Search

Interphase boundaries are planar defects that separate two different minerals, which in general have different compositions and/or crystalline structures; they may play an important role as a pathway for fluids in rocks and affect their physical properties. To completely characterize interphase boundaries, one needs to define the misorientation between adjacent grains and the orientation of the grain boundary plane. The analysis performed here is limited to the misorientation characterization and the trace of the interphase boundary. Although the determination of possible orientation relationships between the two adjacent phases is routinely performed by selected-area electron diffraction in a transmission electron microscope, this method lacks statistical representativeness. With the advent of techniques like electron backscatter diffraction (EBSD), it is possible to calculate orientation relationships not only in single pairs of crystals of the same phase but in full thin sections and between different minerals. The interphase misorientation is calculated from two orientations of two adjacent crystals of different phases. A set of single misorientations is then used to calculate the misorientation distribution function (MDF), from where it is possible to identify a maximum and its crystallographic interpretation. If we then know the misorientation and the unit-cell parameters of the individual phases, the crystallographic relationships between the different phases can be described with the pairs of parallel crystallographic planes and the pairs of crystallographic directions. In this paper, I present examples of the use of interphase misorientation analysis on the transformation of calcite-aragonite, olivine-antigorite, magnetite-hematite, and on the study of orientation relationships between plagioclase-olivine-ilmenite in mid-ocean ridges gabbros (ODP Hole 735). [ABSTRACT FROM AUTHOR]

Antigorite-bearing rocks are thought to contribute significantly to the seismic properties in the mantle wedge of subduction zones. Here we present a detailed study of the microstructures and seismic properties in a sample of antigorite-olivine schist previously studied by Kern et al. (1997, 2015). We have measured crystallographic orientations and calculated the seismic properties in three orthogonal thin sections. Microstructures indicate that deformation is localized in the bands with high antigorite fractions, resulting in strong crystallographic preferred orientations (CPOs) with point maxima of poles to (100) parallel to lineation and poles to (001) to the foliation normal. Olivine CPO suggests deformation under high temperature and low stress, with a [100] fiber texture. The CPO strength varies with grain size, but is strong even in fine-grained antigorite, and larger grains tend to display higher internal misorientation. Orientation relationships between olivine and antigorite are evident in phase boundary misorientation analysis, (100) ol ||(001) atg being more frequent than [001] ol ||[010] atg . Two new orientation relationships between olivine and antigorite have been documented. Seismic velocities decrease while anisotropy increases with increasing antigorite modal content. Antigorite grain shape has a weak effect on seismic velocities, but it is important on anisotropy. Comparison between CPO-derived seismic velocities using Voigt, Reuss, Hill averages and geometric mean only showed good agreement in 1/3 of experimental velocities. If the crack porosity of 1.63% measured experimentally at 600 MPa was used in the self-consistent model with two crack orientations with planes normal to Z and Y, good match with all experimental velocities was achieved. The self-consistent model implies important crack porosity in the foliation plane at 600 MPa that reduces Vp normal to the foliation by 0.3 km/s. [ABSTRACT FROM AUTHOR]

Abstract: Folds with hinge lines subparallel to stretching lineation are common features in shear zones and represent extreme cases of fold modification. However, their microstructures and in particular their kinematic significance remain poorly understood. Here we describe the meso- and micro-scale structures and crystallographic preferred orientation (CPO) of quartz and muscovite from a metric-scale shear-modified fold from Plattjen, Saas Fee, Western Internal Alps. The studied fold is of class-2 and the samples collected around the fold have essentially the same microstructures. Quartz [c]-axes plotted in the tectonic coordinate system exhibit an apparent reverse obliquity on opposing limbs. Muscovite fabrics are controlled essentially by the orientation of {001} planes. If the CPO data are plotted in a common reference frame (e.g. either fold or geographical), all samples exhibit essentially the same patterns for both minerals. In particular, relative to the fold axis and axial surface, the reversal of quartz CPO obliquity on opposing limbs is no longer evident. The small obliquity (~15°) between the intersection lineation of the two planes of {001} of muscovite and the fold axis results in the apparent reversal of shear sense on opposite limbs when viewing the CPO related to the macroscopic fabric. The homogeneity of the microstructures and CPOs of quartz and muscovite across the fold support the interpretation that the fold developed during simultaneous activation of heterogeneous and homogeneous simple shear, with the former responsible for fold development and the latter responsible for fold rotation and the overwhelming of the heterogeneous deformation. These processes led to the observed parallelism between fold hinge line and local stretching lineation. [Copyright &y& Elsevier]

Abstract: Determinations of the LPO-derived seismic properties of iron ores were carried out in five samples with contrasting mineralogy (hematite, magnetite and quartz) and deformed in different conditions. All the samples are seismically quasi-isotropic or weakly anisotropic, which reflect (i) an absent or weak crystallographic preferred orientation (CPO) in some samples; (ii) the high modal content of magnetite, (iii) the relatively weak anisotropy of elastic stiffness of hematite single crystal. Such variables induce a low anisotropic seismic behavior even in high strained iron ores with strong preferred orientation of hematite. A plane of seismic transversal isotropy parallel to the foliation of the aggregates is developed in hematite±magnetite aggregates and in itabirites with strong CPO of hematites. In these high strained aggregates, some relationships between the crystallographic axes of hematite and the propagation velocities can be observed. The magnitudes of P and S-wave velocities derived of hematite CPO are lower than the values experimentally determined in iron ores. Such a difference probably reflects other microstructural variable which were not taken into account in the present contribution (e.g. shape preferred orientation). [Copyright &y& Elsevier]

