¹S. S. Russell et al. (>10)
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70151]
¹Planetary Materials Group, Natural History Museum, London, UK
Published by arrangement with John Wiley & Sons

Aqueously altered carbonaceous astromaterials are dominated by secondary minerals, but a minor fraction of primary, anhydrous silicates and oxides escape alteration, offering insight into the original composition of asteroid parent bodies. We report the mineralogy, petrology, mineral chemistry, and oxygen isotopes of anhydrous minerals—50 olivine grains, 12 pyroxene grains, two spinel grains, and one hibonite grain—in aqueously altered particles returned from asteroid Bennu by the OSIRIS-REx mission. These primordial grains are heterogeneously distributed, being more abundant (up to a few area percent for olivine) in less aqueously altered clasts. Olivine is typically forsteritic (Fo100–Fo93), but we found three fayalitic examples (Fo80–Fo75). Of the pyroxenes, seven grains are enstatite (En100–En94); two are enstatite with more Fe (En83 and En87); one is Fe-rich, low-Ca clinopyroxene; one is Mg-rich, aluminous low-Ca pyroxene; and one is high-Al, Ca-rich clinopyroxene. Chromium is present in olivine grains at high abundances (up to 1.3 wt% Cr2O3), indicating that the grains have not experienced significant thermal metamorphism. The olivines fall into two clusters: one with 16O-rich compositions (δ18O = −43 to −51‰) typical of refractory inclusions in carbonaceous chondrites and one with comparatively 16O-poor compositions (δ18O = −11 to +7.5‰) more typical of chondrules. Pyroxenes all fall in the 16O-poor cluster (δ18O = −2.6 to +13.5‰). For both olivine and pyroxene, the more Fe-rich examples are associated with heavier oxygen isotope compositions. One isolated spinel grain has an oxygen isotope composition typical of Ca-Al–rich inclusions, whereas a spinel–hibonite object has an unusual oxygen isotope composition consistent with fractionated unknown nuclear (FUN) compositions. Together, our data suggest that Bennu’s parent body accreted a population of finer grained (≤10 μm) silicate and oxide grains that as a population are distinct from the anhydrous minerals in most chondrite groups but comparable to those in CI chondrites. The Bennu samples, as well as those from asteroid Ryugu and CI chondrites, appear to have accreted from a distinct reservoir that may have been in the outer protoplanetary disk.

^1,2Cheng-Yu Du, ^1,2Shao-Bing Zhang, ³Hejiu Hui, ¹Ting Liang, ¹Wan-Cai Li, ¹Yong-Fei Zheng
Geochimica et Cosmochimica Acta (in Press) Link to Article [10.1016/j.gca.2026.05.006]
¹State Key Laboratory of Lithospheric and Environmental Coevolution, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
²Deep Space Exploration Laboratory/School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
³State Key Laboratory for Mineral Deposits Research & Lunar and Planetary Science Institute, School of the Earth Sciences and Engineering, Nanjing University, Nanjing, China
Copyright Elsevier

China’s Chang’E-5 (CE-5) lunar mission retrieved the youngest known mare basalt with low magnesium number (Mg#) and high incompatible element contents. However, the nature of its mantle source remains debated. Here we carried out in-situ analyses of major and trace element contents in olivine and pyroxene for CE-5 basalt clasts to constrain the petrogenetic process and mantle source of CE-5 basalt. The results indicate that the pyroxene Ti# (molar Ti/(Ti + Cr)) rose sharply at an early stage into the high-Ti basalt field and subsequently remained constant, implying extensive fractional crystallization of the CE-5 basalt. The near constant Ti content in olivine records an early crystallization of ilmenite at an olivine Mg# of approximately 60. Calculated TiO2 and Cr2O3 contents of the melt equilibrated with the most forsteritic olivine yielded TiO2 content of 6.06 ± 0.77 wt% and Cr2O3 content of 0.516 ± 0.089 wt%. Pyroxene exhibits concurrent decreases in LREE/HREE ratios, Zr/Y ratio and Al content as its Mg# decreases, whereas the early decrease of olivine Zr/Y ratio indicates a primitive melt with high Zr/Y ratio. Integrating these findings with previously reported isotopic data, theoretical modelling supports a hybrid mantle source dominated by orthopyroxene cumulate, with a small proportion of ilmenite-bearing cumulate (IBC, 0.40–0.55%) and KREEP (0.11–0.15%). Such a source region supports a geodynamic mechanism for the young mare magmatism that the IBC-rich hot plume rises from the core-mantle boundary heating the upper mantle that contains small amounts of IBC and KREEP to melt.