¹Jakob Storz,²Thomas Ludwig,¹Addi Bischoff,²Winfried H.Schwarz,²Mario Trieloff
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.05.028]
¹Institut für Planetologie, WWU Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany
²Institut für Geowissenschaften, Klaus-Tschira-Labor für Kosmochemie, Universität Heidelberg, Im Neuenheimer Feld 234-236, D-69120 Heidelberg, Germany
Copyright Elsevier

Carbon is of fundamental interest for constraining the volatile element inventory of terrestrial planets. In some meteorites, like ureilites and enstatite chondrites, graphite is the major carbon-carrier. Here, we report the in-situ analyses of graphite in 19 ureilites, 11 enstatite chondrites, and 3 graphite-bearing clasts in ordinary chondrites by secondary ion-mass spectrometry (SIMS). In coarse-grained ureilites the obtained carbon-compositions of graphite range from –9.2‰ to –0.1‰ (δ13C). The carbon-composition tends to be homogeneous within a sample and correlates with the Fa content in olivine. In contrast, fine-grained ureilites exhibit considerable intra-sample heterogeneity, and graphite tends towards 13C-enriched compositions (up to +10.4‰). Isotopic and petrographic differences are presumably a result of post-igneous shock processing, including annealing during impact smelting. Enstatite chondrites host a variety of graphite morphologies, occurring in two distinct assemblages: Silicate-associated graphite (SAG) and metal-associated graphite (MAG). These assemblages show diverging carbon-compositions: SAG consistently exhibits δ13C in a narrow range between –4‰ to 1‰, very similar to the bulk silicate Earth value. In contrast, diverse compositions from –19.7‰ to +13.7‰ were observed for MAG. These differences are likely pre-accretionary in origin and potentially point towards isotopically distinct precursors. If Earth accreted from enstatite-chondrite-like material, carbon potentially hosted by Earth́s core may have an isotopically light signature when compared to the mantle. Although graphite-bearing clasts in unequilibrated ordinary chondrites (UOCs) are extraordinarily rare, these clasts are of particular interest as they might represent materials, not corresponding to known meteorites. Graphite from these clasts show coinciding carbon-compositions with a mean δ13C close to

–1‰. Although the coinciding compositions might argue for a genetic relationship among the clasts, petrographic evidence suggests they have experienced distinct thermal histories.

¹YunhuavWu,²徐伟彪(Weibiao Hsu),³Qiu-Li Li,⁴Xiaochao Che,²Shiyong Liao
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.05.011]
¹Planetary Environmental and Astrobiological Research Laboratory, School of Atmospheric Sciences, Sun Yat-sen University, Zhuhai 519082, China
²CAS Center for Excellence in Comparative Planetology, Purple Mountain Observatory, Nanjing 210034, China
³State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
⁴Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 102206, China
Copyright Elsevier

Shergottites were derived from mantle reservoirs through various magmatic processes, recording geochemical signatures of the martian mantle. But the U-Pb isotopic system of shergottites remains obscured including the geological significance of Pb isotope composition, the role of martian Pb contamination, and other factors. Here we present in situ U-Pb and/or Pb-Pb analyses on minerals of the basaltic shergottite Northwest Africa (NWA) 8653 after detailed petrological and mineralogical studies. The aims of the project are to evaluate the formation process, the crystallization age as well as the characteristic Pb isotope composition of NWA 8653. Texture, major and trace element composition plus geochemical modeling suggest that NWA 8653 is an enriched shergottite derived from mixing of depleted (e.g., fractionated Yamato 980459) and enriched components (e.g., NWA 1068) in the mantle, instead of crustal assimilation. U-Pb and Pb-Pb isotopes of baddeleyite reveal a young crystallization age (187.6 ± 8.0 Ma). Pb isotope compositions of maskelynite, feldspathic intergrowth, and the majority of phosphate cluster near the predicted initial Pb and a 4.1 Ga isochron. For these minerals, calculations suggest that mixing of Pb from different reservoirs with μ (238U/204Pb) varying from 1.4 to 4.7 could explain the apparent 4.1 Ga isochron in young shergottites. Variable extents of mixing among mantle sources could further increase the isotopic heterogeneity of shergottites. Our results demonstrate that NWA 8653 was derived from a heterogeneous mantle source in terms of trace element and isotope composition. Mixing of Pb from different reservoirs in the mantle plays an important role in shaping Pb in minerals with negligible U. This study provides additional geochemical evidence for a highly heterogeneous martian mantle.

