[Show abstract][Hide abstract] ABSTRACT: We report on the possible correlation between localized (μm-scale) aqueous alteration and the spatial variation of presolar grain abundances in CO3.0 chondrites.
46th Lunar and Planetary Science Conference, The Woodlands, Texas; 03/2015
[Show abstract][Hide abstract] ABSTRACT: A simple procedure for solution-based self-assembly of C60 fullerene nanorods on graphene substrates is presented. Using a combination of electron microscopy, X-ray diffraction and Raman spectroscopy, it is shown that the size, shape and morphology of the nanorods can be suitably modified by controlling the kinetics of self-assembly.
Chemical Communications 12/2014; 51(10). DOI:10.1039/C4CC09362C · 6.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To better understand the formation conditions of ferromagnesian chondrules from the Renazzo-like carbonaceous (CR) chondrites, a systematic study of 210 chondrules from 15 CR chondrites was conducted. The texture and composition of silicate and opaque minerals from each observed FeO-rich (type II) chondrule, and a representative number of FeO-poor (type I) chondrules, were studied to build a substantial and self-consistent data set. The average abundances and standard deviations of Cr2O3 in FeO-rich olivine phenocrysts are consistent with previous work that the CR chondrites are among the least thermally altered samples from the early solar system. Type II chondrules from the CR chondrites formed under highly variable conditions (e.g., precursor composition, redox conditions, cooling rate), with each chondrule recording a distinct igneous history. The opaque minerals within type II chondrules are consistent with formation during chondrule melting and cooling, starting as S- and Ni-rich liquids at 988–1350 °C, then cooling to form monosulfide solid solution (mss) that crystallized around olivine/pyroxene phenocrysts. During cooling, Fe,Ni-metal crystallized from the S- and Ni-rich liquid, and upon further cooling mss decomposed into pentlandite and pyrrhotite, with pentlandite exsolving from mss at 400–600 °C. The composition, texture, and inferred formation temperature of pentlandite within chondrules studied here is inconsistent with formation via aqueous alteration. However, some opaque minerals (Fe,Ni-metal versus magnetite and panethite) present in type II chondrules are a proxy for the degree of whole-rock aqueous alteration. The texture and composition of sulfide-bearing opaque minerals in Graves Nunataks 06100 and Grosvenor Mountains 03116 suggest that they are the most thermally altered CR chondrites.
[Show abstract][Hide abstract] ABSTRACT: Here we report microchemical and microstructural features indicative of space weathering in a particle returned from the surface of asteroid Itokawa by the Hayabusa mission. Space weathering features include partially and completely amorphous rims, chemically and structurally heterogeneous multilayer rims, amorphous surface islands, vesiculated rim textures, and nanophase iron particles. Solar-wind irradiation is likely responsible for the amorphization as well as the associated vesiculation of grain rims. The multilayer rims contain a nanocrystalline outer layer that is underlain by an amorphous inner layer, and both have compositions that are distinct from the underlying, crystalline orthopyroxene grain. The multilayer rim features could be derived from either radiation-induced sputter deposition or vapor deposition from micrometeorite impact events. The amorphous islands on grain surfaces have a distinctive morphology and composition suggesting that they represent surface deposits of melt derived from micrometeorite impact events. These observations indicate that both irradiation damage and micrometeorite impacts play a role in surface modification and space weathering on asteroid Itokawa.
[Show abstract][Hide abstract] ABSTRACT: The adsorption of simple organic compounds onto minerals that are known to occur in the early solar nebula such as olivine, spinel and water-ice, is examined using first-principles density functional theory. The calculations show that electron-rich organics and organics containing cyanide, amine and carboxylic functional groups can strongly bind to low-index surfaces of olivine and spinel. Based on the surface coverage as obtained from these calculations, it can be inferred that an estimated amount of 1013 kg1013 kg of organics could have been delivered to early Earth via direct adsorption mechanisms, thereby providing an endogenous source of planetary organics. In addition, adsorption of organic compounds on magnesite, a carbonate phase believed to have formed via aqueous processes on asteroidal bodies, is also studied. The adsorption behavior of the organics is observed to be similar in both cases, i.e., for minerals that formed during the earliest stages of nebular evolution through condensation (spinel and olivine) or other processes and for those that formed via hydration processes on asteroidal bodies (magnesite). These results suggest that direct incorporation via adsorption is an important delivery mechanism of organics to both asteroidal bodies and terrestrial planets.
