Ar-Ar and U-Pb ages of Chelyabinsk and a re-evaluation of its impact chronology

1,2,3,4Sky P. Beard,2,3Timothy D. Swindle,5Thomas J. Lapen,2,3David A. Kring
Meteoritics & Planetary Science (in Press) Link to Article []
1State Key Laboratory in Lunar and Planetary Science, Macau University of Science and Technology, Avenida Wai Long, Macau, Taipa, 999078 P.R. China
2NASA Solar System Exploration Research Virtual Institute, Moffett Field, California, 94035 USA
3Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, 85721 USA
4CNSA Macau Center for Space Exploration and Science, Macau, P.R. China
5University of Houston, Houston, Texas, 77004 USA
Published by arrangement with John Wiley & Sons

The LL5 chondrite Chelyabinsk has had numerous isotopic studies since its fall in 2013. These data have been used to suggest ~8 impact events recorded from multiple isotopic systems (e.g., Ar-Ar, U–Pb, Sm-Nd, Rb-Sr, among others). We report details of Ar-Ar and U-Pb results and re-evaluate the geochronology of Chelyabinsk. Argon has the youngest Ar-Ar age recorded in meteorites, 25 ± 11 Ma, and an older resetting event at ~2550 Ma. The U-Pb analysis has an upper concordia age of 4456 ± 23 Ma and a lower concordia age of 184 ± 200 Ma. The lower concordia intercept represents a later thermal event (e.g., an impact), the most recent time that lead loss occurred, and could represent resetting by the youngest event recorded by Ar-Ar. Combining our data with literature results, we find strong evidence of at least four impact events (~4450, 2550, 1700, 25 Ma), with some evidence for two additional impacts (~3700, 1000 Ma).

Analysis of the daylight fireball of July 15, 2021, leading to a meteorite fall and find near Antonin, Poland, and a description of the recovered chondrite

1Lukáš Shrbený,2Agata M. Krzesińska,1Jiří Borovička,1Pavel Spurný,3Zbigniew Tymiński,4Kryspin Kmieciak
Meteoritics & Planetary Science (in Press) Open Access Link to Article []
1Astronomical Institute of the Czech Academy of Sciences, Ondřejov, 251 65 Czech Republic
2Centre for Earth Evolution and Dynamics, Department of Geosciences, University of Oslo, Oslo, N-0371 Norway
3National Centre for Nuclear Research, POLATOM Radioisotope Centre, Otwock, 05-400 Poland
4Meteorite Finder
Published by arrangement with John Wiley & Sons

We present the description of an observation of a fireball recorded during the sunrise on July 15, 2021. Atmospheric trajectory, impact area, and heliocentric orbit were determined on the basis of three instrumental video records. The terminal part of the fireball was not instrumentally recorded due to clouds. Based on our computations, one meteorite was found in the predicted impact area by Polish searchers. The specimen was, soon after recovery, analyzed for the presence of short-lived radionuclides and the measurement confirms a very fresh fall, coinciding with the time of the fireball event. The recovered meteorite, Antonin, is an unbrecciated L5 chondrite with shock stage S3, weathering grade W0, and bulk density of 3.42 g cm−3. Unusual for L chondrites, it contains assemblages composed of metal and two sulfides, troilite and mackinawite. We interpret these assemblages to have been formed as products of shock metamorphism and post-shock annealing on the parent body. This suggests that the thermal and collisional history of the Antonin parent body was complex.

Birth and decline of magma oceans in planetesimals. Part 2: Structure and thermal history of early accreted small planetary bodies

1Cyril Sturtz,1Angela Limare,1Stephen Tait,1Édouard Kaminski
Journal of Geophysical Research (Planets) (in Press) Link to Article []
1Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, Paris, F-75005 France
Published by arrangement with John Wiley & Sons

