Topical Field III Deformation assisted re-equilibration phenomenation
Project III.1: The mechanical and chemical behavior of solid inclusions during host mineral deformation
Int. Coop.: -
Rationale: Inclusion domains in porphyroblasts may preserve assemblages of relatively early evolutionary stages of natural metamorphic rocks. We intend to study the interplay between mineral reactions in inclusion domains and host mineral deformation during metamorphic overprinting of pre-existing phase assemblages in polymetamorphic or polyphase metamorphic metapegmatites, metapelites or eclogites. Comparison of the compositional, microstructural and textural behavior of natural host-inclusion systems with results from thermodynamic, mechanical and diffusion modeling is expected to provide information on the chemo-mechanical feedback during metamorphism.
Methods: optical microscopy, EPMA, EBSD, CL, SEM, FIB, 3D reconstruction of microstructures, thermodynamic modelling, mechanical finite element modelling
Project III.2: The effect of strain heterogeneities on Rb-Sr and Sm-Nd isotopic re-equilibration in minerals and rocks
Rationale: Understanding the influence of deformation on isotopic rejuvenation is of utmost importance for the proper interpretation of age information from bulk sample isotopic analysis. Rb-Sr and Sm-Nd characteristics of minerals and rocks across outcrop scale strain gradients will be studied upon the Vinschgau Shear Zone in the Southern Ötztal-Stubai Complex (Italy). The Rb-Sr isotopic record of muscovite, biotite, apatite and feldspar as well as the Sm-Nd isotopic record of garnet, apatite and feldspar from samples with differing strain intensity will be investigated. In combination with major element mineral compositions and microstructures, possibly combined with (LA-)ICP-MS trace element data we intend to obtain information on the influence of the deformation intensity and prevailing deformation mechanisms on the extent of chemical and isotopic equilibration.
Methods: Microscopy; EPMA; TIMS (Rb-Sr & Sm-Nd isotope analysis); ICP-MS; SEM-FIB
Project III.3: Rejuvenation effects during plastic deformation / Microchemistry and microstructure/texture of accessory minerals and the effect of deformation on the retention of radiogenic isotopes
Rationale: Plastic deformation is able to induce isotope exchange processes which result in unpredictable changes in the radiogenic and stable isotope content of minerals. This unambiguously hampers our ability to reliably interpret absolute age data derived from deformed minerals. Therefore, knowledge, in both a qualitative and quantitative way, of the role of plastic deformation in this respect is vital for a geologically meaningful interpretation of the age data. We will investigate the influence of plastic deformation on rejuvenation and alteration effects on high-temperature geochronometers (U-Th-Pb) and stable isotopes (O, Hf) in accessory minerals (zircon, xenotime, monazite) in relation to the respective host minerals, degree of plastic deformation, strain amount, and primary age systematics. The results will allow us to obtain a better understanding of the underlying isotope exchange processes in accessory minerals during plastic deformation. This in turn will allow us to more directly identify individual crystals and crystal domains which should deliver reliable absolute age data.
Methods: Quantitative microstructures, in-situ and mineral separates: EBSD; Quantitative microchemistry, in-situ and mineral separates: EPMA (crystal chemistry), LA-MC-ICP-MS, SIMS (U, Th, Pb, Hf, O isotope systematics); integrated chemical stability: chemical abrasion (only mineral separates)