Interstellar graphite in meteorites: Isotopic compositions and structural properties of single graphite grains from Murchison

Abstract— One hundred forty-three carbon grains, ranging in size from 2 to 8 μm, from two chemical and physical separates from the Murchison CM2 chondrite, were analyzed by ion microprobe mass spectrometry for their C- and N-isotopic compositions. Both separates are enriched in the exotic noble gas component Ne-E(L). Ninety grains were also analyzed for their H and O contents and 118, for Si. Thirteen grains were analyzed by micro-sampling laser Raman spectroscopy. Round grains have large C-isotopic anomalies with ¹²C/¹³C ratios ranging from 7 to 4500 (terrestrial ratio = 89). Nitrogen in these grains is also anomalous but shows much smaller deviations from the terrestrial composition, ¹⁴N/¹⁵N ratios ranging from 193 to 680 (terrestrial ratio = 272). Spherulitic aggregates and non-round compact grains have normal C-isotopic ratios but ¹⁵N excesses (up to 35%). Raman spectra of the analyzed grains indicate varying degrees of crystalline disorder of graphite with estimated in-plane crystallite dimensions varying from 18 Å (highly disordered, similar to terrestrial kerogen) to ～750 Å (well-crystallized graphite). Element contents of H, O, and Si are correlated with one another, and H and O are probably present in the form of organic molecules. On the basis of morphology, the round grains fall into two groups: grains with smooth, shell-like surfaces (“onions”) and grains that appear to be dense aggregates of small scales (“cauliflowers”). “Onions” tend to have lower trace element contents, isotopically light C (¹²C/¹³C > 89) and a high degree of crystalline order, whereas “cauliflowers” have a larger spread in trace element contents and C-isotopic ratios (they range from isotopically light to heavy) but tend to have a low degree of crystalline order. However, these differences exist only on average, and no clear distinction can be made for individual grains. A few limited conclusions can be drawn about the astrophysical origin of the carbon grains of this study. The ¹⁵N excesses in spherulitic aggregates and non-round grains can be explained as the result of ion-molecule reactions in molecular clouds. The round grains, on the other hand, must have formed in stellar atmospheres (circumstellar grains). Grains with isotopically light C must have formed in stellar environments characterized by He-burning, either in the atmosphere of Wolf-Rayet stars during the WC phase or in the He-burning, ¹²C-rich zone of a massive star, ejected by a supernova explosion. Isotopically heavy C is produced by H-burning in the CNO cycle. Possible sources for grains with heavy C are carbon stars (AGB stars during the thermally pulsing phase) or novae, but the detailed distribution of ¹²C/¹³C ratios agree neither with the distribution observed in carbon stars nor with theoretical predictions for these two types of stellar sources.     

Interstellar Grains in Primitive Meteorites: Diamond,Silicon Carbide,and Graphite

Abstract— Primitive meteorites contain a few parts per million (ppm) of pristine interstellar grains that provide information on nuclear and chemical processes in stars. Their interstellar origin is proven by highly anomalous isotopic ratios, varying more than 1000-fold for elements such as C and N. Most grains isolated thus far are stable only under highly reducing conditions (C/O > 1), and apparently are “stardust” formed in stellar atmospheres. Microdiamonds, of median size ～ 10 Å, are most abundant (～ 400–1800 ppm) but least understood. They contain anomalous noble gases including Xe-HL, which shows the signature of the r- and p-processes and thus apparently is derived from supernovae. Silicon carbide, of grain size 0.2–10 μm and abundance ～ 6 ppm, shows the signature of the s-process and apparently comes mainly from red giant carbon (AGB) stars of 1–3 solar masses. Some grains appear to be ≥10⁹ a older than the Solar System. Graphite spherules, of grain size 0.8–7 μm and abundance <2 ppm, contain highly anomalous C and noble gases, as well as large amounts of fossil ²⁶Mg from the decay of extinct ²⁶Al. They seem to come from at least three sources, probably AGB stars, novae, and Wolf-Rayet stars.     

Alpine and Pacific styles of Phanerozoic mountain building: subduction-zone petrogenesis of continental crust

