The temporal evolution of three magmatic systems in the West Eifel volcanic field, Germany

Abstract

The Quaternary West Eifel volcanic field is the surface expression of a mantle plume that is thought to contain ∼1% melt. The volcanoes range in age from 0.7 Ma to ∼10 000 yr BP and the average recurrence rate of volcanism is around 3.5×10 −4 eruptions per year, giving a mean time between eruptions of 2875 years, assuming that the field is monogenetic. The presence of mafic and ultramafic xenoliths is taken as evidence for rapid, single-stage emplacement of magmas at velocities around 0.5 km/h. However, the presence of clinopyroxene that preserves evidence of polybaric crystallisation belies this simple model. This study uses variations in the Fo content of olivine in mantle-derived xenoliths and in olivine xenocrysts enclosed in cumulate xenoliths to calculate the time scale of evolution of three volcanic centres in the Eifel field. Core–rim variations in olivine are interpreted to result from Fe–Mg interdiffusion between it and a surrounding melt. Using experimental determinations of the interdiffusion coefficient and approximations of magma temperature, contact times have been calculated using planar and radial diffusion models. The three volcanic centres studied were Meerfelder Maar, Gemündener Maar and Baarley. The cumulate xenoliths at Meerfelder Maar formed at a depth of ∼13 km and record a magma emplacement event at between 1 and 3 years prior to eruption. Two suites of mantle-derived olivine clinopyroxenites record a further two events at 1–8 days and ∼3 h prior to eruption. The final event is interpreted to record the eruption of this volcano and the 3-h contact time records the time required for transport of peridotite xenoliths from the mantle to surface, giving an average emplacement velocity of around 15 km/h. The cumulate xenoliths at Gemündener Maar crystallised at depths of between 4 and 22 km. There are four distinct events recorded by the zoned olivine grains in the clinopyroxene-rich cumulates from this locality. The maximum residence time is similar to that at Meerfelder Maar. However, in this case the final magma resided in the chamber for between 23 days and 2.7 months prior to eruption, allowing enough time for any entrained peridotite xenoliths to be filtered out. The cumulate xenoliths from the Baarley locality record a much longer history than those at the other localities. In this case, initially peridotitic olivine has been completely re-equilibrated over >175 years prior to eruption. In common with the other two localities, there is evidence of multiple intrusive events, at depths up to 7 km, prior to final eruption. The duration of the final eruption event and the interpreted depth to source of the xenoliths indicate an emplacement velocity of around 3 km/h. The data derived from modelling of diffusion profiles in olivine from cumulate xenoliths indicate that the magmatic systems feeding these three volcanoes were dynamic and very short-lived. In addition, these data can be used to place constraints on the rate of cumulate formation in subvolcanic magma chambers. On the basis of the data presented here, it would seem that an appreciable thickness of cumulates can form over relatively short time spans (days to a few years).