Geological history of the Winchcombe meteorite - A new cm Chrondrite fall

Introduction: The Mighei-like (CM) carbonaceous chondrites are the largest class of hydrated meteorites, representing collisionally derived fragments of water-rich asteroids [1,2]. Most (>95%) are breccias, whose clasts sample a range of aqueous alteration extents [3]. They can therefore act as “snapshots” recording the progression of fluidrock interaction on the CM parent body. Conversely, analysis of the material between clasts (termed cataclastic matrix) provides an opportunity to study the post-hydration history of the CM parent body, specifically its fragmentation and re-accretion. Here, we investigate both aspects of the CM chondrites’ geological history through study of the newly recovered fall: Winchcombe [4, 5]. Methods: Sixteen polished sections with a total area of 190 mm2 were generated for this work. They were studied under scanning electron microscopy (SEM) using backscattered electron (BSE) imaging, energy dispersive X-ray spectroscopy (EDX) and electron microprobe analysis (EMPA). These sections sample the two largest masses (the main mass [320 g] and the agricultural field stone [152 g]) recovered from the Winchcombe strewn field [4]. Results: Winchcombe is a breccia, composed of lithological clasts held within a cataclastic matrix. We identified eight distinct lithologies. Their aqueous alteration extents vary between intensely altered CM2.0 and moderately altered CM2.6 [6]. Although no lithology dominates, three rock types represent >70% of the studied area. Several lithologies contain abundant tochilinite-cronstedtite intergrowths (TCIs). Type-II forms with zoned textures are most common, typically they have Fe-rich rims (“FeO”/SiO2 wt.%: 1-5) and Mg-rich cores (“FeO”/SiO2 wt.%: < 1), however, forms with hollow cores or cores containing a mix of phyllosilicate and calcite or phyllosilciates and anhydrous silicate are also found. The cataclastic matrix represents ~15% of the studied area. It has a coarse, heterogenous texture and includes abundant subangular fragments. Fragments include the full range of CM chondrite components (e.g. Fe-sulphides, whole chondrules with or without fine-grained rims, olivine and pyroxene grains, serpentine, carbonate grains, TCI clusters, as well as coherent blocks of fine-grained matrix). The cataclastic matrix is, therefore, a complex mix of components, with both heavily altered and mildly altered phases found in close association. Another striking feature is the apparent low abundance (< 3 area%) of identifiable whole chondrules. Discussion and conclusions: Our data suggest that both anhydrous silicates and carbonates (T1a calcites) act as precursor phases for type-II TCI formation. Cross-cutting relationships allow the sequence of mineralization to be reconstructed. Initially, inward dissolution by Fe-rich and S-rich fluids forms rims composed of intermixed tochilinite and cronstedtite. In the intermediate stages of type-II TCI formation, further dissolution continues without concurrent precipitation, resulting in the formation of hollow structures. These voids were later infilled, most often by Mg-rich phyllosilicates. As alteration advanced, early-formed secondary phases became unstable and were either dissolved (e.g. T1a calcites) or chemically altered (e.g. TCI rims). The presence of numerous lithological clasts with variable aqueous alteration extents and abrupt boundaries found in close juxtaposition indicates that the cataclastic matrix formed by the deposition of fines, alongside larger fragments (the clasts), on or near the surface of the parent asteroid. Furthermore, the composition of the cataclastic matrix is consistent with formation by fragmentation and mixing of debris derived from the entire clast population. The cataclastic matrix is, therefore, interpreted as an impact-derived fallback breccia. Analysis of grain size and texture suggests that disruption of the original parent asteroid responded by intergranular fracture at grain sizes <100 μm, while larger phases, such as whole chondrules, splintered apart. Re-accretion formed a poorly lithified rubble-pile body. During atmospheric entry, the meteoroid broke apart with new fractures preferentially cutting through the weaker cataclastic matrix and thereby separating the Winchcombe meteoroid into its component- lithological clasts. Thus, the strength of the cataclastic matrix imparts a significant control on the survival of CM chondrite meteoroids. References: [1] McSween, 1979. GCA, 43:1761-1770. [2] Suttle et al. 2021. GCA, 299:219-256. [3] Bischoff, et al. 2017, 80th MetSoc. (Abstr.#6089), [4] Meteoritical Bulletin Database, Winchcombe entry (available at: https://www.lpi.usra.edu/meteor/metbull.php?code=74388). [5] Daly et al., (this meeting). [6] Rubin et al. 2007,GCA, 71:2361-2382

