Peridotite xenoliths from Stöpfling in Hessian Depression (Germany) revisited

The Hessian Depression in Germany represents the northern continuation of the Upper Rhine Graben and is known for Cenozoic alkaline basaltic lavas. Many of these carry peridotite xenoliths of mantle origin, which were studied mainly in the  80-ies of the XX century (Hartmann & Wedepohl 1990 and references therein). These studies documented mantle lithosphere melting followed by metasomatism. Here we describe the xenoliths from the Stöpfling quarry near Homberg upon Efze. The quarry has been recultivated and sampling is not possible now, our samples come from the archival collection of the Department of Geochemistry of the University of Göttingen. In this abstract, we give an overview of newly collected major- and trace-element mineral-chemical data from 11 xenoliths.

The xenoliths from Stöpfling are spinel-facies lherzolites and harzburgites. They consist of aggregates of few coarse (typically 4-6 mm across) grains of olivine and orthopyroxene embedded in fine-grained matrix of olivine, ortho- and clinopyroxene and spinel. Coarse-grained aggregates represent fragments of protogranular texture and are volumetrically prevailing. Spinel is commonly interstitial and has amaeboidal morphology. Locally, centimetre-thick layers of websterites cross-cut the peridotites. Hartmann & Wedepohl (1990) report traces  (<1 vol. %) of phlogopite in 2 of 12 lherzolites they studied.

The major element composition of olivine is strikingly homogeneous in all studied rocks (91±0.5 % forsterite and 0.40 wt. %. NiO), Ca content is < 500 ppm. Orthopyroxene is mildly aluminous (0.10-0.17 atoms of Al per formula unit, [pfu]) as is clinopyroxene (0.12-0.25 atoms Al pfu). Spinel Cr# [= Cr/(Cr+Al)] varies from 0.18 to 0.46. Clinopyroxene coexisting with spinel of low Cr# is Al-rich and contains 1600-2200 ppm Ti, whereas that coexisting with spinel of higher Cr# is less aluminous and contains 600-1200 ppm Ti. Clinopyroxene coexisting with spinel of Cr# 0.46 is extremely impoverished in Ti (50 ppm). The REE patterns of clinopyroxene in most samples are above the primitive-mantle (PM) level, are LREE-enriched and flat at MREE-HREE. Those extremely depleted in Ti show a decrease from HREE towards MREE, the contents of which are below PM level, and are strongly LREE-enriched.

Peridotites from Stöpfling consist of olivine which chemical homogeneity  across the xenolith suite supposedly records melt depletion. The variable content of Al in orthopyroxene from different samples probably is due to subsequent refertilization event(s) involving silicate melt, whereas the REE characteristics of clinopyroxene suggests that it was additionally cryptically  metasomatized. The unaffected olivine composition indicates low ratio of metasomatic agent to protolith.

Acknowledgements. JP is grateful to G. Wörner for enabling access to the xenoliths from Stöpfling that were originally collected by H. Wedepohl and are now archived at the Geochemistry and Isotope Geology Division of the Geoscience Center at University Göttingen (GZG).

Funding. This study originated thanks to the project of Polish National Centre of Research 2021/41/B/ST10/00900 to JP.

References:

Hartmann G., Wedepohl. K.H. (1990): Metasomatically altered peridotite xenoliths from the Hessian Depression (Nortwest Germany). Geochim. Cosmochim. Acta 54: 71-86.