Hyperspectral mapping of alteration assemblages within a hydrothermal vug at the Haughton impact structure,Canada

Meteorite impacts on Earth and Mars can generate hydrothermal systems that alter the primary mineralogies of rocks and provide suitable environments for microbial colonization. We investigate a calcite–marcasite‐bearing vug at the ~23 km diameter Haughton impact structure, Devon Island, Nunavut, Canada, using imaging spectroscopy of the outcrop in the field (0.65–1.1 μm) and samples in the laboratory (0.4–2.5 μm), point spectroscopy (0.35–2.5 μm), major element chemistry, and X‐ray diffraction analyses. The mineral assemblages mapped at the outcrop include marcasite; marcasite with minor gypsum and jarosite; fibroferrite and copiapite with minor gypsum and melanterite; gypsum, Fe³⁺ oxides, and jarosite; and calcite, gypsum, clay, microcline, and quartz. Hyperspectral mapping of alteration phases shows spatial patterns that illuminate changes in alteration conditions and formation of specific mineral phases. Marcasite formed from the postimpact hydrothermal system under reducing conditions, while subsequent weathering oxidized the marcasite at low temperatures and water/rock ratios. The acidic fluids resulting from the oxidation collected on flat‐lying portions of the outcrop, precipitating fibroferrite + copiapite. That assemblage then likely dissolved, and the changing chemistry and pH resulting from interaction with the calcite‐rich host rock formed gypsum‐bearing red coatings. These results have implications for understanding water–rock interactions and habitabilities at this site and on Mars.     

A vacancy‐rich,partially inverted spinelloid silicate, (Mg,Fe,Si)2(Si,□)O4, as a major matrix phase in shock melt veins of the Tenham and Suizhou L6 chondrites

A new high‐pressure silicate, (Mg,Fe,Si)₂(Si,□)O₄ with a tetragonal spinelloid structure, was discovered within shock melt veins in the Tenham and Suizhou meteorites, two highly shocked L6 ordinary chondrites. Relative to ringwoodite, this phase exhibits an inversion of Si coupled with intrinsic vacancies and a consequent reduction of symmetry. Most notably, the spinelloid makes up about 30–40 vol% of the matrix of shock veins with the remainder composed of a vitrified (Mg,Fe)SiO₃ phase (in Tenham) or (Mg,Fe)SiO₃‐rich clinopyroxene (in Suizhou); these phase assemblages constitute the bulk of the matrix in the shock veins. Previous assessments of the melt matrices concluded that majorite and akimotoite were the major phases. Our contrasting result requires revision of inferred conditions during shock melt cooling of the Tenham and Suizhou meteorites, revealing in particular a much higher quench rate (at least 5 × 10³ K s^?1) for veins of 100–500 μm diameter, thus overriding formation of the stable phase assemblage majoritic garnet plus periclase.     

Quartz–coesite–stishovite relations in shocked metaquartzites from the Vredefort impact structure,South Africa

Coesite and stishovite are developed in shock veins within metaquartzites beyond a radius of ~30 km from the center of the 2.02 Ga Vredefort impact structure. This work focuses on deploying analytical field emission scanning electron microscopy, electron backscattered diffraction, and Raman spectrometry to better understand the temporal and spatial relations of these silica polymorphs. α-Quartz in the host metaquartzites, away from shock veins, exhibits planar features, Brazil twins, and decorated planar deformation features, indicating a primary (bulk) shock loading of >5 < 35 GPa. Within the shock veins, coesite forms anhedral grains, ranging in size from 0.5 to 4 μm, with an average of 1.25 μm. It occurs in clasts, where it displays a distinct jigsaw texture, indicative of partial reversion to a less dense SiO₂ phase, now represented by microcrystalline quartz. It is also developed in the matrix of the shock veins, where it is typically of smaller size (<1 μm). Stishovite occurs as euhedral acicular crystals, typically <0.5 μm wide and up to 15 μm in length, associated with clast–matrix or shock vein margin–matrix interfaces. In this context, the needles occur as radiating or subparallel clusters, which grow into/over both coesite and what is now microcrystalline quartz. Stishovite also occurs as more blebby, subhedral to anhedral grains in the vein matrix (typically <1 μm). We propose a model for the evolution of the veins (1) precursory frictional melting in a microfault (~1 mm wide) generates a molten matrix containing quartz clasts. This is followed by (2) arrival of the main shock front, which shocks to 35 GPa. This generates coesite in the clasts and in the matrix. (3) On initial shock release, the coesite partly reverts to a less dense SiO₂ phase, which is now represented by microcrystalline quartz. (4) With continued release, stishovite forms euhedral needle clusters at solid–liquid interfaces and as anhedral crystals in the matrix. (5) With decreasing pressure–temperature, the matrix completes crystallization to yield a microcrystalline quasi-igneous texture comprising quartz–coesite–stishovite–kyanite–biotite–alkali feldspar and accessory phases. It is possible that the shock vein represents the locus of a thermal spike within the bulk shock, in which case there is no requirement for additional pressure (i.e., the bulk shock was ≃35 GPa). However, if that pressure was not realized from the main shock, then supplementary pressure excursions within the vein would have been required. These could have taken the form of localized reverberations from wave trapping, or implosion processes, including pore collapse, phase change–initiated volume reduction, and melt cavitation.     