[Show abstract][Hide abstract] ABSTRACT: Introduction: Because of their higher susceptibility to heating and alteration processes than other types of circumstellar grains, presolar silicates can be a useful tool for understanding secondary processes (e.g., thermal metamorphism, aqueous alteration) acting in the early solar system history . Using a multi-technique approach, we are studying the effect of aqueous alteration on the abundances and elemental compositions of presolar silicates in distinct fine-grained areas in CO3.0 chondrites. These meteorites have experienced only minimal secondary processing [2, 3] and, thus, provide direct information on the processes affecting fine-grained material in the early solar system. Experimental methods: We have carried out additional NanoSIMS raster ion imaging of 12,13 C and 16,17,18 O in multicollection mode in LAP 031117 and DOM 08006 to identify presolar grains. We mapped a total of 12,800 µm 2 in LAP 031117 and 14,900 µm 2 in DOM 08006 (each image = 10×10 µm 2). We will use the Auger Nanoprobe to determine the elemental composition of the presolar grains identified. We will also carry out FIB-TEM analysis as described in  to characterize the mineralogy and degree of alteration of several fine-grained areas. Results and discussion: We identified a total of 51 O-anomalous grains and six C-anomalous grains in the matrix of DOM 08006, corresponding to abundances of 216 ± 30 ppm and 31 ± 13 ppm, respectively. Our estimate is consistent with a previous estimate by  (240 ± 25 ppm) but significantly higher than the abundance in a fine-grained chondrule rim (FGCR, 45 ± 23 ppm) in this meteorite. We previously reported the observation of systematic differences in the O-anomalous grain abundances between the matrix and FGCRs in CO3.0 chondrites (LAP 031117, ALHA77307 and DOM 08006) . Further comparison of the presolar grain abundances between distinct matrix areas in LAP 031117 shows that there are also large variations of the O-anomalous grain abundances (up to ∼200 ppm) between distinct matrix areas. We carried out initial FIB-TEM analysis of two areas to try to understand the reason for those variations. Our initial TEM analysis of a matrix area and a FGCR in LAP 031117 indicated that, while the matrix area is mostly composed of anhydrous amorphous material, the FGCR shows clear evidence of aqueous alteration, with the presence of phyllosilicates. Those results provide a good explanation for the variations in presolar grain abundances, with a higher abundance in the more pristine matrix area (291 ± 54 ppm) than in the aqueously altered FGCR (98 ± 25 ppm). These observations suggest that the spatial variation of presolar grain abundances in LAP 031117 might reflect various degrees of aqueous alteration. We will acquire FIB-TEM data in additional areas to confirm this initial observation.
77th Annual Meteoritical Society Meeting, Casablanca, Morocco; 09/2014
[Show abstract][Hide abstract] ABSTRACT: Insight into the presolar and interstellar grain inventory of the CO3 chondrite Queen Alexandra Range (QUE) 97416 is gained through correlated secondary ion mass spectrometry (SIMS), transmission electron microscopy (TEM), and synchrotron-based X-ray absorption near-edge structure spectroscopy (XANES). Only one presolar silicate grain [O17/O16=(9.96±0.75)×10−4; O18/O16=(19.49±0.96)×10−4] that may have formed in a low-mass Red Giant or Asymptotic Giant Branch star occurs in the coarse-grained matrix of QUE 97416. No other presolar grains were identified. Although presolar grains are rare in QUE 97416, numerous (898±259 ppm898±259 ppm) 15N-rich domains (δN15∼+1447‰ to +3069‰+3069‰) occur in the thin section. Based on TEM of an extracted section, two 15N-rich domains are amorphous, C-bearing, and texturally uniform, and they are embedded in a ferromagnesian silicate matrix with varied grain sizes. The individual 15N-rich organic regions with high δN15 (+2942±107‰+2942±107‰ and +2341±140‰+2341±140‰) exhibit diverse carbon functional groups, such as aromatic, vinyl-keto, amidyl, and carboxylic functionality, while the nitrogen XANES reveals traces of nitrile functionality. QUE 97416 appears to have escaped aqueous alteration based on the absence of hydrated minerals but is thermally altered, which could have resulted in the destruction of presolar grains. However, this process at >400 °C metamorphic temperatures was inefficient in destroying the carriers of N isotope anomalies, which may indicate the resistant nature of the organic carriers and/or the limited extent of thermal metamorphism on the QUE 97416 parent body.
