¹Kun Wang (王昆), ¹Stein B. Jacobsen, ¹Fatemeh Sedaghatpour, ²Heng Chen, ²Randy L. Korotev
¹Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA
²Department of Earth and Planetary Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA

Recent high-precision isotopic measurements show that the isotopic similarity of Earth and Moon is unique among all known planetary bodies in our Solar System. These observations provide fundamental constraints on the origin of Earth–Moon system, likely a catastrophic Giant Impact event. However, in contrast to the isotopic composition of many elements (e.g. , O, Mg, Si, K, Ti, Cr, and W), the Fe isotopic compositions of all lunar samples are significantly different from those of the bulk silicate Earth. Such a global Fe isotopic difference between the Moon and Earth provides an important constraint on the lunar formation – such as the amount of Fe evaporation as a result of a Giant Impact origin of the Moon. Here, we show through high-precision Fe isotopic measurements of one of the oldest lunar rocks (4.51±0.10 Gyr4.51±0.10 Gyr dunite 72 415), compared with Fe isotope results of other lunar samples from the Apollo program, and lunar meteorites, that the lunar dunite is enriched in light Fe isotopes, complementing the heavy Fe isotope enrichment in other lunar samples. Thus, the earliest olivine accumulation in the Lunar Magma Ocean may have been enriched in light Fe isotopes. This new observation allows the Fe isotopic composition of the bulk silicate Moon to be identical to that of the bulk silicate Earth, by balancing light Fe in the deep Moon with heavy Fe in the shallow Moon rather than the Moon having a heavier Fe isotope composition than Earth as a result of Giant Impact vaporization.

Reference
Wang (王昆) K, Jacobsen SB, Sedaghatpour F, Chen H, Korotev RL (2015) The earliest Lunar Magma Ocean differentiation recorded in Fe. Earth and Planetary Science Letters 430, 202–208
Link to Article [doi:10.1016/j.epsl.2015.08.019]
Copyright Elsevier

¹Alexandra Abrajevitch, ²Eric Font, ³Fabio Florindo, ⁴Andrew P. Roberts
¹Institute of Tectonics and Geophysics, Kim-Yu-Chen 65, Khabarovsk 680000, Russia
²IDL-FCUL, Instituto Dom Luís, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016, Portugal
³Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143-Rome, Italy
⁴Research School of Earth Sciences, Australian National University, Canberra, Acton 0200, Australia

The respective roles of an asteroid impact and Deccan Traps eruptions in biotic changes at the Cretaceous–Paleogene (K–Pg) boundary are still debated. In many shallow marine sediments from around the world, the K–Pg boundary is marked by a distinct clay layer that is often underlain by a several decimeter-thick low susceptibility zone. A previous study of the Gubbio section, Italy (Lowrie et al., 1990), attributed low magnetization intensity in this interval to post-depositional dissolution of ferrimagnetic minerals. Dissolution was thought to be a consequence of downward infiltration of reducing waters that resulted from rapid accumulation of organic matter produced by mass extinctions after the K–Pg event. We compare the magnetic properties of sediments from the Gubbio section with those of the Bidart section in southern France. The two sections are similar in their carbonate lithology and the presence of a boundary clay and low susceptibility zone. When compared to background Cretaceous sediments, the low susceptibility zone in both sections is marked by an absence of biogenic magnetite, a decrease in total ferrimagnetic mineral content, and a preferential loss of magnetite with respect to hematite – features that are consistent with reductive dissolution. However, unlike the Gubbio section, where the low susceptibility zone starts immediately below the boundary clay, the low susceptibility zone and the clay layer at Bidart are separated by a ∼4-cm carbonate interval that contains abundant biogenic magnetite. Such separation casts doubt on a causal link between the impact and sediment bleaching. More likely, the low susceptibility layer marks a different environmental event that preceded the impact. An episode of increased atmospheric and oceanic acidity associated with Deccan Traps volcanism that occurred well before the K–Pg impact is argued here to account for the distinct magnetic properties of the low susceptibility intervals.

