¹Takaharu Saito,¹Hiroshi Hidaka,²Seung-Gu Lee
The Astrophysical Journal 877, 73 Link to Article [https://doi.org/10.3847/1538-4357/ab1aa5]
¹Department of Earth and Planetary Sciences, Nagoya University, Nagoya 464-8601, Japan
²Geological Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Republic of Korea

Howardite–eucrite–diogenite meteorites are believed to originate in the crust of the asteroid 4 Vesta, whose differentiation processes are still controversial. In this study, the first 176Lu–176Hf isotopic data of nine diogenites are presented together with their 87Rb–87Sr isotopic compositions and rare earth element (REE) abundances to investigate the differentiation process of diogenites. The 176Lu–176Hf data sets of nine diogenites revealed the significantly higher initial 176Hf/177Hf ratio of diogenites than that of eucrites, while there are no resolvable differences between their ages. Based on the high initial ratio and the early formation of diogenites, their source material is estimated to be the Vestan mantle. The 87Rb–87Sr systematics of nine diogenites are entirely disturbed probably due to impact events on Vesta. The significant variation observed in the REE abundances of nine diogenites suggests their crystallization from compositionally diverse melts. Based on the mantle origin and compositional diversity of diogenites, we propose the crystallization of diogenites from partial melts of the Vestan mantle. The variation of the trace element abundances of diogenites can be explained by the variation of the degree of the partial melting. The timescale between the crystallization and partial melting of the Vestan mantle is estimated to be ~100–600 Ma from the 176Lu–176Hf isotopic data of nine diogenites, while a heat source for the partial melting is uncertain.

¹Bernd Helber,^1,2Bruno Dias,^1,3,4Federico Bariselli,¹Luiza F. Zavalan,⁵Lidia Pittarello,⁶Steven Goderis,⁶Bastien Soens,^6,7,8Seann J. McKibbin,⁶Philippe Claeys,¹Thierry E. Magin
The Astrophysical Journal 876, 120 Link to Article [https://doi.org/10.3847/1538-4357/ab16f0]
¹Aeronautics and Aerospace Department, von Karman Institute for Fluid Dynamics, Rhode-Saint-Genèse, Belgium
²Institute of Mechanics, Materials and Civil Engineering, Université catholique de Louvain, Louvain-la-Neuve, Belgium
³Research Group Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Brussels, Belgium
⁴Dipartimento di Scienze e Tecnologie Aerospaziali, Politecnico di Milano, Milano, Italy
⁵Department of Lithospheric Research, University of Vienna, Vienna, Austria
⁶Analytical, Environmental, and Geo-Chemistry, Vrije Universiteit Brussel, Brussels, Belgium
⁷Institute of Earth and Environmental Science, University of Potsdam, Potsdam-Golm, Germany
⁸Geowissenschaftliches Zentrum, Georg-August-Universität Göttingen, Göttingen, Germany

Meteoroids largely disintegrate during their entry into the atmosphere, contributing significantly to the input of cosmic material to Earth. Yet, their atmospheric entry is not well understood. Experimental studies on meteoroid material degradation in high-enthalpy facilities are scarce and when the material is recovered after testing, it rarely provides sufficient quantitative data for the validation of simulation tools. In this work, we investigate the thermo-chemical degradation mechanism of a meteorite in a high-enthalpy ground facility able to reproduce atmospheric entry conditions. A testing methodology involving measurement techniques previously used for the characterization of thermal protection systems for spacecraft is adapted for the investigation of ablation of alkali basalt (employed here as meteorite analog) and ordinary chondrite samples. Both materials are exposed to a cold-wall stagnation point heat flux of 1.2 MW m−2. Numerous local pockets that formed on the surface of the samples by the emergence of gas bubbles reveal the frothing phenomenon characteristic of material degradation. Time-resolved optical emission spectroscopy data of ablated species allow us to identify the main radiating atoms and ions of potassium, calcium, magnesium, and iron. Surface temperature measurements provide maximum values of 2280 K for the basalt and 2360 K for the chondrite samples. We also develop a material response model by solving the heat conduction equation and accounting for evaporation and oxidation reaction processes in a 1D Cartesian domain. The simulation results are in good agreement with the data collected during the experiments, highlighting the importance of iron oxidation to the material degradation.

