[Show abstract][Hide abstract] ABSTRACT: Calcium–aluminum-rich inclusions (CAIs) from the metal-rich (CH/CB-like) carbonaceous chondrite Isheyevo are mineralogically pristine and show no evidence for postcrystallization alteration. Many of them are composed of very refractory minerals, such as hibonite (CaAl 12 O 19), grossite (CaAl 4 O 7), aluminum-rich pyroxene, and perovskite (CaTiO 3). Twenty-eight out of 35 studied CAIs from Isheyevo have oxygen isotopic compositions similar to those of CAIs from the CM and CR carbonaceous chondrites (Δ 17 O ∼ −20‰). Five igneous CAIs are 16 O-depleted to a level observed in Isheyevo chondrules (Δ 17 O −10‰), suggesting remelting and isotope exchange in an 16 O-poor gaseous reservoir. Two CAIs, WA9 and B1, show the highest enrichment in 16 O (δ 17 O ∼ −68‰, δ 18 O ∼ −66‰, Δ 17 O ∼ −34‰) ever observed among refractory inclusions. In the context of the self-shielding model for the evolution of oxygen isotopes in the solar accretion disk, these CAIs may have recorded the initial oxygen isotopic composition of the solar system, and hence of the Sun.
The Astrophysical Journal 12/2015; 698:18-22. · 6.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The lunar magma ocean model is a well-established theory of the early evolution of the Moon. By this model, the Moon was initially largely molten and the anorthositic crust that now covers much of the lunar surface directly crystallized from this enormous magma source. We are undertaking a study of the geochemical characteristics of anorthosites from lunar meteorites to test this model. Rare earth and other element abundances have been measured in situ in relict anorthosite clasts from two feldspathic lunar meteorites: Dhofar 908 and Dhofar 081. The rare earth elements were present in abundances of approximately 0.1 to approximately 10× chondritic (CI) abundance. Every plagioclase exhibited a positive Eu-anomaly, with Eu abundances of up to approximately 20×CI. Calculations of the melt in equilibrium with anorthite show that it apparently crystallized from a magma that was unfractionated with respect to rare earth elements and ranged in abundance from 8 to 80×CI. Comparisons of our data with other lunar meteorites and Apollo samples suggest that there is notable heterogeneity in the trace element abundances of lunar anorthosites, suggesting these samples did not all crystallize from a common magma source. Compositional and isotopic data from other authors also suggest that lunar anorthosites are chemically heterogeneous and have a wide range of ages. These observations may support other models of crust formation on the Moon or suggest that there are complexities in the lunar magma ocean scenario to allow for multiple generations of anorthosite formation.
Philosophical transactions. Series A, Mathematical, physical, and engineering sciences. 09/2014; 372(2024).
[Show abstract][Hide abstract] ABSTRACT: Seven particles captured by the Stardust Interstellar Dust Collector and returned to Earth for laboratory analysis have features consistent with an origin in the contemporary interstellar dust stream. More than 50 spacecraft debris particles were also identified. The interstellar dust candidates are readily distinguished from debris impacts on the basis of elemental composition and/or impact trajectory. The seven candidate interstellar particles are diverse in elemental composition, crystal structure, and size. The presence of crystalline grains and multiple iron-bearing phases, including sulfide, in some particles indicates that individual interstellar particles diverge from any one representative model of interstellar dust inferred from astronomical observations and theory.
[Show abstract][Hide abstract] ABSTRACT: We report a petrographic and mineralogical survey of Paris, a new CM chondrite considered to be the least-altered CM identified so far (Hewins et al. 2014). Compared to other CMs, Paris exhibits (1) a higher concentration of Fe-Ni metal beads, with nickel contents in the range 4.1–8.1 wt%; (2) the systematic presence of thin lamellae and tiny blebs of pentlandite in pyrrhotite grains; and (3) ubiquitous tochilinite/cronstedtite associations with higher FeO/SiO2 and S/SiO2 ratios. In addition, Paris shows the highest concentration of trapped 36Ar reported so far for a CM chondrite (Hewins et al. 2014). In combination with the findings of previous studies, our data confirm the reliability of (1) the alteration sequence based on the chemical composition of tochilinite/cronstedtite associations to quantify the fluid alteration processes and (2) the use of Cr content variability in type II ferroan chondrule olivine as a proxy of thermal metamorphism. In contrast, the scales based on (1) the Fe3+ content of serpentine in the matrix to estimate the degree of aqueous alteration and (2) the chemical composition of Fe-Ni metal beads for quantifying the intensity of the thermal metamorphism are not supported by the characteristics of Paris. It also appears that the amount of trapped 36Ar is a sensitive indicator of the secondary alteration modifications experienced by chondrites, for both aqueous alteration and thermal metamorphism. Considering Paris, our data suggest that this chondrite should be classified as type 2.7 as it suffered limited but significant fluid alteration and only mild thermal metamorphism. These results point out that two separated scales should be used to quantify the degree of the respective role of aqueous alteration and thermal metamorphism in establishing the characteristics of CM chondrites.
