Texture Evolution of Amphiboles - a Case Study from the Mamonia Complex, Cyprus

<p>Amphibole’s ubiquitous occurrence in the lower crust and subduction zones together with its anisotropic elastic and rheological properties makes its texture evolution essential for assessing the past and current tectonic regimes. Amphibole often display a typical crystallographic preferred orientation (CPO) where the crystals [001] axes align with lineation and the [100] axes align with the normal to the foliation plane. However, this common CPO was attributed to numerous different deformation mechanisms, such as rigid body rotation, dislocation creep, or dissolution precipitation, and there yet to be found a distinct relation between amphibole CPO attributes and the prevailing deformation mechanism. Here, we present a microstructural analysis using electron backscatter diffraction (EBSD) of a highly deformed amphibolite from the metamorphic sole of Mamonia complex in Cyprus in order to investigate texture evolution in amphibole-rich samples. Samples from two localities ~40 km from each other were analyzed: ‘Agia Varvara’ (AV), and ‘Bath of Aphrodite’ (BOA). The two amphibolites show well-foliated microstructure, comprised mainly of hornblende (50-70%), and plagioclase (20-30%) grains under similar calculated P-T conditions of ~600 °C and 6 kbar. Despite the similar compositions and conditions, there are significant differences in the overall texture between the two samples. Samples from AV show strongly clustered amphibole CPO, with the [001] axis forming a strong point maximum parallel to the lineation (X-axis) and the [100] axis aligned perpendicular to foliation (Z-axis). In addition, amphiboles are aligned with the lineation with relatively curved boundaries and moderate aspect ratio (~2). For samples from BOA, amphiboles grains show two distinct CPO types: axial [001], where the [001] is aligned parallel to the shear direction while [100] and [010] oriented along the Y-Z plane, and orthorhombic, where the [001] and [100] are aligned with the lineation and normal to foliation, respectively. In addition, amphibole are tabular-shaped, elongated grains with distinctively straight boundaries and high aspect ratio of ~3.5. Comparison between the AV and BOA grains with average misorientation spread of >1° shows higher fraction for AV (35%) than BOA (13%). We interpret the textural and microstructural analysis of the amphibolites to reflect different deformation mechanisms for AV and BOA. The lack of compositional zoning within hornblende grains suggests no significant deformation by dissolution precipitation for both AV or BOA. For AV, the strong CPO, curved grains boundaries, and high ratio of grains with intragrain misorientations suggest deformation through dislocation creep. Differently, in BOA, the observations of tabular-shaped amphibole grains, the low amount of intra-grain misorientations, along with shape and crystal orientations that vary together with [001] as the rotation angle suggest deformation by rigid body rotation.</p>

Steady Rigid-Body Rotation of Cholesteric Droplets and Their Dumbbell-Shaped Aggregates Driven by Heat Flux along the Helical Axes

We investigated the steady unidirectional rotation of cholesteric (Ch) droplets driven by a heat flux. The droplets coexisted with the isotropic (Iso) phase and possessed a helical molecular arrangement. When...

Closed-form quaternion representations for rigid body rotation: application to error assessment in orientation algorithms of strapdown inertial navigation systems

Closed-form analytical representations of the rigid body orientation quaternion, angular velocity vector and the external moment vector satisfying kinematic equations and equations of motion are derived. In order to analyze errors of orientation algorithms for strapdown inertial navigation systems, reference models for specific rigid body rotation cases are formulated. Based on solutions, analytical expressions for ideal signals of angular velocity sensors in the form of quasi-coordinates are derived. For several sets of parameters, numerical implementations of the reference models are performed and trajectories in the configuration space of orientation parameters are presented. Numerical analysis of the drift error for the third-order orientation algorithm is performed. The results show that the value of the accumulated drift error using the derived two-frequency models exceeds the value of the accumulated drift error in the conventional case of a regular precession.

Structures of the substrate-binding protein YfeA in apo and zinc-reconstituted holo forms

In the structural biology of bacterial substrate-binding proteins (SBPs), a growing number of comparisons between substrate-bound and substrate-free forms of metal atom-binding (cluster A-I) SBPs have revealed minimal structural differences between forms. These observations contrast with SBPs that bind substrates such as amino acids or nucleic acids and may undergo >60° rigid-body rotations. Substrate transfer in these SBPs is described by a Venus flytrap model, although this model may not apply to all SBPs. In this report, structures are presented of substrate-free (apo) and reconstituted substrate-bound (holo) YfeA, a polyspecific cluster A-I SBP from Yersinia pestis. It is demonstrated that an apo cluster A-I SBP can be purified by fractionation when co-expressed with its cognate transporter, adding an alternative strategy to the mutagenesis or biochemical treatment used to generate other apo cluster A-I SBPs. The apo YfeA structure contains 111 disordered protein atoms in a mobile helix located in the flexible carboxy-terminal lobe. Metal binding triggers a 15-fold reduction in the solvent-accessible surface area of the metal-binding site and reordering of the 111 protein atoms in the mobile helix. The flexible lobe undergoes a 13.6° rigid-body rotation that is driven by a spring-hammer metal-binding mechanism. This asymmetric rigid-body rotation may be unique to metal atom-binding SBPs (i.e. clusters A-I, A-II and D-IV).