Grain‐scale dependency of metamorphic reaction on crystal plastic strain

The Breaksea Orthogneiss in Fiordland, New Zealand preserves water‐poor intermediate and mafic igneous rocks that were partially recrystallized to omphacite granulite and eclogite, respectively, at P ≈ 1.8 GPa and T ≈ 850°C. Metamorphic reaction consumed plagioclase and produced grossular‐rich garnet, jadeite‐rich omphacite, clinozoisite and kyanite. The extent of metamorphic reaction, identified by major and trace element composition and microstructural features, is patchy on the grain and outcrop scale. Domains of re‐equilibration coincide with areas that exhibit higher strain suggesting a causal link between crystal plastic strain and metamorphic reaction. Quantitative orientation analysis (EBSD) identifies gradual and stepped changes in crystal lattice orientations of igneous phenocrysts that are surrounded by homophase areas of neoblasts, characterized by high grain boundary to volume ratios and little to no internal lattice distortion. The narrow, peripheral compositional modification of less deformed garnet and omphacite phenocrysts reflects limited lattice diffusion in areas that lacked three‐dimensional networks of interconnected low‐angle boundaries. Low‐angle boundaries acted as elemental pathways (pipe diffusion) that enhanced in‐grain element diffusion. The scale of pipe diffusion is pronounced in garnet relatively to clinopyroxene. Strain‐induced mineral transformation largely controlled the extent of high‐T metamorphic reaction under relatively fluid‐poor conditions

Cooling rates of chondrules from diffusion profiles in relict olivine grains

Chondrule cooling rates are one of the important constraints on chondrule formation and can be used to distinguish between different chondrule formation mechanisms. Here we have modelled diffusion profiles observed across the boundary between forsteritic-olivine relict grains and more fayalitic overgrowth. We show that the cooling of chondrules is complex and good model fits are produced with non-linear cooling rates, offering additional scope for constraining the origin of chondrules

Boom boom pow: Shock-facilitated aqueous alteration and evidence for two shock events in the Martian nakhlite meteorites

Nakhlite meteorites are ~1.4 to 1.3 Ga old igneous rocks, aqueously altered on Mars ~630 Ma ago. We test the theory that water-rock interaction was impact driven. Electron backscatter diffraction demonstrates that the meteorites Miller Range 03346 and Lafayette were heterogeneously deformed, leading to localized regions of brecciation, plastic deformation, and mechanical twinning of augite. Numerical modeling shows that the pattern of deformation is consistent with shock-generated compressive and tensile stresses. Mesostasis within shocked areas was aqueously altered to phyllosilicates, carbonates, and oxides, suggesting a genetic link between the two processes. We propose that an impact ~630 Ma ago simultaneously deformed the nakhlite parent rocks and generated liquid water by melting of permafrost. Ensuing water-rock interaction focused on shocked mesostasis with a high density of reactive sites. The nakhlite source location must have two spatially correlated craters, one ~630 Ma old and another, ejecting the meteorites, ~11 Ma ago

Redox-freezing and nucleation of diamond via magnetite formation in the Earth’s mantle

Diamonds and their inclusions are unique probes into the deep Earth, tracking the deep carbon cycle to >800 km. Understanding the mechanisms of carbon mobilization and freezing is a prerequisite for quantifying the fluxes of carbon in the deep Earth. Here we show direct evidence for the formation of diamond by redox reactions involving FeNi sulfides. Transmission Kikuchi Diffraction identifies an arrested redox reaction from pyrrhotite to magnetite included in diamond. The magnetite corona shows coherent epitaxy with relict pyrrhotite and diamond, indicating that diamond nucleated on magnetite. Furthermore, structures inherited from h-Fe3O4 define a phase transformation at depths of 320–330 km, the base of the Kaapvaal lithosphere. The oxidation of pyrrhotite to magnetite is an important trigger of diamond precipitation in the upper mantle, explaining the presence of these phases in diamonds

Can the magmatic conditions of the Martian nakhlites be discerned via investigation of clinopyroxene and olivine intra-crystalline misorientations?

