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GeoPeP 2001 Basics

At the beginning of our project we discussed the previous works on the Eifel done since 1975 to get a background of the geological setting. We focussed on the mantle source and on the most mafic alkaline magmas. In the Eifel the alkaline magmas can be divided petrographically into two suites (Mertes and Schmincke, 1985) :

1) The F-suite including leucitites, nephelinites, olivine-free foidites and melilite-free sodalite-bearing foidites.

2) The ONB-suite including olivine nephelinites and basanites as well as the differentiates tephrites and phonolites. The trace element difference between these two suite leads to the conclusion that the F-suite magmas derived from phlogopite-bearing mantle source and the ONB-suite from an amphibole-bearing source which lies at a depth of about 60 km while the F-suite magmas come from 90 km depth (Mertes and Schmincke, 1985). These two suite have no spatial and temporal separation. This is an evidence for the heterogeneity of the mantle in the order of tens of kilometres.

How could this heterogeneity evolve?

It is observed that some elements (lit. cit.) are relativly enriched in comparison to the depleted mantle. These elements are mobile and therefore significant for involved fluids. So the process to form this heterogeneity is mantle source metasomatism.

The time needed to generate the observed isotopic heterogeneity can be calculated by the Rb/Sr- (and the Sm/Nd-)System which give a solution of about 90 Ma for the source of the leucitites (F-group) and 500 Ma for the source of the basanites based on the data by Wörner et al. (1986).

There is strong evidence that the earth`s mantle has not been homogenised on geological timescales, and that a strong heterogeneity is preserved. This could be due to two factors:

1) slow convective velocities, meaning that mixing is not possible on geological timescales. Also, convection would not be very effective, even if it would go faster.

2) Diffusion times are also too slow (Fig. 2.1).

Fig.2.1: Diffusion calculations for different diffusion coefficients vs time. Red: region of interest.

Why is there volcanism in the Eifel?

There is a mantle plume!!!! That´s it and trust us, we know it!!

The mantle under the western part of Europe is hot. These temperature anomalies cause a plume which is situated under the lithosphere (Ritter et al., 2001). And maybe even the Alps helped!

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