Garnet Breakdown, Symplectite Formation and Melting in Basanite-hosted Peridotite Xenoliths from Zinst (Bavaria, Bohemian Massif)

Abstract

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/fluids both prior to entrainment of the xenoliths and during their ascent. Based on microstructures and crystallographic fabric, major and trace element mineral chemistry, four distinct concentric zones were defined in various types of pseudomorphs: I) coarse grained (≤ 1 mm) aggregate of orthopyroxene+clinopyroxene+spinel with a granular structure, II) fine-to medium grained (order of 10-100 µm) orthopyroxene+spinel symplectite, III) fine grained (5-300 µm), radially fibrous orthopyroxene+spinel symplectite with interstitial anorthite, and, IV) ultrafine grained (≤ 1 µm) orthopyroxene+spinel+anorthite symplectite with an internal domain sub-structure. 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 lead to largely isochemical, fluidmediated 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/fluids suggests that this phase of garnet breakdown occured largely prior to formation of the xenolith, preceding the emplacement of the basanite magma. Finally, after xenolith formation, and associated with the rapid, isochemical, decompression during ascent, 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 to garnet peridotites worldwide. Their existence at Zinst is explained by an extremely short time span between the formation of the kelyphite, Na-carbonate metasomatism and the final garnet breakdown on the basanite eruption, allowing for rapid quenching of the multiple advancing reaction fronts.

Application of thermodynamic modelling to natural mantle xenoliths: examples of density variations and pressure–temperature evolution of the lithospheric mantle

“…Furthermore, the very high Al 2 O 3 content typically observed in pyroxenes in kelyphites (5-15 wt%; cf. Mukhopadhyay 1991; Foley et al 2006;Špaček et al 2013) is also reproduced by our model (Fig. 6b).…”

Experimental synthesis of isochemical kelyphite — a preliminary report

“…It is quite plausible, therefore, that the latter assemblage occurred in the Na-bearing environment brought about by the introduction of the metasomatic fluid, whereas the former assemblage was stable at the same pressure and temperature condition in the absence of such metasomatism. The assemblage Ol + Sp + Pl is commonly observed in natural non-isochemical (i.e., metasomatic) kelyphites (e.g., Obata, 1994;Špaček et al, 2013;Scott et al, 2013).…”

“…Nevertheless, isochemical kelyphite, i.e., kelyphite that retains the composition of its precursor garnet, has been reported to occur rarely. These kelyphites occur mostly in maficgranulite or pyroxenite xenoliths brought up from the depths by volcanic eruptions (Keankeo et al, 2000;Sapienza et al, 2001;Dégi et al, 2010;Špaček et al, 2013). Obata et al (2013) provided the first firm report of such kelyphite from an orogenic garnet peridotite, in which isochemical breakdown of garnet occurred after 'normal' kelyphite formed by reaction between garnet and olivine.…”

Upper Cretaceous to Pleistocene melilitic volcanic rocks of the Bohemian Massif: petrology and mineral chemistry

“…Konečný et al 2006;Ackerman et al 2007Ackerman et al , 2013Ackerman et al , 2014Špaček et al 2013). The Mg-rich olivine may therefore represent xenocrystic cores being overgrown by Mg-poorer olivine crystallizing from the melt.…”