1997/04/01 by Gudrun Witt-Eickschen, G. Witt-Eickschen, Ulrich Kramm +1 · 2 citations
Earth and Planetary Sciences · #Geological and Geochemical Analysis #Geology and Paleoclimatology Research #earthquake and tectonic studies
paper · doi:10.1093/petroj/38.4.479
Abstract Clinopyroxenes in protogranular, spinel–pyroxene-cluster bearing perdotite xenoliths from the Rhön (Germany) display (1) extremely depleted REE patterns inconsistent with their relatively primitive modal composition and (2) peculiarly high Nd isotopic signatures (143Nd/144Nd=0.5148) and Sm/Nd ratios (147Sm/144Nd=0.82−0.92), which have not been reported for clinopyroxene before. The high 143Nd/144Nd 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°C to 850–950°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 143Nd/144Nd vs 147Sm/144Nd 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 Hercynia age or may be older.