^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