Constraining chondrule formation conditions in ordinary, enstatite and Kakangari chondrites

Chondrites are primitive meteorites made from two major components: chondrules, small rocky spherules, embedded within fine-grained matrix. Both components formed in the protoplanetary disk, however, their formation mechanisms are not well understood. There are two fundamental questions that have not yet been answered: what process(es) formed chondrules, and what is the genetic relationship between chondrules and matrix? The purpose of my thesis is to constrain the conditions of chondrule and matrix formation. To do so, I obtained a comprehensive dataset, containing petrographic and chemical data of chondrules and matrix in ordinary (OC), enstatite (EC) and Kakangari (K) chondrites. I used this dataset to examine the textural characteristics and bulk chemistries of chondrules. A large fraction of chondrules in all chondrites are mineralogically zoned. These chondrules have olivine cores, surrounded by low-Ca pyroxene rims. Average 2D fractions are high in carbonaceous chondrites (CC; 78%), intermediate in Rumuruti (R; 41%) and OC (39%), and rather low in EC (28%) and K chondrites (19%). Due to 2D sectioning effects, 3D zoned chondrule fractions are systematically higher by factor 1.24 in CC, 1.29 in OC and 1.62 in EC. These results show that mineralogically zoned chondrules are the dominant chondrule type in most chondrites. They formed when chondrule melts interacted with surrounding nebula gas, and material from the gas was added to the chondrules. By comparing the bulk compositions of chondrules that are mineralogically zoned with those that are not, I show that gas-melt interaction was a ubiquitous process during chondrule formation in all chondrites. This process explains the origin of chondrule textures and the large variability observed in chondrule bulk compositions. Recent studies identified chondrule-matrix complementarities as key characteristics of CC and R chondrites. Various element and isotope ratios are different in chondrules and matrix, but, at the same time, solar in the bulk meteorite. This requires joint formation of chondrules and matrix from a single solar reservoir. In this thesis, the study of complementarity was expanded to Kakangari chondrites. Chondrules, matrix and bulk Kakangari have identical (solar) Mg/Si ratios as a likely result of chondrule-gas interaction, as well as element exchange between chondrules and matrix during parent body metamorphism. While not strictly complementary, I show that Kakangari chondrules and matrix are genetically linked, thereby supporting the concept of complementarity. Another chapter of this thesis examines a unique compound object found in an ordinary chondrite. It consists of a barred olivine chondrule trapped within a large, Ca,Al-rich host object. The results indicate that this object could represent a macrochondrule that formed from collisions and merging of normal-sized chondrules. It might, therefore, provide first direct evidence for a genetic link between compound chondrules and macrochondrules. Major constraints for chondrule formation conditions were specified in this thesis: chondrules were open systems and interacted with their environment, and each other. Furthermore, chondrules and matrix are genetically linked and formed in a common reservoir. Any proposed model of chondrule formation must meet these constraints

Abundance, Major Element Composition and Size of Components and Matrix in CV, CO and Acfer 094 Chondrites

The relative abundances and chemical compositions of the macroscopic components or "inclusions" (chondrules and refractory inclusions) and fine-grained mineral matrix in chondritic meteorites provide constraints on astrophysical theories of inclusion formation and chondrite accretion. We present new techniques for analysis of low count per pixel Si, Mg, Ca, Al, Ti and Fe x-ray intensity maps of rock sections, and apply them to large areas of CO and CV chondrites, and the ungrouped Acfer 094 chondrite. For many thousands of manually segmented and type-identified inclusions, we are able to assess, pixel-by-pixel, the major element content of each inclusion. We quantify the total fraction of those elements accounted for by various types of inclusion and matrix. Among CO chondrites, both matrix and inclusion Mg to Si ratios approach the solar (and bulk CO) ratio with increasing petrologic grade, but Si remains enriched in inclusions relative to matrix. The oxidized CV chondrites with higher matrix-inclusion ratios exhibit more severe aqueous alteration (oxidation), and their excess matrix accounts for their higher porosity relative to reduced CV chondrites. Porosity could accommodate an original ice component of matrix as the direct cause of local alteration of oxidized CV chondrites. We confirm that major element abundances among inclusions differ greatly, across a wide range of CO and CV chondrites. These abundances in all cases add up to near-chondritic (solar) bulk abundance ratios in these chondrites, despite wide variations in matrix-inclusion ratios and inclusion sizes: chondrite components are complementary. This "complementarity" provides a robust meteoritic constraint for astrophysical disk models

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

Mineralogy and bulk chemistry of chondrules and matrix in petrologic type 3 chondrites : implications for early solar system processes

