^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.

¹Ted L.Roush,^1,2Luis F.A.Teodoro,³David T.Blewett,³Joshua T.S.Cahill
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114331]
¹NASA Ames Research Center, Planetary Systems Branch, MS 245-3, Moffett Field, CA 94035-0001, USA
²Bay Area Environmental Research Institute, P.O. Box 25, Moffett Field, CA 94035-0001, USA
³Planetary Exploration Group, Johns Hopkins University Applied Physics Laboratory, MS 200-W2320, 11100 Johns Hopkins Rd., Laurel, MD 20723, USA
Copyright Elsevier

We use radiative transfer (RT) models, based upon the Hapke (1993) model, to estimate the imaginary refractive index of magnetite from laboratory reflectance measurements. We used a RT program coupled with a least-squares algorithm to fit measured reflectance data using complex refractive indices of magnetite estimated here and literature values. We included differing representations of the grain size distribution for modeling the measured reflectance of the magnetite samples. Best-fitting models were obtained when using the complex indices of refraction estimated from a specific grain size fraction to fit the same grain size of reflectance data. Magnetite complex refractive indices taken from reported literature studies resulted in the poorest fits to the measured reflectance data.

We investigated the multiple-scattering behavior of magnetite using Fresnel’s equation and complex refractive indices from literature values and our own estimates. For both we found the reflection coefficient is <1% after four reflections suggesting that multiple scattering is minimal. We also calculated the transmission via the Beer-Lambert law using the same sets of refractive indices. For both, the initial interface transmission had a comparable value near 80%. However, as the distance through the material increases the discrepancy between the two refractive indices had substantial influence. For the literature values the transmission was reduced to <1% after a distance of 8 μm at all wavelengths, whereas for the estimated values the transmission remained ≥75% at this distance. Magnetite, when viewed in a petrographic thin section (~30 μm thick), is opaque. This suggests that the optical constants estimated via the Hapke approach are not realistic. We compared the calculated Fresnel reflectance using one literature value to the measured reflectances and found that the overall spectral shape was similar to the magnetite diffuse reflectance measurements. However, the magnetite diffuse reflectance is only 30–40% of the calculated Fresnel reflectance. We speculate this may be due to the granular surfaces scattering light into a non-specular angle. Hapke-like models have been successfully applied for estimating optical constants of transparent materials. However, the present study finds that such models may not be appropriate for determining the optical constants of low-reflectance, opaque materials, as the results are not comparable to values of optical constants reported in the literature.

¹Charles A. Geiger,¹Noreen M. Vielreicher,¹Edgar Dachs
American Mineralogist 106, 317-321 Link to Article [DOI: https://doi.org/10.2138/am-2021-7764CCBY]
¹Department of Chemistry and Physics of Materials, Section Materials Science and Mineralogy, Salzburg University, Jakob Haringer Strasse 2a, A-5020 Salzburg, Austria
Copyright: The Mineralogical Society of America

It is not known if the thermodynamic behavior of some minerals and their synthetic analogues are quantitatively the same. Olivine is an important rock-forming substitutional solid solution consisting of the two end-members forsterite, Mg2SiO4, and fayalite, Fe2SiO4. We undertook thefirst heat capac-ity, CP, measurements on two natural olivines between 2 and 300 K; nearly end-member fayalite and a forsterite-rich crystal Fo0.904Fa0.096. Their CP(T) behavior is compared to that of synthetic crystals of similar composition, as found in the literature. The two natural olivines are characterized by X-ray powder diffraction and 57Fe Mössbauer spectroscopy. The X-ray results show that the crystals are well crystalline. The Mössbauer hyperfine parameters, obtained from a fit with two Fe2+ quadrupole split doublets, are similar to published values measured on synthetic olivines. There are slight differences in the absorption line widths (i.e., FWHM) between the natural and synthetic crystals. CP (2 to 300 K) is measured by relaxation calorimetry. The CP results of the natural nearly end-member fayalite and published values for two different synthetic Fa100 samples are in excellent agreement. Even CP result-ing from a Schottky anomaly and a paramagnetic-antiferromagnetic phase transition with both arising from Fe2+ are similar. There are slight differences in the Néel temperature between the natural 63 K and synthetic ~65 K fayalites. This is probably related to the presence of certain minor elements (e.g., Mn2+) in the natural crystal. The third-law entropy, S°, value is 151.6 ± 1.1 J/(mol·K). CP behavior of the natural forsterite, Fo0.904Fa0.096, and a synthetic olivine, Fo90Fa10, are in excellent agreement between about 7 and 300 K. The only difference lies at T < 7 K, as the former does not show Debye T3 behavior, but, instead, a plateauing of CP values. The S° value for the natural forsterite is 99.1 ± 0.7 J/(mol·K).

