^1,2Alan E. Rubin,^2,3Tasha L. Dunn,³Kyla Garner,³Malena Cecchi,³Mitchel Hernandez
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14046]
¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California, USA
²Maine Mineral & Gem Museum, Bethel, Maine, USA
³Department of Geology, Colby College, Waterville, Maine, USA
Published by arrangement with John Wiley & Sons

In general, barred olivine (BO) chondrules formed from completely melted precursors. Among BO chondrules in unequilibrated ordinary chondrites, there are significant positive correlations among chondrule diameter, bar thickness, and rim thickness. In the nebula, smaller BO precursor droplets cooled faster than larger droplets (due to their higher surface area/volume ratios) and grew thinner bars and rims. There is a bimodal distribution in the olivine FeO content in BO chondrules, with a hiatus between 11 and 19 wt% FeO. The ratio of (FeO rich)/(FeO poor) BO chondrules decreases from 12.0 in H to 1.6 in L to 1.3 in LL. This is the opposite of the case for porphyritic chondrules: the mean (FeO rich)/(FeO poor) modal ratio increases from 0.8 in H to 1.8 in L to 2.8 in LL. During H chondrite agglomeration, most precursor dustballs were small with low bulk FeO/(FeO + MgO) ratios and moderately high melting temperatures. The energy available for chondrule melting from flash heating was relatively low, capable of completely melting many ferroan dusty precursors (to form FeO-rich BO chondrules), but incapable of completely melting many magnesian dusty precursors (to form FeO-poor BO chondrules). When L and LL chondrites agglomerated somewhat later, significant proportions of precursor dustballs were relatively large and had moderately high bulk FeO/(FeO + MgO) ratios. The energy available from flash heating was higher, capable of completely melting higher proportions of magnesian dusty precursors to form FeO-poor BO chondrules. These differences may have resulted from an increase in the amplitude of lightning discharges in the nebula caused by enhanced charge separation.

¹Hadrien Pirotte,²Camille Cartier,³Olivier Namur,⁴Anne Pommier,³Yishen Zhang,⁵Jasper Berndt,⁵Stephan Klemme,¹Bernard Charlier
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2023.115699]
¹Department of Geology, University of Liège, 4000 Sart Tilman, Belgium
²Centre de Recherches Pétrographiques et Géochimiques, Université de Lorraine, 54501 Vandœuvre-lès-Nancy, France
³Department of Earth and Environmental Sciences, KU Leuven, 3001 Leuven, Belgium
⁴Carnegie Institution for Science, Earth and Planets Laboratory, Washington, DC 20015, USA
⁵Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Münster 48149, Germany
Copyright Elsevier

Understanding the behavior of elements under highly reduced conditions is fundamental to explain the differentiation, crust formation, and volatile budget of Mercury. Here we report experiments on a synthetic composition representative of the bulk silicate Mercury (BSM), at pressure up to 3 GPa, temperature up to 1720 °C, and under highly reduced conditions (~IW − 8 to ~IW − 1, with IW the iron-wüstite oxygen fugacity buffer). We determined partition coefficients for >30 minor and trace elements between silicate melt, metal melt (Fesingle bondSi), sulfide melt (FeS), and MgS solid sulfides. Based on these results and published literature, we modeled the behavior of heat-producing elements (HPE: U, Th, and K) during Mercury’s early differentiation and mantle partial melting and estimated their concentrations in the mantle and crust. We found that U, K and especially Th are principally concentrated in the BSM and did not partition into the core because they are not siderophile elements. Uranium is chalcophile under highly reduced conditions, and so our model suggests that an FeS layer at the core-mantle boundary formed during Mercury’s primordial differentiation would likely have incorporated large amounts of U, significantly increasing the Th/U ratio of the BSM. However, this is inconsistent with the chondritic or slightly sub-chondritic Th/U ratios of Mercury’s lavas. In addition, the likely presence of mantle sulfides, such as MgS, would have also fractionated U and Th, increasing the mantle Th/U. It is possible to have an FeS layer if Mercury formed under less reduced conditions, or if the building blocks of Mercury had Th/U ratios close to the lower end of chondritic data. If, as suggested by our model, no FeS layer formed during differentiation, it means that the majority of HPE are concentrated in Mercury’s thin silicate part. Based on the compatibility of U, Th and K, we also show that surface K/Th and K/U ratios are respectively 2–4 times and 3–6 times lower than expected for initial K/Th and K/U ratios similar to enstatite chondrites, implying that the planet suffered an important volatile loss via mechanisms that remain undetermined.

