¹Knut Metzler
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13091]
¹Institut für Planetologie, Westfälische Wilhelms‐Universität Münster, , Münster, Germany
Published by arrangement with John Wiley & Sons

In order to characterize the relation between apparent chondrule sizes (2D) and true chondrule sizes (3D), three ordinary chondrites of the H, L, and LL group have been analyzed. The diameters of a large number of chondrule cut faces in thin sections (2D; n = 2037) and of separated chondrules from the same meteorites (3D: n = 2061) have been measured. The obtained 2D/3D mean chondrule sizes (μm) for the H, L, and LL chondrite are 450/490, 500/610, and 690/830; the corresponding median values (μm) are 370/420, 450/530, and 580/730. The data show that there is a cutoff for small chondrule sizes in each sample. Possibly characteristic minimum sizes exist for the various groups, increasing in the (3D) sequence H (~90 μm) <L (~180 μm) <LL (~240 μm). No systematics were found for the maximum chondrule sizes. The investigated samples show very similar chondrule volume (mass) distributions relative to the mode (peak) of their size‐frequency distributions. About 2.6–2.9% and 97.1–97.4% of the total chondrule volume (mass) is present in chondrule sizes smaller and larger than the mode, respectively. It was found that 2D sectioning consistently results in a shift of the true 3D size‐frequency distributions toward smaller sizes. This effect leads to the underestimation of the values for (1) the true mean chondrule size by 8–18%, (2) the true chondrule median value by 12–21%, and (3) the true mode value of the size‐frequency distributions by 12–17% (50 μm binning). This is the opposite of what popular 2D/3D correction models predict (e.g., Eisenhour 1996).

¹Alexander Hubbard, ¹Mordecai‐Mark Mac Low, ²Denton S. Ebel
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13101]
¹Department of Astrophysics, American Museum of Natural History, New York, New York, USA
²Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA
Published by arrangement with John Wiley & Sons

Meteoritical and astrophysical models of planet formation make contradictory predictions for dust concentration factors in chondrule‐forming regions of the solar nebula. Meteoritical and cosmochemical models strongly suggest that chondrules, a key component of the meteoritical record, formed in regions with solids‐to‐gas mass ratios orders above the solar nebula average. However, models of dust grain dynamics in protoplanetary disks struggle to surpass concentration factors of a few except during very short‐lived stages in a dust grain’s life. Worse, those models do not predict significant concentration factors for dust grains the size of chondrule precursors. We briefly develop the difficulty in concentrating dust particles in the context of nebular chondrule formation and show that the disagreement is sufficiently stark that cosmochemists should explore ideas that might revise the concentration factor requirements downward.

¹Rebecca Winkler, ²Robert Luther, ¹Michael H. Poelchau, ²Kai Wünnemann, ¹Thomas Kenkmann
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13080]
¹Institute of Earth and Environmental Sciences—Geology, Albert‐Ludwigs‐Universität Freiburg (ALU), , Freiburg, Germany
²Museum für Naturkunde Berlin, Leibniz Institute for Evolution and Biodiversity Science, , Berlin, Germany
Published by arrangement with John Wiley and Sons

Two impact cratering experiments on nonporous rock targets were carried out to determine the influence of target composition on the structural mechanisms of subsurface deformation. Projectiles of 2.5 mm diameter were accelerated to ~5 km s⁻¹and impacted onto blocks of marble or quartzite. Subsurface deformation was mapped and analyzed on the microscale using thin sections of the bisected craters. Additionally, both experiments were modeled and the calculated strain zones underneath the craters were compared to experimental deformation features. Microanalysis shows that the formation of radial, tensile, and intragranular cracks is a common response of both nonporous materials to impact cratering. In the quartzite target, the subsurface damage is additionally characterized by highly localized deformation along shear bands with intense grain comminution, surrounded by damage zones. In contrast, the marble target shows closely spaced calcite twinning and cleavage activation. Crater diameter and depth as well as the damage lens underneath the crater are unexpectedly smaller in the marble target compared to the quartzite target, which is in contradiction to the marble’s much weaker compressive and tensile strengths. However, numerical models result in craters that are similar in size as well as in strain accumulation at the end of transient crater formation, indicating that current models should still be viewed cautiously when compared to experimental details.