Forsteritic olivine and magnesium-rich orthopyroxene materials measured by Chang’e-4 rover

1,2Sheng Gou et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113776]
1State Key Laboratory of Remote Sensing Science, Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100101, China
2State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, China
Copyright Elsevier

China’s Chang’e-4 probe achieved the first soft landing within the South Pole-Aitken (SPA) basin, which is the oldest, largest, and deepest basin on the lunar farside. The deployed Chang’e-4 rover made in situ spectral measurements along the rover traverse during a nominal three-month mission period. Spectral analyses imply that materials at the Chang’e-4 landing site have a forsteritic olivine (OL) and magnesium (Mg)-rich orthopyroxene (OPX) assemblage in almost equal fractions. The Chang’e-4 rover measured materials, which were essentially mixture of multiple sources, were primarily the weathering products of Finsen crater ejecta. Plagioclase (PLG), which has significant implication for the provenance, is often spectrally transparent or featureless in the wavelength range (450–2395 nm) of the in situ measured spectrum. Depending on the possible absence or presence of abundant PLG, the materials likely originated from a differentiated SPA impact melt pool or from an Mg-suite pluton in the lunar lower crust, and were less likely to originate from the lunar upper mantle.

A refractory inclusion with solar oxygen isotopes and the rarity of such objects in the meteorite record

1,2Levke Kööp,3Kazuhide Nagashima,1,2,4Andrew M. Davis,1Alexander N. Krot
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13434]
1Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois, 60637 USA
2Chicago Center for Cosmochemistry, The University of Chicago, Chicago, Illinois, 60637 USA
3Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Mānoa, Honolulu, Hawai’i, 96822 USA
4Enrico Fermi Institute, The University of Chicago, Chicago, Illinois, 60637 USA
Published by arrangement with John Wiley & Sons

NASA’s Genesis mission revealed that the Sun is enriched in 16O compared to the Earth and Mars (the Sun’s Δ17O, defined as δ17O–0.52×δ18O, is –28.4 ± 3.6‰; McKeegan et al. 2011). Materials as 16O‐rich as the Sun are extremely rare in the meteorite record. Here, we describe a Ca‐Al‐rich inclusion (CAI) from a CM chondrite that is as 16O‐enriched as the Sun (Δ17O = –29.1 ± 0.7‰). This CAI also has large nucleosynthetic anomalies in 48Ca and 50Ti (δ‐values are –8.1 ± 3.3 and –11.7 ± 2.4‰, respectively) and shows no clear evidence for incorporation of live 26Al; (26Al/27Al)0 = (0.03 ± 0.11) × 10–5. Due to their anomalous isotopic characteristics, the rare CAIs consistent with the Genesis value could be among the first materials that formed in the solar system. In contrast to the CAI studied here, the majority of CAIs formed in or interacted with a reservoir characterized by a Δ17O value near –23.5‰. Combined with 26Al‐26Mg systematics, the oxygen isotopic compositions of FUN (fractionation and unidentified nuclear effects), UN, and normal CAIs suggest that nebular conditions were favorable for solids to inherit this value for an extended period of time. Many later‐formed materials, such as chondrules, planetesimals, and terrestrial planets, formed in reservoirs with Δ17O near 0‰. The distribution could be easier to explain if the common CAI value of –23.5‰, which is consistent with the Genesis value within 3σ, represented the average composition of the protoplanetary disk.

Formation of Libyan Desert Glass: Numerical simulations of melting of silica due to radiation from near‐surface airbursts

1,2Vladimir Svetsov,1,2Valery Shuvalov,1Igor Kosarev
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13470]
1Institute for Dynamics of Geospheres, Russian Academy of Sciences, Moscow, 119334 Russia
2Moscow Institute of Physics and Technology, 9 Institutskiy per., Dolgoprudny, Moscow Region, 141701 Russia
Published by arrangement with John Wiley & Sons

Libyan Desert Glass contains meteoritic material and, therefore, its origin is most likely associated with an impact event. However, the impact crater has not been found. We performed numerical simulations of impacts of stony and cometary bodies in order to confirm the version that this glass was formed from silica heated by radiation from aerial bursts near the ground. Asteroids were treated as strengthless bodies from dunite with a density of 3.3 g cm−3, and comets as icy bodies with a density of 1 g cm−3. The simulations based on hydrodynamic equations included the equations of radiation transfer. Melting and vaporization of a silica target under action of radiation incident on a planar surface were modeled using a one‐dimensional hydrodynamic equation of energy and equations of radiation transfer in two‐flux approximation. We selected those variants of simulations in which a crater is not formed, a fireball touches the earth surface, and the area of a molten target corresponds to the area of the Libyan Desert Glass strewn field. Appropriate options include the impact of an asteroid with a diameter of 300 m, an entry speed of 35 km s−1, and an entry angle of 8°, and cometary bodies with diameters from 150 to 300 m, speeds of 50–70 km s−1, and entry angles from 15° to 45°. Impact options with crater formation are also discussed. The maximum depth of molten silica at ground zero reaches 10 cm with the cometary impacts and 3–4 cm with the asteroidal impact. Melting occurs during a period of time from 50 to 400 s.