This is the second of two companion papers that present a theoretical and experimental study of the thermal history of planetesimals in which heating by short-lived radioactive isotopes generates an internal magma ocean and the subsequent cooling and crystallization thereof. We study the conditions required to form and preserve basal cumulates and flotation crusts, and the implications for the thermal evolution of planetary bodies. Our model predicts that planetesimals larger than 30km can reach 1300oC and a melt fraction of 40 vol%, producing a solid-like to liquid-like rheological transition that triggers an internal magma ocean. In the magma ocean regime core-mantle differentiation occurs very quickly and the mantle convects under a relic of chondritic material whose thickness is controlled by the temperature of rheological transition. We show that the magma ocean episode is associated with time-dependent crystal segregation and no re-entrainment. Segregation of crystals is essentially constrained by their size and by their density difference with respect to the melt, the latter being fully determined by the planetesimal’s initial composition. Olivine cumulates are likely to form at the core-mantle boundary. Under certain particular conditions, a flotation crust can also form, which reduces the efficiency of heat evacuation by convection, thereby enhancing the magma ocean’s lifetime and the efficiency of crystal segregation. Two types of large-scale mantle structure are possible outcomes: a well-mixed upper mantle above an olivine cumulate, or a more finely layered ”onion-shell” structure.

Hydrogen solubility in FeSi alloy phases at high pressures and temperatures

1Suyu Fu,2Stella Chariton,2Vitali B. Prakapenka,3Andrew Chizmeshya,1Sang-Heon Shim
American Mineralogist 107, 2307-2314 Link to Article []
1School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, U.S.A.
2Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, U.S.
Copyright: The Mineralogical Society of America

Light elements alloying with metallic Fe can change the properties and therefore play a key role
in the structure and dynamics of planetary cores. Hydrogen and silicon are possible light elements in
the rocky planets’ cores. However, hydrogen storage in Fe-Si alloy systems remains unclear at high
pressures and high temperatures because of experimental difficulties. Taking advantage of pulsed laser
heating combined with high-energy synchrotron X‑ray diffraction, we studied reactions between FeSi
and H in laser-heated diamond-anvil cells (LHDACs) up to 61.9 GPa and 3500 K. We found that under
H-saturated conditions the amount of H alloying with FeSi (0.3 and <0.1 wt% for the B20 and B2 structures, respectively) is much smaller than that in pure Fe metal (>1.8 wt%). Our experiments also
suggest that H remains in the crystal structure of FeSi alloy when recovered to 1 bar. Further density
functional theory (DFT) calculations indicate that the low-H solubility likely results from the highly
distorted interstitial sites in the B20 and B2 structures, which are not favorable for H incorporation.
The recovery of H in the B20 FeSi crystal structure at ambient conditions could open up possibilities
to understand geochemical behaviors of H during core formation in future experiments. The low-H
content in FeSi alloys suggests that if a planetary core is Si-rich, Si can limit the ingassing of H into
the Fe-rich core.

Trace element composition of silicate minerals in the porphyritic and nonporphyritic chondrules of Elenovka (L5) and Knyahinya (L/Ll5) meteorites

1Kristina Sukhanova,1,2Sergey Skublov,1Olga Galankina,2ElenaKotova
Geochemistry (Chemie der Erde) (in Press) Link to Article []
1Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, Makarova emb. 2, 199034 St.-Petersburg, Russia
2Saint Petersburg Mining University, 21st Line 2, 199106 St.-Petersburg, Russia
Copyright Elsevier

The results of SIMS and EPMA studies on the silicate minerals and bulk compositions (SEM-EDS) of porphyritic and nonporphyritic chondrules from Elenovka and Knyahinya meteorites are reported. The trace element composition of silicate minerals (olivine, low-Са pyroxene) in equilibrated ordinary chondrites (EOC) has not been affected considerably by thermal metamorphism on the chondritic parent bodies. Therefore, equilibrated chondrites can be used for chondrule-forming processes studies. Low-Са pyroxene in nonporphyritic chondrules contains higher REE, Ba, Sr concentrations than that in porphyritic chondrules at similar trace element concentrations in the olivine of chondrules. The data obtained indicate that the formation of non-porphyritic chondrules was triggered by an increase in the cooling rate of chondrules upon the formation of pyroxene, rather than a difference in the initial conditions of chondrule formation. Higher refractory incompatible element (Nb, LREE) concentrations in the olivine of chondrules than those in the olivine of the matrix and contrasting trace element (Zr, Sr, Cr, REE) concentrations in the low-Са pyroxene of the chondrules and the matrix suggest that the matrix and chondrules of the meteorites formed in one reservoir under different physico-chemical conditions (density, redox state, rotation speed, homogeneity, temperature, shocks, electrical discharge, etc.).