¹Makiko K. Haba,²Keisuke Nagao
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13660]
¹Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama, Tokyo, 152-8551 Japan
²Korea Polar Research Institute, 26 Songdomirae-ro, Yeonsu-gu, Incheon, 21990 South Korea
Published by arrangement with John Wiley & Sons

Zirconium produces cosmogenic Kr through spallation reactions with cosmic rays. Meteoritic zircons (ZrSiO4) therefore possibly contain a significant amount of cosmogenic Kr in addition to other cosmogenic nuclides. Detection of cosmogenic nuclides from meteoritic zircons would make it possible to determine precise cosmic ray exposure (CRE) ages without knowing the whole rock chemistry because of the robust nature of zircons and limited target elements that produce cosmogenic nuclides in a zircon crystal. Herein, we report the noble gas compositions of zircons separated from the Estherville mesosiderite in addition to those of the silicate and metal parts. The zircons contain cosmogenic noble gas nuclides, and more importantly, cosmogenic 81Kr (t1/2 = 2.29 × 105 years) was successfully detected in the zircons. The 81Kr-Kr exposure age of the zircons was calculated to be 76 ± 5 million years (Ma). This age corresponds to the CRE ages obtained from cosmogenic 3He and 21Ne (82 ± 8 and 88 ± 9 Ma, respectively) of the silicate part and the previously reported 36Cl-36Ar age of the metal part (77 ± 9 Ma). The consistent CRE ages using different dating methods demonstrate that the 81Kr-Kr dating using meteoritic zircons is a new promising tool for determining the CRE age of meteorites. Moreover, based on the 81Kr-Kr age of the zircons, the production rates of cosmogenic 3He and 21Ne in a meteoritic zircon were estimated to be (15 ± 2) × 10−9 and (0.69 ± 0.04) × 10−9 cm3 STP g−1 Ma−1, respectively.

¹M.B.Neuland,²K.Mezger,¹A.Riedo,¹M.Tulej,¹P.Wurz
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2021.105251]
¹University of Bern, Physics Institute, Space Research and Planetary Sciences, Sidlerstrasse 5, CH – 3012 Bern, Switzerland
²University of Bern, Institute of Geological Sciences, Baltzerstrasse 3, CH – 3012 Bern, Switzerland

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^1,2V.G.Shevchenko et al. (>10)
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2021.105248]
¹Institute of Astronomy of V.N. Karazin Kharkiv National University, Kharkiv 61022, 4 Svobody sq., Ukraine
²Department of Astronomy and Space Informatics of V.N. Karazin Kharkiv National University, Kharkiv 61022, 4 Svobody sq., Ukraine

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¹Yuji Matsumoto,²Yasuhiro Hasegawa,³Nozomi Matsuda,³Ming-Chang Liu
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114538]
¹Institute of Astronomy and Astrophysics, Academia Sinica, No.1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
³Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA, USA
Copyright Elsevier

Chondrules are often surrounded by fine-grained rims or igneous rims. The properties of these rims reflect their formation histories. While the formation of fine-grained rims is modeled by the accretion of dust grains onto chondrules, the accretion should be followed by the growth of dust grains due to the shorter growth timescale than the accretion. In this paper, we investigate the formation of rims, taking into account the growth of porous dust aggregates. We estimate the rim thickness as a function of the chondrule fraction at a time when dust aggregate accretion onto chondrules is switched to collisions between these chondrules. Our estimations are consistent with the measured thicknesses of fine-grained rims in ordinary chondrites. However, those of igneous rims are thicker than our estimations. The thickness of igneous rims would be enlarged in remelting events.

¹Andreas Bechtold,²Franz Brandstätter,²Lidia Pittarello,²Ludovic Ferrière,³Richard C. Greenwood,¹Christian Koeberl
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13659]
¹Department of Lithospheric Research, University of Vienna, Althanstrasse 14, Vienna, 1090 Austria
²Natural History Museum Vienna, Burgring 7, Vienna, 1010 Austria
³Planetary and Space Sciences, School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA UK
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 11962 is a lunar regolith breccia composed of a wide range of different clasts. The lunar origin of this meteorite is confirmed by oxygen isotope analysis and the Fe/Mn ratio in pyroxene and olivine. In the present study, the clasts and the matrix of NWA 11962 are characterized by optical and electron microscopy along with electron microprobe analyses and micro‐Raman spectroscopy. The meteorite has a glassy impact melt matrix, which accounts for 35% of the surface area in the two thin sections examined, and which contains a very large variety of different lithic clasts, monomineralic clasts, and glass fragments. The presence of volcanic and impact‐related glass spherules led to the classification of this meteorite as a regolith breccia. Lithic clasts include numerous fragments of quartz monzogabbro and lunar felsite, which are quite rare lithologies in the lunar alkali suite. However, the most abundant components in the breccia are gabbroic clasts. The mineral chemistry of the pyroxenes in the gabbroic clasts and the chemistry of various types of glass fragments in the breccia indicate an origin of the regolith from an area with low‐Ti to high‐Ti mare basalt volcanism. In addition to the peculiar petrographic characteristics of NWA 11962, the possible pairing relationships with other lunar meteorites are discussed.

