¹T.Noguchi et al. (>10)*
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2017.03.034]
¹Faculty of Arts and Science, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
*Find the extensive, full author and affiliation list on the publishers website
Copyright Elsevier

Micrometeorites (MMs) recovered from surface snow near the Dome Fuji Station, Antarctica are almost free from terrestrial weathering and contain very primitive materials, and are suitable for investigation of the evolution and interaction of inorganic and organic materials in the early solar system. We carried out a comprehensive study on seven porous and fluffy MMs [four Chondritic porous (CP) MMs and three fluffy fine-grained (Fluffy Fg) MMs] and one fine-grained type 1 (Fg C1) MM for comparison with scanning electron microscope, transmission electron microscope, X-ray absorption near-edge structure analysis, and secondary ion mass spectrometer.

They show a variety of early aqueous activities. Four out of the seven CP MMs contain glass with embedded metal and sulfide (GEMS) and enstatite whiskers/platelets and do not have hydrated minerals. Despite the same mineralogy, organic chemistry of the CP MMs shows diversity. Two of them contain considerable amounts of organic materials with high carboxyl functionality, and one of them contains nitrile (C≡N) and/or nitrogen heterocyclic groups with D and 15N enrichments, suggesting formation in the molecular cloud or a very low temperature region of the outer solar system. Another two CP MMs are poorer in organic materials than the above-mentioned MMs. Organic material in one of them is richer in aromatic C than the CP MMs mentioned above, being indistinguishable from those of hydrated carbonaceous chondrites. In addition, bulk chemical compositions of GEMS in the latter organic poor CP MMs are more homogeneous and have higher Fe/(Si+Mg+Fe) ratios than those of GEMS in the former organic-rich CP MMs. Functional group of the organic materials and amorphous silicate in GEMS in the organic-poor CP MMs may have transformed in the earliest stage of aqueous alteration, which did not form hydrated minerals.

Three Fluffy Fg MMs contain abundant phyllosilicates, showing a clear evidence of aqueous alteration. Phyllosilicates in thee MMs are richer in Fe than those in hydrated IDPs, typical fine-grained hydrated (Fg C1) MMs, and hydrated carbonaceous chondrites. One of the Fluffy Fg MMs contains amorphous silicate, which is richer in Fe than GEMS and contains little or no nanophase Fe metal but contains Fe sulfide. Because the chemical compositions of the amorphous silicate are within the compositional field of GEMS in CP IDPs, the amorphous silicate may be alteration products of GEMS. The entire compositional field of GEMS in the CP MMs and the amorphous silicate in the Fluffy Fg MM matches that of the previously reported total compositional range of GEMS in IDPs.

One Fluffy Fg MM contains Mg-rich phyllosilicate along with Fe-rich phyllosilicate and Mg-Fe carbonate. Mg-rich phyllosilicate and Mg-Fe carbonate may have been formed through the reaction of Fe-rich phyllosilicate, Mg-rich olivine and pyroxene, and water with C-bearing chemical species.

These data indicate that CP MMs and Fluffy Fg MMs recovered from Antarctic surface snow contain materials that throw a light on the earliest stages of aqueous alteration on very primitive solar system bodies. Because mineralogy and isotopic and structural features of organic materials in D10IB009 are comparable with isotopically primitive IDPs, its parent body could be comets or icy asteroids showing mass ejection (active asteroids). By contrast, organic-poor CP MMs may have experienced the earliest stage of aqueous alteration and Fluffy Fg MMs experienced weak aqueous alteration. The precursor materials of the parent bodies of Fluffy Fg MMs probably contained abundant GEMS or GEMS-like materials like CP IDPs, which is common to fine-grained matrices of very primitive carbonaceous chondrites such as CR3s. However, highly porous nature of organic-poor CP MMs and Fluffy Fg MMs suggests that parent bodies of these MMs must have been much more porous than the parent bodies of primitive carbonaceous chondrites. Given no phyllosilicate among the returned samples of 81P/Wild 2 comet, the MMs may have been derived from porous icy asteroids such as active asteroids as well as P- and D-type asteroids rather than comets.

¹Elishevah M.M.E. van Kooten, , ¹Martin Schiller, ¹Martin Bizzarro
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2017.03.033]
¹Centre for Star and Planet Formation and Natural History Museum of Denmark, University of Copenhagen, DK-1350 Copenhagen, Denmark
Copyright Elsevier