Precious and structural metals on asteroids

1,2Kevin M.Cannon,3Matt Gialich,3Jose Acain
Planetary and Space Science (in Press) Open Access Link to Article []
1Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO, 80401, USA
2Space Resources Program, Colorado School of Mines, Golden, CO, 80401, USA
3AstroForge Inc., Huntington Beach, CA, 92649, USA

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Elemental composition of manganese- and phosphorus-rich nodules in the Knockfarril Hill member, Gale crater, Mars

1S.J.Van Bommel et al. (>10)
Icarus (in Press) Link to Article []
1McDonnell Center for the Space Sciences, Department of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, MO 63130, USA
Copyright Elsevier

The Mars Science Laboratory rover Curiosity encountered nodules rich in manganese and phosphorus while exploring the Knockfarril Hill member of Gale crater on Mars. Deconvolution of X-ray spectroscopy data acquired by the Alpha Particle X-ray Spectrometer (APXS) at the spectral level indicate P2O5 concentrations possibly in excess of 18 wt% and MnO exceeding 8 wt%. The nodules occur intermittently in ~mm-thick layers concordant with the sedimentary laminae, extending up to ~10 cm laterally. Calcium sulfate associated with the nodules is interpreted as having precipitated from fluid that infiltrated between the nodule-bearing bedding planes in a separate and subsequent fluid event. Though the Mn- and P-bearing phase(s) was(were) not definitively identified by X-ray diffraction, evolved gas analyses show that the oxidation state of Mn is most likely 2 + .

Sample studies and SELENE (Kaguya) observations of purest anorthosite (PAN) in the primordial lunar crust for future sample return mission

1Hiroshi Nagaoka,2Makiko Ohtake,3Yuzuru Karouji,4Masahiro Kayama,3Yoshiaki Ishihara5 Satoru Yamamoto,6Risa Sakai
Icarus (in Press) Link to Article []
1Cluster for Pioneering Research, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
2The University of Aizu, Ikki Machi, Tsuruga, Aizu Wakamatsu City 965-8580, Japan
3JAXA Space Exploration Center, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara 252-5210, Japan
4Department of General Systems Studies, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan
5Geological Survey of Japan (GSJ), National Institute of Advanced Industrial Science and Technology (AIST), Central 7, Higashi 1-1-1, Tsukuba 305-8567, Japan
6Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan
Copyright Elsevier

Purest anorthosite (PAN), which consists mostly of plagioclase (≥98%), provides key information for understanding the deep crust of the Moon, because PAN observed at central peaks of large impact craters was uplifted from a deep-seated layer. For future sample return of PAN, we examined the visible and near-infrared spectra of lunar meteorites and the mineralogical and petrological studies for Apollo FAN 60015. Sample analyses for lunar meteorites and Apollo FAN 60015 showed the existence of PAN in lunar samples. However, PAN clasts in lunar meteorites were so small that not enough sampled material could be secured for multiple analyses, such as determining their crystallization ages. The lunar meteorites were also heavily brecciated by multiple impacts on the surface. The brecciation and recrystallization on the surface may have disturbed the original information (i.e., age, texture, etc). Therefore, sample return of PAN rocks that have recently fallen from the central peaks of the large craters is required for analysis to determine the parent magma composition and timing of PAN formation. We investigated the PAN distributions in Jackson crater on the lunar farside and found that PAN rocks are widely distributed over the central peak and parts of the crater wall, using SELENE (Kaguya) observational data to locate where PAN would best be collected from the lunar surface. Based on the slope that a rover can manage, we recommend two areas appropriate for collecting samples of PAN rocks that have separated from the central peak.

