^1,2Janet R. Muhling,¹Alexandra A. Suvorova,²Birger Rasmussen

¹Centre for Microscopy, Characterisation, and Analysis, The University of Western Australia, Crawley, Western Australia 6009, Australia
²Department of Applied Geology, Curtin University, Kent St, Bentley, Western Australia 6102, Australia

Chevkinite-(Ce) and perrierite-(Ce) are the most common members of the chevkinite group of minerals. They are dimorphs, and both have the general formula A₄BC₂D₂Si₄O₂₂, where A = REE, Y, Ca, Sr, Th; B = Fe²⁺, (Mn, Mg); C = Ti, Al, Fe³⁺, Fe²⁺, Cr, Mn, Mg, Zr, Hf, Nb; and D = Ti. Both have been reported from a wide range of igneous, metamorphic, and hydrothermal rocks types, but occurrences in mafic rocks are rare, with minimal chemical and crystallographic documentation. Chevkinite-(Ce) and/or perrierite-(Ce) occur with other Ti-, Zr-, and REE-bearing accessory phases in eight suites of tholeiitic dolerite from Western Australia, and in lunar mare basalt 10047. They are more abundant than has been recognized previously in mafic igneous rocks, and they are significant hosts of incompatible elements. Chevkinite-(Ce) and perrierite-(Ce) from mafic rocks have distinctive chemical compositions with higher Zr than recorded in examples from most other common rock types. Among mafic rocks, two groups are recognized based on total Fe contents in electron microprobe analyses: crystal structural analysis by electron diffraction indicates that the high-Fe group (>8 wt% FeO) is chevkinite-(Ce), while the low-Fe group (<8 wt% FeO) is consistent with perrierite-(Ce), and both minerals can occur within a single hand specimen. A previously proposed chemical discriminant is not applicable to chevkinite-group minerals from typical mafic igneous rocks and crystal structural information is required to unequivocally distinguish between the two dimorphs.

Reference
Muhling JR, Suvorova AA, Rasmussen B (2014) The occurrence and composition of chevkinite-(Ce) and perrierite-(Ce) in tholeiitic intrusive rocks and lunar mare basalt. American Mineralogist 99, 1911-1921
Link to Article: [doi: 10.2138/am-2014-4690]

Copyright: The Mineralogical Society of America

¹Devin L. Schrader,²Jemma Davidson,³Richard C. Greenwood,³Ian A. Franchi,³Jenny M. Gibson
¹Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, 10th & Constitution NW, Washington, DC 20560-0119, USA
²Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, DC 20015-1305, USA
³Planetary and Space Sciences, Department of Physical Sciences, Open University, Milton Keynes MK7 6AA, UK

To better understand the effects of aqueous alteration in the Renazzo-like carbonaceous (CR) chondrite parent asteroid, a minor body in the early Solar System, we studied the petrology and O-isotope compositions of fine-grained matrix from 14 different CR chondrites. The O-isotope compositions of matrix from Queen Alexandra Range 99177 confirm that this sample is the least aqueously altered CR chondrite, provides the best approximation of the primary anhydrous matrix, and suggests matrix is not a byproduct of chondrule formation. Matrix O-isotope compositions within individual CR chondrites are heterogeneous, varying up to ∼5‰∼5‰ in both View the MathML sourceδO18 and View the MathML sourceδO17, as a result of the heterogeneous nature of the matrix and diverse range of aqueous alteration recorded by each sample. Aqueous alteration resulted in matrix that is progressively more 16O-depleted and Ca-carbonate rich. Due to the fine-grained nature of matrix its O-isotope composition is a more sensitive indicator of a chondrite’s overall degree of aqueous alteration than whole-rock O-isotope compositions, which are typically dominated by the compositions of type I (FeO-poor) chondrule phenocrysts. Petrographic signatures correlate with the degree of aqueous alteration and the wide range of matrix O-isotope compositions indicate that some regions of the CR chondrite parent asteroid were relatively dry, while others were heavily hydrated with water. The O-isotope composition of aqueously altered matrix is consistent with asteroidal water being near View the MathML sourceΔO17∼0‰, which suggests an inner Solar System origin for the water. The diverse range of aqueous alteration recorded by a single asteroid has a range of implications for spectral studies of the asteroid belt, and the arrival of Dawn at 1 Ceres, Hayabusa-2 at 162173 1999 JU3, and OSIRIS-REx at 101955 Bennu.