Polymict ureilites are meteoritic breccias that provide insights into the differentiation history of the ureilite parent body.We have sampled a total of 24 clasts from the polymict ureilite Dar al Gani 319, representing a variety of lithologies such as mantle residues, cumulates and crustal fragments that are genetically related to monomict ureilites.In addition, we sampled four non-indigenous dark clasts and two chondrule-containing clasts from the same meteorite.We report on the petrology and the bulk mass-dependent and mass-independent magnesium and chromium isotope systematics of these clasts.The DaG 319 polymict ureilite consists predominantly of clasts related to Main Group ureilite residues (MG clasts) with varying Mg#s (0.74-0.91), as well as a significant fraction of olivine-orthopyroxene clasts related to Hughes Type ureilites (HT clasts) with consistently high Mg#s (∼∼0.89).In addition, DaG 319 contains less abundant feldspathic clasts that are thought to represent melts derived from the ureilite mantle.A significant mass-dependent Mg-isotope fractionation totaling ΔΔμμ2525Mg = ∼∼450 ppm was found between isotopically light feldspathic clasts (μμ2525Mg = –305±±25 to 15±±12 ppm), MG clasts (μμ2525Mg = –23±±51 ppm) and HT clasts (μμ2525Mg = 157±±21 ppm).We suggest that this isotopic offset is the result of equilibrium isotope fractionation during melting in the presence of an isotopically light magnesite component.We propose Mg-carbonates to be stable in the upper ureilite mantle, and pure carbon phases such as graphite to be stable at higher pressures.This is consistent with HT clasts lacking carbon-related phases, whereas MG clasts contain abundant carbon.The timing of differentiation events for the ureilitic clasts are constrained by high precision 5353Mn-5353Cr systematics and 2626Al-2626Mg model ages.We show that a dichotomy of ages exist between the differentiation of main group ureilite residues and HT cumulates rapidly after CAI formation and later remelting of cumulates with corresponding feldspathic melts, at 3.8±±1.3 Myr after CAI formation.Assuming an initial 2626Al/2727Al abundance[(2626Al/2727Al)0 = View the MathML source1.33-0.18+0.21×× 10−5] similar to the angrite parent body, the early melting event is best explained by heat production from 2626Al whereas the late event is more likely caused by a major impact. Variations in 5454Cr between MG clasts and HT clasts agree with a carbonaceous chondrite impactor onto the ureilite parent body. This impactor may be represented by abundant dark clasts found in polymict ureilites, which have μμ26Mg∗26Mg∗ and μμ5454Cr signatures similar to CI chondrites. Similar volatile-rich dark clasts found in other meteorite breccias provide insights into the timing of volatile influx to the accretion region of the terrestrial planets.

¹Frédéric Girault, ¹Frédéric Perrier, ¹Manuel Moreira, ²Brigitte Zanda, ³Pierre Rochette, ^1,4Yoram Teitler
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2017.03.031]
¹Institut de Physique du Globe de Paris, Sorbonne Paris Cité, University Paris Diderot, CNRS UMR 7154, F-75005 Paris, France
²Laboratoire de Minéralogie et de Cosmochimie du Muséum, Muséum National d’Histoire Naturelle, CNRS UMR 7202, F-75005 Paris, France
³Centre Européen de Recherche et d’Enseignement de Géosciences de l’Environnement, CNRS UMR 7330, Aix-Marseille University, BP80 F-13545 Aix en Provence Cedex 4, France
⁴Currently at: Centre de Recherches Pétrographiques et Géochimiques, Université de Nancy, CNRS UMR 7358, Vandœuvre-lès-Nancy, France
Copyright Elsevier

The analysis of noble gases in meteorites provides constraints on the early solar system and the pre-solar nebula. This requires a better characterization and understanding of the capture, production, and release of noble gases in meteorites. The knowledge of transfer properties of noble gases for each individual meteorite could benefit from using radon-222, radioactive daughter of radium-226. The radon-222 emanating power is commonly quantified by the effective radium-226 concentration (ECRa), the product of the bulk radium-226 concentration and of the emanation coefficient E, which represents the probability of one decaying radium-226 to inject one radon-222 into the free porous network. Owing to a non-destructive, high-sensitivity accumulation method based on long photomultiplier counting sessions, we are now able to measure ECRa of meteorite samples, which usually have mass smaller than 15 g and ECRa<0.5 Bq kg^-1. We report here the results obtained from 41 different meteorites, based on 129 measurements on 70 samples using two variants of our method, showing satisfactory repeatability and a detection limit below 10^-2 Bq kg^-1 for a sample mass of 1 g. While two meteorites remain below detection level, we obtain for 39 meteorites heterogeneous EC_Ra values with mean (min–max range) of ca. 0.1 (0.018–1.30) Bq kg^-1. Carbonaceous chondrites exhibit the largest EC_Ra values and eucrites the smallest. Such values are smaller than typical values from most terrestrial rocks, but comparable with those from Archean rocks (mean of ca. 0.18 Bq kg^-1), an end-member of terrestrial rocks. Using uranium concentration from the literature, E is inferred from EC_Ra for all the meteorite samples. Values of E for meteorites (mean 40±4 %) are higher than E values for Archean rocks and reported values for lunar and Martian soils. Exceptionally large E values likely suggest that the ²³⁸U-²²⁶Ra pair would not be at equilibrium in most meteorites and that uranium and/or radium are most likely not uniformly distributed. EC_Ra of meteorites is correlated with E and seems to mainly reflect the gas permeability of the meteorite, which could be one important property, preserved in the meteorite, of its parent body, characterizing its history in space, possibly modified by alteration, shock metamorphism, and eventually weathering on Earth. Larger radon emanation values are associated with larger concentrations of the heaviest noble gases (argon, krypton, xenon), and larger ²⁰Ne/²²Ne and ³⁶Ar/³⁸Ar ratios, suggesting Earth’s atmosphere contamination or solar wind implantation, and probably a similar carrier phase such as Q phase. An unclear correlation is observed with ⁴⁰Ar, which may rule out a purely radiogenic effect on radon emanation. Thus, larger radon emanation suggests a larger capacity of collecting solar and terrestrial gases, which should imply higher loss of gases generated in the meteorite and larger dispersion of Pb/U ratios for age determination. This study provides the first quantification of natural radon-222 loss from meteorites and opens promising perspectives to quantify the relationship between pore space connectivity and the transfer properties for noble gases in meteorites and other extraterrestrial bodies.