We used trace element analyses of plagioclase from Mg-suite troctolite 76535 to estimate the Rare Earth Element (REE) concentrations of its parental liquid and assess the feasibility of an urKREEP contribution to the Mg-suite parental liquid. We measured 33 trace elements in 76535 plagioclase separates. Our measurements revealed enrichments in incompatible elements consistent with previous analyses. Using the measured REE concentrations, we estimated the REE concentrations of the unfractionated Mg-suite parental liquid using a RhyoliteMELTS-based forward model. Compared to chondritic concentrations, the Mg-suite parental liquid is ~100 times more enriched in light REEs and ~10 times more enriched in heavy REEs. We sought to explore the feasibility of reproducing these enrichments in the parental liquid through assimilation of urKREEP by a partial melt of rising LMO cumulates during cumulate mantle overturn. We show that these enrichments can be reproduced by a 30%–50% addition of fully molten urKREEP to the LMO cumulate melt, if the LMO cumulate melt and urKREEP are in thermal equilibrium with each other. However, the Mg# of these mixtures (57–68) is too low to produce the most Mg-rich olivine (Fo 91) observed in Mg-suite troctolites. Alternatively, assuming that the LMO cumulate melt and urKREEP are in thermal disequilibrium, we reproduced both the REE abundances and Mg# of the Mg-suite parental liquid with only a 10% addition of the urKREEP partial melt. These results support the feasibility of urKREEP assimilation as a mechanism for generating the incompatible element enrichments in Mg-suite magmas while preserving their major element chemistry.
This study presents a comprehensive analysis of the mineralogical, geochemical properties, and in situ Sr-Nd-Pb isotopic systematics of a newly discovered unbrecciated lunar basaltic meteorite NWA 14526 (NWA refers to northwest Africa). Bulk composition derived through both mineral modes and impact melt vein classifies NWA 14526 as a low-Ti, low-Al, and low-K mare basalt. In situ Pb isotopic analyses define a Pb–Pb isochron yielding an age of 3009 ± 43 Ma, representing the meteorite's crystallization age. In situ Rb-Sr isotopic analyses of plagioclase and maskelynite provide an initial 87Sr/86Sr ratio of 0.69969 ± 0.00024 (2σ), while phosphate and mesostasis in situ Sm-Nd analyses yield an initial εNd value of +10.7 ± 2.1 (2σ). Although NWA 14526 shares comparable mineralogical, bulk-rock composition, and Sr isotopic characteristics with contemporaneous lunar basaltic meteorites (NWA 4734, LAP 02205, NWA 14137, and NWA 10597), its significantly elevated εNd values preclude genetic pairing with these specimens. Isotopic modeling indicates minimal KREEP component contribution (<0.5%) in its mantle source. Our compilation of lunar Sr-Nd isotopic data reveals two distinct evolutionary trends corresponding to depleted lunar mantle and urKREEP reservoirs, respectively. Notably, no temporal correlation between basalt source KREEP enrichment and eruption age is observed, suggesting that the KREEP component did not necessarily play a decisive role in driving late-stage lunar magmatism and volcanism.