A broad continuum exists between two distinct end-member types of mountain building. Alpine-type orogenic belts develop during subduction of an ocean basin between two continental blocks, resulting in collision. They are characterized by an imbricate sequence of oceanward verging nappes; some Alpine belts exhibit superimposed late-stage backthrusting. Sediments are chiefly platform carbonates and siliciclastics, in some cases associated with minor amounts of bimodal volcanics; pre-existing granitic gneisses and related continental rocks constitute an autochthonous–parautochthonous basement. Metamorphism of deeply subducted portions of the orogen ranges from relatively high-pressure (HP) to ultrahigh-pressure (UHP). Calcalkaline volcanic–plutonic rocks are rare, and have peraluminous, S-type bulk compositions. In contrast, Pacific-type orogens develop within and landward from long-sustained oceanic subduction zones. They consist of an outboard oceanic trench–accretionary prism, and an inboard continental margin–island arc. The oceanic assemblage consists of first-cycle, in-part mélanged volcaniclastics, and minor but widespread cherts ± deep-water carbonates, intimately mixed with disaggregated ophiolites. The section recrystallized under HP conditions. Recumbent fold vergence is oceanward. A massive, slightly older to coeval calcalkaline arc is sited landward from the trench complex on the stable, non-subducted plate. It consists of abundant, dominantly intermediate, metaluminous, I-type volcanics resting on old crust; both assemblages are thrown into open folds, intruded by comagmatic I-type granitoids, and metamorphosed locally to regionally under high-T, low-P conditions. In the subduction channel of collisional and outboard Circumpacific terranes, combined extension above and subduction below allows buoyancy-driven ascent of ductile, thin-aspect ratio slices of HP–UHP complexes to midcrustal levels, where most closely approached neutral buoyancy; exposure of rising sheets caused by erosion and gravitational collapse results in moderate amounts of sedimentary debris because exhumed sialic slivers are of modest volume. At massive sialic buildups associated with convergent plate cuSPS (syntaxes), tectonic aneurysms may help transport HP–UHP complexes from mid- to upper-crustal levels. The closure of relatively small ocean basins that typify many intracratonic suture zones provides only limited production of intermediate and silicic melts, so volcanic–plutonic belts are poorly developed in Alpine orogens compared with Circumpacific convergent plate junctions. Generation of a calcalkaline arc mainly depends on volatile evolution at the depth of magma generation. Phase equilibrium studies show that, under typical subduction-zone P–T trajectories, clinoamphibole ± Ca–Al hydrous silicates constitute the major hydroxyl-bearing phases in deep-seated metamorphic rocks of MORB composition; other hydrous minerals are of minor abundance. Ca and Na clinoamphiboles dehydrate at pressures of above approximately 2 GPa, but low-temperature devolatilization may be delayed by pressure overstepping; thus metabasaltic blueschists and amphibolites expel H₂O at melt-generation depths, and commonly achieve stable eclogitic assemblages. Partly serpentinized mantle beneath the oceanic crust dehydrates at roughly comparable conditions. For reasonable subduction-zone geothermal gradients however, white micas ± biotites remain stable to pressures >3 GPa. Accordingly, attending descent to depths of >100 km, mica-rich quartzofeldspathic lithologies that constitute much of the continental crust fail to evolve substantial amounts of H₂O, and transform incompletely to stable eclogite-facies assemblages. Underflow of amphibolitized oceanic lithosphere thus generates most of the deep-seated volatile flux, and the consequent partial melting to produce the calcalkaline suite, along and above a subduction zone; where large volumes of micaceous intermediate and felsic crustal materials are carried down to great depths, volatile flux severely diminishes. Thus, continental collision in general does not produce a volcanic–plutonic arc whereas in contrast, the long-continued contemporaneous underflow of oceanic lithosphere does.     

A menagerie of graphite morphologies in the Acapulco meteorite with diverse carbon and nitrogen isotopic signatures: Implications for the evolution history of acapulcoite meteorites