Reference
Schrader DL, Davidson J, Greenwood RC, Franchi IA, Gibson JM (2014) A water–ice rich minor body from the early Solar System: The CR chondrite parent Asteroid. Earth and Planetary Science Letters 407, 48-60
Link to Article [DOI: 10.1016/j.epsl.2014.09.030]

Copyright Elsevier

^1,2Vladimir A. Krasnopolsky
¹Department of Physics, Catholic University of America, Washington, DC 20064, USA
²Moscow Institute of Physics and Technology, Dolgoprudny, Russia

X-rays from comets originate in charge exchange between heavy ions of the solar wind and cometary species. Spectra of nine comets observed by the Chandra X-Ray Observatory (CXO) are analyzed using the time-dependent instrument sensitivity and the energy-dependent spectral resolution. X-ray emissions are extracted from the spectra in the range of 150 to 1100 eV using the χ2-fitting. Production of X-rays varies in the observed comets by a factor of 500 from 4.4×1013 erg s-1 in comet 73P to 2.2×1016 erg s-1 in comet Ikeya-Zhang. The measured solar wind flow varies within a factor of 20, being the weakest in comet 73P and the strongest in 9P/Tempel 1. The retrieved X-ray line intensities vary within a factor of 5×104. These lines above 300 eV are attributed to emissions of the H- and He-like ions, and laboratory data on the excitation cross sections for these emissions (Greenwood et al., 2000, Astrophys. J. 533, L175-L178) are used to convert the observed emissions into abundances of heavy ions in the solar wind. Continuity equations for charge exchange in comets are solved analytically and result in relationships between the X-ray emissions and the ion fluxes. The flux of O7+ scaled to 1 AU varies within a factor of 35 with a mean value of 1.6×104 cm-2 s-1. The retrieved ratios of O8+/O7+, C6+/C5+, Ne10+/Ne9+, C6+/O7+, N6+/O7+, and Ne9+/O7+ demonstrate significant variations, while their mean values for O, C, and N agree with those recommended by Schwadron and Cravens (2000, Astrophys. J. 544, 558-566) for the slow and fast solar wind. (Data on Ne9+ and Ne10+ are lacking in Scwadron and Cravens (2000).) The results are compared with the ion ratios from Bodewits et al. (2007, Astron. Astrophys. 469, 1183-1195) that were obtained from the same CXO spectra of comets, and some significant differences are briefly discussed. CXO X-ray spectroscopy of comets is a diagnostic tool to study the composition of the solar wind and its variations.

Reference
Krasnopolsky VA (2014) CXO X-Ray Spectroscopy of Comets and Abundances of Heavy Ions in the Solar Wind. Icarus (in Press)
Link to Article: [DOI: 10.1016/j.icarus.2014.09.026]

Copyright Elsevier

¹Andreas Nathues et al. (>10)*
¹Max-Planck-Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
*Find the extensive, full author and affiliation list on the publishers website

