Mantle Upwelling and Metasomatism beneath Central Europe: Geochemical and Isotopic Constraints from Mantle Xenoliths from the Rhon (Germany)

Abstract

Clinopyroxenes in protogranular, spinel-pyroxene-cluster bearing peridotite xenoliths from the Rhon (Germany) display (1) extremely depleted REE patterns inconsistent with their relatively primitive modal composition and (2) peculiarly high Nd isotopic signatures (Nd-143/Nd-144 = 0.5148) and Sm/Nd ratios (Sm-147/Nd-144= 0.82-0.92), which have not been reported for clinopyroxene before. The high Nd-143/Nd-144 ratios require long-term high Sm/Nd ratios found, so far, only in garnet. Therefore, it is suggested that the isotopic signature reflects the former existence of a precursor garnet. When the clinopyroxene-hosting peridotites entered the spinel stability field, as a consequence of mantle uplift, the garnet crystals broke down, forming spinel-pyroxene clusters and forced their geochemical and isotopic signature upon the clinopyroxenes. A polybaric fractional melting model (beginning in the garnet and continuing into the spinel stability field), calculated from REE abundances, implies formation of partial melts during mantle upwelling. The P-T conditions, derived from equilibrated mineral phases, indicate that the peridotites were finally transported into the upper lithosphere (30-40 km) where they cooled from a minimum temperature of 1030 degrees C to 850-950 degrees C. In this new position, parts of the upper mantle were affected by a cryptic, chromatographic fractionation controlled metasomatism that eliminated to variable degrees the geochemical fingerprint of the precursor garnet with respect to trace elements. If the linear correlation observed in a Nd-143/Nd-144 vs Sm-147/Nd-144 plot for different clinopyroxene specimens, all affected by these processes, has an age significance and is not a mixing line, then the last isotopic homogenization occurred during Hercynian times. This implies that the last stage of melt extraction and the metasomatic event are of Hercynian age or may be older