¹Gert Nolze,²Klaus Heide
Geochemistry (Chemie der Erde) (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.125538]
¹Dept 5.1, Federal Institute for Materials, Research and Testing (BAM), Unter den Eichen 87, 12205 Berlin, Germany
²Institut für Geowissenschaften (IGW), Friedrich-Schiller-Universität Jena (FSU), Germany
Copyright Elsevier

Roaldite – Fe4N – has been identified in the São Julião de Moreira iron meteorite using electron backscatter diffraction (EBSD) and simultaneously acquired energy-dispersive x-ray spectroscopy (EDS). Mean-periodic-number images derived from raw EBSD patterns confirm this phase by an even higher spatial resolution compared to EDS.
Roaldite appears in the form of systematically and repetitively aligned plates. Despite the locally heavy plastic deformation, it is shown that the origin of the oriented precipitation of roaldite is linked to the orientation of the kamacite matrix. Roaldite can be considered to be precipitated from kamacite using an inverse Kurdjumov-Sachs (K-S) or Nishiyama-Wassermann (N-W) orientation relationship. A more accurate discrimination is impossible due to the accumulated shock deformation, which blurs the local reference orientation of kamacite. The habit plane of roaldite is found to be {112}R, which is most likely parallel to {120}K of kamacite. Some of the roaldite plates contain two orientation variants which repeatedly alternate. Their misorientation angle is about 12°.

^1,2,3E.S.Steenstra,²J.Berndt, ¹S.Klemme,¹A.Rohrbach,¹E.S.Bullock,³W.van Westrenen
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.10.006]
¹The Geophysical Laboratory, Carnegie Institution of Science, Washington D.C., the United States of America
²Institute of Mineralogy, University of Münster, Germany
³Faculty of Science, Vrije Universiteit Amsterdam, The Netherlands
Copyright Elsevier

The geochemistry of asteroidal magmas provides fundamental clues to the processes involved in the origin and early evolution of planetary bodies. Although sulfides are important reservoirs for a diverse suite of major and trace elements, it is currently unclear whether the interiors of asteroid Vesta and the Angrite Parent Body were sulfide liquid saturated during petrogenesis of non-cumulate eucrites and volcanic angrites. To assess the potential of sulfide liquid saturation in the interiors of these bodies, high pressure (P) – temperature (T) experiments were used to quantify the sulfur concentrations at sulfide saturation (SCSS) for volcanic angrites and non-cumulate eucrites. The sulfide-silicate partitioning behavior of various trace elements was simultaneously quantified to study their geochemical behavior at sulfide liquid saturation.

It was found that the measured SCSS values agree well with the SCSS values predicted from a previous thermodynamic model for high-FeO* melts. To assess the possibility of sulfide liquid saturation of the source regions of non-cumulate eucrites and angrites, their S abundances were compared with the calculated SCSS values for their source regions. Results show that if eucritic and angritic source regions were saturated with FeS liquid, significant degassing (> 50–80%) of S must have occurred during or following their magmatic emplacement. Such loss is inconsistent with the S, Cl, Zn and Rb isotopic compositions of non-cumulate eucrites. Sulfide liquid saturation of eucrite and angrite source regions is also excluded from the strongly incompatible behavior of Cu and HSE in non-cumulate eucrites and angrites (Riches et al., 2012).

Additional calculations were performed to further explore the timing and extent of S loss during crystallization of the Vestan magma ocean. The assumption of chondritic bulk S abundances of bulk Vesta would correspond with extremely high S contents of the eucrite source region(s), even after consideration of depletion of S due to core formation. In light of the S, Cl, Zn and Rb stable isotopic compositions of eucrites, the S abundances in eucrites are most consistent with the hypothesis that the Vestan mantle was already strongly depleted in S (>70–80 %) by the time of Vestan magma ocean crystallization, resulting in more realistic S contents of the eucrite source region(s). The depletion of S could have been established during initial accretion of Vesta or it could simply reflect accretion of volatile depleted components that experienced incomplete condensation (Wu et al., 2018). Modeling of the new experimentally determined sulfide-silicate partition coefficients and previously reported Vestan mantle depletions of the various chalcophile and siderophile elements suggests that sulfide liquid segregation during early Vestan magma ocean crystallization is also unlikely. The lack of sulfide liquid saturation in the source regions of non-cumulate eucrites and angrites, as well as during early Vestan magma ocean solidification, shows that current geochemical models of core formation and late accretion remain valid for these bodies.