Mineral reactions and deformation in host-inclusion settings

Stranded reactions in grain interiors are used to study the parameters controlling re-equilibration processes in metamorphic rocks. We focus on mineral host/inclusion arrangements, which were affected by re-equilibration under changing ambient conditions (P, T, X, stress) to variable degree. Using available sample material from Austroalpine basement units in the Eastern Alps, we intend to compare inclusion-domains that behaved as essentially closed systems with those which were open for material exchange with the matrix. Special focus will be placed on the interrelation of mineral reactions, chemical mass transport and deformation. Constraining the influence of structural heterogeneities such as microfractures, (sub)grain boundaries and phase boundaries on material rheology, microstructure development and mineral reactions is supposed to yield new information on the behaviour of complex reactive systems under tectonic load. The intended research relies on the characterization of microstructures, textures and mineral compositions with an emphasis on high resolution analytical techniques (electron microprobe analysis, scanning electron microscopy, electron backscatter diffraction, cathodoluminescence, dual focused ion beam technique). These methods will be applied to host/inclusion boundaries, host internal reaction fronts and potential passageways for material transport in strained and unstrained host domains. State of the art material characterization will provide the basis for thermodynamic modelling of phase equilibria, diffusion modelling and mechanical modelling in order to identify and quantify the controlling parameters during metamorphism and their interplay.

Mineral inclusions inside host crystals are widely used to infer information about evolution of rocks affected by changing conditions in the earth. Due to shielding by the host crystal, inclusions may retain information about relatively early events and conditions that may have been erased in the rock outside the host. However, information stored in inclusions can still be erased by adjustment of the inclusion properties in response to later events, a process called re-equilibration.In the joint project Nanoscale Processes and Geomaterials properties we investigated relationships between host and inclusion crystals in different kinds of rocks, in order to understand how and under which circumstances information recorded by inclusions (e.g. their chemical and isotopic composition, their shapes and sizes, and the orientation of their crystal lattices) can be altered by re-equilibration. Special attention was paid to the feedbacks between the way host crystals had been deformed and alteration of the inclusion properties listed above. We found that knowing the behavior of a host crystal during deformation is important for understanding whether the inclusions within it have experienced re-equilibration, and that the effects of re-equilibration can help us to understand the timing of the deformation and the conditions at which it occurred.Studying garnet crystals from deformed and altered igneous rocks in the Koralpe region of Austria, we found localized damage zones in the host crystal caused by cracking. Inclusions within these damaged zones adjusted their size and shape much faster than other inclusions according to changed conditions, even though the cracks themselves had sealed up before. This means that cracks in the host can have an important influence on the re-equilibration of nearby inclusions, even if they healed a long time ago. Work carried out on samples from the Alps and Greece identified the specific types of deformation happening in the rocks studied and the effects of each type on chemistry and isotope ratios inside deformed crystals. Isotope ratios are used to date rocks, so future studies can use these results to test if their deformed samples can provide age-information and to identify samples where it may be possible to date the timing of deformation itself.The orientation of the crystal lattices and shapes of inclusions relative to the orientation of the host crystal lattice has often been used to infer the origins of inclusions. Our study of crystallographic orientation relationships (CORs) in the Koralpe garnets revealed new, complex CORs, only detectable using large datasets. Unlike well-known specific CORs, which imply a fully fixed crystal orientation of the inclusions with respect to the host, statistical CORs allow for dispersion of inclusion orientations by rotation or clustering around host lattice directions. These CORs may have previously been overlooked, and with further research may provide new information on conditions during inclusion formation.