High-pressure mechanical and sonic properties of a Devonian shale from West Virginia

Static mechanical properties and sonic velocities were determined on each of four members of the Devonian shale from Columbia Gas Transmission's well 20403, Huntington, West Virginia. They were: Pressure - volume data to 4.0 GPa; Compressive strength at confining pressures up to 300 MPa, both parallel and perpendicular to bedding. Extensile strength at 100 to 700 MPa confining pressure, both parallel and perpendicular to bedding. Loading and unloading path in uniaxial strain at 20 to 500 MPa confining pressure, both parallel and perpendicular to bedding. Tensile strength at ambient pressure, parallel and perpendicular to bedding. Shear and compressional wave velocities at confining pressures up to 1000 MPa parallel, at 45/sup 0/, and perpendicular to bedding. Results are presented and discussed. 32 refs., 10 figs., 10 tabs

Permeability of generic repository rocks at simulated in situ conditions. [Comparison of Westerly granite and White Lake genissic granite]

New laboratory data are reported on the effect of confining (lithostatic) pressure, pore-water pressure, and principal stress difference on permeability of Westerly granite and White Lake gneissic granite. Permeabilities as low as 10/sup -19/ cm/sup 2/ (10/sup -11/ D) have been measured successfully, using a transient technique. Principal strains, electrical conductivity, and compressional velocity are determined simultaneously. Applied loads on the 15-cm diameter by 28-cm long test sample are controlled automatically and all data are taken by a microcomputer. Results on the gneissic granite indicate permeabilities of 10/sup -18/ to 10/sup -19/ cm/sup 2/ that appear to be unaffected either by effective pressure or by stress. The granite yields permeabilities of 4x10/sup -16/ cm/sup 2/ that decrease by a factor of two with pressure and vary by a factor of two with stress. When compared to the initial value, compressional velocities increase by 4% and conductivity decreases by 50% as pressure is increased to 50 MPa in the gneissic granite. In granite, these become 3% and 58%, respectively. At pressure, loading of the granite of 0.5 of failure stress increases conductivity by about 20%

INELASTIC PROPERTIES OF SEVERAL HIGH PRESSURE CRYSTALLINE PHASES OF H2O : ICES II, III AND V

Des cylindres polycristallins de H2O ont été déformés à des températures entre 178 K et 257 K, et pressions atteignant 500 MPa, dans les domaines de stabilité des glaces II, III, et V. La glace II est la plus dure des trois phases, ayant une résistance mécanique dans les conditions expérimentales, équivalente à celle de la glace Ih. La résistance mécanique de la glace V est un peu moindre. Celle de la glace III est extrêmement faible, et pendant des durées géologiques ce matériau se comporte effectivement comme un liquide, limité au dessous par la glace V et au dessus par la glace II ou Ih. Les relations entres ces phases sont compliquées par la métastabilité de certaines d'entres elles, la plus importante étant l'existence de la glace III dans le domaine de la glace II, même après des périodes prolongées. Même pendant la déformation à des températures aussi basses que 211 K (plus de 30 K au dessous de la température théorique d'apparition de la glace III) la transformation de III à II ne peut pas être provoquée en laboratoire.We have performed deformation experiments on cylinders of polycrystalline H2O at temperatures from 178 to 257 K at pressures to 500 MPa in the stability fields of ices II, III, and V. Ice II is the strongest of the phases, having a strength under laboratory conditions roughly comparable to that of ice Ih. Ice V is somewhat weaker than ice II. Ice III is extremely weak and over geologic times must behave essentially as a liquid bounded below by ice V and above by ice II or Ih. Phase relationships are complicated by a number of phase metastabilities, the most important of which is the existence of ice III in the ice II field for extended periods of time. Even under deformation at temperatures as low as 211 K (over 30 K below the ice III field), the transformations from III to II can not be made to happen in the laboratory