The Fukang pallasite: Characterization and implications for the history of the Main‐group parent body

We report the results of a study of the Fukang pallasite that includes measurements of bulk composition, mineral chemistry, mineral structure, and petrology. Fukang is a Main‐group pallasite that consists of semiangular olivine grains (Fo 86.3) embedded in an Fe‐Ni matrix with 9–10 wt% Ni and low‐Ir (45 ppb). Olivine grains sometimes occur in large clusters up to 11 cm across. The Fe‐Ni phase is primarily kamacite with accessory taenite and plessite. Minor phases include schreibersite, chromite, merrillite, troilite, and low‐Ca pyroxene. We describe a variety of silicate inclusions enclosed in olivine that contain phases rarely or not previously reported in Main‐group pallasites, including clinopyroxene (augite), tridymite, K‐rich felsic glass, and an unknown Ca‐Cr silicate. Pressure constraints determined from tridymite (<0.4 GPa), two‐pyroxene barometry (0.39 ± 0.07 GPa), and geophysical calculations that assume pallasite formation at the core–mantle boundary (CMB), provide an upper estimate on the size of the Main‐group parent body from which Fukang originated. We conclude that Fukang originated at the CMB of a large differentiated planetesimal 400–680 km in radius.     

Characterization of iron meteorites by scanning electron microscopy,X-ray diffraction,magnetization measurements,and Mössbauer spectroscopy: Gibeon IVA

Gibeon IVA iron meteorite fragment was characterized using optical microscopy, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), magnetization measurements, and Mössbauer spectroscopy. Optical microscopy and SEM made on the polished section of the meteorite, show the presence of α-Fe(Ni, Co) and γ-Fe(Ni, Co) phases and plessite structures. There are no troilite inclusions observed in the studied section. EDS studies indicate some variations in the Ni concentrations: (i) within the α-Fe(Ni, Co) phase in the range ~5.0 ± 0.1 – ~7.5 ± 0.1 at% and (ii) within the γ-Fe(Ni, Co) phase in the range ~26.0 ± 0.2 – ~36.1 ± 0.2 at%. The latter Ni concentration range indicates the presence of small amount of the paramagnetic γ-phase in addition to the ferromagnetic γ-phase. EDS also shows that Ni content in two plessite structures is varying in the range ~16–37 at%, which can indicate the presence of only the α₂-Fe(Ni, Co) and γ-Fe(Ni, Co) phases in the duplex plessite structure. This may be a result of the γ-phase decomposition with the incomplete martensitic transformation: γ → α₂ + γ due to a faster cooling rate. XRD indicates the presence of ~1.3 wt% of the γ-Fe(Ni, Co) phase in Gibeon VIA. The saturation magnetization moment of 185(2) emu g⁻¹ obtained also confirms the presence of phases with low and high Ni concentrations. The most appropriate fit of the Gibeon IVA Mössbauer spectrum demonstrates the presence of five magnetic sextets and one paramagnetic singlet which are assigned to the ferromagnetic α₂-Fe(Ni, Co), α-Fe(Ni, Co), γ-Fe(Ni, Co), and paramagnetic γ-Fe(Ni, Co) phases. The relative average Fe contents in these phases are: 13.4% in the α₂-Fe(Ni, Co) phase, 78.3% in the α-Fe(Ni, Co) phase, and 8.3% in the ferromagnetic and paramagnetic γ-Fe(Ni, Co) phases.     

Atom‐probe tomography and transmission electron microscopy of the kamacite–taenite interface in the fast‐cooled Bristol IVA iron meteorite

We report the first combined atom‐probe tomography (APT) and transmission electron microscopy (TEM) study of a kamacite–tetrataenite (K–T) interface region within an iron meteorite, Bristol (IVA). Ten APT nanotips were prepared from the K–T interface with focused ion beam scanning electron microscopy (FIB‐SEM) and then studied using TEM followed by APT. Near the K‐T interface, we found 3.8 ± 0.5 wt% Ni in kamacite and 53.4 ± 0.5 wt% Ni in tetrataenite. High‐Ni precipitate regions of the cloudy zone (CZ) have 50.4 ± 0.8 wt% Ni. A region near the CZ and martensite interface has <10 nm sized Ni‐rich precipitates with 38.4 ± 0.7 wt% Ni present within a low‐Ni matrix having 25.5 ± 0.6 wt% Ni. We found that Cu is predominantly concentrated in tetrataenite, whereas Co, P, and Cr are concentrated in kamacite. Phosphorus is preferentially concentrated along the K‐T interface. This study is the first precise measurement of the phase composition at high spatial resolution and in 3‐D of the K‐T interface region in a IVA iron meteorite and furthers our knowledge of the phase composition changes in a fast‐cooled iron meteorite below 400 °C. We demonstrate that APT in conjunction with TEM is a useful approach to study the major, minor, and trace elemental composition of nanoscale features within fast‐cooled iron meteorites.     

