Multi-stage evolution of the lithospheric mantle beneath the Hessian Depression (Germany): Peridotite xenoliths from Stöpfling

Abstract

The Hessian Depression in Germany is the northern continuation of the Upper Rhine Graben, from which it is separated by the Vogelsberg volcanic region. Abundant Cenozoic basaltic eruptions locally brought mantle xenoliths to the surface. The legacy suite of 15 ultramafic xenoliths from the Stöpfling quarry has been studied in detail to decipher nature and evolution of this fragment of European mantle lithosphere.

The xenolith suite comprises harzburgites, clinopyroxene-poor lherzolites and clinopyroxene-rich lherzolites. One of the xenoliths is a harzburgite hosting a centimetric olivine websterite layer. Olivine contains 90.1–91.5 % forsterite, 0.39–0.43 wt%. NiO, 120–500 μg/g Ca. Orthopyroxene (0.10–0.18 atoms of Al per formula unit, pfu), has Mg/(Mg + Fe), Mg#, of 0.90–0.92, similar to that of olivine. Clinopyroxene (0.12–0.25 atoms Al pfu; Mg# 0.90–0.95). Rare Earth Elements (REE) and trace-element patterns are highly variable, but all are enriched in light REE relative to medium/heavy REE. Spinel Cr/(Cr + Al) varies from 17 to 46. The ⁸⁷Sr/⁸⁶Sr ratios measured in situ in clinopyroxene vary from 0.703969(± 37 1σ) to 0.708615(± 149 1σ) and are in part heterogenous at the scale of individual samples and grains.

The peridotites typically have well defined olivine-orthopyroxene or olivine-only crystal preferred orientation, whereas clinopyroxene (or clinopyroxene and orthopyroxene) is oriented independently in most of the rocks. This is taken to reflect secondary clinopyroxene addition to an originally refractory harzburgite protolith. The latter is still preserved in two harzburgites with highly depleted compositions and low clinopyroxene modal abundances. Melt depletion recorded in the most depleted harzburgites was followed by carbonatitic metasomatism, whereas other harzburgites and clinopyroxene-poor lherzolites were metasomatized by carbonated silicate or alkaline silicate melts. The clinopyroxene-rich lherzolites preserved relics of MORB-like refertilization, overprinted by alkaline metasomatism analogous to that affecting the rest of the rocks.

The two-pyroxene equilibration temperatures (940–1050 °C) mostly show excellent agreement between FeMg and REE-based thermometers. This suggests chemical and thermal equilibration after clinopyroxene addition and shows that Stöpfling xenoliths sample a rather narrow ∼8 km depth interval of the mantle lithosphere. The Stöpfling mantle profile probably records two-stage tectono-metasomatic evolution, comprising (1) mantle lithosphere upwelling and melting during flattening of the Variscan orogen root due to late orogenic collapse followed by thermal re-equilibration, and (2) further local lithosphere thinning during Alpine rifting, accompanied by multiple local chromatographic-style metasomatic episodes that culminated in volcanism as well as xenolith entrainment and transport to the surface.