Reference
Abrajevitch A, Font E, Florindo F, Roberts AP (2015) Asteroid impact vs. Deccan eruptions: The origin of low magnetic susceptibility beds below the Cretaceous–Paleogene boundary revisited. Earth and Planetary Science Letters 430, 209–223
Link to Article [doi:10.1016/j.epsl.2015.08.022]
Copyright Elsevier

^1,2Yves Marrocchi, ³Marc Chaussidon
¹CNRS, CRPG UMR 7358, Vandoeuvre-lès-Nancy, F-54501, France
²Université de Lorraine, CRPG UMR 7358, Vandoeuvre-lès-Nancy, F-54501, France
³Institut de Physique du Globe de Paris (IPGP), CNRS UMR 7154, Paris, F-75238, France

Primitive meteorites are characteristically formed from an aggregation of sub-millimeter silicate spherules called chondrules. Chondrules are known to present large three-isotope oxygen variations, much larger than shown by any planetary body. We show here that the systematic of these oxygen isotopic variations results from open-system gas–melt exchanges during the formation of chondrules, a conclusion that has not been fully assessed up to now. We have considered Mg-rich porphyritic chondrules and have modeled the oxygen isotopic effects that would result from high-temperature interactions in the disk between precursor silicate dust and a gas enriched in SiO during the partial melting and evaporation of this dust. This formation process predicts: (i) a range of oxygen isotopic composition for bulk chondrules in agreement with that observed in Mg-rich porphyritic chondrules, and (ii) variable oxygen isotopic disequilibrium between chondrule pyroxene and olivine, which can be used as a proxy of the dust enrichment in the chondrule-forming region(s). Such enrichments are expected during shock waves that produce transient evaporation of dust concentrated in the mid-plane of the accretion disk or in the impact plumes generated during collisions between planetesimals. According to the present model, gas–melt interactions under high PSiO(gas) left strong imprints on the major petrographic, chemical and isotopic characteristics of Mg-rich porphyritic chondrules.

Reference
Marrocchi Y, Chaussidon M (2015) A systematic for oxygen isotopic variation in meteoritic chondrules. Earth and Planetary Science Letters 430, 308–315
Link to Article [doi:10.1016/j.epsl.2015.08.032]
Copyright Elsevier

¹Gerrit Budde, ¹Thomas S. Kruijer, ¹Mario Fischer-Gödde, ²Anthony J. Irving, ¹Thorsten Kleine
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany
²Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA

Determining the timescales of the accretion and chemical differentiation of meteorite parent bodies provides some of the most direct constraints on the formation of planetesimals and the earliest stages of planet formation. We present high-precision Hf–W isotope data for a comprehensive set of ureilites, ultramafic mantle restites derived from a partially melted and incompletely differentiated asteroid. All samples are characterized by strong 182W deficits, indicating that silicate melt extraction on the ureilite parent body at 3.3±0.7 Ma3.3±0.7 Ma after CAI formation postdated core formation in iron meteorite parent bodies by ∼2–3 Ma. Thermal modeling of planetesimal heating by 26Al-decay combined with the new Hf–W data indicates that the ureilite parent body accreted at ∼1.6 Ma after CAI formation and, therefore, more than ∼1 Ma later than iron meteorite parent bodies, but more than ∼0.5 Ma earlier that most chondrite parent bodies. Due to its relatively ‘late’ accretion, the ureilite parent body contained too little 26Al to cause complete melting and, therefore, would have probably remained incompletely differentiated even without exhaustion of 26Al by silicate melt segregation. Our results show that both in terms of degree of differentiation and accretion timescale the ureilite parent body is intermediate between fully differentiated and undifferentiated bodies, implying that there is an inverse correlation between extent of melting and metal–silicate separation versus time of accretion and differentiation.

Reference
Budde G, Kruijer TS, Fischer-Gödde M, Irving AJ, Kleine T (2015) Planetesimal differentiation revealed by the Hf–W systematics of ureilites. Earth and Planetary Science Letters 430, 316–325
Link to Article [doi:10.1016/j.epsl.2015.08.034]
Copyright Elsevier

¹S.M. Reddy, ¹T.E. Johnson, ²S. Fischer, ³W.D.A. Rickard,¹R.J.M. Taylor
¹Department of Applied Geology, Institute for Geoscience Research (TIGeR), Curtin University, GPO Box U1987, Perth, WA 6845, Australia
²Department of Earth and Environmental Sciences, University of St Andrews, St Andrews, Fife KY16 9AL, UK
³Department of Imaging and Applied Physics, Curtin University, GPO Box U1987, Perth, WA 6845, Australia

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

Reference
Reddy SM, Johnson TE, Fischer S, Rickard WDA, Taylor RJM (2015) Precambrian reidite discovered in shocked zircon from the Stac Fada impactite, Scotland. Geology (in Press)
Link to Article [doi: 10.1130/G37066.1]

