¹GianfrancoDi Vincenzo,^2,3Luigi Folco,^2,4Martin D.Suttle,⁵Lauren Brase,⁵Ralph P.Harvey
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.01.046]
¹Istituto di Geoscienze e Georisorse – CNR, via Moruzzi 1, 56124 Pisa, Italy
²Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy
³CISUP, Centro per l’Integrazione della Strumentazione Scientifica dell’Università di Pisa, Lungarno Pacinotti 42, 56126 Pisa, Italy
⁴Planetary Materials Group, Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom
⁵Department of Earth, Environmental and Planetary Sciences, Case Western Reserve University, 112 A.W. Smith Bldg., Cleveland, OH 44106-7216, United States
Copyright Elsevier

Microtektites represent high-velocity/distal meteorite impact ejecta. Demonstrating that microtektites found at several locations throughout East-Antarctica consist of a homogeneous class of geological objects belonging to the Australasian tektite/microtektite strewn field is fundamental to define the actual extent of the largest and youngest known tektite field on Earth produced by an asteroidal impact ∼0.8 Ma ago. This study presents new 40Ar/39Ar analyses performed by multi-collector noble gas mass spectrometry on individual microtektites from two key locations in the Transantarctic Mountains: Miller Butte, in northern Victoria Land, and Mount Raymond, over 1,000 km further south, in the Grosvenor Mountains. Results indicate that particles are heavily contaminated by at least one extraneous Ar component, which is not correlated with size nor with bulk chemical composition, and precludes a straightforward interpretation of 40Ar/39Ar data. Analysis of data from step-heating and total fusion analyses in three-isotope correlation diagrams yielded indistinguishable isochron ages from the two locations, with a combined isochron average of 800±89 ka (95% confidence level). These age results improve by more than one order of magnitude previously published 40Ar/39Ar age determinations and improve by ∼4 times a previous fission track date, thus providing conclusive evidence that microtektites found throughout the Transantarctic Mountains of Antarctica belong to a single source – the Australasian field. This study strengthens the southward extension of the Australasian field (∼4,000 km southward with respect to Australasian microtektites recovered at lower latitudes from deep sea sediments), thus implying a launch distance of nearly 12,000 km from the putative impact location in Indochina. From a broad perspective, results also reveal a contrasting behavior between microtektites from the Transantarctic Mountains, highly contaminated by extraneous Ar, and Australasian macroscopic tektites, weakly or negligibly contaminated. Although future dedicated experimental work, aimed at the definition of physical homogeneity of microtektites at the submicroscale and at the understanding of the true intra-particle spatial distribution of Ar isotopes are necessary, we speculatively hypothesize that the contrasting behavior between tektites and microtektites may reflect displacement in different environments.

^1,2,3Kirtland J.Robinson,²Christiana Bockisch,²Ian R.Gould,⁴Yiju Liao,⁴Ziming Yang,⁵Christopher R.Glein,²Garrett D.Shaver,^2,3Hilairy E.Hartnett,³Lynda B.Williams,^2,3Everett L.Shock
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.01.038]
¹Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, 02543
²School of Molecular Sciences, Arizona State University, Tempe, Arizona, 85287
³School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, 85287
⁴Department of Chemistry, Oakland University, Rochester, Michigan, 48309
⁵Space Science and Engineering Division, Southwest Research Institute, San Antonio, Texas, 78228
Copyright Elsevier

Amide bonds are fundamental products in biochemistry, forming peptides critical to protein formation, but amide bonds are also detected in sterile environments and abiotic synthesis experiments. The abiotic formation of amide bonds may represent a prerequisite to the origin of life. Here we report thermodynamic models that predict optimal conditions for amide bond synthesis across geologically relevant ranges of temperature, pressure, and pH. We modeled acetamide formation from acetic acid and ammonia as a simple analog to peptide bond formation, and tested this model with hydrothermal experiments examining analogous reactions of amides including benzanilide and related structures. We also expanded predictions for optimizing diglycine formation, revealing that in addition to synthesis becoming more favorable at near-ambient pressures (Psat) with increasing temperatures, the strongest thermodynamic drive exists at extremely high pressures (> 15,000 bar) and decreasing temperatures. Beyond implications for life’s origins, the reactants and products involved in simple amide formation reactions can potentially be used as geochemical tracers for planetary exploration of environments that may be habitable.