In this paper we present the results of a global survey of olivine-rich lithologies on (4) Vesta. We investigated Dawn Framing Camera (FC) High Altitude Mapping Orbit (HAMO) color cubes (∼60 m/pixel resolution) by using a method described in Thangjam et al. (2014b). In total we identified 15 impact craters exhibiting olivine-rich (> 40 wt.% ol) outcrops on their inner walls, some showing olivine-rich material also in their ejecta and floors. Olivine-rich sites are concentrated in the Bellicia, Arruntia and Pomponia region on Vesta’s northern hemisphere. From our multi-color and stratigraphic analysis, we conclude that most, if not all, of the olivine-rich material identified is of exogenic origin, i.e. remnants of A- or/and S-type projectiles. The olivine-rich lithologies in the north are possibly ejecta of the ∼90 km diameter Albana crater. We cannot draw a final conclusion on their relative stratigraphic succession, but it seems that the dark material (Nathues et al., 2014b) and the olivine-rich lithologies are of a similar age. The origin of some potential olivine-rich sites in the Rheasilvia basin and at crater Portia are ambiguous, i.e. these are either of endogenic or exogenic origin. However, the small number and size of these sites led us to conclude that olivine-rich mantle material, containing more than 40 wt. % of olivine, is basically absent on the present surface of Vesta. In combination with recent impact models of Veneneia and Rheasilvia (Clenet et al., 2014 and Jutzi et al., 2013), which predict an excavation depth of up to 80 km, we are confident that the crust – mantle depth is significantly deeper than predicted by most evolution models (30 km; Mittlefehldt, 2014) or, alternatively, the olivine-content of the (upper) mantle is lower than our detection limit, which would lead to the conclusion that Vesta’s parent material was already depleted in olivine compared to CI meteorites.

Reference
Nathues et al. (2014) Exogenic Olivine on Vesta from Dawn Framing Camera Color Data. Icarus (in Press)
Link to Article [DOI: 10.1016/j.icarus.2014.09.045]

Copyright Elsevier

^1,2Martin Schmieder,³Winfried H. Schwarz,³Mario Trieloff,¹Eric Tohver,⁴Elmar Buchner, ⁵Jens Hopp,¹Gordon R. Osinski
¹School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
²Western Australian Argon Isotope Facility, Department of Applied Geology and JdL Centre, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
³Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 234-236, D-69120 Heidelberg, Germany
⁴HNU-Neu-Ulm University, Wileystraße 1, D-89231 Neu-Ulm, Germany
⁵Departments of Earth Sciences/Physics and Astronomy, University of Western Ontario, London, ON, N6A 5B7, Canada

The two Clearwater Lake impact structures (Québec, Canada) are generally interpreted as a crater doublet formed by the impact of a binary asteroid. Here, arguments are presented that raise important questions about the proposed double impact scenario.New 40Ar/39Ar dating of two virtually fresh impact melt rock samples from the ⩾36 km West Clearwater Lake impact structure yielded two statistically robust Early Permian plateau ages with a weighted mean of 286.2 ± 2.2 (2.6) Ma (2σ; MSWD = 0.33; P = 0.57). In contrast, 40Ar/39Ar results for two chloritized melt rocks from the ∼26 km East Clearwater Lake impact structure produced disturbed age spectra suggestive of a distinct extraneous argon component. Although individually weakly robust, age spectra corrected for the trapped argon component and inverse isochron plots for the East Clearwater melt rocks consistently yielded apparent ages around ∼460–470 Ma. No Permian signal was found in either of these melt aliquots. Our new 40Ar/39Ar results reproduce earlier 40Ar/39Ar plateau ages (∼283 Ma and ∼465 Ma, respectively) for the two impact structures by Bottomley et al. (1990) and are in conflict with a previous, statistically non-robust Rb-Sr age of 287 [293] ± 26 Ma for East Clearwater. The combined cluster of apparent ages of ∼460–470 Ma, derived from four different samples across the impact melt sheet, is very unlikely to represent a ‘false age effect’ due to the incorporation of extraneous argon into the melt; instead, it strongly favors a Middle Ordovician age for the East Clearwater impact and impact-generated hydrothermal chloritization. Moreover, the Clearwater impact structures are characterized by different natural remanent magnetizations testifying to separate geologic histories, an effect unexpected in the case of a Permian double impact. Whereas the West Clearwater impact affected Ordovician carbonates incorporated into the impact breccia, drill core reports from the 1960s concluded that clasts of Ordovician sedimentary rocks are seemingly absent in the impact breccia lens of the East Clearwater Lake impact structure, which is overlain by >100 m of post-impact sandstones, shales and carbonates. No resolvable impactor contamination has so far been detected in the West Clearwater impact melt rocks, whereas East Clearwater carries a distinct ordinary (possibly L-) chondritic impactor signature in its melt rocks. East Clearwater Lake might thus represent one among a long list of Ordovician impact structures in North America and northern Europe that were presumably generated in response to the L-chondrite asteroid breakup event ∼470 Ma ago. Paleogeographic reconstructions show that the Ordovician East Clearwater impact probably occurred in a near-coastal to shallow marine setting, while the Permian West Clearwater impact hit continental Pangaea. Along with the new 40Ar/39Ar data, the paleomagnetic, sedimentologic, and paleogeographic findings suggest that the close spatial arrangement of the two Clearwater lakes is probably pure coincidence. The two impact structures seem to represent a ‘false doublet’ struck by impacts separated by ∼180 million years in time. The new results for the Clearwater Lake impact structures have major implications for the reliable identification of doublet impact craters and the rate of binary asteroid impacts on Earth and on other planetary bodies in the inner Solar System.

