¹Edouard Kaminski,¹Angela Limare,¹ Balthasar Kenda,¹Marc Chaussidon
Earth and Planetary Science 548, 116469 Link to Article [https://doi.org/10.1016/j.epsl.2020.116469]
¹Université de Paris, Institut de Physique du Globe de Paris, CNRS, 1 rue Jussieu, F-75005 Paris, France
Copyright Elsevier

The timing of formation of 100-300 km size planetesimals in the protoplanetary disk remains largely unconstrained. Recent models show that gravitational collapse of boulders in overdense regions of a dusty accretion disk can overcome the meter-sized barrier and lead to rapid formation of planetesimals with size of several km that further grow by pebble accretion. Hf/W ages indicate that the first large planetesimals to form could be the parent bodies of magmatic iron meteorites. These ages have been so far used to constrain timing of accretion considering (i) instantaneous accretion, and (ii) purely conductive heat transfer in the planetesimal. To relax these hypotheses we model the thermal evolution of a planetesimal in course of accretion and we take into account the possibility of convection onset. Our model is further based on considering the possibility of a common thermal evolution for all the parent bodies of iron meteorites. Within that framework we show that the planetesimals could have grown following a universal accretion law starting at the very beginning of the history of the disk by a nearly instantaneous formation of 60 ± 30 km size nuclei, followed by a growth via pebble accretion at a much slower pace to reach final sizes of 150–300 km in about 3 Myr. In this universal scenario, complete melting and total differentiation are not bound to happen in the parent body due to the continuous accretion of cold pebbles. The model, though calibrated here on iron meteorites, is general and can in principle be applied to other types of planetesimals such as for instance the parent bodies of CV chondrites.

¹Dennis Harries,²Addi Bischoff
Earth and Planetary Science Letters 548, 116506 Link to Article [https://doi.org/10.1016/j.epsl.2020.116506]
¹Institute für Geowissenschaften, Friedrich-Schiller-Universität Jena, Carl-Zeiss-Promenade 10, 07745 Jena, Germany
²Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149, Münster, Germany
Copyright Elsevier

Two samples of a unique achondritic lithology of the Almahata Sitta meteorite (MS-MU-019 and MS-MU-036) contain three coexisting pyroxene species: orthoenstatite, clinoenstatite and augite. The silicate assemblage appears to be the restite after extraction of melts of broadly basaltic and metal-sulfide composition from an enstatite chondrite protolith. Transmission electron microscopy (TEM) provides evidence that clinoenstatite within the lithology formed from earlier protoenstatite. The absence of pigeonite despite the successful nucleation of augite and the persistence of orthoenstatite during cooling suggests that the sub-solidus formation of the three coexisting pyroxenes occurred at a pressure of about 0.1 GPa. Rapid cooling at >1 K/h below 1260°C is documented by the cessation of augite equilibration, preservation of the 3-pyroxene assemblage and a low volume fraction of nanoscale orthoenstatite lamellae formed during the transformation of protoenstatite to clinoenstatite. The pressure implies a diameter of roughly 500 km of the differentiated parent body, putting petrological constraints on the size of planetesimals that may have contributed to the accretion of the terrestrial planets including Earth. The high cooling rate indicates a catastrophic disruption of this large planetesimal early in its history. The lithology studied here underlines that planetesimals which existed in the inner Solar System were more diverse than previously thought, and included potentially large differentiated bodies with very FeO-poor, enstatite-dominated mantles. Remains of these bodies are poorly represented in meteorite collections, which points to efficient accretion in the inner Solar System or to removal and little re-distribution of material into the present-day asteroid belt.

