¹Kazuhiko Ninomiya et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14135]
¹Institute for Radiation Sciences, Osaka University, Toyonaka, Japan
Published by arrangement with John Wiley & Sons

Samples from asteroid Ryugu, brought back by asteroid explorer Hayabusa2, are important for investigating the origin and evolution of the solar system. Here, we report the elemental compositions of a 123-mg Ryugu sample determined with a nondestructive muon elemental analysis method. This method is a powerful tool for determining bulk chemical composition, including light elements such as C, N, and O. From the muonic x-ray spectra with three carbonaceous chondrites, the relationship between the elemental composition and muonic x-ray intensity was determined for each element. Calibration curves showed linearity, and the elemental composition of Ryugu was quantitatively determined. The results reflect the average bulk elemental composition of asteroid Ryugu owing to the large amount of samples. Ryugu has an elemental composition similar to that of Orgueil (CI1) and should be classified as CI1. However, the O/Si ratio of Ryugu is 25% lower than that of Orgueil, indicating that Orgueil may have been seriously contaminated by terrestrial materials after its fall to Earth. These results indicate that the Ryugu sample is more representative than the CI chondrites as a solid material of the solar system.

^1,2Ting Cao,²Huapei Wang,²Shaochen Hu,^3,4Kaixian Qi
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14136]
¹School of Earth Sciences, China University of Geosciences, Wuhan, Hubei, China
²Paleomagnetism and Planetary Magnetism Laboratory, School of Geophysics and Geomatics, China University of Geosciences, Wuhan, Hubei, China
³State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China
⁴College of Earth and Planetary Sciences, University of the Chinese Academy of Sciences, Beijing, China
Published by arrangement with John Wiley & Sons

We present the first rock magnetic and paleomagnetic analyses of the Martian meteorite Grove Mountains (GRV) 020090, a suitable candidate for paleomagnetic study due to its low degree of weathering and shock metamorphism. Petrological and rock magnetic investigation indicates that pyrrhotite is the dominant magnetic mineral in GRV 020090, where it occurs as a primary phase without significant shock metamorphism or alteration. The magnetic grains in GRV 020090 exhibit single-domain behavior that facilitates high-fidelity magnetic recording. We obtained a positive fusion-crust baked contact test, which supports an extraterrestrial origin of the primary remanence in GRV 020090. The nature of the primary remanence was identified as thermoremanence acquired during crystallization of the rock on Mars. Anhysteretic remanent magnetization and isothermal remanent magnetization paleointensity methods indicated paleofield strengths of 1.6 and 2.6 μT, respectively, for the primary remanence. However, the shock pressure that GRV 020090 experienced may have partially demagnetized the primary remanence, leading to underestimated paleointensity values. Therefore, 1.6 μT is regarded as the lower limit on the paleointensity of GRV 020090. This lower limit is higher than the model-predicted surface magnetic field strength in the source region for GRV 020090, suggesting that it may have recorded a small-scale crustal magnetic field previously undetected by orbital magnetic data. This small-scale crustal field is likely generated by the underlying ancient, magnetized layers, as the crustal magnetization of the surficial terrane with lithology similar to GRV 020090 is too weak to produce such a crustal field.

^1,2M. Kimura,^3,4M. K. Weisberg,¹A. Yamaguchi
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14129]
¹National Institute of Polar Research, Tokyo, Japan
²Ibaraki University, Mito, Japan
³Kingsborough College and Graduate Center of the City University of New York, New York City, New York, USA
⁴American Museum of Natural History, New York City, New York, USA
Published by arrangement with John Wiley & Sons

Type 3 chondrites are subdivided into 3.0–3.9. Subtype 3.0 chondrites nearly preserve all of their primitive features. Many criteria have been proposed to distinguish such primitive chondrites. Here, we compiled mineral data and reconsider the petrologic classification criteria for subtype 3.0. Chondrites are classified into subtypes by the minor element distribution of olivine and textural and chemical features of Fe-Ni metal. The []Si4O8 and MgO components of feldspar also distinguish subtype 3.0 from subtypes ≥3.1. Other features, such as the occurrence of near pure chromite, are also indicators of subtype 3.0. It is difficult to distinguish between subtypes 3.0 and ≤2.9 based on mineral chemistry. Therefore, we propose the following criteria to distinguish between subtypes 3.0 and ≤2.9. In type 3.0 chondrites, major silicate (olivine, pyroxene, and plagioclase), oxide, metal, and sulfide minerals do not show aqueous alteration features. Melilite, anorthite, and glass show no or mild aqueous alteration features. Subtype 3.0 has not been identified in all chondrite groups. The absence of subtype 3.0 from some groups mainly reflects differences in the degrees of secondary parent body processes among the chondrite groups.

^1,2F. Campanale,^2,3E. Mugnaioli,^2,3L. Folco,⁴P. Parlanti,⁴M. Gemmi
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14137]
¹Dipartimento di Scienze dell’Ambiente e Della Terra, Università degli Studi di Milano-Bicocca, Milan, Italy
²Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy
³CISUP, Centro per l’Integrazione della Strumentazione dell’Università di Pisa, Pisa, Italy
⁴Centre for Materials Interfaces, Electron Crystallography, Istituto Italiano di Tecnologia, Pontedera, Italy
Copyright Elsevier

TiO2II, a high-pressure polymorph of titanium dioxide, is a diagnostic indicator of shock metamorphism in impact rocks. Due to its typical micro-to-nanometer scale, there are no ab initio structure solutions of natural TiO2II, thereby generating uncertainty about its crystal structure and its known similarity with srilankite (Ti0.67,Zr0.33)O2. Nanoscale electron diffraction investigation of TiO2II from the Australasian tektite strewn field provides the first ab initio structure solution revealing a primitive orthorhombic lattice with cell parameters a = 4.547 Å, b = 5.481 Å, c = 4.891 Å, and space group Pbcn, that is, the same as srilankite and scrutinyite α-PbO2. The linear a and c decrease, and b increase with Ti content indicate TiO2II as Zr-free srilankite endmember in the binary system ZrO2-TiO2. Thereby the name srilankite should be used referring to TiO2II, according to the International Mineralogical Association recommendations. We provide the first evidence for a topotactic subsolidus rutile-to-TiO2II transition, founding their finely intermixing nanocrystals in the same TiO2 crystal, where TiO2II is within the crystal and surrounded by rutile in direct contact. They also show recurrent iso-orientation, with TiO2II [100] parallel to rutile [100], TiO2II [010] parallel to rutile [011], and TiO2II [001] parallel to rutile (0–11). The rutile-TiO2II iso-orientation suggests the compression of rutile (0–11) planes as a possible transition mechanism from rutile to TiO2II, with a consequent shortening of ~0.5 Å per cell. The presence of TiO2II in the distal (~1200 km) impact ejecta from the Australasian tektite strewn field indicates shock pressures of ~12–15 GPa and post-shock temperatures below 500°C followed by rapid quenching.