¹Arshad Ali,²Iffat Jabeen
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13627]
¹Earth Sciences Research Centre (ESRC), Sultan Qaboos University, Al‐Khoudh, Muscat 123, Sultanate of Oman
²Department of Earth Sciences, University of Western Ontario, London, Ontario, N6A 3K7 Canada
Published by arrangement with John Wiley & Sons

The Allende, Eagle Station pallasites (ESP), some ungrouped achondrites (UA), and ungrouped irons (UI) represent different types of meteorites; these are traditionally distinguished as chondrule‐bearing chondrite, chondrule‐free stony irons, stones, and irons, respectively. Oxygen isotopic compositions of meteorites have long been used as a robust tool to identify the parent bodies of these extraterrestrial materials. We revisited the δ18O‐∆17O, along with ε54Cr and ε92Mo, of these meteorites and established a genetic link that possibly suggests that undifferentiated carbonaceous chondrites and their differentiated counterparts belong to a common reservoir. We observed that the differentiated counterparts of Allende CV3 including ESP, ungrouped achondrites, and ungrouped irons collectively provide the oxygen isotope trends comparable to those of the Y&R and the PCM lines, and are likely to represent the primitive isotope reservoirs in the solar nebula. ε54Cr and ε92Mo data of these meteorite types support their oxygen isotope trends. The idea of a common partially differentiated parent body also lends support from natural remnant magnetization in the CV chondrites and Eagle Station olivine. Furthermore, ∆17O‐ε54Cr data suggest that these meteorites originated from carbonaceous planetesimal(s) accreted at different heliocentric distances in the solar nebula in the outer solar system. As proposed earlier, these bodies remained separated from the inner solar system due to formation of Jupiter. Taken together, we propose that the ungrouped irons, ESP, and ungrouped achondrites could possibly represent the differentiated sections, such as core, core–mantle, and mantle of a planetesimal(s), respectively, with the Allende CV3 representing an undifferentiated chondritic crust.

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Specifically, we have just released vacancy notices for one senior lecturer (somewhat similar to associate professor in the US or senior lecturer in the UK, tenure), one associate senior lecturer (equivalent to lecturer/assistant professor, tenure track) and a postdoc (2 year fixed-term).
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^1,2Samuel P. Alpert,^1,2Denton S. Ebel,^1,2,3Michael K. Weisberg,
^1,4Jeremy R. Neiman
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13619]
¹Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, 10024 USA
²Department of Earth and Environmental Sciences, CUNY Graduate Center, New York, New York, 10016 USA
³Department of Physical Sciences, Kingsborough Community College, CUNY, Brooklyn, New York, 11235 USA
⁴Python developer and participant in the American Museum of Natural History “Hack the Solar System” 2019 Hackathon.
Published by arrangement with John Wiley & Sons

Opaque assemblages (OAs) are small (submillimeter) objects composed primarily of metals, sulfides, and oxides that exist in nearly all chondritic meteorite groups as discrete objects in the matrix or associated with chondrules. The size, morphology, and petrology of OAs vary greatly between different chondrite groups, with petrologic grade within a single group, and by their apparent textural setting. Two hypotheses may explain the formation of matrix OAs: (1) they were separated from chondrules via surface tension during heating events, or (2) they formed as free‐floating objects in the solar nebula; however, this is the first comprehensive study of the petrology of OAs in ordinary chondrites (OCs) as a group, which seeks to determine if one hypothesis is sufficient to explain all such objects. Here, we use a newly developed machine learning algorithm to show that all OAs from the least equilibrated OC, Semarkona (LL 3.01), are composed of kamacite, taenite, troilite, pentlandite, magnetite, and other minor phases. These OAs form two distinct groups based on their modal mineralogy: one group in and associated with chondrules, and the other group free‐floating in the matrix. Chondrule OAs exhibit a bimodal distribution between sulfide‐ and metal‐rich endmembers in agreement with previous findings. Matrix OAs cluster at roughly equal abundances of sulfides and metals and universally exhibit magnetite rims. The two populations of chondrule OAs cannot be combined to form the modal mineralogies observed in matrix OAs and some matrix OAs exhibit mineralogical layering consistent with fractional condensation. Both observations support the hypothesis that matrix OAs were not formed by expulsion from chondrules and instead formed as free‐floating objects in the solar nebula; however, chondrule OAs must have formed with their host chondrules during heating events.