¹James M. D. Day
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13544]
¹Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, 92093 USA
Published by arrangement with John Wiley & Sons

Anhedral metal grains of >micrometer size occur in many lunar rock types, including mare basalts, magnesian suite rocks (MGS), ferroan anorthosites (FAN), and impact melt rocks and breccias. Some metal grains are inherited from, or modified by, impactors striking the Moon into crustal materials. These grains have high Ni/Co resulting from the addition of chondritic or iron impactors. Metal grains in mare basalts, FAN, and MGS have Ni/Co ranging from >20 to <1, being generally distinct from impactor compositions. Nickel and Co behave as compatible elements in lunar melts, with parental melts having between ~40–50 ppm Co, ~40–60 ppm Ni, and Ni/Co ~1. These compositions suggest a bulk silicate Moon (BSM) with Ni some three times lower than in bulk silicate Earth. Modeling of Ni and Co during fractional crystallization of mafic mare basalt parental melts originating from a BSM source predicts high Ni/Co metals form during early olivine fractionation. The combined effects of pyroxene ± plagioclase crystallization and increasing but variable compatibility of Ni and Co during basaltic melt evolution can explain the generation of low Ni/Co metals in more differentiated mare basalts. High‐Ti mare basalts have metal with low Ni/Co, but the crystallization of ilmenite and armalcoite restricts the range of Ni and Co in metal. Collectively, these results are consistent with metal grains in mare basalts forming solely through endogenous processes. Measurement of metal grains represents a rapid way for determining endogenous (e.g., lunar interior melts) versus exogenous (e.g., impact contamination) processes acting on lunar samples. In turn, the presence of low Ni/Co metal grains in mare basalts supports their origin as uncontaminated partial melts originating from lunar mantle sources that may have experienced loss of Ni to a small lunar core.

¹Kento Kiryu,¹Yoko Kebukawa,²Motoko Igisu,²Takazo Shibuya,³Michael E. Zolensky,¹Kensei Kobayashi
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13548]
¹Graduate School of Engineering Science, Yokohama National University, 79‐5 Tokiwadai, Hodogaya‐ku, Yokohama, 240‐8501 Japan
²Super‐cutting‐edge Grand and Advanced Research (Sugar) Program, Institute for Extra‐cutting‐edge Science and Technology Avant‐garde Research (X‐star), Japan Agency for Marine‐Earth Science and Technology (JAMSTEC), 2‐15 Natsushima‐cho, Yokosuka, 237‐0061 Japan
³Astromaterials Research and Exploration Science, NASA Johnson Space Center, 2101 NASA Parkway, Houston, Texas, 77058 USA
Published by arrangement with John Wiley & Sons

In order to establish kinetic expressions for Raman spectroscopic parameters of organic matter in chondritic meteorites with heating, a series of heating experiments (at 600–900 °C for 3–48 h) of the Murchison (CM2) meteorite was conducted. For comparison, several carbonaceous chondrites with various metamorphic degrees—Allende (CV3.2), Moss (CO3.6), Yamato (Y‐) 793321 (heated CM2), and Tagish Lake (ungrouped C2)—were also analyzed by the Raman spectrometer. Changes in the full width at half maximum of the D1 band (Γ_D) of heated Murchison correlated well with temperature and time, and showed similar trends of chondrites with various metamorphic degrees. We obtained the kinetic expressions for the changes in Γ_D by heating to estimate the time–temperature history of thermally metamorphosed type 3 chondrites and heated CM chondrites. Our results may also be useful for asteroids which are the targets of Hayabusa2 and OSIRIS‐REx missions.

¹Hope A.Tornabene,¹Connor D.Hilton,¹Katherine R.Bermingham,¹Richard D.Ash,¹Richard J.Walker
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.07.036]
¹Department of Geology, University of Maryland, College Park, Maryland, 20742, USA
Copyright Elsevie

The eight iron meteorites currently classified as belonging to the IIC group were characterized with respect to the compositions of 21 siderophile elements. Several of these meteorites were also characterized for mass independent isotopic compositions of Mo, Ru and W. Chemical and isotopic data for one, Wiley, indicate that it is not a IIC iron meteorite and should be reclassified as ungrouped. The remaining seven IIC iron meteorites exhibit broadly similar bulk chemical and isotopic characteristics, consistent with an origin from a common parent body. Variations in highly siderophile element (HSE) abundances among the members of the group can be well accounted for by a fractional crystallization model with all the meteorites crystallizing between ∼10 and ∼26% of the original melt, assuming initial S and P concentrations of 8 wt.% and 2 wt.%, respectively. Abundances of HSE estimated for the parental melt suggest a composition with chondritic relative abundances of HSE ∼6 times higher than in bulk carbonaceous chondrites, consistent with the IIC irons sampling a parent body core comprising ∼17% of the mass of the body.