^1,2Margrit A. Rindlisbacher,^2,3,4Michael K. Weisberg,^2,4Denton S. Ebel,^2,4Samuel P. Alpert
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13658]
¹Department of Geology, Mount Holyoke College, South Hadley, Massachusetts, 01075 USA
²Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, 10024 USA
³Department of Physical Science, Kingsborough College CUNY, Brooklyn, New York, 11235 USA
⁴Department of Earth and Environmental Science, CUNY Graduate Center, New York, New York, 10016 USA
Published by arrangement with John Wiley & Sons

NWA 8785 is a remarkable, recently identified, unequilibrated enstatite chondrite. It was classified as an EL3 but contains highly unusual characteristics not observed in any other EL3, including a high abundance of FeO‐rich matrix and metal‐rich nodules that are texturally and mineralogically different from those in other EL3s. We characterized the mineral assemblages and compositions of metal‐rich nodules in a thin section of NWA 8785 and compared them to nodules in other EL3s to evaluate models for formation of metal‐rich nodules in EL3s. Of a total of 40 metal‐rich nodules, 10 were selected for detailed study. These metal‐rich nodules vary in their physical structure, texture, and mineral assemblages. Some contain the rare Al‐poor, alkali‐rich silicate mineral roedderite, a first discovery in an EL3, as well as the Cl‐bearing sulfide djerfisherite. The diversity of metal‐rich nodules in NWA 8785 suggests each nodule formed independently and supports their origin by primary processes prior to accretion. The high abundance of FeO‐rich matrix and the unique qualities of its metal‐rich nodules call into question classification of NWA 8785 as an EL3, but the Si content in its kamacite and Cr and Ti content in its troilite, and the presence of alabandite, support its classification as an EL3; thus, it is an EL3‐anomalous. Although alternative hypotheses exist, the presence of roedderite, as well as a magnetite‐rich matrix and sodalite, may provide the first evidence of extensive metasomatic alteration on the EL3 parent body.

^1,2Pranabendu Moitra,^2,3David G.Horvath,²Jeffrey C.Andrews-Hanna
Earth and Planetary Science Letters 567, 116986 Link to Article [https://doi.org/10.1016/j.epsl.2021.116986]
¹Department of Geosciences, University of Arizona, AZ, USA
²Lunar and Planetary Laboratory, University of Arizona, AZ, USA
³Planetary Science Institute, Tucson, AZ, USA
Copyright Elsevier

Volcanic activity on Mars has been dominantly effusive. The existence of a young (∼0.05-1 Ma) and well-preserved possible pyroclastic deposit along a segment of the Cerberus Fossae fissures, overlying the effusive lava flows making up the bulk of Elysium Planitia, provides the motivation and opportunity to explore the dynamics of explosive volcanic eruptions on Mars. Here we investigate the subsurface magmatic processes that may have led to magma fragmentation and the explosivity of the eruption forming the deposit. Using numerical models of magma ascent in a volcanic fissure, we show that the dissolved magmatic water with or without suspended crystals is capable of driving the inferred explosive magma fragmentation and the formation of the deposit. We also explore an alternative eruption scenario and show that an intruded dike explosively interacting with melted ground ice might also have generated the deposit. The close proximity of the proposed pyroclastic deposit (15-35 km) to the similarly aged Zunil impact crater suggests the possibility of an impact-triggered volcanic eruption scenario. Using scaling analysis, we find that the high seismic energy density associated with the impact may have been sufficient to trigger a volcanic eruption if a magma chamber was present in the subsurface. These findings have implications for the generation of similar explosive eruptions on Mars and other bodies, as well as the possibility of ongoing magmatic activity on Mars.

¹Michael Volk,¹Roger Fu,²Anna Mittelholz,³James M.D. Day
Journal of Geophysical Research Planets (in Press) Link to Article [https://doi.org/10.1029/2021JE006856]
¹Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, 02138
²Institute of Geophysics, ETH Zuerich, 8092 Zuerich, Switzerland
³Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, 92093‐0244 USA
Published by arrangement with John Wiley & Sons

The martian dynamo’s strength and duration are essential for understanding Mars’ habitability and deep interior dynamics. Although most northern volcanic terranes were likely emplaced after the martian dynamo ceased, recent data from the InSight mission show stronger than predicted crustal fields. Studying young volcanic martian meteorites offers a precise, complementary method to characterize the strength of the martian crustal field and examine its implications for past dynamo activity. We present the first rock and paleomagnetic study of nine mutually oriented samples from the martian Nakhlite meteorite MIL 03346, which is well‐suited for paleomagnetic analysis due to its well‐known age (1368 ± 83 Ma) and lack of significant aqueous, thermal, and shock overprinting. Rock magnetic analysis, including quantum diamond microscope (QDM) imaging, showed that the natural remanent magnetization (NRM) is carried by Ti‐magnetite crystals containing µm‐scale ilmenite exsolution lamellae, which can accurately record ancient magnetic fields. Demagnetization of the NRM revealed a high coercivity magnetization interpreted to date from the age of eruption based on its intensity, unidirectionality, and a passing fusion crust baked contact test. Paleointensities of four samples reveal a 5.1±1.5 µT paleofield, representing the most reliable martian paleointensity estimates to‐date and stronger than the 2 µT surface fields measured by InSight. Modeling shows that the observed fields can be explained by an older sub‐surface magnetized layer without a late, active dynamo and support a deeply buried, highly magnetized crust in the northern hemisphere of Mars. These results provide corroborating evidence for strong, small scale crustal fields on Mars.