Reference
Schmieder M, Schwarz WH, Trieloff M, Tohver E,Buchner E, Hopp J,Osinski GR (2014) New 40Ar/39Ar dating of the Clearwater Lake impact structures (Québec, Canada) – Not the binary asteroid impact it seems? Geochimica et Cosmoschimica Acta (in Press)
Link to Article [DOI: 10.1016/j.gca.2014.09.037]

Copyright Elsevier

¹Marc Javoy,¹Edouard Kaminski
¹Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ. Paris Diderot, CNRS, F-75005 Paris, France

The Earth’s thermal evolution is controlled by the amount of heat released by the radioactive decay of 40K, 238U, 235U and 232Th. Their crust and upper mantle content is inferred from direct sampling, whereas estimating the lower mantle concentrations requires indirect constraints, such as those brought by primitive chondrites, or by geoneutrinos. Here we follow the framework of “E-Earth” models, based on the isotopic and chemical composition of E-chondrites (EC), to calculate U and Th concentrations in the Earth’s present day mantle, and the corresponding geoneutrinos flux. The model uses a compilation of data of U and Th contents of EC and account for the Earth differentiation and crust extraction. We obtain that the Bulk Silicate Earth (BSE) contains 15.4±1.8 ppb15.4±1.8 ppb of Uranium and 51.3±4.4 ppb51.3±4.4 ppb of Thorium, and has an average Th/U mass ratio of 3.4±0.43.4±0.4, with a peak value around 3.15. The prediction of geoneutrinos events originating from the mantle (i.e., without taking into account the local contribution of the crust) is 5.1±1.05.1±1.0 TNU, with 4.3±0.94.3±0.9 TNU coming from Uranium, and 0.8±0.20.8±0.2 TNU from Thorium. These numbers are in good agreement with the most recent KamLAND detector estimate, and compatible with the (higher) Borexino flux. On the other hand, the KamLAND constraints are not consistent with the high content of heat producing elements in the mantle predicted by the simple application of parameterized convection model to the thermal evolution of the Earth’s mantle. Since the measurement error in the mantle neutrino flux is currently dominated by the crustal contribution, geoneutrinos cannot for now discriminate between CI-based and EH-base models of the Earth’s composition. Further progress is expected if an ocean based geoneutrino detector is deployed.