Deformation is a near ubiquitous process that is observed within nearly all naturally forming rocks. Electron backscatter diffraction (EBSD) is a technique that enables slip-systems (a form of plastic deformation) to be inferred from intra-crystalline misorientations at a comparable scale to representative CPO analysis (≥300 crystals for the nakhlites). Extensive laboratory and studies on naturally occurring samples have identified preferential mantle condition extrinsic parameters for specific slip-system signatures within olivine and clinopyroxene. Intra-crystalline misorientation patterns for olivine and augite (high Ca-clinopyroxene) for 16 different Martian nakhlite meteorites (21 sections) were analysed and assessed against these known parameters. Investigation of high and low deformation regions within the nakhlites revealed a shift in intra-crystalline misorientation patterns for 10 of the 21 sections. Interpreted as both shock (high deformations) and emplacement (low deformation) signatures. The observed variations in deformation patterns for the two main regimes of deformation indicate heterogeneous sampling of the nakhlite ejecta crater. Our findings indicate that shock deformation is prevalent throughout the nakhlites, and that great care needs to be taken when interpreting intra-crystalline misorientations of crystals within apparent lower deformation regions

Characterization of nitride thin films by electron backscatter diffraction and electron channeling contrast imaging

In this paper we describe the use of electron backscatter diffraction (EBSD) mapping and electron channeling contrast imaging-in the scanning electron microscope-to study tilt, atomic steps and dislocations in epitaxial GaN thin films. We show results from epitaxial GaN thin films and from a just coalesced epitaxial laterally overgrown GaN thin film. From our results we deduce that EBSD may be used to measure orientation changes of the order of 0.02 degrees, in GaN thin films. As EBSD has a spatial resolution of approximate to 20 rim, this means we have a powerful technique with which to quantitatively map surface tilt. We also demonstrate that channeling contrast in electron channeling contrast images may be used to image tilt, atomic steps and threading dislocations in GaN thin films

Constraints on the emplacement of Martian nakhlite igneous rocks and their source volcano from advanced micro-petrofabric analysis

The Martian nakhlite meteorites, which represent multiple events that belong to a single magma source region represent a key opportunity to study the evolution of Martian petrogenesis. Here 16 of the 26 identified nakhlite specimens are studied using coupled electron backscatter diffraction (EBSD) and emplacement end-member calculations. EBSD was used to determine shape preferred orientation (SPO) of contained augite (high Ca-clinopyroxene) phenocrysts by considering their crystallographic preferred orientation (CPO). Parameters derived from EBSD, and energy dispersive X-ray spectroscopy (EDS) data were used in basic emplacement models to assess their dominant mechanism against three end-member scenarios: thermal diffusion, crystal settling, and crystal convection. Results from CPO analyses indicate low intensity weak-moderate CPO. In all samples, a consistent foliation within the axes of augite are observed typically coupled with a weaker lineation CPO in one of the other crystallographic axes. These CPO results agree best with crystal settling being the dominant emplacement mechanism for the nakhlites. Modelled crystal settling results identify two distinguishable groups outside of the model’s resolution indicating the presence of secondary emplacement mechanisms. Comparison of the two identified groups against petrofabric, geochemical, and age parameters indicate random variability between individual meteorites. Therefore, coupled petrofabric and emplacement modelling results identify an overarching characteristic of a dominant crystal settling emplacement mechanism for the nakhlite source volcano despite exhibiting random variation with each discharge through time

Characterisation of nano-grains in MgB<inf>2</inf> superconductors by transmission Kikuchi diffraction

© 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. We report the first application of the emerging transmission Kikuchi diffraction technique in the scanning electron microscope to investigate nano-grain structures in polycrystalline MgB2 superconductors. Two sintering conditions were considered, and the resulting differences in superconducting properties are correlated to differences in grain structure. A brief comparison to X-ray diffraction results is presented and discussed. This work focusses more on the application of this technique to reveal grain structure, rather than on the detailed differences between the two sintering temperatures