequilibrium with the magma that brought them to surface. This disequilibrium is reflected in the development of Fe-Mg diffusion profiles from olivine cores to rims. The coefficient of Fe-Mg diffusion has previously been determined, combining this with estimated temperature, oxygen fugacity, and forsterite compositions allows one to determine the length of time of diffusion within xenoliths. Olivine was analyzed in 9 peridotite and 5 high pressure cumulate (clinopyroxenite) xenoliths. Fe-Mg diffusion times in peridotites indicate xenoliths took less than one week to reach the surface whereas olivine from fragmented xenoliths included in high pressure cumulates records a much longer contact time approximately one week to one year. Variations in forsterite compositions and melt mg# indicate the presence of more than one magma in the volcanic plumbing system beneath RVC, possible re-equilibration of olivine, and a vein network of magma interacting with lithospheric mantle ~25 years before magma ascent into crust. Additionally, results indicate that magma was present within the crust below the RVC for up to as much as one year prior to the first eruption. The short transit times for the mantle xenoliths indicate that new batches of magma cross the lithosphere quickly at an average speed of 1.7 m/s or 6 km/hr. If magma input is accompanied by significant seismic activity we can expect precursor events for approximately one year before an eruption. However, the rapid rise of the magma that is erupted indicates that there will be very little warning of the actual eruption itself.