Radiogenic ¹⁸²W abundances of two group IIC irons, corrected for a nucleosynthetic component, indicate a metal-silicate segregation age of 3.2 ± 0.5 Myr subsequent to the formation of Calcium-Aluminum-rich Inclusions (CAI). When this age is coupled with thermal modeling, and assumptions about the Hf/W of precursor materials, a parent body accretion age of 1.4 ± 0.5 Myr (post-CAI) is obtained.

The IIC irons and Wiley have ¹⁰⁰Ru mass independent “genetic” isotopic compositions that are identical to other irons with so-called carbonaceous chondrite (CC) type genetic affinities, but enrichments in ^94,95,97Mo and ¹⁸³W that indicate greater s-process deficits relative to most known CC iron meteorites. If the IIC irons and Wiley are of the CC type, this indicates variable s-process deficits within the CC reservoir, similar to the s-process variability within the NC reservoir observed for iron meteorites. Nucleosynthetic models indicate that Mo and ¹⁸³W s-process variability should correlate with Ru isotopic variability, which is not observed. This may indicate the IIC irons and Wiley experienced selective thermal processing of nucleosynthetic carriers, or are genetically distinct from the CC and NC precursor materials.

¹Borovička, J.,¹Spurný, P.,¹Shrbený, L.
Astronomical Journal 160, 42 Link to Article [DOI: 10.3847/1538-3881/ab9608]
¹Astronomical Institute of the Czech Academy of Sciences, Fričova 298, CZ-25165 Ondrejov, Czech Republic

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¹Busarev, V.V.,²Badyukov, D.D.,³Pronina, N.V.
Geochemistry International 58, 795-801 Link to Article [DOI: 10.1134/S0016702920070058]
¹Moscow State University (MSU), Sternberg State Astronomical Institute, Moscow, 119992, Russian Federation
²Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences (GEOKHI RAS), ul. Kosygina 19, Moscow, 119991, Russian Federation
³Moscow State University, Geological Faculty, Moscow, 119991, Russian Federation

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^1,2Luca Bindi,³Vladimir E. Dmitrienko,⁴Paul J. Steinhardt
American Mineralogist 105, 1121-1125 Link to Article [http://www.minsocam.org/MSA/AmMin/TOC/2020/Abstracts/AM105P1121.pdf]
¹Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via La Pira 4, I-50121 Firenze, Italy
²CNR-Istituto di Geoscienze e Georisorse, Sezione di Firenze, Via La Pira 4, I-50121 Firenze, Italy
³A.V. Shubnikov Institute of Crystallography, FSRC “Crystallography and Photonics” RAS, 119333 Moscow, Russia
⁴Department of Physics, Princeton University, Jadwin Hall, Princeton, New Jersey 08544, U.S.A
Copyright: The Mineralogical Society of America

Until 2009, the only known quasicrystals were synthetic, formed in the laboratory under highly controlled conditions. Conceivably, the only quasicrystals in the Milky Way, perhaps even in the Universe, were the ones fabricated by humans, or so it seemed. Then came the report that a quasicrystal with icosahedral symmetry had been discovered inside a rock recovered from a remote stream in far eastern Russia, and later that the rock proved to be an extraterrestrial, a piece of a rare CV3 carbonaceous chondrite meteorite (known as Khatyrka) that formed 4.5 billion years ago in the pre-solar nebula. At present, the only known examples of natural quasicrystals are from the Khatyrka meteorite. Does that mean that quasicrystals must be extremely rare in the Universe? In this speculative essay, we present several reasons why the answer might be no. In fact, quasicrystals may prove to be among the most ubiquitous minerals found in the Universe.