The New Peruvian Meteorite Carancas: Mössbauer Spectroscopy and X-Ray Diffraction Studies

The Carancas meteorite fell on 15 September 2007 approximately 10 km south of Desaguadero, near Lake Titicaca, Peru, producing bright lights, clouds of dust in the sky and intense detonations. The Carancas meteorite is classified as a H4–5 ordinary chondrite with shock stage S3 and a degree of weathering W0. The Carancas meteorite is characterized by well defined chondrules composed either of olivine or pyroxene. The Mössbauer spectra show an overlapping of paramagnetic and magnetic phases. The spectra show two quadrupole doublets associated to olivine and pyroxene; and two magnetic sextets, associated with the primary phases kamacite/taenite and Troilite (Fe²⁺). Metal particles were extracted from the bulk powdered samples exhibit only kamacite and small amounts of the intergrowth tetrataenite/antitaenite. X-Ray diffractogram shows the primary phases olivine, pyroxene, troilite, kamacite, diopside and albite. Iron oxides has not been detected by Mössbauer spectroscopy or XRD as can be expected for a meteorite immediately recovered after its fall.     

The micrometeorite flux to Earth during the earliest Paleogene reconstructed in the Bottaccione section (Umbrian Apennines), Italy

Based on sediment‐dispersed extraterrestrial spinel grains in the Bottaccione limestone section in Italy, we reconstructed the micrometeorite flux to Earth during the early Paleocene. From a total of 843 kg of limestone, 86 extraterrestrial spinel grains (12 grains > 63 μm, and 74 in the 32–63 μm fraction) have been recovered. Our results indicate that the micrometeorite flux was not elevated during the early Paleocene. Ordinary chondrites dominated over achondritic meteorites similar to the recent flux, but H chondrites dominated over L and LL chondrites (69%, 22%, and 9%, respectively). This H‐chondrite dominance is similar to that recorded within an enigmatic ³He anomaly (70, 27, and 3%) in the Turonian, but different from just before this ³He anomaly and in the early Cretaceous, where ratios are similar to the recent flux (~45%, 45%, and 10%). The K‐Ar isotopic ages of recently fallen H chondrites indicate a small impact event on the H‐chondrite parent body ~50 to 100 Ma ago. We tentatively suggest that this event is recorded by the Turonian ³He anomaly, resulting in an H‐chondrite dominance up to the Paleocene. Our sample spanning the 20 cm above the Cretaceous–Paleogene (K–Pg) boundary did not yield any spinel grains related to the K–Pg boundary impactor.     

Photometric,spectral phase and temperature effects on 4 Vesta and HED meteorites: Implications for the Dawn mission

Phase angle and temperature are two important parameters that affect the photometric and spectral behavior of planetary surfaces in telescopic and spacecraft data. We have derived photometric and spectral phase functions for the Asteroid 4 Vesta, the first target of the Dawn mission, using ground-based telescopes operating at visible and near-infrared wavelengths (0.4–2.5 μm). Photometric lightcurve observations of Vesta were conducted on 15 nights at a phase angle range of 3.8–25.7° using duplicates of the seven narrowband Dawn Framing Camera filters (0.4–1.0 μm). Rotationally resolved visible (0.4–0.7 μm) and near-IR spectral observations (0.7–2.5 μm) were obtained on four nights over a similar phase angle range. Our Vesta photometric observations suggest the phase slope is between 0.019 and 0.029 mag/deg. The G parameter ranges from 0.22 to 0.37 consistent with previous results (e.g., Lagerkvist, C.-I., Magnusson, P., Williams, I.P., Buontempo, M.E., Argyle, R.W., Morrison, L.V. [1992]. Astron. Astrophys. Suppl. Ser. 94, 43–71; Piironen, J., Magnusson, P., Lagerkvist, C.-I., Williams, I.P., Buontempo, M.E., Morrison, L.V. [1997]. Astron. Astrophys. Suppl. Ser. 121, 489–497; Hasegawa, S. et al. [2009]. Lunar Planet. Sci. 40. ID 1503) within the uncertainty. We found that in the phase angle range of 0° < α ? 25° for every 10° increase in phase angle Vesta’s visible slope (0.5–0.7 μm) increases 20%, Band I and Band II depths increase 2.35% and 1.5% respectively, and the BAR value increase 0.30. Phase angle spectral measurements of the eucrite Moama in the lab show a decrease in Band I and Band II depths and BAR from the lowest phase angle 13° to 30°, followed by possible small increases up to 90°, and then a dramatic drop between 90° and 120° phase angle. Temperature-induced spectral effects shift the Band I and II centers of the pyroxene bands to longer wavelengths with increasing temperature. We have derived new correction equations using a temperature series (80–400 K) of HED meteorite spectra that will enable interpretation of telescopic and spacecraft spectral data using laboratory calibrations at room temperature (300 K).     