[Show abstract][Hide abstract] ABSTRACT: Ion beam (Particle Induced X-ray Emission, or PIXE) analysis shows subtle micrometeoroid impactor compositions on samples returned from the Hubble Space Telescope.
[Show abstract][Hide abstract] ABSTRACT: The quantity of methane in Mars' atmosphere, and the potential mechanism(s) responsible for its production, are still unknown. In order to test viable, abiotic, methangenic processes, we experimentally investigated two possible impact mechanisms for generating methane. In the first suite of experiments, basaltic rocks were impacted at 5 km s−1 and the quantity of gases (CH4, H2, He, N2, O2, Ar and CO2) released by the impacts was measured. In the second suite of experiments, a mixture of water ice, CO2 ice and anhydrous olivine grains was impacted to see if the shock induced rapid serpentinization of the olivine, and thus production of methane. The results of both suites of experiments demonstrate that impacts (at scales achievable in the laboratory) do not give rise to detectably enhanced quantities of methane release above background levels. Supporting hydrocode modelling was also performed to gain insight into the pressures and temperatures occurring during the impact events.
International Journal of Astrobiology 01/2014; · 1.45 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Cation and anion impact ionization mass spectra of polypyrrole-coated pyrrhotite cosmic dust analogue particles are analysed over a range of cosmically-relevant impact speeds. Spectra with mass resolutions of 150–300 were generated by hypervelocity impacts of charged particles, accelerated to up to 37 km s−1 in a Van de Graaff electrostatic accelerator, onto a silver target plate in the Large Area Mass Analyzer (LAMA) spectrometer. Ions clearly indicative of the polypyrrole overlayer are identified at masses of 93, 105, 117, 128 and 141 u. Organic species, predominantly derived from the thin (20 nm) polypyrrole layer on the surface of the particles, dominate the anion spectra even at high (>20kms−1) impact velocities and contribute significantly to the cation spectra at velocities lower than this. Atomic species from the pyrrhotite core (Fe and S) are visible in all spectra at impact velocities above 6 km s−1 for 56Fe+, 9 km s−1 for 32S+ and 16 km s−1 for 32S− ions. Species from the pyrrhotite core are also frequently visible in cation spectra at impact speeds at which surface ionisation is believed to dominate (<10kms−1), although the large number of organic peaks complicates the identification of characteristic molecular species. A thin oxidised surface layer on the pyrrhotite particles is indicated by weak spectral features assigned to iron oxides and iron oxy-hydroxides, although the definitive identification of sulfates and hydrated sulfates from the oxidation process was not possible. Silver was confirmed as an excellent choice for the target plate of an impact ionization mass spectrometer, as it provided a unique isotope signature for many target-projectile cluster peaks at masses above 107–109 u. The affinity of Ag towards a dominant organic fragment ion (CN−) derived from fragmentation of the polypyrrole component led to molecular cluster formation. This resulted in an enhanced sensitivity to a particular particle component, which may be of great use when investigating astrobiologically-relevant chemicals, such as amino acids.