Reference
Javoy M, Kaminski E (2014) Earth’s Uranium and Thorium content and geoneutrinos fluxes based on enstatite chondrites. Earth and Planetary Science Letters 407, 1-8
Link to Article [DOI: 10.1016/j.epsl.2014.09.028]

Copyright Elsevier

^1,Carole Cordier, ³Luigi Folco
¹Univ. Grenoble Alpes, ISTerre, F-38041 Grenoble, France
²CNRS, ISTerre, F-38041 Grenoble, France
³Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italia

A long-standing controversy in the micrometeorite community regards the relative contribution of primitive asteroids or comets and of evolved asteroids to the interplanetary dust cloud. We compiled and studied a large set of oxygen isotopic data from the literature on cosmic spherules from different collections covering different influx periods within the last ∼1 Myr. Cosmic spherules (micrometeorites melted during atmospheric entry) are the most abundant micrometeorites in worldwide collections. According to several models, they are representative of the composition and origin of micrometeorites >50 μm in size. Spherule statistics (136 spherules, 50-2280 μm in size) indicate that at least 20% of the micrometeoroid complex is fed by asteroids observed in the inner asteroid belt: the ordinary chondrite and secondarily the HED parent asteroids likely belonging to the S-type and V-type spectral classes, respectively. Another ∼60% (or more) is related to primitive objects of the Solar System with carbonaceous chondrite compositions: either primitive asteroids belonging to the C-, D- or P-type spectral classes in the outer asteroid belt or comets. Contribution from terrestrial planets has not been identified yet. Oxygen isotopes also document that the composition of the micrometeoroid complex is different from that of macroscopic meteoroids, since the latter is dominated by materials from evolved and differentiated asteroids rather than primitive asteroids or comets. Cosmic spherule statistics show that the contribution of ordinary chondrite material to the composition of the micrometeoroid complex increases with micrometeorite size, thereby documenting a continuum between meteorites and micrometeorites. The transition in terms of relative abundance is ∼500 μm in size.

Reference
Cordier C, Folco L (2014) Oxygen isotopes in cosmic spherules and the composition of the near Earth interplanetary dust complex. Geochimica et Cosmochimica Acta (in Press)
Link to Article: [DOI: 10.1016/j.gca.2014.09.038]

Copyright Elsevier

^1,2David J. Gombosi,^1,2Suzanne L. Baldwin,^2,3E. Bruce Watson,^4,2Timothy D. Swindle,^5,2John W. Delano,^6,2Wayne G. Roberge
¹Department of Earth Sciences, Syracuse University, Syracuse, NY 13244
²New York Center for Astrobiology, Rensselaer Polytechnic Institute, Troy, NY 12180
³Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180
⁴Lunar and Planetary Laboratory, The University of Arizona, Tucson, AZ 85721
⁵Department of Atmospheric and Environmental Sciences, University at Albany (SUNY), Albany, NY 12222
⁶Department of Physics, Applied Physics & Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180

The 40Ar/39Ar technique applied to impact glass has been used to date both terrestrial and lunar impact events. The ability to utilize the 40Ar/39Ar technique rests on the assumption that impact glasses are closed to the loss of daughter product, 40Ar∗, after formation. Diffusion experiments were performed on three Apollo 16 lunar impact glasses and yielded activation energies for 39Ar of ∼17 to 20 kcal mol-1 and log10(D0/a2) values of -5.2 to -6.0 s-1. The resulting diffusion coefficients are interpreted as minimum values and the Apollo 16 glass is probably some of the least retentive of lunar glasses, as the degree of non-bridging oxygen is at one end of the range in lunar glasses. At temperatures below the glass transition temperature (i.e., ∼660°C), the data can be explained by volume diffusion from a single diffusion domain. Modeling shows that Apollo 16 composition glass could lose significant quantities of radiogenic argon (40Ar∗) (∼90-100% over 20-40 Myr assuming a diffusion domain size (a) of 75 μm) due to diurnal temperature variations on the lunar surface, although 40Ar∗ loss is highly sensitive to exposure duration and effective diffusion domain size. Modeling shows that loss from transient thermal events (e.g., heating to ∼200°C for 102 yr duration) can also cause partial resetting of apparent 40Ar/39Ar ages. In small (a=75 μm) glasses a maximum of 50-60% of 40Ar∗ is lost over 4 Ga when buried to depths corresponding to temperatures of -15°C. Results indicate that caution should be exercised in interpreting lunar impact glass 40Ar/39Ar ages, as the assumption of closed system behavior may have been violated, particularly in glasses with low fractions of non-bridging oxygen.

