¹S. Alwmark,¹J. Granbom,¹P. Ahlberg,¹M. Calner,¹S. Richoz,¹K. J. Gajewska,¹W. R. Hyde,¹K. Ljung,¹C. Alwmark
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70160]
¹Department of Geology, Lund University, Lund, Sweden
Published by arragement with John Wiley & Sons

Hummeln is a simple impact structure located in south-eastern Sweden. It is approximately 1.2 km in diameter and almost completely covered by a lake. Here, we present the first detailed investigation of impactites and mapping of the 164.25 m deep drill core Hummeln-1 with a focus on impact metamorphism and the impact process. We find that the drilling has penetrated a complex sedimentary succession representing syn- to postimpact crater fill. It consists of (from base to top) lithic impact breccia (Unit 1), overlain by diamictite and graywacke with an overall fining upward trend grading from sandy into silty to clayey turbidites (Units 2, 3), and, lastly, suspension dominated marine clays and limestone (Units 4, 5). The crater fill was deposited mostly as gravity slides and sediment gravity flows (debris flows, (hyper)concentrated density flows and turbidity flows), which transported sediment into the crater as a series of fan lobes prograding toward the crater center. We have identified shocked quartz in 12 samples covering the interval of 160.69–56.60 m in the drill core and in samples of polymict and suevitic breccia obtained during fieldwork. Shocked quartz grains dominantly record planar fractures (PFs), with an average of 1.5–3.5 sets per grain. We measured a total of 122 PF sets in 54 grains, with orientations parallel to the $$, (0001), and $$ orientations being most common (30%, 26%, 21%, respectively). In the same samples, we also measured and indexed 14 sets of planar deformation features (PDFs) in eight grains, oriented parallel to the basal plane (50%), as well as rhombohedral planes $$, $$, and $$ (21%, 21%, and 7%, respectively). Feather features occur associated with PFs in seven of the samples. The quartz grains with shock microstructures in the drill core occur exclusively in beige graywacke–diamictite interbedded with the basal lithic breccia unit and in distinct graywacke–diamictite beds in the late syn- to early postimpact crater fill. We suggest that Hummeln was formed just prior to the deposition of parallel bedded marine mudrock with trilobites of the species Ellipsocephalus polytomus, indicating an early “middle” Cambrian (Wuliuan) age for the impact.

^1,2Zhong-Sha Meng,^1,3Ying-Kui Xu,^1,3Shi-Jie Li,^3,4Dan Zhu,^1,2De-Liang Wang,^1,3Yang Li,^1,3Xiong-Yao Li,^1,3Jian-Zhong Liu
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.70171]
¹Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China
²University of Chinese Academy of Sciences, Beijing, China
³Center for Excellence in Comparative Planetology, Chinese Academy of Sciences, Hefei, 230026, China
⁴State Key Laboratory of Critical Mineral Research and Exploration, Institute of Geochemistry, Chinese Academy of Sciences,Guiyang, 550081, China
Published by arrangement with John Wiley & Sons

Meteorite finds are commonly used to assess the chemical and isotopic compositions of their parent bodies. Among these, lithium (Li) isotopes in ordinary chondrites (OCs) have been applied to infer the Li abundance and isotopic characteristics of their parent bodies. However, Li is highly mobile in aqueous conditions and readily undergoes isotopic fractionation during fluid–mineral interactions. It remains uncertain whether Li isotopic compositions in meteorite finds reliably preserve their original parent-body signatures, particularly after prolonged terrestrial exposure. In this study, we investigated Li isotope behavior in Kumtag 015 (W3, L5) by conducting a series of leaching experiments. The untreated whole-rock sample yields a δ7Li value of +6.1‰, whereas all leachates exhibit heavier δ7Li values, ranging from +8.4‰ to +14.8‰, indicating the presence of weathering-related secondary components enriched in heavy Li isotopes. Combined with the petrographic observations and mass-balance results, these data suggest that the relatively heavy whole-rock δ7Li of Kumtag 015 is mainly related to the addition of heavy-δ7Li surficial fluids during terrestrial weathering, followed by the sequestration of Li into secondary minerals such as carbonates and Fe-(oxyhydr)oxides. This finding is consistent with prior work showing heavy δ7Li in carbonates. We conclude that terrestrial alteration can substantially modify Li isotope compositions in meteorite finds, highlighting the need for caution when using such samples to trace pristine planetary Li inventories.

¹Allan H. Treiman
American Mineralogist 111, 1009-2021 Link to Article [https://doi.org/10.2138/am-2025-9919]
¹Lunar and Planetary Institute (USRA), 3600 Bay Area Boulevard, Houston, Texas 77058, U.S.A.
Copyright: The Mineralogical Society of America

The nakhlite martian meteorites, basaltic rocks with abundant crystals of augite pyroxene, have been interpreted as cumulates, having formed as crystals of augite (and olivine) settled through basalt magma to accumulate at its bottom. Here, I show that the chemical compositions of most nakhlites are best explained if they represent magmas, and not crystal accumulates. The nakhlites Nakhla, Lafayette, and Y000593 are identified as cumulates; the rest are magmas. Among the magma nakhlites, abundances of all igneous incompatible elements are strongly correlated; abundances of highly incompatible elements are linearly correlated with the La/Yb ratio, with R2 values from 0.7 to 0.95. These close correlations are not consistent with the group being cumulates, where the proportion of cumulus augite should be mostly independent of magma composition. Instead, the correlations are consistent with the nakhlites representing magmas, with the range of compositions reflecting processes including: variable enrichment in incompatible elements (i.e., metasomatism) in the source mantle; variable degrees of partial melting of the source mantle; and mixing among such magmas. Significant crustal assimilation (for the tested elements) is excluded by the correlation of Al with La/Yb. The most Al-rich nakhlite would require assimilation of 25% mass crust into the Al-poorest, which is inconsistent with thermochemical constraints. Recognition of such augite-rich, pyroxenitic magmas requires that their mantle source be similarly pyroxenitic, which implies that the martian mantle is more heterogeneous than previously appreciated.