The visible and near infrared module of EChO

The Visible and Near Infrared (VNIR) is one of the modules of EChO, the Exoplanets Characterization Observatory proposed to ESA for an M-class mission. EChO is aimed to observe planets while transiting by their suns. Then the instrument had to be designed to assure a high efficiency over the whole spectral range. In fact, it has to be able to observe stars with an apparent magnitude M_v?=?9–12 and to see contrasts of the order of 10^?4–10^?5 necessary to reveal the characteristics of the atmospheres of the exoplanets under investigation. VNIR is a spectrometer in a cross-dispersed configuration, covering the 0.4–2.5 μm spectral range with a resolving power of about 330 and a field of view of 2 arcsec. It is functionally split into two channels respectively working in the 0.4–1.0 μm and 1.0–2.5 μm spectral ranges. Such a solution is imposed by the fact the light at short wavelengths has to be shared with the EChO Fine Guiding System (FGS) devoted to the pointing of the stars under observation. The spectrometer makes use of a HgCdTe detector of 512 by 512 pixels, 18 μm pitch and working at a temperature of 45 K as the entire VNIR optical bench. The instrument has been interfaced to the telescope optics by two optical fibers, one per channel, to assure an easier coupling and an easier colocation of the instrument inside the EChO optical bench.     

Spectral reflectance properties of carbonaceous chondrites: 3. CR chondrites

Powdered samples of a suite of 14 CR and CR-like chondrites, ranging from petrologic grade 1 to 3, were spectrally characterized over the 0.3–2.5 μm interval as part of a larger study of carbonaceous chondrite reflectance spectra. Spectral analysis was complicated by absorption bands due to Fe oxyhydroxides near 0.9 μm, resulting from terrestrial weathering. This absorption feature masks expected absorption bands due to constituent silicates in this region. In spite of this interference, most of the CR spectra exhibit absorption bands attributable to silicates, in particular an absorption feature due to Fe²⁺-bearing phyllosilicates near 1.1 μm. Mafic silicate absorption bands are weak to nonexistent due to a number of factors, including low Fe content, low degree of silicate crystallinity in some cases, and presence of fine-grained, finely dispersed opaques. With increasing aqueous alteration, phyllosilicate: mafic silicate ratios increase, resulting in more resolvable phyllosilicate absorption bands in the 1.1 μm region. In the most phyllosilicate-rich CR chondrite, GRO 95577 (CR1), an additional possible phyllosilicate absorption band is seen at 2.38 μm. In contrast to CM spectra, CR spectra generally do not exhibit an absorption band in the 0.65–0.7 μm region, which is attributable to Fe³⁺–Fe²⁺ charge transfers, suggesting that CR phyllosilicates are not as Fe³⁺-rich as CM phyllosilicates. CR2 and CR3 spectra are uniformly red-sloped, likely due to the presence of abundant Fe–Ni metal. Absolute reflectance seems to decrease with increasing degree of aqueous alteration, perhaps due to the formation of fine-grained opaques from pre-existing metal. Overall, CR spectra are characterized by widely varying reflectance (4–21% maximum reflectance), weak silicate absorption bands in the 0.9–1.3 μm region, overall red slopes, and the lack of an Fe³⁺–Fe²⁺ charge transfer absorption band in the 0.65–0.7 μm region.     

Metal phases in ordinary chondrites: Magnetic hysteresis properties and implications for thermal history