Three-dimensional imaging of high-velocity-impact induced crack growth in carbonaceous meteorites

1Tatsuhiro Michikami et al. (>10)
Icarus (in Press) Link to Article []
1Faculty of Engineering, Kindai University, Hiroshima Campus, 1 Takaya Umenobe, Higashi-Hiroshima, Hiroshima 739-2116, Japan
Copyright Elsevier

The material strength of meteorites provides useful information on the make-up and history of asteroids. However, the unique determination of the material strength of a meteorite is difficult because of the wide range of strengths many meteorites exhibit. Even within a single sample, complicated textures and mineral granular compositions make measurements difficult. Michikami et al. (2019) investigated the impact-induced crack growth in ordinary (L5) chondrites and indicated that crack growth is largely affected by the strength of individual mineral grains (and/or chondrules). In this study, we examine the strengths of mineral grains in carbonaceous meteorites qualitatively. To this end, we use X-ray microtomography to investigate how chondrules are affected by impact-induced crack growth in carbonaceous meteorites. Spherical alumina projectiles with a diameter of 1.0 mm were fired into the surfaces of seven Allende (CV) meteorite target samples with sizes of ~1 to 2 cm at a nominal impact velocity of 2.0 km/s. In addition, spherical glass projectiles with a diameter 0.8 mm were fired into the target surfaces of two Murchison (CM) and two Aguas Zarcas (CM) meteorite target samples with sizes of ~2 cm at a nominal impact velocity of 4.0 km/s. The results show that most cracks in CV chondrites tend to grow along the boundary surfaces of the chondrules, while most chondrule-related cracks in CM samples grow regardless of the boundary surfaces of the chondrules. This suggests that crack growth is largely affected by the chondrules’ strength as indicated by Michikami et al. (2019). The weaker the strength of chondrules, the more likely crack growth tends to occur regardless of chondrule boundaries. We found that the mesostasis of chondrules in CM meteorite Murchison (and likely Aguas Zarcas) has experienced aqueous alteration and the chondrules have become structurally weak as a whole. This indicates that impact-induced crack propagation in CM chondrites differs from thermal-fatigue induced crack propagation inferred from previous studies. As the sample material to be returned from asteroid Bennu is considered to be related to CM chondrites, we propose that observation of the cracks in chondrules in Bennu samples might tell us whether those cracks are impact- or thermal-fatigue-induced.

Shidian meteorite, a new fall analog of near-Earth asteroid (101955) Bennu

1,2Yan Fan et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article []
1State Key Laboratory of Continental Dynamics and Department of Geology, Northwest University, Xi’an, 710069 China
2Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550081 China
Published by arrangement with John Wiley & Sons

Shidian is a recent meteorite which fell in Yunnan province, China, on November 27, 2017, and has been classified as a CM2 chondrite. Petrography, mineralogy, oxygen and chromium isotopic composition, reflectance spectrum, and density studies of Shidian are reported in this study. Clasts with different aqueous alteration degree, two type 1 clasts with nontypical CM petrography, and one metamorphic clast are observed in Shidian. Mineralogically, Shidian main body consists of phyllosilicates (∼70 vol%), forsterite (∼13 vol%), fayalitic olivine, carbonates, sulfide, high-Ca pyroxene, magnetite framboids, and Fe-Ni metal. The average electron microprobe analysis (EMPA) analytical totals of phyllosilicates are 84.07 ± 1.75 wt%, with average FeO/SiO2 of tochilinite–cronstedtite intergrowths (TCIs) in different clasts ranging from 1.18 to 3.29. The bulk geochemical composition is characterized by flat rare earth element pattern, and by depletion of highly volatile elements. The whole rock oxygen isotopic composition is −0.51 ± 0.73‰, 5.44 ± 1.01‰, and −3.38 ± 0.20‰ for δ17O, δ18O, and Δ17O, respectively, with bulk chromium isotopic composition as ε54Cr = 1.00 ± 0.11. The grain density, bulk density, and porosity are 2.758 ± 0.008 g cm−3, 2.500 ± 0.004 g cm−3, and 9.37 ± 0.59%, respectively. The reflectance spectrum shows “blue” (negative) continuum slope across the visible and near-infrared range, with characteristic absorption features (such as 0.765, 0.923, and 1.160 μm for phyllosilicates). These characteristics indicate that Shidian is an unheated, brecciated CM chondrite and may be an analog of asteroid Bennu.