¹Francis M. McCubbin,²Jessica J. Barnes
The American Mineralogist 105, 1270-1274 Link to Article [http://www.minsocam.org/MSA/AmMin/TOC/2020/Abstracts/AM105P1270.pdf]
¹NASA Johnson Space Center, Mailcode XI, 2101 NASA Parkway, Houston, Texas 77058, U.S.A.
²Lunar and Planetary Laboratory, University of Arizona, 1629 E University Blouvard, Tucson, Arizona 85721, U.S.A
Copyright: The Mineralogical Society of America

We conducted in situ Cl isotopic measurements of apatite within intercumulus regions and within a holocrystalline olivine-hosted melt inclusion in magnesian-suite troctolite 76535 from Apollo 17. These data were collected to place constraints on the Cl-isotopic composition of the last liquid to crystallize from the lunar magma ocean (i.e., urKREEP, named after its enrichments in incompatible lithophile trace elements like potassium, rare earth elements, and phosphorus). The apatite in the olivine-hosted melt inclusion and within the intercumulus regions of the sample yielded Cl-isotopic compositions of 28.3 ± 0.9‰ (2σ) and 30.3 ± 1.1‰ (2σ), respectively. The concordance of these values from both textural regimes we analyzed indicates that the Cl-isotopic composition of apatites in 76535 likely represents the Cl-isotopic composition of the KREEP-rich magnesian-suite magmas. Based on the age of 76535, these results imply that the KREEP reservoir attained a Cl-isotopic composition of 28–30‰ by at least 4.31 Ga, consistent
with the onset of Cl-isotopic fractionation at the time of lunar magma ocean crystallization or shortly thereafter. Moreover, lunar samples that yield Cl-isotopic compositions higher than the value for KREEP are likely affected by secondary processes such as impacts and/or magmatic degassing. The presence of KREEP-rich olivine-hosted melt inclusions within one of the most pristine and ancient KREEP-rich rocks
from the Moon provides a new opportunity to characterize the geochemistry of KREEP. In particular, a broader analysis of stable isotopic compositions of highly and moderately volatile elements could provide an unprecedented advancement in our characterization of the geochemical composition of the KREEP reservoir and of volatile-depletion processes during magma ocean crystallization, more broadly.

Marina A. Ivanova et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13546]
¹Vernadsky Institute of Geochemistry and Analytical Chemistry, Moscow, 119991 Russia
Published by arrangement with John Wiley & Sons

We investigated the metal‐rich chondrite Sierra Gorda (SG) 009, a member of the new G chondrite grouplet (also including NWA 5492, GRO 95551). G chondrites contain 23% metal, very reduced silicates, and rare oxidized mineral phases (Mg‐chromite, FeO‐rich pyroxene). G chondrites are not related to CH‐CB chondrites, based on bulk O, C, and N isotopic compositions, mineralogy, and geochemistry. G chondrites have no fine‐grained matrix or matrix lumps enclosing hydrated material typical for CH‐CB chondrites. G chondrites’ average metal compositions are similar to H chondrites. Siderophile and lithophile geochemistry indicates sulfidization and fractionation of the SG 009 metal and silicates, unlike NWA 5492 and GRO 95551. The G chondrites have average O isotopic compositions Δ17O>0‰ ranging between bulk enstatite (E) and ordinary (O) chondrites. An Al‐rich chondrule from SG 009 has Δ17O>0‰ indicating some heterogeneity in oxygen isotopic composition of G chondrite components. SG 009’s bulk carbon and nitrogen isotopic compositions correspond to E and O chondrites. Neon isotopic composition reflects a mixture of cosmogenic and solar components, and cosmic ray exposure age of SG 009 is typical for O, E, and R chondrites. G chondrites are closely related to O, E, and R chondrites and may represent a unique metal‐rich parent asteroid containing primitive and fractionated material from the inner solar system. Oxidizing and reducing conditions during SG 009 formation may be connected with a chemical microenvironment and possibly could indicate that G chondrites may have formed by a planetesimal collision resulting in the lack of matrix.