¹Stefan Loehle,¹Martin Eberhart,¹Fabian Zander,¹Arne Meindl,²Regina Rudawska,²Detlef Koschny,²Joe Zender,³Ron Dantowitz,⁴Peter Jenniskens
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13622]
¹Institut für Raumfahrtsysteme, Universität Stuttgart, Stuttgart, 70569 Germany
²ESA ESTEC, Noordwijk, 2201 The Netherlands
³Dexter Southfield, Boston, MA02445 Massachusetts, USA
⁴SETI Institute, Mountain View, CA94043 California, USA
Published by arrangement with John Wiley & Sons

The advancement in the acquisition of spectral data from meteors, as well as the capability to analyze meteoritic entries in ground testing facilities, requires the assessment of the performance of software tools for the simulation of spectra for different species. The Plasma Radiation Database, PARADE, is a line‐by‐line emission calculation tool. This article presents the extensions implemented for the simulation of meteor entries with the additional atomic species Na, K, Ti, V, Cr, Mn, Fe, Ca, Ni, Co, Mg, Si, and Li. These atoms are simulated and compared to ground testing spectra and to observed spectra from the CILBO observatory. The diatomic molecules AlO and TiO have now been added to the PARADE database. The molecule implementations have been compared to the results of a simple analytical program designed to approximate the vibrational band emission of diatomic molecules. AlO and TiO have been identified during the airborne observation campaigns of re‐entering man‐made objects WT1190F and CYGNUS OA6. Comparisons are provided showing reasonable agreement between observation and simulation.

^1,2,3Imene Kerraouch et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13620]
¹Institut für Planetologie, University of Münster, Wilhelm‐Klemm Str. 10, Münster, D‐48149 Germany
²Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, Texas, 77058 USA
³Department of Geology, University of Science and Technology Houari Boumediene (USTHB), Bab Ezzouar, 16111 Algeria
Published by arrangement with John Wiley & Sons

On April 23, 2019, a meteorite fall occurred in Aguas Zarcas, Costa Rica. According to the Meteoritical Bulletin, Aguas Zarcas is a brecciated CM2 chondrite dominated by two lithologies. Our X‐ray computed tomography (XCT) results show many different lithologies. In this paper, we describe the petrographic and mineralogical investigation of five different lithologies of the Aguas Zarcas meteorite. The bulk oxygen isotope compositions of some lithologies were also measured. The Aguas Zarcas meteorite is a breccia at all scales. From two small fragments, we have noted five main lithologies, including (1) Met‐1: a metal‐rich lithology; (2) Met‐2: a second metal‐rich lithology which is distinct from Met‐1; (3) a brecciated CM lithology with clasts of different petrologic subtypes; (4) a C1/2 lithology; and (5) a C1 lithology. The Met‐1 lithology is a new and unique carbonaceous chondrite which bears similarities to CR and CM chondrite groups, but is distinct from both based on oxygen isotope data. Met‐2 also represents a new type of carbonaceous chondrite, but it is more similar to the CM chondrite group, albeit with a very high abundance of metal. We have noted some similarities between the Met‐1 and Met‐2 lithologies and will explore possible genetic relationships. We have also identified a brecciated CM lithology with two primary components: a chondrule‐poor lithology and a chondrule‐rich lithology showing different petrologic subtypes. The other two lithologies, C1 and C1/2, are very altered and possibly related to the CM chondrite group. In this article, we describe all the lithologies in detail and attempt a classification of each in order to understand the origin and the history of formation of the Aguas Zarcas parent body.