Morphologies, petrographic settings and carbon and nitrogen isotopic compositions of graphites in the Acapulco meteorite, the latter determined by secondary ionization mass spectrometry, are reported. Seven different graphite morphologies were recognized, the majority of which occur enclosed exclusively in kamacite. Individual graphite grains also rarely occur in the silicate matrix. Kamacite rims surrounding taenite cores of metal grains are separated from the Ni-rich metal cores by graphite veneers. These graphite veneers impeded or prevented Ni-Fe interdiffusion during cooling. In addition, matrix FeNi metal contains considerable amounts of phosphorous (≈ 700 ppm) and silicon (≈ 300 ppm) (Pack et al., 2005 in preparation) thus indicating that results of laboratory cooling experiments in the Fe-Ni binary system are inapplicable to Acapulco metals. Graphites of different morphologies display a range of carbon and nitrogen isotopic compositions, indicating a diversity of source regions before accretion in the Acapulco parent body. The isotopic compositions point to at least three isotopic reservoirs from which the graphites originated: (1) A reservoir with heavy carbon, represented by graphite in silicates (δ¹³C = 14.3 ± 2.4 ‰ and δ¹⁵N = −103.4 ± 10.9 ‰), (2) A reservoir with isotopically light carbon and nitrogen, characteristic for the metals. Its C- and N-isotopic compositions are probably preserved in the graphite exsolutions that are isotopically light in carbon and lightest in nitrogen (δ¹³C = −17 to −23 ‰ δ¹⁵N = −141 to −159 ‰). (3) A reservoir with an assumed isotopic composition (δ¹³C ∼ −5 ‰; δ¹⁵N ∼ −50 ‰). A detailed three-dimensional tomography in reflected light microscopy of the decorations of metal-troilite spherules in the cores of orthopyroxenes and olivines and metal-troilite veins was conducted to clarify their origin. Metal and troilite veins are present only near the fusion crust. Hence, these veins are not pristine to Acapulco parent body but resulted during passage of Acapulco in Earth’s atmosphere. A thorough search for symplectite-type silicate-troilite liquid quench textures was conducted to determine the extent of closed-system partial silicate melting in Acapulco.Metal-troilite spherules in orthopyroxenes and olivines are not randomly distributed but decorate ferromagnesian silicate restite cores, indicating that the metal-spherule decoration around restite silicates took place in a silicate partial melt. Graphite inclusions in these spherules have C- and N- isotopic compositions (δ¹³C = −2.9 ± 2.5 ‰ and δ¹⁵N = −101.2 ± 32 ‰) close to the average values of graphite in metals and in the silicate matrix, thus strongly suggesting that they originated from a mixture of graphite inclusions in metals and silicate matrix graphite during a closed system crystallization process subsequent to silicate-metal-sulfide partial melting. Troilite-orthopyroxene quench symplectite textures in orthopyroxene rims are clear evidence that silicate-sulfide partial melting took place in Acapulco. Due to petrographic heterogeneity on a centimeter scale, bulk REE abundances of individual samples or of individual minerals provide only limited information and the REE abundances alone are not entirely adequate to unravel the formational processes that prevailed in the acapulcoite-lodranite parent body. The present investigations demonstrate the complexity of the evolutionary stages of acapulcoites from accretion to parent body processes.     

Recruitment of the Crabs Eurypanopeus depressus, Rhithropanopeus harrisii, and Petrolisthes armatus to Oyster Reefs: the Influence of Freshwater Inflow

Oyster reefs provide structural habitat for resident crabs and fishes, most of which have planktonic larvae that are dependent upon transport/retention processes for successful settlement. High rates of freshwater inflow have the potential to disrupt these processes, creating spatial gaps between larval distribution and settlement habitat. To investigate whether inflow can impact subsequent recruitment to oyster reefs, densities of crab larvae and post-settlement juveniles and adults were compared in Estero Bay, Florida, over 22 months (2005–2006). Three species were selected for comparison: Petrolisthes armatus, Eurypanopeus depressus, and Rhithropanopeus harrisii. All are important members of oyster reef communities in Southwest Florida; all exhibit protracted spawning, with larvae present throughout the year; and each is distributed unevenly on reefs in different salinity regimes. Recruitment to oyster reefs was positively correlated with bay-wide larval supply at all five reefs examined. Species-specific larval connectivity to settlement sites was altered by inflow: where connectivity was enhanced by increased inflow, stock–recruitment curves were linear; where connectivity was reduced by high inflows, stock–recruitment curves were asymptotic at higher larval densities. Maximum recruit density varied by an order of magnitude among reefs. Although live oyster density was a good indicator of habitat quality in regard to crab density, it did not account for the high variability in recruit densities. Variation in recruit density at higher levels of larval supply may primarily be caused by inflow-induced variation in larval connectivity, creating an abiotic simulation of what has widely been regarded as density dependence in stock–recruitment curves.     

http://www.sciencedirect.com/science/article/pii/S1674987112001041

Large igneous provinces (LIPs) are considered a relevant cause for mass extinctions of marine life throughout Earth’s history. Their flood basalts and associated intrusions can cause significant release of SO₄ and CO₂ and consequently, cause major environmental disruptions. Here, we reconstruct the long-term periodic pattern of LIP emplacement and its impact on ocean chemistry and biodiversity from δ³⁴S_sulfate of the last 520 Ma under particular consideration of the preservation limits of LIP records. A combination of cross-wavelet and other time-series analysis methods has been applied to quantify a potential chain of linkage between LIP emplacement periodicity, geochemical changes and the Phanerozoic marine genera record. We suggest a mantle plume cyclicity represented by LIP volumes (V) of V = ?(350–770) × 10³ km³ sin(2πt/170 Ma) + (300–650) × 10³ km³ sin(2πt/64.5 Ma + 2.3) for t = time in Ma. A shift from the 64.5 Ma to a weaker ～28–35 Ma LIP cyclicity during the Jurassic contributes together with probably independent changes in the marine sulfur cycle to less ocean anoxia, and a general stabilization of ocean chemistry and increasing marine biodiversity throughout the last ～135 Ma. The LIP cycle pattern is coherent with marine biodiversity fluctuations corresponding to a reduction of marine biodiversity of ～120 genera/Ma at ～600 × 10³ km³ LIP eruption volume. The 62–65 Ma LIP cycle pattern as well as excursion in δ³⁴S_sulfate and marine genera reduction suggest a not-yet identified found LIP event at ～440–450 Ma.     

