¹James Mortimer, ²Christophe Lécuyer, ²François Fourel, ³James Carpenter
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2018.05.010]
¹Planetary and Space Sciences, School of Physical Sciences, The Open University, Walton Hall, MK7 6AA, Milton Keynes, Buckinghamshire, United Kingdom
²Laboratoire de Géologie de Lyon, CNRS UMR 5276, Université Claude Bernard Lyon 1, 69622, Villeurbanne, France
³ESA ESTEC, Keplerlaan 1, 2401, AZ, Noordwijk, the Netherlands

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¹R.Brunetto et al. (>10)
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2018.05.008]
¹IAS, UMR 8617, CNRS, Université Paris-Sud, Bât 121, F-91405, Orsay, France

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^1,2Pierre Beck,³A.Maturilli,¹A.Garenne,⁴P.Vernazza,³J.Helbert,¹E.Quirico,¹B.Schmitt
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.05.010]
¹Institut de Planétologie et d’Astrophysique de Grenoble, France.
²Institut Universitaire de France.
³DLR, Berlin, Germany.
⁴Aix Marseille Univ, CNRS, LAM, Laboratoire d’Astrophysique de Marseille, Marseille, France
Copyright Elsevier

In order to determine the controls on the reflectance spectra of hydrated carbonaceous chondrites, reflectance spectra were measured for a series of samples with well-determined mineralogy, water-content, and thermal history. This includes 5 CR chondrites, 11 CM chondrites, and 7 thermally metamorphosed CM chondrites. These samples were characterized over the 0.35 to 150 µm range by reflectance spectroscopy in order to cover the full spectral range accessible from ground based observation, and that will be determined in the near-future by the Hayabusa-2 and Osiris-REx missions. While spectra show absorption features shortward of 35 µm, no strong absorption bands were identified in this suite of samples longward of 35 µm. This work shows that the 0.7-µm band observed in hydrated carbonaceous chondrites is correlated with the total water content as well as with the band depth at 2.7 µm, confirming the suggestion that they are related to Mg-rich, Fe-bearing phyllosilicates. A feature at 2.3 µm, diagnostic of such phyllosilicates was found for all samples with a detectable 0.7-µm band, also indicative of Mg-rich phyllosilicates.
A strong variability is found in the shape of the 3-µm band among CM chondrites, and between CM, CR and thermally metamorphosed CM chondrites. Heavily altered CM chondrites show a single strong band around 2.72 µm while more thermally metamorphosed CM samples show an absorption band at higher wavelength. The CR chondrite GRO 95577 has a 3-µm feature very similar to those of extensively altered CM chondrites while other CR chondrite rather shows goethite-like signatures (possibly due to terrestrial weathering of metals). Thermally metamorphosed CM chondrites all have 3-µm features, which are not purely due to terrestrial adsorbed water. The band shape ranges from heavily altered CM-like to goethite-like.
The overall reflectance was found to be significantly higher for CR chondrites than for CM chondrites. This is also true for the hydrated CR chondrite GRO 95577 whose reflectance spectrum is almost identical to spectra obtained for CM chondrites except that it is brighter by about 40 % in the visible. Another possibility to distinguish hydrated CM from hydrated CR chondrites is to use the combination of band depths at 0.7 and 2.3 µm.
When comparing the spectra obtained with Cg and Cgh spectral end member, it is found that the band depth determined for hydrated chondrites (0.7 and 2.3 µm) are always higher than calculated for these spectral endmembers. If one considers only asteroids with unambiguous hydration detection, band depth at 0.7 µm are of similar values to those measured for hydrated carbonaceous chondrites.

¹Quinn R.Shollenberger,¹Jan Render, ¹Gregory A.Brennecka
Earth and Planetary Science Letters 495, 12-23 Link to Article [https://doi.org/10.1016/j.epsl.2018.05.007]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Straße 10, Münster, 48149 Germany
Copyright Elsevier