A detailed electron microprobe (EMP) study was performed on chondrules of various textural types in four petrologic type 3 chondrites: MET 00526 (L3.05), MET 00426 (CR3.0), Kainsaz (CO3.2) and Kakangari (K3). Bulk compositions of twenty chondrules in each meteorite were determined with modal recombination analysis. This study provides a self-consistent dataset that combines chondrule textures with mineralogy and bulk chemical compositions. It allows us to make comparisons between different chondrite groups. In order to interpret the compositional relationship between chondrules and matrix, bulk matrix compositions were obtained as well, using EMP defocused beam analyses. In Chapter 1, we compare the mineralogy and bulk chemistry of chondrules and matrix in MET 00526 (L), MET 00426 (CR) and Kainsaz (CO). These three chondrites represent some of the most pristine material that formed in the solar nebula. Chondrule characteristics and the complementary relationship between the compositions of chondrules and matrix suggest open system behavior during chondrule formation, in the form of evaporation and recondensation of volatile and siderophile elements. While chondrules of the same textural types (e.g., FeO-poor (type I) and FeO-rich (type II) porphyritic chondrules) are present in all three chondrites and show similar characteristic features, there are also significant differences between the chondrite groups. This indicates that they probably formed in different regions of the solar nebula. One significant difference can be found in the Fe-Mn systematics of FeO-rich porphyritic olivine (type IIA) chondrules (Chapter 2). We also recognized that Fe-Mn systematics can be used to identify relict grains in type IIA chondrules. Chapter 3 deals with the chondrite Kakangari, which has been thought of as a very pristine chondrite in previous studies. Our study reveals that it records a complex series of events including reduction, thermal metamorphism, sulfidization and low-temperature aqueous alteration. Kakangari chondrules, as they are preserved in the meteorite, are quite different from chondrules in unequilibrated ordinary and carbonaceous chondrites. Kakangari appears to have undergone processing similar to that experienced by the enstatite chondrites

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

Formation conditions of plagioclase-bearing type I chondrules in CO chondrites : a study of natural samples and experimental analogs

Chondrites are samples from undifferentiated asteroids that contain components that formed in the early solar system. One of the components found in chondrites is chondrules, which are small igneous spherules that formed from the melting of precursor dust ball assemblages during very short, high-temperature events in the solar nebula. Chondrules typically contain olivine and pyroxene, a glassy mesostasis, Fe-Ni metal, and sulfides. Type I chondrules contain FeO-poor and volatile-poor mineral assemblages. Approximately 10% of type I chondrules, including type IA, IAB, and IB textures, also contain igneous plagioclase. While dynamic cooling experiments have put constraints on the formation conditions of chondrules based on olivine and pyroxene textures and morphologies, plagioclase has not been produced in previous experimental chondrule analogs. In this study, we investigated common chondrule textures in CO chondrites and determined mineral and bulk compositions for plagioclase-bearing type I chondrules in CO chondrites. We also performed one-atmosphere dynamic cooling experiments in order to establish formation conditions for type I chondrules. We attempted to optimize conditions for plagioclase nucleation and growth by conducting experiments at slow cooling rates, low quench temperatures, in the presence of a Na-rich atmosphere, and with anorthite seeds present in the starting material. Experimental run products closely resemble textures of type I chondrules in ordinary and carbonaceous chondrites. Olivine is commonly poikilitically enclosed in euhedral low-Ca pyroxene. Ca-pyroxene appears as overgrowths on larger low-Ca pyroxene grains. Compositions of olivine, pyroxene, and mesostasis from experimental charges are also very similar to those observed in natural chondrules. Therefore, peak temperatures (1500 - 1600°C) and slow cooling rates (5 - 25°C/hr) used are plausible conditions for type I chondrule formation. These conditions are also predicted by the shock wave model for chondrule formation. While our experiments were conducted at conditions that we considered optimized for plagioclase crystallization, plagioclase was not observed in any experiment. Defining the conditions necessary for plagioclase nucleation may place important constraints on chondrule thermal histories

Mineralogical zonation in chondrules and chemical chondrule-matrix complementarities in carbonaceous and Rumuruti chondrites