¹Arthur Goodwin,¹Romain Tartèse,¹Russell J. Garwood,¹Rhodri Jerrett,¹Katherine H. Joy
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14035]
¹Department of Earth and Environmental Sciences, The University of Manchester, Manchester, UK
Published by arrangement with John Wiley & Sons

The Stac Fada Member (Stoer Group) is a ~1.2 Ga melt-rich impact breccia preserved and intermittently exposed along the NW coast of Scotland. Using a combination of x-ray diffraction and micro-Raman spectroscopy, we identify potential coesite that is spatially associated with micron-sized diamonds, as well as disordered carbon phases. Comparing the graphite G-band of disordered carbon phases in the impact breccia to samples from underlying units indicates that most of the carbon in the Stoer Group was ultimately derived from the underlying Lewisian basement. Disordered carbon phases within the Stac Fada Member have been modified by mild heating within a hot ejecta blanket rather than shock pressure. We also report the first evidence for impact diamonds discovered within the Stac Fada Member. These diamonds have an average Raman shift of 1328.5 cm−1 and are present within both the impact breccia and the shocked gneiss clasts that are present in sandstones directly underlying the Stac Fada Member contact, and within sandstone rafts entrapped in the unit. These findings have implications for the timing of deposition of the Stac Fada Member, which must have occurred after ballistic ejection of Lewisian basement clasts during the impact event.

¹Laura B. Seifert,¹Pierre Haenecour,²Tarunika Ramprasad,^1,2Thomas J. Zega
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14040]
¹Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
²Department of Materials Science and Engineering, University of Arizona, Tucson, Arizona, USA
Copyright Elsevier

Here we report in situ structural and chemical analyses of four presolar grains and the matrices of the Meteorite Hills (MET) 00526 L3.05 and Queen Alexandra Range (QUE) 97008 L3.05 unequilibrated ordinary chondrites (UOCs). The presolar grains in MET 00526 include one Fe-rich single crystal olivine, and one olivine grain that contains both amorphous and polycrystalline material. The single crystal olivine likely has origins in the circumstellar envelope (CSE) of a red giant branch (RGB) or asymptotic giant branch (AGB) star, and the amorphous/polycrystalline olivine has an O-isotopic composition consistent with origins in a type II supernova. The presolar grains from QUE 97008 are Fe rich and include one crystalline, stoichiometric olivine that contains a Ca-rich core and one crystalline, stoichiometric pyroxene grain, both of which have O-isotopic compositions consistent with origins in the CSEs of low-mass AGB/RGB stars. The matrices of both UOCs are mineralogically diverse with evidence for unaltered material in the form of amorphous silicates and a C-rich nanoglobule and altered material in the form of Ni-rich sulfides, abundant Fe-rich olivine, and Fe-Mg zoning in matrix silicates. No phyllosilicates were observed. The Fe-rich olivine grains are the dominant alteration phase in both UOCs and likely replaced primary amorphous silicates in the presence of an Fe-rich fluid during parent body alteration. Our work suggests that the ordinary and carbonaceous chondrites received a similar inventory of dust with comparable structures and chemistries.

^1,2R. H. Hewins,³H. Leroux,³D. Jacob,¹S. Pont,¹O. Beyssac,¹V. Malarewicz,⁴J.-P. Lorand,⁵P.-M. Zanetta,¹B. Zanda
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14028]
¹IMPMC, MNHN, UMR CNRS 7590, Sorbonne Université, Paris, France
²Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey, USA
³CNRS, INRAE, Centrale Lille, UMR 8207—UMET—Unité Matériaux et Transformations, Univ. Lille, Lille, France
⁴LPG Nantes, UMR CNRS 6112, Univ. Nantes, Nantes, France
⁵Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
Published by arrangement with John Wiley & Sons

Shergottites have provided abundant information on the volcanic and impact history of Mars. Northwest Africa (NWA) 14672 contributes to both of these aspects. It is a vesicular ophitic depleted olivine–phyric shergottite, with average plagioclase An61Ab39Or0.2. It is highly ferroan, with pigeonite compositions En49-25Fs41-61Wo10-14 like those of basaltic shergottites, for example, NWA 12335. Olivine (Fo53-15) has discrete ferroan overgrowths, more ferroan when in contact with plagioclase than when enclosed by pyroxene. The pyroxene (a continuum of augite, subcalcic augite, and pigeonite) is patchy, with ragged “cores” enveloped or invaded by ferroan pyroxene. Magma mixing may be responsible for capture of olivine and formation of pyroxene mantles. The plagioclase is maskelynite-like in appearance, but the original laths were (congruently) melted and the melt partly crystallized as fine dendrites. Most of the 14% vesicles occur within plagioclase. Olivine, pyroxene, and ilmenite occur in part as fine aggregates crystallized after congruent melting with limited subsequent liquid mixing. There are two fine-grained melt components, barred plagioclase with interstitial Fe-bearing phases, and glass with olivine dendrites, derived by melting of mainly plagioclase and mainly pyroxene, respectively. Rare silica particles contain coesite and/or quartz, and silica glass. The rock has experienced >50% melting, compatible with peak pressure >~65 GPa. It is the most highly shocked shergottite so far, at shock stage S6/7. It may belong to the group of depleted shergottites ejected at ~1 Myr from Tooting Crater.

