Enstatite achondrites as indicators of processes and environments in the early solar system

This thesis investigates the processes and environments of formation of the anomalous enstatite achondrites NWA 4301, Zakłodzie, and NWA 8173 to determine how they relate to other enstatite meteorites (chondrites, impact melt rocks, aubrites, and so-called “primitive enstatite achondrites”). Observations of Zakłodzie and NWA 4301 showed both meteorites may share a common parent body, but experienced different metamorphic conditions. Different zonation patterns in plagioclase, silica polymorphs and associations, and sulfide assemblages indicate Zakłodzie experienced higher metamorphic temperatures and cooled faster than NWA 4301, therefore NWA 4301 was buried deeper in the parent body relative to Zakłodzie. In NWA 8173, mineral assemblages and textures indicate it formed at high temperatures, and underwent subsolidus annealing. Detailed mineral, chemical and structural classification of fluorophlogopite in NWA 8173 was made, to determine the role of halogens in formation mechanisms of enstatite meteorites. Overall findings showed that incipient partial melting and annealing following impact melting can both form the anomalous enstatite meteorites. However, a model is proposed where halogens can act as a fluxing agent in the melt and are represented by NWA 8173 and cause a thermally metamorphosed rock such as Zaklodzie and NWA 4301 to melt, fractionally crystallize and ultimately differentiate

Leave no trace: A non-destructive correlative approach providing new insights into impactites and meteorites

Impact cratering is today recognized as a fundamental geological process on all rocky bodies in the solar system. On Earth, however, processes such as plate tectonics and erosion have eradicated most craters from the geological record, or they may be buried under sediments, oceans, and vegetation. The formation of a hypervelocity impact crater involves extreme pressures and temperatures that induce permanent changes in the target rocks, so-called shock-metamorphic effects, which can be used to identify and confirm impact structures. The research in this thesis focuses on the impact cratering process, both during the formation, and post-impact. A number of terrestrial impactites and meteorites were analyzed using a multi-modal approach, including correlative non-destructive neutron and X-ray imaging, and detailed 2D analysis using scanning electron microscopy and electron backscatter diffraction. The material encompasses (1) impactites from the Mien impact structure, (2) a sample of the Martian Miller Range (MIL) 03346 meteorite, (3) a Chicxulub drill core sample, (4) a sample of Libyan Desert Glass, and (5) a sample of impact melt rock from the Luizi impact structure. The first study investigated shock deformation in zircon grains from the Mien impact structure in Sweden, using electron backscatter diffraction (EBSD). The results show that several of these grains contain evidence of the former presence of a high-pressure phase that is only known from impact structures. These grains would be suitable candidates for refining the age of the impact event. In paper II, combined NCT and XCT were employed to investigate the three-dimensional distribution of hydrogen-rich material in MIL 03346, by utilizing the neutrons’ sensitivity to hydrogen. The results revealed that the hydrogen-rich material occurs in localized clusters, with limited interconnectivity between clusters. This suggests that the fluid source could be small patches of sub-surface ice and that the alteration event likely was short-lived, meaning that the source terrain of this sample was likely not habitable. In Paper III we combined XCT and NCT to test if these methods can be used to locate projectile material in impactites. After careful investigations of the 3D images, an iron-nickel silicide spherule could be pin-pointed in the Libyan Desert glass. The sample was then polished for detailed analysis using scanning electron microscopy. Overall, the non-destructive nature of XCT and NCT makes these methods highly relevant for studying rare samples, such as meteorites and returned samples

Three-dimensional features of chondritic meteorites : applying micro-computed tomography to extraterrestrial material

This work examines the application of X-ray computed tomography (XCT) in meteoritics. This powerful technique uses the attenuation of X-rays passing through a sample to map it in three dimensions, allowing for the imaging and quantification of phases and features without the need for destructive sampling. XCT is a novel method with its applications to planetary science only recently recognised and not extensively explored. As such, this study presents two examples of using XCT to both elucidate its potential, and better understand the constituents of chondritic meteorites and the processes experienced on their parent bodies. To test the reliability of XCT, the data are conjoined with standard analytical techniques. Firstly, the 3D fabric and textural properties of 17 L chondrites of varying petrological type and shock stage are described. Specifically, porosity is imaged, quantified and compared with pycnometry data. For each chondrite, the size distribution and orientations of metal grains are reconstructed and correlated with the degree and direction of anisotropy of magnetic susceptibility in the sample. Both porosity and metal grain fabrics reveal trends with progressive thermal and shock metamorphism. The mechanisms accounting for such correlation are explored. Secondly, XCT is used to survey fragments of the Barwell L6 meteorite to identify and locate igneous inclusions. From this data, several inclusions were then subsampled and further geochemically investigated, including oxygen isotopic compositions, hafnium-tungsten systematics, and trace element analysis. Studied inclusions are found to be similar in composition and age to chondrules, but depleted in metal. A possible formation scenario is proposed and the potential link to chondrule formation is discussed. Using these examples, the factors influencing the accuracy of XCT data acquisition and processing are described. The benefits and limitations of the technique, with respect to the analysis of extraterrestrial material and implications for future use, are also considered