Reference
Gombosi DJ,Baldwin SL,Watson EB,Swindle TD,Delano JW, Roberge WG (2014) Argon diffusion in apollo 16 impact glass spherules: Implications for 40Ar/39Ar dating of lunar impact Events. Geochimica et Cosmochimical Acta (in Press)
Link to Article [DOI: 10.1016/j.gca.2014.09.031]

Copyright Elsevier

¹Alexander N. Krot,¹Kazuhide Nagashima,^1,2Gerald J. Wasserburg,¹Gary R. Huss,^2,3Dimitri Papanastassiou,⁵Andrew M. Davis,⁵Ian D. Hutcheon,⁶Martin Bizzarro
¹Hawai‘i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
¹The Lunatic Asylum, California Institute of Technology, MC 170-25, Pasadena, CA 91125, USA
¹Jet Propulsion Laboratory, Mail Stop 183-335, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹Glenn Seaborg Institute, Lawrence Livermore National Laboratory, Livermore, CA 94551, USA
¹Department of the Geophysical Sciences, Enrico Fermi Institute, and Chicago Center for Cosmochemistry, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637-1433, USA
¹Centre for Star and Planet Formation, Geological Museum, University of Copenhagen, Øster Voldgade 5-7, DK-1350, Denmark

We present a detailed characterization of the mineralogy, petrology, and oxygen isotopic compositions of twelve FUN CAIs, including C1 and EK1-4-1 from Allende (CV), that were previously shown to have large isotopic fractionation patterns for magnesium and oxygen, and large isotopic anomalies of several elements. The other samples show more modest patterns of isotopic fractionation and have smaller but significant isotopic anomalies. All FUN CAIs studied are coarse-grained igneous inclusions: Type B, forsterite-bearing Type B, compact Type A, and hibonite-rich. Some inclusions consist of two mineralogically distinct lithologies, forsterite-rich and forsterite-free/poor. All the CV FUN CAIs experienced postcrystallization open-system iron-alkali-halogen metasomatic alteration resulting in the formation of secondary minerals commonly observed in non-FUN CAIs from CV chondrites. The CR FUN CAI GG#3 shows no evidence for alteration. In all samples, clear evidence of oxygen isotopic fractionation was found. Most samples were initially 16O-rich. On a three-oxygen isotope diagram, various minerals in each FUN CAI (spinel, forsterite, hibonite, dmisteinbergite, most fassaite grains, and melilite (only in GG#3)), define mass-dependent fractionation lines with a similar slope of ∼0.5. The different inclusions have different Δ17O values ranging from ∼ −25‰ to ∼ −16‰. Melilite and plagioclase in the CV FUN CAIs have 16O-poor compositions (Δ17O ∼ −3‰) and plot near the intercept of the Allende CAI line and the terrestrial fractionation line. We infer that mass-dependent fractionation effects of oxygen isotopes in FUN CAI minerals are due to evaporation during melt crystallization. Differences in Δ17O values of mass-dependent fractionation lines defined by minerals in individual FUN CAIs are inferred to reflect differences in Δ17O values of their precursors. Differences in δ18O values of minerals defining the mass-dependent fractionation lines in several FUN CAIs are consistent with their inferred crystallization sequence, suggesting these minerals crystallized during melt evaporation. In other FUN CAIs, no clear correlation between δ18O values of individual minerals and their inferred crystallization sequence is observed, possibly indicating gas-melt back reaction and oxygen-isotope exchange in a 16O-rich gaseous reservoir. After oxygen-isotope fractionation, some FUN CAIs could have experienced partial melting and gas-melt oxygen-isotope exchange in a 16O-poor gaseous reservoir that resulted in crystallization of 16O-depleted fassaite, melilite and plagioclase. The final oxygen isotopic compositions of melilite and plagioclase in the CV FUN CAIs may have been established on the CV parent asteroid as a result of isotope exchange with a 16O-poor fluid during hydrothermal alteration.
We conclude that FUN CAIs are part of a general family of refractory inclusions showing various degrees of fractionation effects due to evaporative processes superimposed on sampling of isotopically heterogeneous material. These processes have been experienced both by FUN and non-FUN igneous CAIs. Generally, the inclusions identified as FUN show larger isotope fractionation effects than non-FUN CAIs. There is a wide spread in UN isotopic anomalies in a large number of CAIs not exhibiting large fractionation effects in oxygen, magnesium, and silicon. We consider the majority of igneous CAIs to be the result of several stages of thermal processing (evaporation, condensation, and melting) of aggregates of solid precursors composed of incompletely isotopically homogenized materials. The unknown nuclear effects in CAIs are common to both FUN and non-FUN CAIs, and are not a special characteristic of FUN inclusions but represent the spectrum of results from sampling a very heterogeneous medium in the accreting Solar System.