^1,2V.V. Svetsov ^1,2V.V. Shuvalov
¹Institute for Dynamics of Geospheres, Leninskiy Prospekt 38-1, Moscow 119334, Russia
²Moscow Institute of Physics and Technology, Institutskiy Per. 9, Dolgoprudny, Moscow Region, 141700, Russia

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

Reference
Svetsov VV, Shuvalov VV (2015) Water delivery to the Moon by asteroidal and cometary Impacts. Planetary and Space Science (in Press)
Link to Article [doi:10.1016/j.pss.2015.09.011]

^1,2A. Garenne,^1,2P. Beck,^3,4,5,6G. Montes-Hernandez, ^1,2O. Brissaud, ^1,2B. Schmitt, ^1,2E. Quirico, ^1,2L. Bonal, ⁷C. Beck, ^8,9K.T. Howard
¹Univ. Grenoble Alpes, IPAG, F-38000 Grenoble, France
²CNRS, IPAG, F-38000 Grenoble, France
³Univ. Grenoble Alpes, ISTerre, F-38000 Grenoble, France
⁴CNRS, ISTerre, F-38000 Grenoble, France
⁵IRD, ISTerre, F-38000 Grenoble, France
⁶IFSTTAR, ISTerre, F-38000 Grenoble
⁷Univ. Savoie, ISTerre Science Institue EARTH, 73376, Le Bourget du Lac, France
⁸Kingsborough Community College, 2001 Oriental Blvd., Brooklyn, NewYork, NT 11235, USA
⁹Department of Earth and Planetary Sciences, American Museum of Natural History.

In this study, we measured bidirectional reflectance spectra (0.5-4.0 μm) of 24 CMs, five CRs, one CI, one CV, and one C2 carbonaceous chondrites. These meteorites are known to have experienced an important variability in their relative degrees of aqueous alteration degree (Rubin et al., 2007 and Howard et al., 2009; 2011; Alexander et al., 2013). These measurements were performed on meteorite powders inside an environmental cell under a primary vacuum and heated at 60°C in order to minimize adsorbed terrestrial water. This protocol allows controlling of atmospheric conditions (i.e. humidity) in order to avoid contamination by terrestrial water. We discuss various spectral metrics (e.g. reflectance, band depth, single-scattering albedo, …) in the light of recent bulk composition characterization (Howard et al., 2009, Howard et al., 2015, Alexander et al., 2012, Beck et al., 2014 and Garenne et al., 2014). This study reveals variability of reflectance among meteorite groups. The reflectance is not correlated with carbon or hydrogen abundance neither with measured grain size distribution. We suggest that it is rather controlled by the nature of accreted components, in particular the initial matrix/chondrule proportion. Band depth, integrated band depth, mean optical path length, normalized optical path length, effective single–particle absorption thickness were calculated on the so called 3-μm band for reflectance spectra and for single scattering albedo spectra. They were compared with hydrated phase proportions from previous study on the same meteorites by thermogravimetric analyses and infrared spectroscopy in transmission. We find that normalized optical path length (NOPL) is the most appropriate to quantify water abundance, with an absolute error of about 5 wt.%. These datasets also reveal a variability of the band shape between 2.8 and 2.9 μm, which is interpreted as reflecting variation in the chemical composition and structure of phyllosilicates. This chemical variation could also be used to quantify the aqueous alteration degree between meteorite groups. The combination of reflectance at 2 μm and the depth of 3-μm band can be combined, to classify carbonaceous chondrites in reflectance in term of primary composition (e.g. matrix/chondrule ratio, carbon content) and secondary processes (e.g. aqueous alteration, thermal metamorphism). This could be used to decipher the nature of aqueous alteration in C-complex asteroids.

Reference
Garenne A, Beck P, Montes-Hernandez G, Brissaud O, Schmitt B, Quirico E, Bonal L, Beck C, Howard KT (2015) Bidirectional reflectance spectroscopy of carbonaceous chondrites: Implications for water quantification and primary composition. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.09.005]
Copyright Elsevier

¹H.M. Kaluna, ²J.R. Masiero, ¹K.J. Meech
¹Institute for Astronomy, University of Hawaii, 2680 Woodlawn Dr., Honolulu-HI-96822, USA
²Jet Propulsion Laboratory/Caltech, Pasadena, CA, USA

We present visible spectroscopic and albedo data of the 2.3 Gyr old Themis family and the 15 km) and small (⩽⩽15 km) Themis members suggest these phyllosilicate feature and albedo trends result from regolith variations as a function of diameter. Observations of the Beagle asteroids show a small, but notable fraction of members with phyllosilicate features. The presence of phyllosilicates and the dynamical association of the main-belt comet 133P/Elst-Pizarro with the Beagle family imply the Beagle parent body was a heterogenous mixture of ice and aqueously altered minerals.