The oldest dated solids in our Solar System, calcium–aluminum-rich inclusions (CAIs), contain isotopic anomalies in a whole suite of elements relative to later formed Solar System materials. Previous work has reported differences in the proportions of nucleosynthetic components between CAIs and terrestrial rocks as a function of mass. However, the nucleosynthetic fingerprint of the CAI-forming region is still lacking significant data in the heavier mass range (A > 154). Therefore, we present the first erbium (Er) and ytterbium (Yb) isotopic data along with hafnium (Hf) isotopic compositions in a wide variety of CAIs derived from a variety of CV and CK chondrites. This work presents new methods for Er and Yb isotopic investigation that were explored using both thermal ionization mass spectrometry (TIMS) and multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). Relative to terrestrial rock standards, CAIs—regardless of host rock, petrologic or chemical classification—have uniform and resolvable Er, Yb, and Hf isotopic compositions. The CAI isotopic patterns correspond to r-process deficits (or s-process excesses) relative to terrestrial values of 9 ppm for Er, 18 ppm for Yb, and 17 ppm for Hf. This new Er, Yb, and Hf data help complete the nucleosynthetic fingerprint of the CAI-forming region, further highlighting the systematic difference between the CAIs and later formed bulk planetary bodies. Such a systematic difference between CAIs and terrestrial rocks cannot be caused by different amounts of any known single presolar phase but is likely the result of a well-mixed reservoir made of diverse stellar sources.

^1,2Kim T.Tait, ²James M.D.Day
Earth and Planetary Science Letters 494, 99-108 Link to Article [https://doi.org/10.1016/j.epsl.2018.04.040]
¹Royal Ontario Museum, 100 Queens Park, Toronto, ON M5S 2C6, Canada
²Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0244, USA
Copyright Elsevier

Mars is considered to have formed as a planetary embryo that experienced extensive differentiation early in its history. Shergottite meteorites preserve evidence for this history, and for late accretion events that affected their mantle sources within Mars. Here we report the first coupled 187Re–187Os, 87Sr/86Sr, highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) and major element abundance dataset for martian shergottites that span a range of MgO contents, from 6.4 to 30.3 wt.%. The shergottites range from picro-basalt to basaltic-andesite compositions, have enriched to depleted incompatible trace-element compositions, and define fractional crystallization trends, enabling the determination of HSE compatibility for martian magmatism in the order: Os > Ir ≥ Ru ≫ Pt ≥ Pd ≥ Re. This order of compatibility is like that defined previously for Earth and the Moon, but the fractionation of strongly compatible Os, Ir and Ru appears to take place at higher MgO contents in martian magmas, due to early onset of sulfide fractionation. In general, enriched shergottites have lower MgO contents than intermediate or depleted shergottites and have fractionated HSE patterns (Re + Pd + Pt > Ru + Ir + Os) and more radiogenic measured 87Sr/86Sr (0.7127–0.7235) and 187Os/188Os (0.140–0.247) than intermediate or depleted shergottite meteorites (87Sr/86Sr = 0.7010–0.7132; 187Os/188Os = 0.127–0.141). Osmium isotope compositions, corrected for crystallization age, define compositions that are implausibly unradiogenic in some enriched shergottites, implying recent mobilization of Re in some samples. Filtering for the effects of alteration and high Re/Os through crystal-liquid fractionation leads to a positive correlation between age-corrected Sr and Os isotope compositions. Mixing between hypothetical martian crustal and mantle reservoirs are unable to generate the observed Sr–Os isotope compositions of shergottites, which require either distinct and discrete long-term incompatible-element depleted and enriched mantle sources, or originate from hybridized melting of deep melts with metasomatized martian lithosphere. Using MgO-regression methods, we obtain a modified estimate of the bulk silicate Mars HSE composition of (in ng g−1) 0.4 [Re], 7.4 [Pd], 9.6 [Pt], 6.2 [Ru], 3.7 [Ir], 4 [Os], and a long-term chondritic 187Os/188Os ratio (∼0.1312). This result does not permit existing models invoking high-pressure and temperature partitioning of the HSE. Instead, our estimate implies 0.6–0.7% by mass of late accretion of broadly chondritic material to Mars. Our results indicate that Mars could have accreted earlier than Earth, but that disproportional accretion of large bodies and a relative constant flux of accretion of available materials in the first 50–100 Ma of Solar System led to the broad similarity in HSE abundances between Earth and Mars.