Reference
Krot AN, Nagashima K,Wasserburg GJ, Huss GR, Papanastassiou D, Davis AM, Hutcheon ID, Bizzarro M (2014)
Calcium-aluminum-rich inclusions with fractionation and unknown nuclear effects (FUN CAIs): I. Mineralogy, petrology, and oxygen isotopic compositions. Geochimica et Cosmochimica Acta (in Press)
Link to Article [DOI: 10.1016/j.gca.2014.09.027]

Copyright Elsevier

¹N.G. Rudraswami,¹M. Shyam Prasad,²E.V.S.S.K. Babu,²T. Vijaya Kumar
¹National Institute of Oceanography (Council of Scientific and Industrial Research), Dona Paula, Goa 403 004, India
²National Geophysical Research Institute, (Council of Scientific and Industrial Research), Hyderabad 500 007, India

Fe–Ni beads are observed to occur in all three (Stony, Glass, Iron) types of cosmic spherules collected from deep sea sediments of the Indian Ocean. Fe–Ni beads in cosmic spherules can provide insights for understanding metal segregation mechanisms and their refractory metal element (RME: Re, Os, W, Ir, Ru, Mo, Pt, Rh including Pd) compositions can help ascertain their precursor meteorites. We measured RME compositions of 55 Fe–Ni beads using LA-ICP-MS in all three basic types of cosmic spherules selected after examining ∼2000 cosmic spherules. The RMEs of Fe–Ni beads provide unique information on formation and differentiation during atmospheric entry. The variability in the concentration of the RMEs depends on the initial mass of the cosmic spherules, volatility, temperature attained and efficiency in metal segregation during entry. The CI chondrite and Os normalized RME compositions of the beads display a pattern that is close to CI chondritic composition. The presence of Pd, a non-refractory metal having condensation temperature similar to Fe, in Fe–Ni beads of all types of cosmic spherules indicates that the heating undergone was below its vaporization temperature. Not all parent bodies lead to the formation of beads, the precursor needs to exceed a certain minimum size and temperature to facilitate the metal to get segregated into beads. The minimum size of a parent particle that could enclose a Fe–Ni bead is estimated to have a size ∼1 mm. This places constraints on the sizes of materials that are ablated during entry, and the accompanying mass loss during entry. Our study further points out that all the three basic types of cosmic spherules have a chondritic origin based on their RME distribution patterns. Only metal-rich carbonaceous chondrites contain the required quantities of metal for the formation of Fe-Ni beads during atmospheric entry and during this process the RMEs are also efficiently segregated into these beads.

Reference
Rudraswami NG, Prasad MS, Babu EVSSK, Kumar TV (2014) Chemistry and petrology of Fe–Ni beads from different types of cosmic spherules: Implication for precursors. Geochimica et Cosmoschimica Acta (in Press)
Link to Article [DOI: 10.1016/j.gca.2014.09.029]

Copyright Elsevier