¹Niclas Schneider,¹Grzegorz Musiolik,¹Jonathan E.Kollmer,¹Tobias Steinpilz,¹Maximilian Kruss,¹Felix Jungmann,¹Tunahan Demirci,¹Jens Teiser,¹Gerhard Wurm
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114307]
¹University of Duisburg-Essen, Faculty of Physics, Lotharstr. 1-21, 47057 Duisburg, Germany
Copyright Elsevier

In protoplanetary disks zones of dense particle configuration promote planet formation. Solid particles in dense clouds alter their motion through collective effects and back reaction to the gas. The effect of particle-gas feedback with an ambient solid-to-gas ratios on the stopping time of particles is investigated. In experiments on board the International Space Station we studied the evolution of a dense granular gas while interacting with air. We observed diffusion of clusters released at the onset of an experiment but also the formation of new dynamical clusters. The solid-to-gas mass ratio outside the cluster varied in the range of about 2.5–60. We find that the concept of gas drag in a viscous medium still holds, even if the medium is strongly dominated in mass by solids. However, a collective factor has to be used, depending on , i.e. the drag force is reduced by a factor 18 at the highest mass ratios. Therefore, flocks of grains in protoplanetary disks move faster and collide faster than their constituents might suggest.

¹C. A. Lorenz,¹M. A. Ivanova,²F. Brandstaetter,¹N. N. Kononkova,³N. G. Zinovieva
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13612]
¹Vernadsky Institute of Geochemistry and Analytical Chemistry, Kosygin St. 19, Moscow, 119991 Russia
²Museum of Natural History, A‐1014 Vien, Burgring 7, Austria
³Lomonosov Moscow State University, Leninskie Gory, Moscow, 119991 Russia
Published by arrangement with John Wiley & Sons

The Pesyanoe aubrite is an essentially polymict regolith breccia comprised by fragments of different highly magnesian pyroxenitic lithologies: albite; anorthoclase and labradorite‐bearing pyroxenites; diopside and magnesian augite pyroxenites; roedderite‐ and forsterite‐bearing pyroxenites; and impact glasses; porphyritic and melt matrix breccia fragments; FeO‐rich chondritic inclusions; and exotic oxidized clasts. The parent magma of Pesyanoe probably was carbon saturated, as suggested by pyroxenite fragments containing igneous‐textured carbon phases, possibly graphite. The composition of feldspar and trapped melt inclusions in enstatite indicates occurrence of at least three metaluminous melt sources with different (K + Na)/Al and K/(K + Na) atomic ratios on the Pesyanoe parent body and has records of K and Na loss from the melt, possibly due to evaporation from the parent body surface. The roedderite‐ and forsterite‐bearing rocks probably crystallized from a peralkaline melt. We propose that peralkaline melt could be formed from a metaluminous melt(‐s) due to gravitational segregation of djerfisherite‐bearing metal‐sulfide liquid in the lower horizon of the magma chamber and following oxidation of the magma. This should lead to enrichment of silicate melt in K2O and Na2O and increasing of (K + Na)/Al > 1, allowing forsterite and roedderite to crystallize. Rocks enriched in K and containing rare K‐bearing minerals were found among both magmatic and melt rocks. This may imply an insignificant role of regolith transport in the process of the breccia’s formation and, therefore, an origin of all of the breccia components from a local region of the Pesyanoe parent body, probably from a single complex igneous massif.