Planetary and Space Science 01/2014; · 2.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: CR chondrites are considered as one of the most primitive classes of
meteorites. Most of them experienced a mild aqueous alteration and show
no evidence of significant effect of thermal metamorphism. We present
here a search for low degree metamorphic effects in CR chondrites. We
studied 15 CR chondrites using different metamorphic indicators: (1)
structure and Ni content of metal grains; (2) hydration state of matrix;
(3) structure and composition of organic matter. The different
metamorphic indicators show that two of the analyzed CR chondrites, GRA
06100 and GRO 03116, experienced thermal metamorphism. Indeed, all of
the metal grains in GRA 06100 and half of the metal grains in GRO 03116
show Ni-rich phases; the matrix of GRA 06100 is almost completely
dehydrated, and the matrix of GRO 03116 is partially dehydrated; Raman
spectra of organic matter in these two meteorites are clearly different
from those obtained for organic matter in the other CR chondrites, which
resemble Raman spectra of organic matter in unmetamorphosed, CM2
meteorites; IR spectra of insoluble organic matter extracted from GRA
06100 and GRO 03116 show lower carbonyl abundance and higher
CH2/CH3 ratio with respect to organic matter of
unmetamorphosed chondrites. The other CR chondrites analyzed here lack
these characteristics and only show a few metal grains with Ni-rich
inclusions. Our results also show that the metamorphic effects observed
in GRA 06100 and GRO 03116 are different from those observed in type 3
chondrites, which experienced long-duration metamorphism of radiogenic
origin. We infer that thermal processing in these two CRs extended over
a short duration and was triggered by impacts.
Geochimica et Cosmochimica Acta 12/2013; · 3.88 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Olivine, (Mg, Fe)2[SiO4], is a common mineral in
extraterrestrial materials, whose Mg-Fe content varies from the
end-members Forsterite (Mg2SiO4: denoted 'Fo') to
Fayalite (Fe2SiO4: denoted 'Fa'), together with
minor quantities of Ca, Cr, Mn and Ni. Olivine is readily identified by
Raman spectroscopy, and the Mg-Fe content can be obtained by precise
measurements of the position of the two strongest Raman peaks. Here we
show that this is not only true for pristine and highly crystalline
olivine, but also for grains which have undergone high pressure shock
processing during hypervelocity impact. We demonstrate that there are
subtle changes to the Raman spectra in grains impacted at 6.1 km
s-1 onto aluminium foil and into low density aerogel. We
quantify these changes, and also show that if no correction is made for
the impact effects, the Fe:Mg molar ratio of the olivine can be
significantly misinterpreted. This study was stimulated by NASA's
Stardust mission to comet 81P/Wild-2, since freshly ejected cometary
dust particles were collected (via impact) onto aluminium foil and into
aerogel cells at 6.1 km s-1 and these samples are being
investigated with Raman spectroscopy. We identify the residue in one
Stardust impact crater on aluminium foil as arising from an olivine with
a composition of Fo97-100.
Geochimica et Cosmochimica Acta 11/2013; · 3.88 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report trace element analyses by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of metal grains from nine different CR chondrites, distinguishing grains from chondrule interior (“interior grains”), chondrule surficial shells (“margin grains”), and the matrix (“isolated grains”). Save for a few anomalous grains, Ni-normalized trace element patterns are similar for all three petrographic settings, with largely unfractionated refractory siderophile elements and depleted volatile Au, Cu, Ag, S. All three types of grains are interpreted to derive from a common precursor approximated by the least-melted, fine-grained objects in CR chondrites. This also excludes recondensation of metal vapor as the origin of the bulk of margin grains. The metal precursors were presumably formed by incomplete condensation, with evidence for high-temperature isolation of refractory platinum-group-element (PGE)-rich condensates before mixing with lower temperature PGE-depleted condensates. The rounded shape of the Ni-rich, interior grains shows that they were molten and that they equilibrated with silicates upon slow cooling (1–100 K h−1), largely by oxidation/evaporation of Fe, hence their high Pd content, for example. We propose that Ni-poorer, amoeboid margin grains, often included in the pyroxene-rich periphery common to type I chondrules, result from less intense processing of a rim accreted onto the chondrule subsequent to the melting event recorded by the interior grains. This means either that there were two separate heating events, which formed olivine/interior grains and pyroxene/margin grains, respectively, between which dust was accreted around the chondrule, or that there was a single high-temperature event, of which the chondrule margin records a late “quenching phase,” in which case dust accreted onto chondrules while they were molten. In the latter case, high dust concentrations in the chondrule-forming region (at least three orders of magnitude above minimum mass solar nebula models) are indicated.