¹J.I.Simon,²J.N.Cuzzi^1,2K.A.McCain,⁴M.J.Cato,⁵P.A.Christoffersen,⁶K.R.Fisher⁷P.Srinivasan⁸A.W.Tait⁹D.M.Olson²J.D.Scargle
Earth and Planetary Science Letters 494, 69-82 Link to Article [https://doi.org/10.1016/j.epsl.2018.04.021]
¹Center for Isotope Cosmochemistry and Geochronology, ARES, EISD-XI3, NASA Johnson Space Center, Houston, TX 77058, USA
²NASA Ames Research Center, Moffett Field, CA 94035, USA
³The University of Chicago, Chicago, IL 60637, USA
⁴Western Carolina University, Cullowhee, NC, 28723, USA
⁵St. Lawrence University, Canton, NY, 13617, USA
⁶University of Cincinnati, Cincinnati, OH, 45219, USA
⁷Rutgers University, Piscataway, NJ, 08854, USA
⁸Monash University, Clayton, 3168, VIC, Australia
⁹BAERI, inc., Petaluma, CA 94952, USA
Copyright Elsevier

Magnesium-rich silicate chondrules and calcium-, aluminum-rich refractory inclusions (CAIs) are fundamental components of primitive chondritic meteorites. It has been suggested that concentration of these early-formed particles by nebular sorting processes may lead to accretion of planetesimals, the planetary bodies that represent the building blocks of the terrestrial planets. In this case, the size distributions of the particles may constrain the accretion process. Here we present new particle size distribution data for Northwest Africa 5717, a primitive ordinary chondrite (ungrouped 3.05) and the well-known carbonaceous chondrite Allende (CV3). Instead of the relatively narrow size distributions obtained in previous studies (Ebel et al., 2016, Friedrich et al., 2015, Paque and Cuzzi, 1997, and references therein), we observed broad size distributions for all particle types in both meteorites. Detailed microscopic image analysis of Allende shows differences in the size distributions of chondrule subtypes, but collectively these subpopulations comprise a composite “chondrule” size distribution that is similar to the broad size distribution found for CAIs. Also, we find accretionary ‘dust’ rims on only a subset (∼15–20%) of the chondrules contained in Allende, which indicates that subpopulations of chondrules experienced distinct histories prior to planetary accretion. For the rimmed subset, we find positive correlation between rim thickness and chondrule size. The remarkable similarity between the size distributions of various subgroups of particles, both with and without fine grained rims, implies a common size sorting process. Chondrite classification schemes, astrophysical disk models that predict a narrow chondrule size population and/or a common localized formation event, and conventional particle analysis methods must all be critically reevaluated. We support the idea that distinct “lithologies” in NWA 5717 are nebular aggregates of chondrules. If ≥cm-sized aggregates of chondrules can form it will have implications for planet formation and suggests the sticking stage is where the preferential size physics is operating.

¹Timo Hopp, ¹Thorsten Kleine
Earth and Planetary Science Letters 494, 50-59 Link to Article[https://doi.org/10.1016/j.epsl.2018.04.058]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
Copyright Elsevier

Elevated abundances of highly siderophile elements in Earth’s mantle are thought to reflect the late accretion of primitive material after the cessation of core formation, but the origin of this material, and whether or not it can be linked to specific types of meteorites remain debated. Here, mass-dependent Ru isotopic data for chondrites and terrestrial peridotites are reported to evaluate the chemical nature and type of the late-accreted material. After correction for nucleosynthetic Ru isotope anomalies, enstatite, ordinary and carbonaceous chondrites all have indistinguishable mass-dependent Ru isotopic compositions. Thus, neither distinct formation conditions in the solar nebula nor parent body processes resulted in significant mass-dependent Ru isotope fractionation. All five terrestrial peridotites analyzed have mass-dependent Ru isotopic compositions that are indistinguishable from each other and from the composition of chondrites. The chondritic mass-dependent Ru isotopic composition of Earth’s mantle is difficult to reconcile with prior suggestions that the late accretionary assemblage was a mixture of chondrites with a chemically evolved metal component. Although this mixture can reproduce the suprachondritic Ru/Ir inferred for Earth’s mantle, it consistently predicts a heavy Ru isotopic composition of Earth’s mantle with respect to chondrites. This is because metal components with elevated Ru/Ir are also enriched in heavy Ru isotopes, resulting from isotope fractionation during core crystallization. Thus, if late accretion involved impacts of differentiated protoplanetary bodies, then the projectile cores must have been either homogenized upon impact, or added to Earth’s mantle completely, because otherwise Earth’s mantle would have inherited a non-chondritic mass-dependent Ru isotopic composition from the unrepresentative sampling of core material.