¹D.I.Foustoukos,¹C.M.O’D.Alexander,¹G.D.Cody
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.01.021]
¹Earth & Planets Laboratory, Carnegie Institution of Washington, 5241 Broad Branch Rd. NW, Washington DC 20015, USA
Copyright Elsevier

A series of experiments was performed to constrain the chemical and isotope evolution of insoluble organic material (IOM) during hydrothermal alteration at temperatures ranging from 250 °C to 450 °C at 50 MPa. Experiments involved IOM that was extracted from the Murchison (CM2) meteorite or synthesized by aqueous carbonization of dextrose. Flash (dry) pyrolysis experiments at 400 – 1000 °C were also conducted with Murchison-IOM to distinguish between the effects of hydrothermal and thermal degradation. Extended reaction times (up to 3905 h) were employed to establish D/H equilibria between IOM and H2O. The H isotope compositions of the H2O used in the experiments ranged from δD = -447 ‰ to 3259 ‰. Results revealed that the extent of the IOM H isotope evolution strongly depends on the δD composition of the coexisting H2O with minimal temperature effects. The empirical relationship that describes the isotope exchange between IOM and H2O is as follows:

δDIOM (‰) = 0.643 (± 0.007) * δDH2O (‰) – 86 (± 8) (‰)

Based on this empirical relationship, two models are proposed for the H2O-IOM H exchange. The first assumes that all H in IOM is exchangeable and that the redistribution of H-bearing moieties with experiment temperature results in an “apparent” εorganics-H2O= -357 ‰. The second model considers a higher εorganics-H2O (-131 ‰), in accordance with theoretical studies, and assumes the presence of two H reservoirs, one that undergoes H isotope exchange with H2O and one that does not. In this case, 74 % of the H in IOM is exchangeable with H2O.

In our experiments, the hydrothermally altered Murchison-IOM lost labile 15N enriched N-H moieties. Experiments that included 15N-labelled NH3(aq) found that there was only minor N exchange with IOM. Furthermore, the experimental data show that the extent of H and N loss is temperature and process dependent. This results in the decoupling of N/C and H/C atomic ratio systematics between hydrothermal alteration and flash (dry) pyrolysis, with much more limited changes in H/C and N/C after flash pyrolysis.

In the light of the experiments, two models for the range of bulk and IOM H isotope compositions of the aqueously altered CI, CM, and CR chondrites are explored. The very D-rich IOM compositions, relative to the bulk compositions, cannot be explained by a fully exchangeable IOM with a reasonable value for εorganics-H2O (i.e., <0 ‰). Instead, a two-component IOM model is invoked in which the initial bulk and non-exchangeable IOM have δD = 3650 ‰. The estimated ranges of Fexchange, including uncertainties in εorganics-H2O, are 0.59-0.75 and 0.13-0.30 for CMs and CRs, respectively. Most values of Fexchange are significantly lower than in the experiments, perhaps because the alteration temperatures in the chondrites were << 250 °C. An apparent relationship between Fexchange and the IOM δ15N suggests an endmember composition of ∼ 300 ‰. For the CMs, alone, however, the initial δ15N is projected to ∼ 137 ‰.

¹Zhengbin Deng,¹Marc Chaussidon,^1,2,3Denton S.Ebel,⁴Johan Villeneuve,¹Julien Moureau,¹Frédéric Moynier
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.01.015]
¹Université de Paris, Institut de physique du globe de Paris, CNRS, UMR 7154, Paris 75005, France
²Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA
³Department of Earth and Environmental Sciences, Columbia University, New York, USA
⁴Centre de Recherches Pétrographiques et Géochimiques, Université de Lorraine, CNRS 7358, Vandoeuve-lès-Nancy, France
Copyright Elsevier

Improvements in our understanding of the formation of chondrules requires a better knowledge of the thermal histories and the nature of their solid precursors. We present an in situ nanosecond laser ablation multi-collector inductively-coupled-plasma mass-spectrometry (LA-MC-ICP-MS) technique to measure simultaneously mass-dependent Mg isotopic fractionations and radiogenic 26Mg in chondritic components, thus allowing us to investigate within a chronological framework the thermal processes redistributing Mg in chondrules and their precursors. The internal 26Al-26Mg isochrons provide initial 26Al/27Al ratios from 5.46 (± 0.38) × 10−5 to 6.14 (± 0.92) × 10−5 for amoeboid olivine aggregates (AOAs) and Ca-, Al-rich inclusions (CAIs), and from 0.16 (± 0.08) × 10−5 to 1.87 (± 0.92) × 10−5 for chondrules from Allende and Leoville chondrites, which are consistent with the previously reported values. The combination of these values with up to 2.5‰ variation of the 25Mg/24Mg ratio within the studied chondrules shows that: (i) AOAs and the precursors of chondrules were likely formed via condensation of rapid-cooling gas reservoirs, and (ii) Mg stable isotopes are probably at disequilibrium between olivines and mesostases in some chondrules, likely due to Mg loss by vaporization during chondrule formation. We use these new observations to propose that Mg isotopes can likely serve as a tracer for the thermal histories of chondrules. We present here a scenario taking into account Mg loss by vaporization from chondrule melt and Mg gain into the melt by olivine dissolution. The existing Mg isotopic observations in chondrule melts and olivines can be explained in a scenario with a homogeneous distribution of Mg isotopes and initial 26Al in the accretion disk, provided that chondrule precursors have been heated up to sufficiently high peak temperatures (up to 2123 K) and stayed above 1800 K for several tens of minutes to allow for significant Mg evaporation. These conditions are most consistent with a shock wave model for the origin of chondrules.

