Rb/Sr isotopic and compositional retentivity of muscovite during deformation.

Permian metapegmatite muscovite from the Upper-Austroalpine Matsch Unit in Southern Tyrol (Italy) was investigated regarding its Rb/Sr and compositional retentivity during Cretaceous Upper-greenschist facies deformation. The data imply that microstructurally relic Permian magmatic muscovite largely maintained its major and trace element compositions during deformation, whereas the Rb/Sr geochronometer is strongly affected by a net loss of Sr. Lower Sr concentrations of muscovite correlate with higher 87Rb/86Sr and 87Sr/86Sr ratios. In most samples, the muscovite grain size- and magnetic-fractions with the lowest 87Rb/86Sr and 87Sr/86Sr ratios preserve a Permo-Triassic muscovite-whole rock Rb/Sr apparent age interpreted as to reflect formation during or cooling after pegmatite emplacement. Contrastingly, muscovite fractions with higher 87Rb/86Sr and 87Sr/86Sr ratios are arranged along a roughly linear array with a positive correlation of the 87Rb/86Sr and 87Sr/86Sr ratios in the 87Rb/86Sr vs 87Sr/86Sr space. They yield successively lower muscovite-whole rock Rb/Sr apparent ages. We explain the variations in the Rb/Sr isotopic character of microstructurally relic muscovite by a, presumably deformation-related, loss of Sr during the Cretaceous event. Contemporaneously, only very limited amounts of isotopically different Sr from the matrix reservoir might possibly have entered the muscovite. Consequently, the Rb/Sr of the relic muscovite is affected by a net loss of Sr. The results imply that at temperatures of < 500 °C, deformation is supposed to be the predominant factor in controlling the Rb/Sr geochronometer of relic muscovite, by significantly reducing the characteristic length scale for volume diffusion. However, variations of the major and trace element compositions within Permian relic muscovite are interpreted to rather reflect primary compositional instead of deformation-related variations.

Structure evolution of h.c.p./c.c.p. metal oxide interfaces in solid-state reactions.

The structure of crystalline interfaces plays an important role in solid-state reactions. The Al2O3/MgAl2O4/MgO system provides an ideal model system for investigating the mechanisms underlying the migration of interfaces during interface reaction. MgAl2O4 layers have been grown between Al2O3 and MgO, and the atomic structure of Al2O3/MgAl2O4 interfaces at different growth stages was characterized using aberration-corrected scanning transmission electron microscopy. The oxygen sublattice transforms from hexagonal close-packed (h.c.p.) stacking in Al2O3 to cubic close-packed (c.c.p.) stacking in MgAl2O4. Partial dislocations associated with steps are observed at the interface. At the reaction-controlled early growth stages, such partial dislocations coexist with the edge dislocations. However, at the diffusion-controlled late growth stages, such partial dislocations are dominant. The observed structures indicate that progression of the Al2O3/MgAl2O4 interface into Al2O3 is accomplished by the glide of partial dislocations accompanied by the exchange of Al3+ and Mg2+ cations. The interface migration may be envisaged as a plane-by-plane zipper-like motion, which repeats along the interface facilitating its propagation. MgAl2O4 grains can adopt two crystallographic orientations with a twinning orientation relationship, and grow by dislocations gliding in opposite directions. Where the oppositely propagating partial dislocations and interface steps meet, interlinked twin boundaries and incoherent Σ3 grain boundaries form. The newly grown MgAl2O4 grains compete with each other, leading to a growth selection and successive coarsening of the MgAl2O4 grains. This understanding could help to interpret the interface reaction or phase transformation of a wide range of materials that exhibit a similar h.c.p./c.c.p. transition.