Magnetic properties are sensitive proxies to characterize FeNi metal phases in meteorites. We present a data set of magnetic hysteresis properties of 91 ordinary chondrite falls. We show that hysteresis properties are distinctive of individual meteorites while homogeneous among meteorite subsamples. Except for the most primitive chondrites, these properties can be explained by a mixture of multidomain kamacite that dominates the induced magnetism and tetrataenite (both in the cloudy zone as single‐domain grains, and as larger multidomain grains in plessite and in the rim of zoned taenite) dominates the remanent magnetism, in agreement with previous microscopic magnetic observations. The bulk metal contents derived from magnetic measurements are in agreement with those estimated previously from chemical analyses. We evidence a decreasing metal content with increasing petrologic type in ordinary chondrites, compatible with oxidation of metal during thermal metamorphism. Types 5 and 6 ordinary chondrites have higher tetrataenite content than type 4 chondrites. This is compatible with lower cooling rates in the 650–450 °C interval for higher petrographic types (consistent with an onion‐shell model), but is more likely the result of the oxidation of ordinary chondrites with increasing metamorphism. In equilibrated chondrites, shock‐related transient heating events above approximately 500 °C result in the disordering of tetrataenite and associated drastic change in magnetic properties. As a good indicator of the amount of tetrataenite, hysteresis properties are a very sensitive proxy of the thermal history of ordinary chondrites, revealing low cooling rates during thermal metamorphism and high cooling rates (e.g., following shock reheating or excavation after thermal metamorphism). Our data strengthen the view that the poor magnetic recording properties of multidomain kamacite and the secondary origin of tetrataenite make equilibrated ordinary chondrites challenging targets for paleomagnetic study.     

Characterizing Io’s Pele,Tvashtar and Pillan plumes: Lessons learned from Hubble

Hubble Space Telescope/Wide Field and Planetary Camera 2 (HST/WFPC2) images of Io obtained between 1995 and 2007 between 0.24 and 0.42 μm led to the detection of the Pele plume in reflected sunlight in 1995 and 1999; imaging of the Pele plume via absorption of jovian light in 1996 and 1999; detection of the Prometheus-type Pillan plume in reflected sunlight in 1997; and detection of the 2007 Pele-type Tvashtar plume eruption in reflected sunlight and via absorption of jovian light. Based on a detailed analysis of these observations we characterize and compare the gas and dust properties of each of the detected plumes. In each case, the brightness of the plumes in reflected sunlight is less at 0.26 μm than at 0.33 μm. Mie scattering analysis of the wavelength dependence of each plume’s reflectance signature suggests that range of particle sizes within the plumes is quite narrow. Assuming a normal distribution of particle sizes, the range of mean particle sizes is ～0.035–0.12 μm for the 1997 Pillan eruption, ～0.05–0.08 μm for the 1999 Pele and 2007 Tvasthar plumes, and ～0.05–0.11 μm for the 1995 Pele plume, and in each case the standard deviation in the particle size distribution is <15%. The Mie analysis also suggests that the 2007 Tvashtar eruption released ～10⁹ g of sulfur dust, the 1999 Pele eruption released ～10⁹ g of SO₂ dust, the 1997 Pillan eruption released ～10¹⁰ g of SO₂ dust, and the 1995 Pele plume may have released ～10¹⁰ g of SO₂ dust. Analysis of the plume absorption signatures recorded in the F255W filter bandpass (0.24–0.28 μm) indicates that the opacity of the 2007 Tvashtar plume was 2× that of the 1996 and 1999 Pele plume eruptions. While the sulfur dust density estimated for the Tvashtar from the reflected sunlight data could have produced 61% of the observed plume opacity, <10% of the 1999 Pele F255W plume opacity could have resulted from the SO₂ dust detected in the eruption. Accounting for the remaining F255W opacity level of the Pele and Tvasthar plumes based on SO₂ and S₂ gas absorption, the SO₂ and S₂ gas density inferred for each plume is almost equivalent corresponding to ～2–6 × 10¹⁶ cm^?2 and 3–5 × 10¹⁵ cm^?2, respectively, producing SO₂ and S₂ gas resurfacing rates ～0.04–0.2 cm yr^?1 and 0.007–0.01 cm yr^?1; and SO₂ and S₂ gas masses ～1–4 × 10¹⁰ g and ～2–3 × 10⁹ g; for a total dust to gas ratio in the plumes ～10^?1–10^?2. The 2007 Tvashtar plume was detected by HST at ～380 ± 40 km in both reflected sunlight and absorbed jovian light; in 1999, the detected Pele plume altitude was 500 km in absorbed jovian light, but in reflected sunlight the detected height was ～2× lower. Thus, for the 1999 Pele plume, similar to the 1979 Voyager Pele plume observations, the most efficient dust reflections occurred in the region closest to the plume vent. The 0.33–0.42 μm brightness of the 1997 Pillan plume was 10–20× greater than the Pele or Tvashtar plumes, exceeding by a factor of 3 the average brightness levels observed within 200 km of 1979 Loki eruption vent. But, the 0.26 μm brightness of the 1997 Pillan plume in reflected sunlight was significantly lower than would be predicted by the dust scattering model. Presuming that the 0.26 μm brightness of the 1997 Pillan plume was attenuated by the eruption plume’s gas component, then an SO₂ gas density ～3–6 × 10¹⁸ cm^?2 is inferred from the data (for S₂/SO₂ ratios ?4%), comparable to the 0.3–2 × 10¹⁸ cm^?2 SO₂ density detected at Loki in 1979 (Pearl, J.C. et al. [1979]. Nature 280, 755; Lellouch et al., 1992), and producing an SO₂ gas mass ～3–8 × 10¹¹ g and an SO₂ resurfacing rate ～8–23 cm yr^?1. These results confirm the connection between high (?10¹⁷ cm^?2) SO₂ gas content and plumes that scatter strongly at nearly blue wavelengths, and it validates the occurrence of high density SO₂ gas eruptions on Io. Noting that the SO₂ gas content inferred from a spectrum of the 2003 Pillan plume was significantly lower ～2 × 10¹⁶ cm^?2 (Jessup, K.L., Spencer, J., Yelle, R. [2007]. Icarus 192, 24–40); and that the Pillan caldera was flooded with fresh SO₂ frost/slush just prior to the 1997 Pillan plume eruption (Geissler, P., McEwen, A., Phillips, C., Keszthelyi, L., Spencer, J. [2004a]. Icarus 169, 29–64; Phillips, C.B. [2000]. Voyager and Galileo SSI Views of Volcanic Resurfacing on Io and the Search for Geologic Activity at Europa. Ph.D. Thesis, Univ. of Ariz., Tucson); we propose that the density of SO₂ gas released by this volcano is directly linked to the local SO₂ frost abundance at the time of eruption.     

