Sodium-potassium interdiffusion in potassium-rich alkali feldspar II: Composition- and temperature-dependence obtained from cation exchange experiments

No abstract availabl

Chemically induced fracturing in alkali feldspar

Kein Abstract verfügba

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

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, 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/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

Microstructure and texture evolution during growth of magnesio-aluminate spinel at corundum-periclase interfaces under uniaxial load: The effect of stress concentration on reaction progress

Reaction rims of magnesio-aluminate spinel were grown at the contacts between periclase and corundum at temperatures of 1250 °C to 1350 °C and under uniaxial load of 0.026 and 0.26 kN per 9 mm² of initial contact area. Single crystals of periclase with [100] and of corundum with [0001] perpendicular to the polished reaction interface as well as polycrystalline corundum were used as starting materials. Immediate application of the load before heating resulted in deformation twinning and fracturing of corundum introducing stress concentration and lateral variations in the quality of physical contact at the reaction interface. The tight contacts are characterized by enhanced reaction progress which together with the positive volume change of the reaction and limits on plasticity of the studied phases led to the opening of void spaces along the reaction interface and large lateral variations in rim thickness occur. Spinel shows strong topotactic relations to the reactant phases including full topotaxy between spinel and periclase, partial topotaxy with (111)ₛₚᵢ‖(0001)cor and {101}ₛₚᵢ‖{10-10}cor, and axiotaxy with (111)ₛₚᵢ‖(0001)cor between spinel and corundum. Oriented nucleation and selective growth were the main mechanism of texture formation. Stress concentrations and tight physical contacts across the reaction interface may enhance nucleation of topotactic grains. The respective spinel–periclase and spinel–corundum reaction interfaces are mostly semi-coherent with sets of line dislocations accounting for the lattice misfit. The systematic occurrence of porosity along the semi-coherent and its absence along the incoherent sections of the spinel–periclase interface reflect the different capacities of the (semi)-coherent and incoherent interface sections for annihilating the vacancies that were emitted from the advancing spinel–periclase reaction interface

Localization of submicron inclusion re-equilibration at healed fractures in host garnet

Microstructures in Permian inclusion-bearing meta-pegmatite garnets from the Koralpe (Eastern Alps, Austria) reveal re-equilibration by coarsening of abundant submicron-sized inclusions (1 µm – 2 nm diameter) at the site of healed brittle cracks. The microstructures developed during Cretaceous eclogite facies deformation and the related overprinting of the host-inclusion system. Trails of coarsened inclusions (1 – 10 µm diameter) crosscut the garnet, defining traces of former fractures with occasional en-echelon overlaps. Trails are flanked by 10 – 100 µm wide ‘bleaching zones’ characterized by the absence of ≤1 µm sized inclusions in optical and SE images. FEG-microprobe data show that trails and bleaching zones can form isochemically, although some trails exhibit non-isochemical coarsening. Cross correlation EBSD reveals subtle garnet lattice rotation of up to 0.45° around consistent misorientation axes, spatially correlated with bleaching zones. Elevated dislocation density within these zones is confirmed by TEM observations. Brittle fracture enhanced diffusion rates in the lattice adjacent to crack planes, priming these areas to behave differently to the bulk of the garnet during Cretaceous metamorphism and facilitating localized coarsening of inclusions. The preferred mechanism for this is increased dislocation density near former cracks, with the dislocations interpreted as a plastic wake originating from crystal plastic deformation at the crack tip. This partially closed host-inclusion system clearly records the influence of deformation mechanisms on re-equilibration and contributes to a wider understanding of the interaction between deformation and chemical reaction during metamorphism