¹M.D.Suttle,^1,2A.J.King,¹P.F.Schofield,^1,3H.Bates,¹S.S.Russell
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.01.014]
¹Planetary Materials Group, Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, UK
²Planetary and Space Sciences, Open University, Walton Hall, Milton Keynes, MK7 6AA, U.K
³Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford OX1 3PU, UK
Copyright Elsevier

The CM chondrites are samples of primitive water-rich asteroids formed during the early solar system. They record significant interaction between liquid water and silicate rock, resulting in a mineralogy dominated by hydrated secondary phases. Their similarity to the near-Earth asteroids Bennu and Ryugu – targets of current sample return space missions – makes the analysis of CM chondrites essential to the interpretation of these enigmatic bodies. Here, we review the aqueous alteration history of the CM chondrite group.

Initially, amorphous silicate, metal and sulphides within the matrix were converted into Fe-cronstedtite and tochilinite. Later, the serpentinization of refractory coarse-grained inclusions led to the addition of Mg to the fluid phase. This is reflected in the cation composition of secondary phases which evolved from Fe-rich to Mg-rich. Although most CM meteorites are classified as CM2 chondrites and retain some unaltered anhydrous silicates, a few completely altered CM1s exist (∼4.2% [Meteoritical Bulletin, 2021]).

The extent of aqueous alteration can be quantified through various techniques, all of which trace the progression of secondary mineralization. Early attempts employed petrographic criteria to assign subtypes – most notably the Browning and Rubin scales have been widely adopted. Alternatively, bulk techniques evaluate alteration either by measuring the ratio of phyllosilicate to anhydrous silicate (this can be with X-ray diffraction [XRD] or infrared spectroscopy [IR]) or by measuring the combined H abundance/δD compositions. The degree of aqueous alteration appears to correlate with petrofabric strength (most likely arising due to shock deformation). This indicates that aqueous alteration may have been driven primarily by impact rather than by radiogenic heating. Alteration extent and bulk O-isotope compositions show a complex relationship. Among CM2 chondrites higher initial water contents correspond to more advanced alteration. However, the CM1s have lighter-than-expected bulk compositions. Although further analyses are needed these findings could suggest either differences in alteration conditions or initial isotopic compositions – the latter scenario implies that the CM1 chondrites formed on a separate asteroid from the CM2 chondrites.

Secondary phases (primarily calcite) act as proxies for the conditions of aqueous alteration and demonstrate that alteration was prograde, with an early period at low temperatures (<70°C), while later alteration operated at higher temperatures of 100-250°C. Estimates for the initial water-to-rock ratios (W/R) vary between 0.2-0.7. They are based either on isotopic mass balance or mineral stoichiometry calculations – variability reflects uncertainties in the primordial water and protolith compositions and whether alteration was open or closed system. Some CM chondrites (<36%) experienced a later episode of post-hydration thermal metamorphism, enduring peak temperatures <900°C and resulting in a dehydrated mineralogy and depleted volatile element abundances. Heating was likely short-duration and caused by impact events. The presence of CM chondrite material embedded in other meteorites, their prominence among the micrometeorite flux and the link between CMs and rubble-pile C-type near-Earth asteroids (e.g. Bennu and Ryugu) implies that the CM parent body was disrupted, leaving second-generation CM asteroids to supply material to Earth.