Hidden secrets of deformation: Impact-induced compaction within a CV chondrite

The CV3 Allende is one of the most extensively studied meteorites in worldwide collections. It is currently classified as S1—essentially unshocked—using the classification scheme of Stöffler et al. (1991), however recent modelling suggests the low porosity observed in Allende indicates the body should have undergone compaction-related deformation. In this study, we detail previously undetected evidence of impact through use of Electron Backscatter Diffraction mapping to identify deformation microstructures in chondrules, AOAs and matrix grains. Our results demonstrate that forsterite-rich chondrules commonly preserve crystal-plastic microstructures (particularly at their margins); that low-angle boundaries in deformed matrix grains of olivine have a preferred orientation; and that disparities in deformation occur between chondrules, surrounding and non-adjacent matrix grains. We find heterogeneous compaction effects present throughout the matrix, consistent with a highly porous initial material. Given the spatial distribution of these crystal-plastic deformation microstructures, we suggest that this is evidence that Allende has undergone impact-induced compaction from an initially heterogeneous and porous parent body. We suggest that current shock classifications (Stöffler et al., 1991) relying upon data from chondrule interiors do not constrain the complete shock history of a sample

The Sequence of Carbonate Mineralization in the CM Carbonaceous Chondrites Revealed by the Winchcombe Meteorite

No abstract available

Using crystal-lattice distortion data for geological investigations: the weighted Burgers vector method

Distorted crystals carry useful information on processes involved in their formation, deformation and growth. The distortions are accommodated by geometrically necessary dislocations, and therefore characterising those dislocations is an informative task, to assist in, for example, deducing the slip systems that produced the dislocations. Electron backscatter diffraction (EBSD) allows detailed quantification of distorted crystals and we summarise here a method for extracting information on dislocations from such data. The weighted Burgers vector (WBV) method calculates a vector at each point on an EBSD map, or an average over a region. The vector is a weighted average of the Burgers vectors of dislocation lines intersecting the map surface. It is weighted towards dislocation lines at a high angle to the map but that can be accounted for in interpretation. The method is fast and does not involve specific assumptions about dislocation types; it assumes only that elastic strains have little effect on the calculation. It can be used, with care, to analyse subgrain walls (sharp orientation changes) as well as gradational orientation changes within individual grains. There are four linked parts to this contribution. We describe the mathematical background to the WBV and then how it is modified to deal with spaced, discrete orientation measurements. EBSD orientation data have angular errors, and so does the WBV. We present a new analysis of these angular errors, showing there is a trade-off between directional accuracy and area sampled. Angular errors can now be accounted for during testing of hypotheses about dislocation types. We present new studies on olivine and plagioclase to illustrate how to use the method. We discuss published studies on ice and titanite to further illustrate the method. We note that the methods discussed here are applicable to any crystalline material encompassing minerals (including ice), metals and ceramics

A Morphologic and Crystallographic Comparison of CV Chondrite Matrices

Meteoritic matrices are commonly classified by their modal mineralogy, alteration, and shock levels. Other “textural” characteristics are not generally considered in classification schemes, yet could carry important information about their genesis and evolution. Terrestrial rocks are routinely described by grain morphology, which has led to morphology-driven classifications, and identification of controlling processes. This paper investigates three CV chondrites—Allende (CV3.2oxA), Kaba (CV3.0oxB), and Vigarano (CV3.3red)—to determine the morphologic signature of olivine matrix grains. 2D grain size and shape, and crystallographic preferred orientations (CPOs) are quantified via electron backscatter diffraction mapping. Allende contains the largest and most elongate olivine grains, while Vigarano contains the least elongate, and Kaba contains the smallest grains. Weak but notable CPOs exist in some regions proximal to chondrules and one region distal to chondrules, and CPO geometries reveal a weak flattening of the matrix grains against the edge of chondrules within Allende. Kaba contains the least plastically deformed grains, and Allende contains the most plastically deformed grains. We tentatively infer that morphology is controlled by the characteristics of the available population of accreting grains, and aqueous and thermal alteration of the parent body. The extent of overall finite deformation is likely dictated by the location of the sample with respect to compression, the localized environment of the matrix with respect to surrounding material, and the post deformation temperature to induce grain annealing. Our systematic, quantitative process for characterizing meteorite matrices has the potential to provide a framework for comparison within and across meteorite classes, to help resolve how parent body processing differed across and between chondritic asteroids

A Morphologic and Crystallographic Comparison of CV Chondrite Matrices

No abstract available

Transmission Kikuchi Diffraction Applied to Primitive Grains in Meteorites

No abstract available

Transmission Kikuchi Diffraction Applied to Primitive Grains in Meteorites

No abstract available

Evidence for Flow and Gravity Settling in the Parent Lavas of the Nakhlite (Martian) Meteorites from Crystal Textures and Fabrics

The nakhlite meteorites are Martian igneous rocks that sample at least four lava flows erupted between ~1,416 and 1,332 Ma1 . They contain phenocrysts of augite and olivine, between which is a finely crystalline mesostasis. The augite phenocrysts are coarse and prismatic, and so may be ideal for recording processes that operated during emplacement. Here we evaluated the evidence for flow and gravitational settling by quantifying the crystallographic preferred orientations of phenocrysts via large area electron backscatter diffraction (LA-EBSD) mapping of thin sections of three nakhlites: Nakhla, Governador Valadares and Miller Range (MIL) 03346. LA-EBSD analyses reveal a moderate alignment of the long axis (<c>) of augite phenocrysts in a plane forming a magmatic foliation in all samples, as well as a moderate linear alignment with maxima of the <a>, <b> and <c> axes in the general foliation plane. The linear alignment is most pronounced in those meteorites with a low ratio of mesostasis to phenocrysts: alignment increases from MIL 03346 to Grosvenor Valadares to Nakhla. Lineations defined by elongate minerals in lavas can be generated by shear stresses associated with flow2 , while foliations are attributed to gravity settling3 and compaction. Here we have observed good evidence for both processes, thus confirming that the nakhlites do represent lava flows, with an associated gravitational component

Crystallography of Refractory Metal Nuggets Identified Within Ca-Al-Rich Inclusions, Chondrules and Matrix of Carbonaceous Chondrites

No abstract available