CM Murchison : nebular formation of fine-grained chondrule rims followed by impact processing on the CM parent body

We examine the primitive carbonaceous chondrite, CM Murchison, to infer details concerning its formation and subsequent processing on the CM parent body. We use X-ray computed tomography (XCT) to measure the 3D morphology and spatial relationship of fine-grained rims (FGRs) of Type I chondrules and find that the relationship between FGR volume and interior core radius is well described by a power law function as proposed for FGR accretion in a turbulent nebula by Cuzzi (2004). We also find evidence that the rimmed chondrules were slightly larger than Kolmogorov-stopping-time nebular particles. Evidence against parent body FGR formation includes a positive correlation between rim thickness and chondrule size and no correlation between interior chondrule roughness (used as a proxy for degree of aqueous alteration) and FGR volume. We find that the chondrules are foliated and that the FGRs are compressed in the direction of maximum stress, resulting in rims that are consistently thicker in the plane of foliation. After accretion to the CM parent body, the material within Murchison experienced brittle deformation, porosity loss, and aqueous alteration. XCT reveals that partially altered chondrules define a prominent foliation and weak lineation. The presence of a lineation and evidence for a component of rotational, noncoaxial shear suggest that the deformation was caused by impact. Olivine optical extinction indicates that the sample is classified as shock stage S1 and electron microscopy reveals that plastic deformation was minimal and that brittle deformation was the dominant microstructural strain-accommodating mechanism. Evidence such as serpentine veins parallel to the foliation fabric and crosscutting alteration veins strongly suggest that some aqueous alteration post-dated or was contemporaneous with the deformation and that multiple episodes of fracturing and mineralization occurred. Finally, using the deformed shape of the chondrules we estimate the strain and infer that the original bulk porosity of Murchison before deformation was 32.2 – 53.4%. Our findings suggest that significant porosity loss, deformation, and compaction from impact can occur on chondrite parent bodies whose samples record only a low level of shock, and that significant chondrule deformation can result from brittle processes and does not require plastic deformation of grains.Geological Science

The Influence of SPetrology of Chondrule Precursors and Sorting of Particles in Ordinary Chondrites

This dissertation is an investigation of two processes of fundamental cosmochemical importance: chondrule formation and chondrule sorting. The first two parts address chondrule formation, while the second two address chondrule sorting. The four parts are each self-contained papers that have been or are in the process of being published. In Part 1, experimental work on the ordinary chondrite QUE97008 is used to develop a set of textural criteria by which a chondrule’s degree of partial melting can be qualitatively determined and to test the validity of quantitative measures of degree of melting. In Part 2 the textural criteria developed in Part 1 are used to inventory chondrule precursors by finding natural chondrules that have experienced minimal degrees of partial melting. We show that chondrule precursors are similar mineralogically and chemically to the general chondrule population, implying that chondrule recycling was ubiquitous in the presolar nebula. In Part 3, X-ray computed technology (CT) data are used to develop a dataset of size and shape measurements for chondrules and metal/sulfide grains in ordinary chondrites. We show that chondrules are in general not spherical, and compare size and shape measurements of chondrules and metal grains to those of other authors. The dataset developed in Part 3 is applied to the study of chondrule sorting in Part 4. We test hypotheses of mass, photophoretic, and aerodynamic sorting in the nebula and assess the relationship between size sorting of chondrules and metal-silicate fractionation, one of the most fundamental fractionations in cosmochemistry

Characterising primitive chondrite components

Primitive chondrite components in six carbonaceous chondrites, Bencubbin, HaH 237, Gujba, Isheyevo, Acfer 209 and Acfer 094 were studied to examine the complex thermal histories of individual particles. Significant information about the origin and evolution of the solar nebula is contained within primitive chondrite components including FeNi metals, sulphides, matrix material and calcium aluminium inclusions, allowing conclusions to be drawn about the conditions which prevailed in the early nebula. This thesis describes the analysis of meteoritic metal and other components in carbonaceous chondrites using a suite of complementary techniques including secondary electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), secondary ion mass spectrometry (nanoSIMS), grain-size frequency distribution (GSFD) and computed tomography. Metal is chosen as the primary comparative component as it is a common feature in carbonaceous chondrites and is an indication of the extent to which a sample has been exposed to thermal, metamorphic and alteration processes. EBSD results reveal a variation between chondrule-associated metal and matrix metal in CR chondrite Acfer 209 and the ungrouped chondrite Acfer 094 indicating a difference in formation and subsequent processing. TEM results demonstrated that evidence for aqueous alteration occurs on a sub-μm scale on the rims of FeNi metal grains in Acfer 094. FeNi metallic rims displayed regions of pitting corrosion and an enrichment in O and Ni accompanied by depletion in Fe. These features indicate interaction with an aqueous fluid. Grain-size frequency distribution analyses revealed a strong and common mode in the metal grain aspect ratios of three samples from the CB group of chondrites indicating a common deformational event. The presence of adjacent primitive components with varying chemical and crystallographic textures reveals that these samples were subject to a complex thermal history. Fine-grained matrix material in HaH 237 is heavily hydrated and shows no complementarity to chondrules which escaped aqueous alteration consistent with the X-wind model. In contrast, matrix material does show compositional complementarity to chondrules in Acfer 094 and Acfer 209. This suggests material for both components formed in the same region of a nebula conforming to the shock model where material formed on the disk