Physical constraints on impact melt properties from Lunar Reconnaissance Orbiter Camera images

Impact melt flows exterior to Copernican-age craters are observed in high spatial resolution (0.5 m/pixel) images acquired by the Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC). Impact melt is mapped in detail around 15 craters ranging in diameter from 2.4 to 32.5 km. This survey supports previous observations suggesting melt flows often occur at craters whose shape is influenced by topographic variation at the pre-impact site. Impact melt flows are observed around craters as small as 2.4 km in diameter, and preliminary estimates of melt volume suggest melt production at small craters can significantly exceed model predictions. Digital terrain models produced from targeted NAC stereo images are used to examine the three-dimensional properties of flow features and emplacement setting, enabling physical modeling of flow parameters. Qualitative and quantitative observations are consistent with low-viscosity melts heated above their liquidii (superheated) with limited amounts of entrained solids.     

Detection and Classification of Phytoplankton Deposits Along an Estuarine Gradient

Phytoplankton deposition onto sediments affects trophic structures, sedimentary nutrient fluxes, and dissolved oxygen concentrations in coastal ecosystems. Deposition can occur as distinct events that are highly variable over space and time, necessitating detection methods that have similarly high resolution. We present an assessment of a novel rapid detection method that combines water-column and benthic fluorometry with surficial sediment sampling to identify phytoplankton deposition, as implemented in a 2-year study of a Florida estuary (24?monthly collections at 14 locations). Maximum water-column chlorophyll concentration, average benthic chlorophyll fluorescence, and the proportion of centric vs. pennate diatoms at the sediment?Cwater interface were each fitted to sine functions to represent phytoplankton bloom cycles. The phase offsets among the three fitted sine functions were varied to maximize fit to the 336 observations. The fitted cycles were divided into four classes that separate dominance by benthic microalgae from early, late, and post-phytoplankton depositional states. Best-fitting cycles for the proportion of centric diatoms were consistently offset from water-column chlorophyll cycles, indicating peak deposition occurred after peak phytoplankton blooms. Phytoplankton deposition dominated the upstream region of the studied estuary and was associated with reduced dissolved oxygen concentrations. Benthic algae dominated in downstream regions, particularly during low freshwater flow conditions when light absorption by colored dissolved organic matter was low. This approach produced repeatable and consistent patterns that agreed with expected relationships and was practical for sampling with high spatial and temporal resolution.     

Physical characterization of a suite of Buzzard Coulee H4 chondrite fragments

On November 20, 2008, the Buzzard Coulee H4 chondrite fell to Earth outside of Lloydminster, Alberta, Canada. Eighteen fresh samples obtained by the National Meteorite Collection of Canada, ranging from 8.80 to 109.14 g, were investigated in this study. Physical properties of the samples were first obtained using a suite of nondestructive techniques. The bulk density (Archimedean bead method: 3.48 ± 0.04 g cm^?3; 3‐D laser imaging: 3.46 ± 0.03 g cm^?3; micro‐computed tomography: 3.44 ± 0.03 g cm^?3), porosity (6.2 ± 0.1%), bulk magnetic susceptibility (log χ: 5.364 ± 0.056 × 10^?9 m³ kg^?1 at 825 Hz; 5.329 ± 0.052 × 10^?9 m³ kg^?1 at 19,000 Hz), and other derived magnetic properties (frequency dependence: 8.7 ± 6.2%; degree of anisotropy A%: 22.0 ± 2.0%; ellipsoid shape B%: ?18.7 ± 8.7%) are typical of H chondrites. The coefficient of variation associated with the properties measured directly was low (0.10–1.15%), indicating that the samples are homogenous at the interfragment scale. The study then proceeded with detailed analyses at the intrafragment scale. Visual inspection of micro‐computed tomographic images allowed the identification of an anomalous large clast with low metal content in a fragment. Another fragment exhibited macroscopic shock veins that warranted further examination. These fragments were cut and polished thin sections prepared for petrological analysis by optical and scanning electron microscopy. Based on mineralogical and textural similarities with several chondrules, the large clast was interpreted to be a macrochondrule. In a larger context, this study proposes a protocol for the systematic investigation of extraterrestrial material that can be exported to other new meteorite falls and finds, and specimens from sample return mission.