We show that naturally-deformed hematite from the Quadrilátero Ferrífero Province, Minas Gerais, Brazil, develops CPOs by dislocation creep, strongly influenced by basal plane parallel glide, even when this is not the favored slip system. Characterization of microstructure and texture, particularly intragranular misorientations, of naturally deformed hematite aggregates by EBSD allowed us to determine the importance of different slip systems, and confirm dislocation creep as the dominant deformation mechanism. Viscoplastic self-consistent (VPSC) models were constructed to constrain the slip systems required to operate for the observed CPO to develop, and its rheological implications. Changes in the CRSS ratio of hematite prism and basal slip systems and deformation regime lead to the development of distinct patterns of hematite crystallographic orientations. The basal slip-dominated simple shear model is the only one that can develop quasi-single-crystal CPO of the kind observed in highly deformed rocks from Quadrilátero Ferrífero. Comparison between naturally deformed hematite aggregates and VPSC models shows that CPO development of hematite is strongly influenced by a highly viscoplastic anisotropy through dislocation creep on hematite basal plane. Nonetheless, our results demonstrate that even the unfavorable slip systems should be regarded when the bulk rheology of mineral aggregates is evaluated. [ABSTRACT FROM AUTHOR]

Abstract Accurate and precise dating of terrestrial impact craters is a critical requirement for correlating impacts with events such as mass extinctions. A number of isotopic systems have been applied to impact chronology but it is important to understand what an age actually represents and, thus, if it accurately represents the 'true' impact age and is suitable for use in correlation. Here we report imaging, microstructural characterisation and high spatial resolution ion microprobe U-Pb analysis of shocked zircon from the approximately 23 km-in-diameter Lappajärvi impact structure, Finland, for which a well-established 40Ar/39Ar framework exists. Microstructural analysis identified two distinct styles of shock recrystallisation: (i) granular zircon that displays multiple domains of similarly oriented neoblasts, some of which are interpreted to be the product of reversion from the high-pressure ZrSiO 4 polymorph, reidite, and (ii) granular zircon composed entirely of similarly oriented neoblasts. Only the former gave concordant U-Pb data interpreted to record the age of the impact. The U-Pb 'concordia age' reported here, 77.85 ± 0.78 Ma (1.0%; MSWD = 0.60; probability = 0.87; n = 8; 2σ; full external uncertainty), is resolvable from the previously published 'best estimate' 40Ar/39Ar age for impact melt rock (76.20 ± 0.29 Ma) and 40Ar/39Ar K-feldspar ages as young as 75.11 ± 0.36 Ma, and is therefore interpreted to more accurately reflect the age of the impact event. The resolvable disparity between the zircon U-Pb and the 40Ar/39Ar data indicates that even the oldest statistically robust 40Ar/39Ar ages obtained at medium- and large-sized impact craters may not accurately record the timing of an impact event at a kyr level. The offset between the U-Pb and 40Ar/39Ar data is interpreted to be, at least in part, a result of the zircon data recording a higher isotopic closure temperature, and the younger 40Ar/39Ar ages recording the progressive cooling of different domains of the impact structure. The Lappajärvi impact structure is the first Phanerozoic impact structure dated by U-Pb analysis of shock-recrystallised zircon to better than, or equal to, 1.0% uncertainty. This further demonstrates that well-characterised granular zircon grains are likely to have wide utility in the accurate and precise dating of terrestrial impact events. [ABSTRACT FROM AUTHOR]

Based on samples from the major high-temperature Três Rios-Além Paraíba-Pádua transpressive shear zone in the Ribeira orogenic belt, Brazil, we discuss the applicability of TitaniQ geothermometry to constrain peak temperature conditions during high-temperature mylonitization of quartzofeldspatic rocks, and explore the microfabrics formed at these conditions. We discuss various aspects of the TitaniQ method and conclude that deformation occurred at temperatures ranging from 612 to 740 ± 20 °C in the studied segment of the shear zone. This high-temperature deformation resulted in relatively large grain size, quartz ribbons and abundant intracrystalline deformation. However, the CPO fabrics are weak, and microstructures suggest that quartz deformation was accommodated by dynamic recrystallization involving grain boundary migration with subsequent grain growth, and later some subgrain rotation during exhumation. We relate the weak fabrics to diffusion processes during or immediately after dynamic recrystallization and dislocation creep, and to the effect of competing slip systems during deformation. In terms of rheology, evidence for Dauphiné twinning in our samples suggest strain softening during mylonitization, and we suggest that such twinning may add to the rheologically weak nature of quartzo-feldspatic portions of hot middle to lower crust. [ABSTRACT FROM AUTHOR]