Chondritic meteorites (´chondrites`) are primitive early solar system materials; the composition of chondrites—especially of CI chondrites—represent the average solar system composition. The two main components of chondrites are (i) chondrules, µm to mm-sized silicatic melt droplets, and (ii) matrix, an opaque and fine-grained unequilibrium mineral assemblage. The origin of these two constituents, especially for the heat source required for melting chondrules, is still enigmatic. In this work, the genetic link between chondrules and matrix was studied. A mineralogical zonation with olivine minerals dominating the cores and low-Ca pyroxenes at the margins are present in at least 75% of all chondrules studied in chapter 2. In total, 256 chondrules of 16 different carbonaceous and Rumuriti chondrites (R chondrites) were studied. The low-Ca pyroxene rims were formed by addition of Si to the chondrules (or their precursor) from the surrounding nebula gas, which later condensed to form matrix. Hence, chondrules were open systems and gained 3-15 wt.% material by this process. In chapters 3 and 4, bulk chondrule, matrix and bulk meteorite compositions of the recently found CM chondrite Jbilet Winselwan (JW) and of three different R chondrites were studied. Bulk chondrule and matrix compositions were obtained with the electron microprobe and bulk meteorite compositions with X-ray spectrometry. Jbilet Winselwan and the R chondrites show chemical complementarities. Thus, although bulk meteorite compositions are CI chondritc (=solar), chondrules and matrix have different compositions. The amount of matrix in the studied chondrites are at least 50 vol.%. All chondrites have higher than bulk ratios of Fe/Mg, Si/Mg, Al/Ti, Al/Ca in the matrices and vice versa in chondrules. Bulk chondrite ratios are (except Si/Mg in R chondrites) CI chondritic. These complementarities, together with the solar bulk composition of the meteorites, can only be explained when chondrules and matrix formed from a single reservoir. In chapter 4, complementarity is, to my knowledge, for the first time reported in non-carbonaceous chondrites. It is assumed that carbonaceous and non-carbonaceous chondrites formed in distinct regions within the solar system. A joint formation of chondrules and matrix is required for both reservoirs

Microtextural Studies of Feldspar in Ordinary Chondrites

Ordinary chondrites contain an important record of events that took place during the earliest period of solar system evolution. These include primary processes, such as chondrule formation, and secondary processes, those that affected asteroids after accretion and modified primary components. Secondary processes include aqueous alteration, thermal metamorphism, and shock effects from impact events. Secondary minerals can provide insight into the chemical and physical conditions that affected their parent asteroids. Feldspar is known to be a secondary mineral that crystallized during thermal metamorphism. The goal of this work is to use the formation and evolution of feldspar to elucidate the conditions of secondary processing on the ordinary chondrite parent asteroids. I show the common occurrence of primary feldspar in chondrules and reveal ubiquitous evidence for widespread metasomatism recorded by feldspar, which has not been fully recognized previously. Chapter 1 provides an overview of the effects of metamorphism in the L group of ordinary chondrites, as observed in feldspar as well as secondary phosphate minerals. I show that metamorphism of secondary minerals is similar in H, L, and LL groups of ordinary chondrites. Chapter 2 presents a study of the minimally metamorphosed ordinary chondrite Semarkona. I show that primary igneous plagioclase, with a wide range of compositions, is present within chondrules. Chapter 3 then follows the formation and alteration of feldspar in chondrules through the metamorphic sequence. I observe abundant evidence for metasomatism in feldspar, particularly in altered calcic plagioclase, crystallized secondary albite, and exsolved of K-feldspar from primary and secondary albite. I present a three-stage model of metasomatism involving prograde hydrous alteration, dehydration near peak metamorphism, and late-stage infiltration of anhydrous fluids. In Chapter 4, I examine fine-scale exsolution lamellae of K-feldspar in albite to determine cooling rates. I find fast cooling rates at high temperatures and discuss implications for thermal histories of ordinary chondrite parent bodies. Chapter 5 explores the development of porosity in chondrules. I show that pores in chondrules are the result of dissolution of feldspar and mesostasis glass, and that their existence facilitated fluid flow and chemical transport between chondrules and surrounding matrix

Hydrodynamics

The phenomena related to the flow of fluids are generally complex, and difficult to quantify. New approaches - considering points of view still not explored - may introduce useful tools in the study of Hydrodynamics and the related transport phenomena. The details of the flows and the properties of the fluids must be considered on a very small scale perspective. Consequently, new concepts and tools are generated to better describe the fluids and their properties. This volume presents conclusions about advanced topics of calculated and observed flows. It contains eighteen chapters, organized in five sections: 1) Mathematical Models in Fluid Mechanics, 2) Biological Applications and Biohydrodynamics, 3) Detailed Experimental Analyses of Fluids and Flows, 4) Radiation-, Electro-, Magnetohydrodynamics, and Magnetorheology, 5) Special Topics on Simulations and Experimental Data. These chapters present new points of view about methods and tools used in Hydrodynamics