¹Giulio F.D.Solferino, ²Gregor J.Golabek
Earth and Planetary Science Letters 504, 38-52 Link to Article [https://doi.org/10.1016/j.epsl.2018.09.027]
¹Department of Earth Sciences, Royal Holloway University of London, TW20 0EX Egham, United Kingdom
²Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
Copyright Elsevier

The origin of pallasites has been the focus of a number of recent studies. Yet, their formation process remains elusive, while the mechanism leading to the genesis of the sub-group termed ‘mixed type’ pallasites (containing polygonal, rounded, and fragmental olivines simultaneously) is unclear. Here we test the hypothesis of mixing of olivine fragments with Fe–Ni–S after a non-destructive impact followed by annealing employing both experimental analogues and numerical models.
The experimental series evidenced that the addition of sulfur to olivine + Fe–Ni accelerates olivine grain growth, though the growth rate is reduced when Fe–Ni–S is not fully molten. This is shown to be the consequence of competing growth of olivine and Fe–Ni grains.
Numerical models satisfying available formation constraints from natural samples indicate that planetesimals with radii ≥200 km are favorable for the genesis of rounded olivine-bearing pallasites by annealing of fragments in partially molten Fe–Ni–S. Moreover, early mixing in the planetesimal can form regions containing olivine grains with different grain sizes that could explain the formation of mixed-type pallasites.

¹B. J. Tkalcec, ¹F. E. Brenker
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13204]
¹Institute of Geoscience, Goethe University, Frankfurt am Main, Germany
Published by arrangement with John Wiley & Sons

Shock is often given as the cause for many observations in meteorites due to the assumed previous exposure of most meteorites to at least one impact event that ultimately led to their ejection from their parent body. Here we present electron backscatter diffraction (EBSD) results on a substantially shocked dunitic achondrite, chassignite Northwest Africa (NWA) 8694, and question the general culpability of shock exposure for the formation of preferred orientation fabrics of meteoritic olivine crystals. Despite the ubiquitous presence of substantial shock indicators, the EBSD results for NWA 8694 reveal an absence of preferred orientation of olivine crystals, displaying instead an overall random fabric. We propose that the passage of shock waves through olivine crystals within a solid, crystalline, dunitic rock does not produce an overall preferred orientation, nor is it likely to actively form a whole‐rock random fabric but instead has likely no bearing on the formation of olivine orientation fabrics.

¹Laurette Piani, ¹Yves Marrocchi
Earth & Planetary Science Letters 504, 64-71 Link to Article [https://doi.org/10.1016/j.epsl.2018.09.031]
¹CRPG, UMR 7358 CNRS, Université de Lorraine, 54500 Vandoeuvre-lès-Nancy, France
Copyright Elsevier

Among the different groups of carbonaceous chondrites, variable concentrations of hydrous minerals and organic matter are observed that might be related to the time and/or place of formation of their asteroidal parent bodies. However, the precise distribution of these volatile-bearing components between chondrite groups and their chemical and isotopic compositions remain fairly unknown. In this study, we used a novel secondary ion mass spectrometry analytical protocol to determine the hydrogen isotopic composition of water-bearing minerals in CV-type carbonaceous chondrites. This protocol allows for the first time the D/H ratio of CV chondrite hydrous minerals to be determined without hindrance by hydrogen contributions from adjacent organic material. We found that water in the altered CV chondrites Kaba, Bali, and Grosnaja has an average D/H ratio of D/HCV-water = [144−21+8] × 10−6 (or δDCV-water = ‰−77−131+54‰, 2σ), significantly higher than water in most CM-type carbonaceous chondrites (D/HCM-water = [101 ± 6] × 10−6 or δDCM-water = −350 ± 40‰, 2σ). We show that because organic matter in CV chondrites is depleted in deuterium compared to that in CM chondrites, such differences could result from isotopic exchange between water and organics. Another possibility is that the CM and CV parent bodies sampled different reservoirs of water ice and organics characterized by variable isotopic compositions due to their different time and/or place of accretion.