Coesite in suevites from the Chesapeake Bay impact structure

The occurrence of coesite in suevites from the Chesapeake Bay impact structure is confirmed within a variety of textural domains in situ by Raman spectroscopy for the first time and in mechanically separated grains by X‐ray diffraction. Microtextures of coesite identified in situ investigated under transmitted light and by scanning electron microscope reveal coesite as micrometer‐sized grains (1–3 μm) within amorphous silica of impact‐melt clasts and as submicrometer‐sized grains and polycrystalline aggregates within shocked quartz grains. Coesite‐bearing quartz grains are present both idiomorphically with original grain margins intact and as highly strained grains that underwent shock‐produced plastic deformation. Coesite commonly occurs in plastically deformed quartz grains within domains that appear brown (toasted) in transmitted light and rarely within quartz of spheroidal texture. The coesite likely developed by a mechanism of solid‐state transformation from precursor quartz. Raman spectroscopy also showed a series of unidentified peaks associated with shocked quartz grains that likely represent unidentified silica phases, possibly including a moganite‐like phase that has not previously been associated with coesite.     

Electron microscopy study of the iron meteorite Santa Catharina

Abstract— Characterization of the microstructural features of the metal of the Santa Catharina meteorite was performed using a variety of electron optical techniques. Sample USNM#6293 is chemically homogeneous on the micron scale and has a Ni content of 28.2 wt.%. Its microstructure is similar to that of the Twin City ataxite and contains clear taenite II, i.e., fcc taenite with domains of tetrataenite, < 10 nm in size. Sample USNM#3043 is a more typical Santa Catharina specimen with dark and light regions as observed with the light optical microscope. The dark regions are inhomogeneous and contain 45–50 wt.% Ni and 7–12 wt.% O. The light regions are homogeneous and contain 35 wt.% Ni and no detectable oxygen. The microstructure is that of cloudy zone, i.e., islands of tetrataenite, ～20 nm in size, in a honeycomb matrix. The honeycomb phase contains Ni rich oxide in the dark regions and contains metal, fcc taenite, in the light regions. The original metal structure of USNM#3043 is cloudy zone which formed during cooling into the low temperature miscibility gap of the Fe-Ni phase diagram. The dark regions were developed from the metal by selective corrosion of the honeycomb structure, transforming it into Ni containing oxides, possibly non-stoichiometric Fe₂NiO₄ while retaining the tetrataenite islands. Using the results of this study, many of the existing discrepancies concerning the microstructure of Santa Catharina can be explained.     

The first samples from Almahata Sitta showing contacts between ureilitic and chondritic lithologies: Implications for the structure and composition of asteroid 2008 TC3

Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008 TC₃). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70–80% are ureilites (achondrites) and 20–30% are various types of chondrites. Asteroid 2008 TC₃ was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 TC₃. AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (<10 μm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metal‐sulfide, as well as chondrules (~130–600 μm) and chondrule fragments. The C1 material consists of fine‐grained phyllosilicates (serpentine and saponite) and amorphous material, magnetite, breunnerite, dolomite, fayalitic olivine (Fo 28‐42), an unidentified Ca‐rich silicate phase, Fe,Ni sulfides, and minor Ca‐phosphate and ilmenite. It has similarities to CI1 but shows evidence of heterogeneous thermal metamorphism. Its bulk oxygen isotope composition (δ¹⁸O = 13.53‰, δ¹⁷O = 8.93‰) is unlike that of any known chondrite, but similar to compositions of several CC‐like clasts in typical polymict ureilites. Its Cr isotope composition is unlike that of any known meteorite. The enclosed clasts and chondrules do not belong to the C1 lithology. The olivine (Fo 75‐88), pyroxenes (pigeonite of Wo ~10 and orthopyroxene of Wo ~4.6), plagioclase, graphite, and some metal‐sulfide are ureilitic, based on mineral compositions, textures, and oxygen isotope compositions, and represent at least six distinct ureilitic lithologies. The chondrules are probably derived from type 3 OC and/or CC, based on mineral and oxygen isotope compositions. Some of the metal‐sulfide clasts are derived from EC. AhS 91A and AhS 671 are plausible representatives of the bulk of the asteroid that was lost. Reflectance spectra of AhS 91A are dark (reflectance ~0.04–0.05) and relatively featureless in VNIR, and have an ~2.7 μm absorption band due to OH^? in phyllosilicates. Spectral modeling, using mixtures of laboratory VNIR reflectance spectra of AhS stones to fit the F‐type spectrum of the asteroid, suggests that 2008 TC₃ consisted mainly of ureilitic and AhS 91A‐like materials, with as much as 40–70% of the latter, and <10% of OC, EC, and other meteorite types. The bulk density of AhS 91A (2.35 ± 0.05 g cm^?3) is lower than bulk densities of other AhS stones, and closer to estimates for the asteroid (~1.7–2.2 g cm^?3). Its porosity (36%) is near the low end of estimates for the asteroid (33–50%), suggesting significant macroporosity. The textures of AhS 91A and AhS 671 (finely comminuted clasts of disparate materials intimately mixed) support formation of 2008 TC₃ in a regolith environment. AhS 91A and AhS 671 could represent a volume of regolith formed when a CC‐like body impacted into already well‐gardened ureilitic + impactor‐derived debris. AhS 91A bulk samples do not show a solar wind component, so they represent subsurface layers. AhS 91A has a lower cosmic ray exposure (CRE) age (~5–9 Ma) than previously studied AhS stones (11–22 Ma). The spread in CRE ages argues for irradiation in a regolith environment. AhS 91A and AhS 671 show that ureilitic asteroids could have detectable ~2.7 μm absorption bands.     

Spectral reflectance properties of carbonaceous chondrites: 6. CV chondrites

Multiple reflectance spectra of 11 CV chondrites have been measured to determine spectral–compositional relationships for this meteorite class and to aid the search for CV parent bodies. The reflectance of CV chondrite spectra is variable, ranging from ～5% to 13% at 0.56 μm, and ～5% to 15% at the 0.7 μm region local reflectance maximum. Overall slopes range from slightly blue to red for powders, while slab spectra are strongly blue-sloped. With increasing average grain size and/or removal of the finest fraction, CV spectra generally become more blue-sloped. CV spectra are characterized by ubiquitous absorption features in the 1 and 2 μm regions. The 1 μm region is usually characterized by a band centered near 1.05–1.08 μm and a band or shoulder near 1.3 μm that are characteristic of Fe-rich olivine. Band depths in the 1 μm region for powdered CVs and slabs range from ～1% to 10%. The 2 μm region is characterized by a region of broad absorption that extends beyond 2 μm and usually includes band minima near 1.95 and 2.1 μm; these features are characteristic of Fe²⁺-bearing spinel. The sample suite is not comprehensive enough to firmly establish whether spectral differences exist between CV_R, CV_OxA, and CV_OxB subclasses, or as a function of metamorphic grade. However, we believe that the mineralogic and petrologic differences that exist between these classes, and with varying petrologic subtype (CV3.0–>3.7), may not be significant enough to result in measurable spectral differences that exceed spectral variations within a subgroup, within an individual meteorite, or as a function of grain size. Terrestrial weathering seems to affect CV spectra most noticeably in the visible region, resulting in more red-sloped spectra for finds as compared to falls. The search for CV parent bodies should focus on the detection of olivine and spinel absorption bands, specifically absorption features near 1.05, 1.3, 1.95, and 2.1 μm, as these are the most commonly seen spectral features of CV chondrites.     

