A large planetary body inferred from diamond inclusions in a ureilite meteorite

1,2Farhang Nabiei, 1James Badro, 2,5Teresa Dennenwaldt, 2Emad Oveisi, 2Marco Cantoni, 2,5Cécile Hébert, 6Ahmed El Goresy, 7Jean-Alix Barrat, 1Philippe Gillet
Nature Communications 9, 1327 Link to Article [doi:10.1038/s41467-018-03808-6]
1Earth and Planetary Science Laboratory (EPSL), Institute of Physics, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
2Interdisciplinary Center for Electron Microscopy (CIME), Ecole 3Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
4Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Paris, France
5Electron Spectrometry and Microscopy Laboratory (LSME), Institute of Physics, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
6Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, Germany
7Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, Plouzané, France

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From 2D to 3D chondrule size data: Some empirical ground truths

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).

Dust concentration and chondrule formation

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.

Subsurface deformation of experimental hypervelocity impacts in quartzite and marble targets

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.

Fractional crystallization of the lunar magma ocean: Updating the dominant paradigm

1J. F. Rapp,2D. S. Draper
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13086]
1Jacobs JETS, NASA Johnson Space Center, Houston, Texas, USA
2Astromaterials Research Office, EISD, NASA Johnson Space Center, Houston, Texas, USA
Published by arrangement with John Wiley & Sons

We report results of systematic experimental simulation of fractional crystallization of a lunar magma ocean (LMO) with the Lunar Primitive Upper Mantle bulk composition. These results complement prior work that simulated equilibrium crystallization. In contrast to previous numerical models for investigating magma ocean solidification processes and implications, our combined program simulates these processes directly using petrologic experimentation. Our experiments mimic LMO crystallization that is fractional throughout the process, rather than switching from initially equilibrium to fractional crystallization partway through. To do this, we adopted an iterative approach in which the starting material for each run is synthesized using the composition of the melt phase from the prior run. We compare our results to those from long‐standing numerical models of LMO crystallization and show that although some features of those models are broadly reproduced, there are key differences in liquid lines of descent and the cumulate lithologies generated. Our results can be used to estimate the possible thickness of a primordial lunar crust formed from flotation of plagioclase during magma ocean solidification. Our estimate is greater than that from the recent Gravity Recovery and Interior Laboratory (GRAIL) mission, but consistent with the criteria on which the starting bulk composition was originally calculated. It assumes perfectly efficient separation of all plagioclase formed from the crystallizing magma ocean, which is likely not the case. We also demonstrate that a non‐chondritic bulk composition, with respect to trace elements, is not required in order to generate a KREEP (potassium, rare earth elements, and phosphorus) signature from magma ocean crystallization.

Cathodoluminescence of high‐pressure feldspar minerals as a shock barometer

1,2Masahiro Kayama,3,4Toshimori Sekine,5Naotaka Tomioka,6Hirotsugu Nishido,3Yukako Kato,6Kiyotaka Ninagawa,7Takamichi Kobayashi,8,9Akira Yamaguchi
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13092]
1Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai, Japan
2Creative Interdisciplinary Research Division, Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai, Japan
3Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi‐Hiroshima, Japan
4Center for High Pressure Science and Technology Advanced Research, , Shanghai, China
5Kochi Institute for Core Sample Research, Japan Agency for Marine‐Earth Science and Technology, , Nankoku City, Kochi, Japan
6Department of Biosphere‐Geosphere Science, Okayama University of Science, , Okayama, Japan
7National Institute for Materials Science, , Tsukuba, Ibaraki, Japan
8National Institute of Polar Research, Tachikawa, Tokyo, Japan
9Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Hayama, Tokyo, Japan
Published by arrangement with John Wiley & Sons

Cathodoluminescence (CL) analyses were carried out on maskelynite and lingunite in L6 chondrites of Tenham and Yamato‐790729. Under CL microscopy, bright blue emission was observed in Na‐lingunite in the shock veins. Dull blue‐emitting maskelynite is adjacent to the shock veins, and aqua blue luminescent plagioclase lies farther away. CL spectroscopy of the Na‐lingunite showed emission bands centered at ~330, 360–380, and ~590 nm. CL spectra of maskelynite consisted of emission bands at ~330 and ~380 nm. Only an emission band at 420 nm was recognized in crystalline plagioclase. Deconvolution of CL spectra from maskelynite successfully separated the UV–blue emission bands into Gaussian components at 3.88, 3.26, and 2.95 eV. For comparison, we prepared K‐lingunite and experimentally shock‐recovered feldspars at the known shock pressures of 11.1–41.2 GPa to measure CL spectra. Synthetic K‐lingunite has similar UV–blue and characteristic yellow bands at ~550, ~660, ~720, ~750, and ~770 nm. The UV–blue emissions of shock‐recovered feldspars and the diaplectic feldspar glasses show a good correlation between intensity and shock pressure after deconvolution. They may be assigned to pressure‐induced defects in Si and Al octahedra and tetrahedra. The components at 3.88 and 3.26 eV were detectable in the lingunite, both of which may be caused by the defects in Si and Al octahedra, the same as maskelynite. CL of maskelynite and lingunite may be applicable to estimate shock pressure for feldspar‐bearing meteorites, impactites, and samples returned by spacecraft mission, although we need to develop more as a reliable shock barometer.

Chondritic ingredients: II. Reconstructing early solar system history via refractory lithophile trace elements in individual objects of the Leoville CV3 chondrite

1Andrea Patzer, 2Dominik C. Hezel, 2Verena Bendel, 2Andreas Pack
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13084]
1Geowissenschaftliches Zentrum, Universität Göttingen, , Göttingen, Germany
2Institut für Geologie und Mineralogie, Universität zu Köln, , Köln, Germany
Published by arrangement with John Wiley & Sons

We performed a LA‐ICP‐MS study of refractory lithophile trace elements in 32 individual objects selected from a single section of the reduced CV3 chondrite Leoville. Ingredients sampled include ferromagnesian type I and II chondrules, Al‐rich chondrules (ARCs), calcium‐aluminum‐rich inclusions (CAIs), a single amoeboid olivine aggregate (AOA), and matrix. The majority of rare earth element (REE) signatures identified are either of the category “group II” or they are relatively flat, i.e., more or less unfractionated. Data derived for bulk Leoville exhibit characteristics of the group II pattern. The bulk REE inventory is essentially governed by those of CAIs (group II), ARCs (flat or group II), type I chondrules (about 90% flat, 10% group II), and matrix (group II). Leoville matrix also shows a superimposed positive Eu anomaly. The excess in Eu is possibly due to terrestrial weathering. The group II pattern, however, testifies to volatility‐controlled fractional condensation from a residual gas of solar composition at still relatively high temperature. In principle, this signature (group II) is omnipresent in all types of constituents, suggesting that the original REE carrier of all components was CAI‐like dust. In addition, single‐element anomalies occasionally superimposing the group II signature reveal specific changes in redox conditions. We also determined the bulk chemical composition of all objects studied. For Mg/Si, Mg/Fe, and Al/Ca, Leoville’s main ingredients—type I chondrules and matrix—display a complementary relationship. Both components probably formed successively in the same source region.