We report on the mineralogy, petrography, and oxygen and aluminum-magnesium isotopic systematics of the corundum-bearing Ca,Al-rich inclusions (CAIs) from the CK3 (Karoonda-type) carbonaceous chondrites NWA (Northwest Africa) 4964-#1 and -Homer, NWA 5343-#1, and LAR (Larkman Nunatak) 12002-#1. These CAIs experienced extensive metasomatic alteration: melilite and possibly anorthite and AlTi-diopside are nearly completely replaced by secondary corundum, grossular, CaNa-plagioclase, FeAl-diopside, and FeO-rich spinel; perovskite is largely replaced by ilmenite. Two types of corundum grains occur in the NWA 4964 CAIs: (1) compact, FeO-poor grains zoned in cathodoluminescent (CL) images and (2) FeO-bearing (up to 1.5 wt% FeO), porous grains showing no detectable CL; the porous corundum grains overgrow the compact ones. Corundum grains in CAIs from LAR 12002 and NWA 5343 belong to the first and second types, respectively. Hibonite, primary spinel, and rare perovskite inclusions in spinel retained the original, 16O-rich compositions (Δ17O ~ −24 ± 2‰), whereas melilite, most perovskite grains, and secondary corundum and spinel are 16O-depleted (Δ17O ~ −5 ± 2‰). Hibonite and melilite have excesses of radiogenic 26Mg (26Mg*) corresponding to approximately the canonical initial 26Al/27Al ratio [(26Al/27Al)0] of ~5 × 10−5 suggesting that corundum-bearing CAIs studied belong to a population of the canonical inclusions, dominant in most chondrite groups. Corundum grains in LAR 12002-#1, NWA 4964-#1, NWA 4964-Homer, and NWA 5343-#1 show resolvable 26Mg* correlated with 27Al/24Mg ratio which corresponds to much lower than the canonical (26Al/27Al)0: (3.10 ± 0.48) × 10−6, (3.03 ± 0.23) × 10−6, (2.72 ± 0.19) × 10−6, and (3.5 ± 1.2) × 10−7, respectively. Porous Fe-bearing corundum grains in NWA 4964 CAIs Homer and #1 have low 26Mg* not correlated with 27Al/24Mg ratio. We conclude that compact corundum grains in the CK3 CAIs studied are secondary parent body products that resulted from metasomatic alteration of the host inclusions by hydrothermal fluid ~3−5 Ma after their crystallization. Porous corundum grains may have formed by dehydration of diaspore [AlO(OH)] during subsequent thermal metamorphism.
Löpönvaara is a rare new phosphorus-rich iron meteorite find from Löpönvaara, Finland. The ~164 g meteorite was discovered in 2017 from the same area as the ungrouped Lieksa pallasite. Löpönvaara was classified as an ungrouped iron meteorite due to its unusually high concentration of P (>4 wt%), coupled with a moderate concentration of Ni (~11 wt%), and Ga–Ge abundances in the “III” range. The meteorite consists of ~75 vol% kamacite and ~22 vol% schreibersite, with accessory troilite (<0.1 vol%), and minor terrestrial weathering products. The kamacite in Löpönvaara occurs as three different types: (1) rare, large 2–5 mm partially resorbed clasts; (2) round, ≤0.5 mm partially resorbed clasts; and (3) small, several tens of μm to sub-μm exsolution blebs and globules in the matrix. Schreibersite occurs solely as microscopic matrix material in between the type (1) and (2) kamacite clasts. The lack of taenite and the overall compositional and textural features of Löpönvaara suggest that it retained its composition possibly from a P-rich portion of immiscible melt at late stages of fractional crystallization, but its textural features suggest that the meteorite suffered impact-related metamorphism. The meteorite has no close textural or compositional affinities, which makes it unique and an important target for future studies.
Silica polymorphs in meteorites provide critical constraints on crystallization processes associated with thermal activity in the early solar system. A detailed investigation of silica polymorphs in eucrites (the largest group of achondrites) using cathodoluminescence imaging and laser-Raman spectroscopy revealed significant variations in the relative abundance of silica polymorphs. Based on these variations, the eucrites were divided into four “Si-groups” according to their dominant silica phase: Si-0 (cristobalite-dominant eucrites), Si-I (quartz-dominant eucrites), Si-II (quartz and tridymite-dominant eucrites), and Si-III (tridymite-dominant eucrites). In studied eucrites, tridymite and cristobalite form lathy euhedral shapes, while quartz is anhedral, coexistent with opaques and phosphates, suggesting that silica polymorphs were crystallized from different stages and formation processes. We propose a new model that explains the formation pathways of silica minerals in eucrites and accounts for the distinct formation histories represented by each Si-group: tridymite crystallizes from alkali-rich immiscible melts (starting at ≥ ~1060°C), cristobalite crystallizes from quenched melts (~1060°C), and quartz crystallizes from extremely differentiated melts and/or by solid-state transformation from tridymite and cristobalite through interactions with sulfur-rich vapor below ~1025°C. This model explains the occurrences of silica polymorphs in eucrites without requiring secondary heating or shock processes.
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.
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 δ18Owater 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.
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.
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