¹K. Metzler,²D. C. Hezel,³J. Nellesen
The Astrophysical Journal 887, 230 Link to Article [DOI
https://doi.org/10.3847/1538-4357/ab58d0]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany
²Institut für Geologie und Mineralogie, University of Cologne, Zülpicher Straße 49a, D-50674 Köln, Germany
³Fakultät Maschinenbau, Technische Universität Dortmund, Leonhard-Euler-Str. 2, D-44227 Dortmund, Germany

Chondrules are approximately millimeter-sized beads of crystallized silicate melt. They formed mainly in the first ~3 Ma of the Sun’s protoplanetary disk and are the main constituents of chondritic asteroids. Here we report on the size–frequency distributions (2D and 3D) of chondrules in the brecciated ordinary chondrite (OC) Northwest Africa (NWA) 5205. We investigated three large (centimeter- to decimeter-sized) chondritic lithic clasts of a particular textural type (“cluster chondrite”) with eye-catching different chondrule sizes. One clast shows the largest mean chondrule size (~1.5 mm) ever measured in a chondrite. As in the other OCs, we find a positive correlation between the minimum and mean chondrule size, which we consider as an argument for chondrule size sorting. Chondrule size–frequency distributions in the clasts are distinctly more symmetric than the about log-normal distributions in other OCs. Furthermore, we find a co-enrichment of chondrule types with a priori small mean sizes (type I, porphyritic) in clasts with overall small mean chondrule sizes. We consider this as the fingerprint of an additional/second size-sorting process, which acted later on these chondrule populations. This process possibly subdivided a typical LL-type chondrule population into several subpopulations with different mean chondrule sizes. We speculate that this second sorting occurred in a unidirectional gas stream or headwind, e.g., by settling of chondrules through an asteroidal atmosphere or interaction with an expanding impact plume. Possibly, fine-grained matrix was almost completely removed by this, and the size-sorted chondrule subpopulations accreted in a hot state separately in different regions of the asteroid.

^1,2Guillaume Florin,¹Béatrice Luais,² Tracy Rushmer,^2,3Olivier Alard
Geochimica et Cosmochimica Acta 269, 270-291 Link to Article [https://doi.org/10.1016/j.gca.2019.10.038]
¹Centre de Recherches Pétrographiques et Géochimiques, CRPG-CNRS – UMR 7358, Université de Lorraine, 15 Rue Notre Dame des Pauvres, 54500 Vandœuvre-lès-Nancy, France
²Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia
³Géosciences Montpellier, UMR 5243, CNRS & Université Montpellier, 34095 Montpellier, France
Copyright Elsevier

Ordinary chondrites (OCs) are classified into three groups, according to their oxidation state, which increases from the H to L to LL groups. This is demonstrated by the decrease in metal content (H = ∼8 vol%, L = ∼4 vol%, and LL = ∼2 vol%), and by a positive correlation between Δ¹⁷O and %Fa through the OC sequence. Compared to other chondrites, OCs exhibit the largest variation in oxidation state, but there is an ongoing debate on the processes that control this variation. To constrain the causes of the variations in the oxidation state with respect to the associated nebular versus parent bodies processes, we investigated the elemental and isotopic variations of germanium (moderately siderophile and volatile) in the bulk sample, as well as in the metal, silicate and sulfide phases, over a range of petrographic types for the H, L, and LL ordinary chondrites.

We found that δ^74/70Ge_metal is a proxy for the δ^74/70Ge_bulk composition and that each OC group is distinguishable by their δ^74/70Ge_metal, which increases from −0.51 ± 0.09‰ for H chondrites, −0.31 ± 0.06‰ for L chondrites, and, finally, to −0.26 ± 0.09‰ for LL chondrites (2σ SD). Additionally, the OC sequence exhibited a positive correlation, from H to L to LL, between δ^74/70Ge_metal and %Fa, as well as oxygen isotopes (δ¹⁷O, δ¹⁸O and Δ¹⁷O), that was not a consequence of a “size sorting effect” on chondrules (i.e., chondrule mixing) or metamorphic processes in the parent bodies but, rather, was the result of nebular processes. We propose that the correlation between the δ^74/70Ge values and %Fa, Δ¹⁷O, δ¹⁸O can be explained by an increasing proportion of accreted hydrated phyllosilicates, from the H, L to LL groups, with high δ^74/70Ge and Δ¹⁷O. We found that 10 to 15% of phyllosilicates, with a composition of [Ge] = 4–7 ppm and δ^74/70Ge = 3–2.5‰, is needed to change the δ^74/70Ge from H to LL, which corresponds to a Δ¹⁷O ≈ 8–7‰. This value agrees with the Δ¹⁷O ≈ 7‰ composition of the accreted nebular component reported by Choi et al. (1998). During thermal metamorphism, phyllosilicates destabilize, liberating germanium that will be incorporated in the metal, then leading to its high δ^74/70Ge signature.

High-temperature metamorphism can explain the lack of δ^74/70Ge_metal variation with the petrologic type in the OC, even for the type 3 chondrites (T ≈ 675 °C), implying a complete reaction even at low petrologic types. In addition, metal-silicate re-equilibration in response to thermal metamorphism results in a decrease in Δ^74/70Ge_{metal-silicate} from 0.33‰ to 0.06‰, within the H chondrite group, which is interpreted as the result of δ^74/70Ge_silicate variation. The mean positive Δ^74/70Ge_{metal-silicate} fractionation factor of +0.22 ± 0.36‰ (error propagation on individual error) also displays a remarkable similarity to the direction of isotopic fractionation with other germanium isotopic metal-silicate datasets, such as the magmatic iron meteorites, the Earth silicate reservoirs. We propose that the Δ^74/70Ge_{metal-silicate} and the negative δ^74/70Ge values of OCs are inherited from metal-silicate melting and partial exchange before planetesimal accretion in a light isotope-enriched gas. Finally, the δ^74/70Ge_metal-Δ¹⁷O_silicate correlation between the IIE iron meteorites and OCs, provides new evidence for the existence of a highly reduced HH group.