Mechanical properties of Blair dolomite

Pressure-volume, uniaxial stress loading to failure, uniaxial strain, and acoustic velocity determinations were made on Blair dolomite at confining pressures ranging to 3.5 GPa (Pa = Paschals where 10/sup 5/ Pa = 1 bar or 0.1 GPa = 1 kbar). The bulk modulus K, rapidly rises from an initial 10.4 GPa (at atmospheric pressure) to 102.0 GPa at 1 GPa pressure. At higher pressures, K remains essentially constant (110 GPa). The maximum volume change on loading is 3.9% at 3.5 GPa; the unloading closely follows the loading path. Comparison of uninxial stress tests in compression to 0.7 GPa and extension to 2.1 GPa confining pressure demonstrates that the characteristic shear stress at failure as well as the transition from brittle fracture to ductile flow is strongly dependent upon both the value of the intermediate principal stress sigma /sub 2/ and the rate of strain. The onset of dilatancy as determined in uniaxial compression occurs at about two-thirds of the failure stress. The uniaxial strain loading path is well below the failure envelope in compression. In uniaxial stress loading (compression), Young' s modulus (E) and shear modulus ( mu ) are demonstrated to be very sensitive to both confining pressure and to the level of shear stress. For example, at pressures of 0.1 MPa to 0.5 GPa, both E and mu first increase up to shear stresses of 0.05 to 0.15 GPa and then decrease at all higher stress values. These moduli are shown to be very sensitive indicators of the onset of dilatancy. Elastic moduli as derived from acoustic velocity measurements also increase with confining pressure (to 1 GPa), with the major change occurring below 0.1 GPa. All of the observations made at nonhydrostatic conditions are consistent with the closure of preexisting cracks at low pressures and low shear stresses followed by an increasing rate of crack growth as stress is increased, even at the higher corfining pressures. However, some cracks, which would normally close with hydrostatic pre ssure, remain open under uniaxial stress loading at similar mean pressures. (auth

INELASTIC PROPERTIES OF ICE Ih AT LOW TEMPERATURES AND HIGH PRESSURES

Le but de notre programme de recherche est d'étudier le comportement rhéologique de glaces soumises aux mêmes conditions que celles existant à l'intérieur de satellites des planètes extérieures afin de connaître leurs lois de déformation. Pour cela, nous avons effectué 100 essais de compression à vitesse de déformation constante pour des pressions allant jusqu'à 500 MPa et pour des températures aussi basses que 77 K. Pour P > 30 MPa, la glace Ih se fracture par instabilité de cisaillement produisant des fautes dans la direction du maximum de contrainte de cisaillement et la contrainte de fissuration est indépendante de la pression. Ce comportement inhabituel peut-être associé à des transformations de phases localisées dans les zones de cisaillement. La résistance en régime stationnaire suit des lois rhéologiques thermiquement activées décrites par des lois de puissance, avec différents paramètres d'écoulement dépendant des gammes de températures étudiées. Les lois d'écoulement seront discutées en relation avec les divers mécanismes de déformation déduits des microstructures observées optiquement et en comparaison avec d'autres travaux.The aim of our research programme is to explore the rheological behavior of H2O ices under conditions appropriate to the interiors of the icy satellites of the outer planets in order to give insight into their deformation. To this end, we have performed over 100 constant-strain-rate compression tests at pressures to 500 MPa and temperatures as low as 77 K. At P > 30 MPa, ice Ih fails by a shear instability producing faults in the maximum shear stress orientation and failure strength typically is independent of pressure. This unusual faulting behavior is thought to be connected with phase transformations localized in the shear zones. The steady-state strength follows rheological laws of the thermally-activated power-law type, with different flow law parameters depending on the range of test temperatures. The flow laws will be discussed with reference to the operating deformation mechanisms as deduced from optical-scale microstructures and comparison with other work