ToScA North America (6 – 8 June 2017, The University of Texas, Austin, TX) Program

ToScA North America will address key areas of science, including Multi-modal Imaging, Geosciences, Forensics, Increasing Contrast, Educational Outreach, Data, Materials Science and Medical and Biological Science.University of Texas High-Resolution X-ray CT Facility (UTCT); Jackson School of Geosciences, The University of Texas at Austin; Natural History Museum (London); Royal Microscopical Society (Oxford, UK)Geological Science

Multimodal x-ray and electron microscopy of the Allende meteorite.

Multimodal microscopy that combines complementary nanoscale imaging techniques is critical for extracting comprehensive chemical, structural, and functional information, particularly for heterogeneous samples. X-ray microscopy can achieve high-resolution imaging of bulk materials with chemical, magnetic, electronic, and bond orientation contrast, while electron microscopy provides atomic-scale spatial resolution with quantitative elemental composition. Here, we combine x-ray ptychography and scanning transmission x-ray spectromicroscopy with three-dimensional energy-dispersive spectroscopy and electron tomography to perform structural and chemical mapping of an Allende meteorite particle with 15-nm spatial resolution. We use textural and quantitative elemental information to infer the mineral composition and discuss potential processes that occurred before or after accretion. We anticipate that correlative x-ray and electron microscopy overcome the limitations of individual imaging modalities and open up a route to future multiscale nondestructive microscopies of complex functional materials and biological systems

Evidence for impact induced pressure gradients on the Allende CV3 parent body: Consequences for fluid and volatile transport

Carbonaceous chondrites, such as those associated with the Vigarano (CV) parent body, exhibit a diverse range of oxidative/reduced alteration mineralogy (McSween, 1977). Although fluids are often cited as the medium by which this occurs (Rubin, 2012), a mechanism to explain how this fluid migrates, and why some meteorite subtypes from the same planetary body are more oxidized than others remains elusive. In our study we examined a slab of the well-known Allende (CV3OxA) meteorite. Using several petrological techniques (e.g., Fry's and Flinn) and Computerized Tomography (CT) we discover it exhibits a strong penetrative planar fabric, resulting from strain partitioning among its major components: Calcium–Aluminum-rich Inclusions (CAIs) (64.5%CT) > matrix (21.5%Fry) > chondrules (17.6%CT). In addition to the planar fabric, we found a strong lineation defined by the alignment of the maximum elongation of flattened particles interpreted to have developed by an impact event. The existence of a lineation could either be non-coaxial deformation, or the result of a mechanically heterogeneous target material. In the later case it could have formed due to discontinuous patches of sub-surface ice and/or fabrics developed through prior impact compaction (MacPherson and Krot, 2014), which would have encouraged preferential flow within the target material immediately following the impact, compacting pore spaces. We suggest that structurally controlled movement of alteration fluids in the asteroid parent body along pressure gradients contributed to the formation of secondary minerals, which may have ultimately lead to the different oxidized subtypes

Rapid core formation in terrestrial planets by percolative flow: in-situ imaging of metallic melt under high pressure/temperature conditions

Core formation has left a lasting geochemical signature on the Earth. In order to constrain the composition of the Earth we must fully understand the processes by which newly formed Earth, and the bodies which accreted to it, differentiated. Percolation of iron-rich melt through solid silicate has been invoked as a mechanism for differentiation and core formation in terrestrial bodies in the early solar system. However, to date the contribution of percolation to core formation cannot be assessed due to the absence of data on Fe-rich melt migration velocities. Here we use a novel experimental design to investigate textural changes in an analog system, Au melt in polycrystalline h-BN, at 3 GPa, relevant to core formation in the early solar system. Using a combination of high resolution, in-situ X-ray tomography and fast 2-D radiographic imaging, we obtain the first direct data on melt migration velocities at high PT. Melt migration is highly variable and episodic, driven by variations in differential pressure during melt migration and matrix compaction. Smaller scale melt processes, representing migration of melt along pre-existing melt networks, give comparatively fast velocities of 0.6–60 μms−1. Ex-situ experiments are used to compare melt networks in analog systems to Fe-rich melt in silicates. Two competing processes for melt migration are percolation of melt along grain boundaries, and hydraulic fracturing induced by melt injection. Typically, both processes are noted in experimental and natural systems, although the relative importance of each mechanism is variable. Using a simple model for melt flow through a porous media, migration velocities determined here account for full differentiation of Earth-sized bodies within 101–103 Myr, for submicron diameter melt bands, or within a few Myr or micron-sized melt bands. This is consistent with rapid timescales inferred from geochemistry for core formation in planetesimals, implying that percolation may have had an important contribution to core differentiation in the Earth