Reference
Kaluna HM, Masiero JR, Meech KJ (2015) Space Weathering Trends Among Carbonaceous Asteroids. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.09.007]
Copyright Elsevier

¹Romy D. Hanna,¹Richard A. Ketcham,²Mike Zolensky,¹Whitney Behr
¹Jackson School of Geosciences, University of Texas, Austin TX 78712, USA
²Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, TX 77058, USA

X-ray computed tomographic scanning of a 44 g Murchison stone (USNM 5487) reveals a preferred alignment of deformed, partially altered chondrules, which define a prominent foliation and weak lineation in 3D. The presence of a lineation and evidence for a component of rotational, noncoaxial shear suggest that the deformation was caused by impact. Olivine optical extinction indicates that the sample can be classified as shock stage S1, and electron backscatter diffraction (EBSD) and electron microscopy reveal that plastic deformation within the chondrules was minimal and that brittle deformation in the form of fracturing, cataclasis, and grain boundary sliding was the dominant microstructural strain-accommodating mechanism. Textural evidence such as serpentine veins parallel to the foliation fabric and crosscutting alteration veins strongly suggest that some aqueous alteration post-dated or was contemporaneous with the deformation and that multiple episodes of fracturing and mineralization occurred. Finally, using the deformed shape of the chondrules we estimate that the strain experienced by Murchison was 17-43%. This combined with the current measured porosity of Murchison suggests that the original bulk porosity of Murchison prior to its deformation was 32.2 – 53.4% and likely at the upper end of this range due to chondrule compressibility, providing a unique estimate of pre-deformation porosity for a carbonaceous chondrite. Our findings suggest that significant porosity loss, deformation, and compaction from impact can occur on chondrite parent bodies whose samples may record only a low level of shock, and that significant chondrule deformation resulting in a chondrite foliation fabric can occur primarily through brittle processes and does not require plastic deformation of grains.

Reference
Hanna RD, Ketcham RA, Zolensky M, Behr W (2015) Impact-induced brittle deformation, porosity loss, and aqueous alteration in the Murchison CM chondrite. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.09.005]
Copyright Elsevier

¹Frans J. M. Rietmeijer
¹Department of Earth and Planetary Sciences, MSC 03-2040, 1-University of New Mexico, Albuquerque, New Mexico, USA

The bulbous Stardust track #80 (C2092,3,80,0,0) is a huge cavity. Allocations C2092,2,80,46,1 nearest the entry hole and C2092,2,80,47,6 about 0.8 mm beneath the entry hole provide evidence of highly chaotic conditions during capture. They are dominated by nonvesicular low-Mg silica glass instead of highly vesicular glass found deeper into this track which is consistent with the escape of magnesiosilica vapors generated from the smallest comet grains. The survival of delicate (Mg,Al,Ca)-bearing silica glass structures is unique to the entry hole. Both allocations show a dearth of surviving comet dust except for a small enstatite, a low-Ca hypersthene grain, and a Ti-oxide fragment. Finding scattered TiO2 fragments in the silica glass could support, but not prove, TiO2 grain fragmentation during hypervelocity capture. The here reported dearth in mineral species is in marked contrast to the wealth of surviving silicate and oxide minerals deeper into the bulb. Both allocations show Fe-Ni-S nanograins dispersed throughout the low-Mg silica glass matrix. It is noted that neither comet Halley nor Wild 2 had a CI bulk composition for the smallest grains. Using the analogs of interplanetary dust particles (IDPs) and cluster IDPs it is argued that a CI chondritic composition requires the mixing of nonchondritic components in the appropriate proportions. So far, the fine-grained Wild 2 dust is biased toward nonchondritic ferromagnesiosilica materials and lacking contributions of nonchondritic components with Mg-Fe-Ni-S[Si-O] compositions. To be specific, “Where are the GEMS”? The GEMS look-alike found in this study suggests that evidence of GEMS in comet Wild 2 may still be found in the Stardust glass.

Reference
Rietmeijer FJM (2015) The smallest comet 81P/Wild 2 dust dances around the CI composition. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12510]
Published by arrangement with John Wiley&Sons