Ordinary chondrite metallography: Part 2. Formation of zoned and unzoned metal particles in relatively unshocked H,L, and LL chondrites

Abstract— We studied the metallography of Fe‐Ni metal particles in 17 relatively unshocked ordinary chondrites and interpreted their microstructures using the results of P‐free, Fe‐Ni alloy cooling experiments (described in Reisener and Goldstein 2003). Two types of Fe‐Ni metal particles were observed in the chondrites: zoned taenite + kamacite particles and zoneless plessite particles, which lack systematic Ni zoning and consist of tetrataenite in a kamacite matrix. Both types of metal particles formed during metamorphism in a parent body from homogeneous, P‐poor taenite grains. The phase transformations during cooling from peak metamorphic temperatures were controlled by the presence or absence of grain boundaries in the taenite particles. Polycrystalline taenite particles transformed to zoned taenite + kamacite particles by kamacite nucleation at taenite/taenite grain boundaries during cooling. Monocrystalline taenite particles transformed to zoneless plessite particles by martensite formation and subsequent martensite decomposition to tetrataenite and kamacite during the same cooling process. The varying proportions of zoned taenite + kamacite particles and zoneless plessite particles in types 4–6 ordinary chondrites can be attributed to the conversion of polycrystalline taenite to monocrystalline taenite during metamorphism. Type 4 chondrites have no zoneless plessite particles because metamorphism was not intense enough to form monocrystalline taenite particles. Type 6 chondrites have larger and more abundant zoneless plessite particles than type 5 chondrites because intense metamorphism in type 6 chondrites generated more monocrystalline taenite particles. The distribution of zoneless plessite particles in ordinary chondrites is entirely consistent with our understanding of Fe‐Ni alloy phase transformations during cooling. The distribution cannot be explained by hot accretion‐autometamorphism, post‐metamorphic brecciation, or shock processing.     

The binary collision-induced second overtone band of gaseous hydrogen: modelling and laboratory measurements

Collision-induced absorption (CIA) is the major source of the infrared opacity of dense planetary atmospheres which are composed of nonpolar molecules. Knowledge of CIA absorption spectra of H₂–H₂ pairs is important for modelling the atmospheres of planets and cold stars that are mainly composed of hydrogen. The spectra of hydrogen in the region of the second overtone at 0.8 μm have been recorded at temperatures of 298 and 77.5 K for gas densities ranging from 100 to 800 amagats. By extrapolation to zero density of the absorption coefficient measured every 10 cm⁻¹ in the spectral range from 11,100 to 13,800 cm⁻¹, we have determined the binary absorption coefficient. These extrapolated measurements are compared with calculations based on a model that was obtained by using simple computer codes and lineshape profiles. In view of the very weak absorption of the second overtone band, we find the agreement between results of the model and experiment to be reasonable.     

The Erbisberg drilling 2011: Implications for the structure and postimpact evolution of the inner ring of the Ries impact crater

The 26 km diameter Nördlinger Ries is a complex impact structure with a ring structure that resembles a peak ring. A first research drilling through this “inner crystalline ring” of the Ries was performed at the Erbisberg hill (SW Ries) to better understand the internal structure and lithology of this feature, and possibly reveal impact‐induced hydrothermal alteration. The drill core intersected the slope of a 22 m thick postimpact travertine mound, before entering 42 m of blocks and breccias of crystalline rocks excavated from the Variscan basement at >500 m depth. Weakly shocked gneiss blocks that show that shock pressure did not exceed 5 GPa occur above polymict lithic breccias of shock stage Ia (10–20 GPa), with planar fractures and planar deformation features (PDFs) in quartz. Only a narrow zone at 49.20–50.00 m core depth exhibits strong mosaicism in feldspar and {102} PDFs in quartz, which are indicative of shock stage Ib (20–35 GPa). Finally, 2 m of brecciated Keuper sediments at the base of the section point to an inverse layering of strata. While reverse grading of clast sizes in lithic breccias and gneiss blocks is consistent with lateral transport, the absence of diaplectic glass and melt products argues against dynamic overthrusting of material from a collapsing central peak, as seen in the much larger Chicxulub structure. Indeed, weakly shocked gneiss blocks are rather of local provenance (i.e., the transient crater wall), whereas moderately shocked polymict lithic breccias with geochemical composition and ⁸⁷Sr/⁸⁶Sr signature similar to Ries suevite were derived from a position closer to the impact center. Thus, the inner ring of the Ries is formed by moderately shocked polymict lithic breccias likely injected into the transient crater wall during the excavation stage and weakly shocked gneiss blocks of the collapsing transient crater wall that were emplaced during the modification stage. While the presence of an overturned flap is not evident from the Erbisberg drilling, a survey of all drillings at or near the inner ring point to inverted strata throughout its outer limb. Whether the central ring of the Ries represents remains of a collapsed central peak remains to be shown. Postimpact hydrothermal alteration along the Erbisberg section comprises chloritization, sulfide veinlets, and strong carbonatization. In addition, a narrow zone in the lower parts of the polymict lithic breccia sequence shows a positive Eu anomaly in its carbonate phase. The surface expression of this hydrothermal activity, i.e., the travertine mound, comprises subaerial as well as subaquatic growth phases. Intercalated lake sediments equivalent to the early parts of the evolution of the central crater basin succession confirm a persistent impact‐generated hydrothermal activity, although for less time than previously suggested.