Rachel S. KIRBY^1,2,3, Penelope L. KING¹, and Andrew G. TOMKINS²
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.70040]
¹Sesearch School of Earth Sciences, The Australian National University, Acton, Australian Capital Territory, Australia
²School of Earth, Atmosphere and Environment, Monash University, Melbourne, Victoria, Australia
³Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, Perth, Western Australia, Australia
Published by arrangement with John Wiley & Sons

It has been proposed that IIE iron meteorites formed through impact processes on a parent body that was composed of either the H chondrites or a much-debated fourth ordinary chondrite group, the HH chondrites. To resolve this debate, we have compiled a large dataset for the ordinary chondrites, low-fayalite ungrouped chondrites, and IIE irons, and undertaken a statistical analysis to determine if: (1) the current classification of ordinary chondrite groups is statistically appropriate; and (2) the IIE irons are related to H chondrites or if they represent a distinct group that formed on a “HH” chondrite parent body. We demonstrate that the current classification system based on petrography and olivine and orthopyroxene chemistry is appropriate for the H, L, and LL chondrites. We define a fourth “F chondrite” group consisting of eight, previously ungrouped, very low-Fa Type 3 and 4 chondrites. Statistical analysis of Δ¹⁷O data alone cannot distinguish between the H chondrites and IIE irons, nor between the L and LL chondrites. Furthermore, statistical analyses are unable to distinguish H chondrites from IIE irons in all measures (mineral chemistry, chondrule size, bulk Δ¹⁷O, Ge and Mo isotopic compositions, and bulk siderophile element abundances in metal); there is no evidence for a “HH” chondrite group. These results are consistent with formation of IIE iron meteorites through impact melting and near-surface metal segregation on the H chondrite parent body. This genetic link between H chondrites and IIE irons allows us to understand the geochemical and petrological changes that occurred during planetary formation and evolution.

M. D. SUTTLE^1,2, R. FINDLAY^1,3, I. A. FRANCHI¹, C. BIAGIONI^2,4, X. ZHAO¹,  F. A. J. ABERNETHY¹, L. RICHES¹, and L. FOLCO² 
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.70043]
¹School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, UK
²Dipartimento di Scienze della Terra, Universit`a di Pisa, Pisa, Italy
<sup>3</sup>Department of Earth Sciences, University of Cambridge, Cambridge, UK
<sup>4</sup>CISUP, Centro per l’Integrazione della Strumentazione dell’Universit`a di Pisa, Pisa, Italy
Published by arrangement with John Wiley & Sons

We report the first oxygen isotope measurements of tochilinite (δ¹⁷O: 11.0 ± 2.1‰, δ¹⁸O: 23.5 ± 4.0‰ and Δ¹⁷O: −1.1 ± 1.2‰) in a CM chondrite (Winchcombe, lithology C [CM2.2/2.3]). We analyzed type-I tochilinite-cronstedtite intergrowths (TCIs)—formed by pseudomorphic replacement of kamacite. Alongside T1 and T2 calcite and magnetite, these secondary phases define a linear trendline in δ¹⁷O-δ¹⁸O isotope space with a slope of 0.50, slightly shallower than the mass-dependent slope (0.52). This demonstrates that, in addition to dominant mass-dependent fractionation (controlled by mineral-specific and temperature-dependent equilibrium processes), mass-independent mixing between ¹⁶O-rich anhydrous silicates, and ¹⁶O-poor water influenced the evolving Δ¹⁷O composition of alteration fluids. Petrographic evidence shows tochilinite and T1 calcite formed early and are closely associated in the alteration sequence. Assuming isotopic equilibrium between these phases, we estimate formation temperatures of approximately 135°C and a δ¹⁸O_water value of 28‰. These findings align with previous hydrothermal synthesis experiments and underscore the value of multi-phase isotopic measurements for reconstructing the fluid history of chondritic parent bodies.

Zhi-Ming CHEN^1,2,3, Le ZHANG^1,3, Cheng-Yuan WANG^1,3, Ya-Nan YANG¹, Peng-Li HE¹, Hai-Yang XIAN^1,3,4, Xiao-Ping XIA⁵, Jian-Xi ZHU^1,3,4, and Yi-Gang XU^1,3
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.70042]
¹State Key Laboratory of Deep Earth Processes and Resources, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China
²University of Chinese Academy of Sciences, Beijing, China
³Center for Advanced Planetary Science (CAPS), Guangzhou Institute of Geochemistry, Chinese Academy of Sciences,
Guangzhou, China
⁴Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese
Academy of Sciences, Guangzhou, China
⁵College of Resources and Environment, Yangtze University, Wuhan, China
Published by arrangement with John Wiley & Sons

Numerous studies of basalt clasts in regolith samples returned by the Chang’E-5 (CE-5) mission have provided constraints on the timing and nature of the youngest magmatism on the Moon. However, there have been far fewer studies of breccias, one of the main constituents of regolith. Here, we present a comprehensive investigation of the mineralogy, petrology, and U-Pb geochronology of two CE-5 regolith breccia samples, which are composed of lithic clasts, agglutinates, glass particles, and mineral fragments. In contrast to the high level of maturity of CE-5 regolith, the regolith breccias are immature, as judged by their low agglutinate (~11 vol%) and moderate to low matrix contents (~49 vol%). The CE-5 regolith breccias comprise mainly mare (~90 vol%) and non-mare (~10 vol%) materials. A low-Ti mare component of late Imbrian to early Eratosthenian age is identified, in addition to the predominant late Eratosthenian basalts in mare components. Non-mare components include Mg-suite norite, highland impact melt clasts, glass particles, and minor fragmented minerals. The glass particles in the CE-5 regolith breccias are compositionally variable and can be divided into five types, that is, basaltic (mare), KREEP-rich, feldspathic (highland), Si-poor, and Si-K-rich glasses. Among these glasses, most (65%) are compositionally exotic to the site. The diverse provenance of these “exotic” materials in the CE-5 breccias is consistent with the multiple ages of Zr-bearing phases at 3.97–3.92 Ga, ~3.2 Ga, 2.93–2.40 Ga, and ~2.0 Ga, in which early Eratosthenian ages are reported for the first time from returned lunar samples. The contrast in the level of maturity and in glass composition between CE-5 regolith and regolith breccias can be reconciled if CE-5 regolith breccias represent an ancient soil and were excavated from a buried stratigraphic sequence by later impacts. The duration of exposure of this old soil was short (<250 Myr), and its maturation was interrupted by late Eratosthenian basaltic magmatism.