[Show abstract][Hide abstract] ABSTRACT: Four samples (5b, 11h, 11i, and 11v) from the pristine collection of the Tagish Lake meteorite, an ungrouped C2 chondrite, were studied to characterize and understand its alteration history using ,, and . We determined that samples 11h and 11i have a relatively smaller proportion of amorphous silicate material than sample 5b, which experienced low-temperature hydrous parent-body alteration conditions to preserve this indigenous material. The data suggest that lithic fragments of 11i experienced higher degrees of aqueous alteration than the rest of the matrix, based on its low porosity and high abundance of coarse- and fine-grained sheet silicates, suggesting that 11i was present in an area of the parent body where alteration and brecciation were more extensive. We identified a coronal, "flower"-like, microstructure consisting of a fine-grained serpentine core and coarse-grained saponite-serpentine radial arrays, suggesting varied fluid chemistry and crystallization time scales. We also observed pentlandite with different morphologies: an exsolved morphology formed under nebular conditions; a nonexsolved pentlandite along grain boundaries; a "bulls-eye" sulfide morphology and rims around highly altered chondrules that probably formed by multiple precipitation episodes during low-temperature aqueous alteration (≥100 °C) on the parent body. On the basis of petrologic and mineralogic observations, we conclude that the Tagish Lake parent body initially contained a heterogeneous mixture of anhydrous precursor minerals of nebular and presolar origin. These materials were subjected to secondary, nonpervasive parent-body alteration, and the samples studied herein represent different stages of that hydrous alteration, i.e., 5b (the least altered) < 11h < 11i (the most altered). Sample 11v encompasses the petrologic characteristics of the other three specimens.
[Show abstract][Hide abstract] ABSTRACT: Organic nanoglobules are microscopic spherical carbon-rich objects present in chondritic meteorites and other astromaterials. We performed a survey of the morphology, organic functional chemistry, and isotopic composition of 184 nanoglobules in insoluble organic matter (IOM) residues from seven primitive carbonaceous chondrites. Hollow and solid nanoglobules occur in each IOM residue, as well as globules with unusual shapes and structures. Most nanoglobules have an organic functional chemistry similar to, but slightly more carboxyl-rich than, the surrounding IOM, while a subset of nanoglobules have a distinct, highly aromatic functionality. The range of nanoglobule N isotopic compositions was similar to that of nonglobular 15N-rich hotspots in each IOM residue, but nanoglobules account for only about one third of the total 15N-rich hotspots in each sample. Furthermore, many nanoglobules in each residue contained no 15N enrichment above that of bulk IOM. No morphological indicators were found to robustly distinguish the highly aromatic nanoglobules from those that have a more IOM-like functional chemistry, or to distinguish 15N-rich nanoglobules from those that are isotopically normal. The relative abundance of aromatic nanoglobules was lower, and nanoglobule diameters were greater, in more altered meteorites, suggesting the creation/modification of IOM-like nanoglobules during parent-body processing. However, 15N-rich nanoglobules, including many with highly aromatic functional chemistry, likely reflect preaccretionary isotopic fractionation in cold molecular cloud or protostellar environments. These data indicate that no single formation mechanism can explain all of the observed characteristics of nanoglobules, and their properties are likely a result of multiple processes occurring in a variety of environments.