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The Sr-Nd-Pb isotope compositions of primitive Tertiary-Quaternary mafic volcanics within Western/Central Europe and contemporaneous magmatism in the Mediterranean and East Atlantic (Wilson and Downes, 1991; Hoernle et al., 1995; Cebriá and Wilson, 1995) suggest that the primary magmas are mixtures of partial melts derived from a heterogeneous lithospheric end-member and a common HIMU-like asthenospheric mantle source. The HIMU mantle source beneath Europe, which we term the European Asthenospheric Reservoir (EAR) (Cebriá and Wilson, 1995), has a relatively well constrained isotopic composition of 87Sr/86Srª0.7030, 143Nd/144Ndª0.5130, 206Pb/204Pbª19.90, 207Pb/204Pbª15.65, and 208Pb/204Pbª39.30 (Cebriá and Wilson, 1995). The major and trace element and mineralogical composition of the EAR is, however, unknown. By analogy with current models for the origin of HIMU mantle sources, it may consist of a mixture of recycled ancient subducted oceanic crust mixed with a more depleted mantle protolith. Modelling the trace element characteristics of near-primary partial melts derived from the EAR source provides important constraints with which to test this hypothesis.
The two main problems in this modelling approach are: 1) Selection of appropriate samples, and 2) Modelling of the process itself. As suggested by the isotopic variation of the volcanic rocks of Western/Central Europe, most of the primitive magmas have suffered variable degrees of mixing with small-degree partial melts of enriched lithospheric mantle. The selection of samples for trace element modelling must then be restricted to those lavas, least affected by lithospheric contamination, which have the closest isotopic composition to the proposed EAR source. The data set we use includes representative samples from the Eifel (Germany), Massif Central (France), Ohre Rift (Czech Republic) and Calatrava and Olot (Central and NE Spain, respectively). This data set is not intended to represent the relative volumes of magma for each region nor the relative abundance of each petrologic group, but the compositional variation due to partial melting. For the trace element modelling of partial melting processes we have used the method of Cebriá & López-Ruiz (1996). Preliminary results of the modelling (Fig. 1) suggest that the trace element compositions of the primitive mafic magmas from Western/Central Europe could result from variable degrees of partial melting (F=4-14%) of a homogeneous garnet lherzolite source variably enriched in incompatible elements (up to x18 chondrite for the highly incompatible elements and x3-4 chondrite for the moderately incompatible elements). The distinctive negative K and Rb anomalies exhibited by all EAR melts combined with the highly incompatible behaviour of Sr suggest the presence of phlogopite as a residual phase in the mantle source. More complex partial melting models involving a binary mixture of ultramafic and mafic components in the EAR source will be discussed.
Cebriá, J.M. & López-Ruiz, J., Geochim. Cosmochim. Acta in press (1996).
Cebriá, J.M. & Wilson, M., Terra Abstracts 7, 162 (1995).
Hoernle, K.; Zhang, Y-S. & Graham, D., Nature 374, 34-39 (1995).
Wilson, M. & Downes, H., J. Petrology 32, 811-849 (1991).
Fig. 1: Mantle normalized trace element compositions for primitive mafic volcanics from Western/Central Europe and results of the partial melting modelling.