¹M. Murphy Quinlan,²A.M. Walker,¹C.J. Davies
Earth and Planetary Science Letters 618, 118284 Link to Article [https://doi.org/10.1016/j.epsl.2023.118284]
¹School of Earth and Environment, University of Leeds, Leeds, UK
²Department of Earth Sciences, University of Oxford, Oxford, UK
Copyright Elsevier

Pallasite meteorites contain evidence for vastly different cooling timescales: rapid cooling at high temperatures (K/yrs) and slow cooling at lower temperatures (K/Myrs). Pallasite olivine also shows contrasting textures ranging from well-rounded to angular and fragmental, and some samples record chemical zoning. Previous pallasite formation models have required fortuitous changes to the parent body in order to explain these contrasting timescales and textures, including late addition of a megaregolith layer, impact excavation, or parent body break-up and recombination. We investigate the timescales recorded in Main Group Pallasite meteorites with a coupled multiscale thermal diffusion modelling approach, using a 1D model of the parent body and a 3D model of the metal-olivine intrusion region, to see if these large-scale changes to the parent body are necessary. We test a range of intrusion volumes and aspect ratios, metal-to-olivine ratios, and initial temperatures for both the background mantle and the intruded metal. We find that the contrasting timescales, textural heterogeneity, and preservation of chemical zoning can all occur within one simple ellipsoidal segment of an intrusion complex. These conditions are satisfied in 13% of our randomly generated models (2200 model runs), with small intrusion volumes (with a mean radius ≲100 m) and colder background mantle temperatures (≲1200 K) favourable. Large rounded olivine can be explained by a previous intrusion of metal into a hotter mantle, suggesting possible repeated bombardment of the parent body. We speculate that the formation of pallasitic zones within planetesimals may have been a common occurrence in the early Solar System, as our model shows that favourable pallasite conditions can be accommodated in a wide range of intrusion morphologies, across a wide range of planetesimal mantle temperatures, without the need for large-scale changes to the parent body. We suggest that pallasites represent a late stage of repeated injection of metal into a cooling planetesimal mantle, and that heterogeneity observed in micro-scale rounding or chemical zoning preservation in pallasite olivine can be explained by diverse cooling rates in different regions of a small intrusion.

¹Brouwers, Marc G.,¹Bonsor, Amy,^2,3Malamud, Uri
Monthly Notices of the Royal Astronomical Society 519, 2646-2662 Open Access Link to Article [DOI
10.1093/mnras/stac3316]
¹Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, United Kingdom
²Department of Physics, Technion − Israel Institute of Technology, Technion City, Haifa, 3200003, Israel
³School of the Environment and Earth Sciences, Tel Aviv University, Ramat Aviv, Tel Aviv, 6997801, Israel

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¹Zamiatina, Daria A.,¹Zamyatin, Dmitry A.,¹Mikhalevskii, Georgii B.,¹Chebikin, Nikolai S.
Minerals 13, 686 Link to Article [DOI 10.3390/min13050686]
¹The Zavaritsky Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences, Ekaterinburg, 620016, Russian Federation

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¹Ray D.,¹Shukla A.D.,¹Bhardwaj, Anil
Current Science 124, 1138-1139 Link to Article [ISSN00113891]
¹Physical Research Laboratory, Ahmedabad, 380 009, India

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¹Briony Horgan et al. (>10)
Journal of Geophysical Research (Planets)(in Press) Link to Article [https://doi.org/10.1029/2022JE007612]
¹Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West 19 Lafayette, IN 47906
Published by arrangement with John Wiley & Sons

The first samples collected by the Perseverance rover on the Mars 2020 mission were from 60 the Maaz formation, a lava plain that covers most of the floor of Jezero crater. Laboratory 61 analysis of these samples back on Earth would provide important constraints on the petrologic 62 history, aqueous processes, and timing of key events in Jezero crater. However, interpreting 63 these samples requires a detailed understanding of the emplacement and modification history of 64 the Maaz formation. Here we synthesize rover and orbital remote sensing data to link outcrop-65 scale interpretations to the broader history of the crater, including Mastcam-Z mosaics and 66 multispectral images, SuperCam chemistry and reflectance point spectra, RIMFAX ground 67 penetrating radar, and orbital hyperspectral reflectance and high-resolution images. We show 68 that the Maaz formation is composed of a series of distinct members corresponding to basaltic to 69 basaltic-andesite lava flows. The members exhibit variable spectral signatures dominated by 70 high-Ca pyroxene, Fe-bearing feldspar, and hematite, which can be tied directly to igneous 71 grains and altered matrix in abrasion patches. Spectral variations correlate with morphological 72 variations, from recessive layers that produce a regolith lag in lower Maaz, to weathered 73 polygonally fractured paleosurfaces and crater-retaining massive blocky hummocks in upper 74 Maaz. The Maaz members were likely separated by one or more extended periods of time, and 75 were subjected to variable erosion, burial, exhumation, weathering, and tectonic modification. 76 The two unique samples from the Maaz formation are representative of this diversity, and 77 together will provide an important geochronological framework for the history of Jezero crater.