ALBERT

Description: Complex, symplectite-bearing pseudomorphs after garnet recently found in unique basanite-hosted peridotite xenoliths from Zinst, Bavaria, allow the study of the interaction between garnet peridotite and melts or fluids both prior to entrainment of the xenoliths and during their ascent. Based on microstructures and crystallographic fabric, and major and trace element mineral chemistry, four distinct concentric zones were defined in various types of pseudomorph: Zone I, coarse-grained (≤1 mm) aggregate of orthopyroxene + clinopyroxene + spinel with a granular structure; Zone II, fine- to medium-grained (order of 10–100 µm) orthopyroxene + spinel symplectite; Zone III, fine-grained (5–300 µm), radially fibrous orthopyroxene + spinel symplectite with interstitial anorthite; Zone IV, ultrafine-grained (≤1 µm) orthopyroxene + spinel + anorthite symplectite with an internal domain substructure. Zones III and IV have bulk compositions of pyrope-rich garnet. All zones exhibit perfect inter-sample correlation and document the discontinuous evolution of peridotite under changing conditions with successively increasing rates of garnet breakdown. Based on thermometry and microstructural relations, a sequence of three pre- and syn-volcanic events is discerned. The first traceable event corresponds to regional heating in the uppermost mantle probably related to the early stages of Tertiary rifting, which triggered the reaction between garnet and olivine (Zone I) leading to a partial re-equilibration of the rock at 1040–1080°C within the spinel peridotite stability field. Subsequently a short period of heating by ~100–250°C led to largely isochemical, fluid-mediated in situ melting of garnet and to the formation of kelyphite by crystallization from the melt (Zone III). The subsequent metasomatic alteration by external, Na-rich, K-poor, carbonate-bearing melts or fluids suggests that this phase of garnet breakdown occurred largely prior to formation of the xenolith, preceding the emplacement of the basanite magma. Finally, after xenolith formation, and associated with rapid, isochemical, decompression during exhumation, the garnet relics were transformed into microsymplectite (Zone IV). The positive volume change associated with this reaction caused fracturing, producing radial cracks that emanate from Zone IV and extend into the adjacent peridotite, allowing infiltration of basanite-derived melt components. The well-developed and clearly separated symplectite zones indicating the isochemical breakdown of garnet are uncommon in garnet peridotites worldwide. Their existence at Zinst is explained by an extremely short time span between the formation of the kelyphite, metasomatism by Na- and carbonate-rich agents and the final garnet breakdown during the host basanite eruption, allowing for rapid quenching of the multiple advancing reaction fronts.

Description: Peridotite xenoliths hosted by Cenozoic basanite lava flows (21·2–23·5 Ma) occur at several localities in the western continuation of the Ohre/Eger rift (Central European Volcanic Province) in NE Bavaria, Bohemian Massif. Detailed petrography, major and trace element compositions of whole-rock samples and selected mineral phases as well as Sr–Nd–Li isotopic compositions for a suite of mantle xenoliths from Zinst, Hirschentanz and Teichelberg document variable degrees of partial melting and metasomatism. Melting models based on whole-rock major element composition and Cr# of spinel indicate ~6–30% melting in the spinel stability field. Subsequent metasomatism by alkaline and carbonate-rich melts resulted in modal and cryptic metasomatism, expressed by the presence of carbonate-bearing silicate melt pockets with complex secondary mineral assemblages and by enrichment in light rare earth elements, Li, Rb, U, Pb, high field strength elements and P. The carbonate is most probably associated with fractionation of the Na-rich silicate melt. High P contents, variable but low to negative 7 Li values from +2·5 to –9·7, coupled with 87 Sr/ 86 Sr ratios between ~0·7032 and ~0·7041 may reflect a significant contribution of recycled crustal material such as eclogite in the infiltrating melts responsible for metasomatism, although the Li isotope compositions may reflect kinetic modifications through diffusion. The trace element geochemistry of clinopyroxene, carbonate and melt pockets suggests that clinopyroxene plays a very important role in fractionation of the rare earth elements, high field strength elements and Sr, whereas carbonate does not host large quantities of incompatible trace elements except for Sr, Ba and, to a lesser extent, Th and U.