Emma S. Bullock

Recent evidence from primitive achondrites and achondrites suggests that early‐formed melted planetesimals are depleted in hydrogen relative to the bulk silicate Earth. Nevertheless, evidence from angrites, the lone oxidized group of achondrites investigated to date, suggests that oxidized planetesimals may be H‐rich relative to their reduced counterparts. Additionally, we have limited constraints, derived from a few ungrouped samples, on the H budgets of outer solar system planetesimals that underwent melting. Therefore, in order to provide additional constraints on the H budgets of oxidized and outer solar system planetesimals that experienced melting, we measured the H contents of silicate minerals in brachinite and tafassite group meteorites, respectively. We found that olivine, pyroxene, and plagioclase across both groups are essentially devoid of H (<~2.6 μg/g H ₂ O _T ; total H as H ₂ O equivalents). Based upon the thermal histories of the tafassite and brachinite parent bodies as well as their petrology and mineralogy, we argue that both bodies were essentially anhydrous prior to their disruption. This result is consistent with prior work on primitive achondrites and achondrites and requires that Earth's H budget be accounted for by accretion of thermally primitive materials, such as chondrites, comets, and ices or capture of nebular gas.

The nature of chemical exchange between Earth’s core and mantle is fundamental to understanding their evolution. Tungsten-182 and helium-3 anomalies in volcanic rocks from deeply sourced mantle plumes have been attributed to core-mantle exchange. Hydrogen (H) is potentially abundant in the core. Therefore, H may also be a sensitive tracer of core-mantle exchange. We measured ² H/ ¹ H ratios (reported as δD) in olivine-hosted basaltic melt inclusions from a Baffin Island lava to test whether mantle plumes contain H from the core. The average δD value (−144 ± 24 per mil) is lower than some estimates for the average depleted upper mantle (δD ≈ −60 ± 20 per mil). The low δD composition likely derives from isotopic diffusion or H leakage from the core, not isotopic fractionation during magmatism or crustal contamination. Over geologic time, core-mantle exchange of H may have overprinted the isotopic composition of mantle plume source regions and much of the upper mantle.

As part of the Apollo Next‐Generation Sample Analysis (ANGSA) program, we provide bulk composition, mineralogy, petrology, and noble gas assays of lunar particles and soils from the top half of the 73001/2 double‐drive tube (i.e., 73002) studied by the ANGSA consortium. The particles are derived from lithologies of the South Massif, including anorthosites, anorthositic breccias, high‐Ti basalts, noritic impact‐melt breccias, agglutinates, and regolith breccias. The impact‐melt breccias are distinguished by their inclusion of high‐Mg spinel that may extend the compositional family of pink spinel troctolites. The noble gas data provide cosmic‐ray exposure ages representing the emplacement of the light mantle unit at 60 Ma preceded by soil exposure in an avalanche‐emplaced surface around 100 Ma. These measurements provide additional insight into the geologic evolution of the Apollo 17 site and the light mantle unit sampled by the double‐drive tube.

The germanosilicide Na4-x GeySi16-y (0.4 <= x <= 1.1, 4.7 <= y <= 9.3) was synthesized under high-pressure, high-temperature conditions. The novel guest-host compound comprises a unique tetrel framework with dual channels housing sodium and smaller, empty (Si,Ge)9 units. The arrangement represents a new structure type with an overall structural topology that is closely related to a hypothetical carbon allotrope. Topological analysis of the structure revealed that the guest environment space cannot be tiled with singular polyhedra as in cage compounds (e.g., clathrates). The analysis of natural tilings provides a convenient method to unambiguously compare related tetrel-rich structures and can help elucidate new possible structural arrangements of intermetallic compounds.

The Mighei‐like carbonaceous (CM) chondrites, the most abundant carbonaceous chondrite group by number, further our understanding of processes that occurred in their formation region in the protoplanetary disk and in their parent body/bodies and provide analogs for understanding samples returned from carbonaceous asteroids. Chondrules in the CMs are commonly encircled by fine‐grained rims (FGRs) whose origins are debated. We present the abundances, sizes, and petrographic observations of FGRs in six CMs that experienced varying intensities of parent body processing, including aqueous and thermal alteration. The samples studied here, in approximate order of increasing thermal alteration experienced, are Allan Hills 83100, Murchison, Meteorite Hills 01072, Elephant Moraine 96029, Yamato‐793321, and Pecora Escarpment 91008. Based on observations of these CM chondrites, we recommend a new average apparent (2‐D) chondrule diameter of 170 μm, which is smaller than previous estimates and overlaps with that of the Ornans‐like carbonaceous (CO) chondrites. Thus, we suggest that chondrule diameters are not diagnostic for distinguishing between CM and CO chondrites. We also argue that chondrule foliation noted in ALH 83100, MET 01072, and Murchison resulted from multiple low‐intensity impacts; that FGRs in CMs formed in the protoplanetary disk and were subsequently altered by both aqueous and thermal secondary alteration processes in their parent asteroid; and that the heat experienced by some CM chondrites may have originated from solar radiation of their source body/bodies during close solar passage as evidenced by the presence of evolved desiccation cracks in FGRs that formed by recurrent wetting and desiccation cycles.