The timing of lunar solidification and mantle overturn recorded in ferroan anorthosite 62237

1C.K.Sio,1L.E.Borg,1W.S.Cassata
Earth and Planetary Science Letters 538, 116219 Link to Article [https://doi.org/10.1016/j.epsl.2020.116219]
1Lawrence Livermore National Laboratory, 7000 East Ave. L-231, Livermore, CA 94550, USA
Copyright Elsevier

Ferroan anorthosite suite (FAS) rocks are widely interpreted to represent primordial lunar crust. Despite their importance in pinpointing the timing of lunar crust formation, robust chronological investigations for this rock type are scarce. Here, we report the Ar-Ar, Rb-Sr, and Sm-Nd isotopic systematics for the FAS troctolitic anorthosite 62237. The Ar-Ar isotopic system has been reset by a thermal event at 3710 ± 48 Ma, and the Rb-Sr isotopic systematics has been disturbed such that a Rb-Sr isochron age cannot be determined. However, an internal isochron for the Sm-Nd isotopic system has yielded an age of 4350 ± 73 Ma (MSWD = 2.0) with an initial NdCHUR of −0.53 ± 0.26. The mineral and whole-rock fractions of 62237 plot on the same internal isochron as FAS sample 60025. The combined datasets define an age of 4372 ± 35 Ma (MSWD = 4.0) with an initial NdCHUR of −0.17 ± 0.22. Literature Sm-Nd data for FAS and Mg-suite whole-rocks also plot on the 60025-62237 isochron. The coherence of data from both FAS and Mg-suite rocks examined thus far suggests that both rock suites formed contemporaneously from identical, or nearly identical, sources. In addition, the concordance of FAS and Mg-suite ages suggests that primordial crust solidification either involved both magmatic suites, or that Mg-suite magmatism was contemporaneous with FAS magmatism within resolution of the Sm-Nd chronometer. The ages for FAS and Mg-suite also coincide with the formation ages of the mare basalt source regions and urKREEP. Ferroan anorthosite suite rocks and urKREEP are thought to represent primordial LMO solidification products, whereas Mg-suite and the mare basalt source regions are argued to represent mixtures of various LMO crystallization products that were formed during density-driven overturn of the LMO. The concordance of ages implies that the 4372 ± 35 Ma Sm-Nd isochron records the age of mantle overturn, and that overturn occurred during, or shortly after, solidification of the LMO.

Subsolar Al/Si and Mg/Si ratios of non-carbonaceous chondrites reveal planetesimal formation during early condensation in the protoplanetary disk

1A.Morbidelli,1G.Libourel,2H.Palme,3 S.A.Jacobson,4D.C.Rubie
Earth and Planetary Science Letters 538, 116220 Link to Article [https://doi.org/10.1016/j.epsl.2020.116220]
1Laboratoire Lagrange, UMR7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, Boulevard de l’Observatoire, 06304 Nice Cedex 4, France
2Senckenberg, world of biodiversity, Sektion Meteoritenforschung, Senckenberganlage 25, 60325 Frankfurt am Main, Germany
3Northwestern University, Dept. of Earth and Planetary Sciences, Evanston, 60208 IL, United States of America
4Bayerisches Geoinstitut, University of Bayreuth, 95440, Bayreuth, Germany
Copyright Elsevier

The Al/Si and Mg/Si ratios in non-carbonaceous chondrites are lower than the solar (i.e., CI-chondritic) values, in sharp contrast to the non-CI carbonaceous meteorites and the Earth, which are enriched in refractory elements and have Mg/Si ratios that are solar or larger. We show that the formation of a first generation of planetesimals during the condensation of refractory elements implies the subsequent formation of residual condensates with strongly sub-solar Al/Si and Mg/Si ratios. The mixing of residual condensates with different amounts of material with solar refractory/Si element ratios explains the Al/Si and Mg/Si values of non-carbonaceous chondrites. To match quantitatively the observed ratios, we find that the first-planetesimals should have accreted when the disk temperature was ∼1,330–1,400 K depending on pressure and assuming a solar C/O ratio of the disk. We discuss how this model relates to our current understanding of disk evolution, grain dynamics, and planetesimal formation. We also extend the discussion to moderately volatile elements (e.g., Na), explaining how it may be possible that the depletion of these elements in non-carbonaceous chondrites is correlated with the depletion of refractory elements (e.g., Al). Extending the analysis to Cr, we find evidence for a higher than solar C/O ratio in the protosolar disk’s gas when/where condensation from a fractionated gas occurred. Finally, we discuss the possibility that the supra-solar Al/Si and Mg/Si ratios of the Earth are due to the accretion of ∼40% of the mass of our planet from the first-generation of refractory-rich planetesimals.

A possible high-temperature origin of the Moon and its geochemical consequences

1,2,3E.S.Steenstra,3J.Berndt,3S.Klemme,1Y.Fei,2W.van Westrenen
Earth and Planetary Science Letters 538, 116222 Link to Article [https://doi.org/10.1016/j.epsl.2020.116222]
1The Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC, USA
2Faculty of Science, VU Amsterdam, Amsterdam, the Netherlands
3Institute of Mineralogy, University of Münster, Münster, Germany
Copyright Elsevier

The formation of the Moon is thought to be the result of a giant impact between a Mercury-to-proto-Earth-sized body and the proto-Earth. However, the initial thermal state of the Moon following its accretion is not well constrained by geochemical data. Here, we provide geochemical evidence that supports a high-temperature origin of the Moon by performing high-temperature (1973–2873 K) metal-silicate partitioning experiments, simulating core formation in the newly-formed Moon. Results indicate that the observed lunar mantle depletions of Ni and Co record extreme temperatures (>2600–3700 K depending on assumptions about the composition of the lunar core) during lunar core formation. This temperature range is within range of the modeled silicate evaporation buffer in a synestia-type environment. Our results provide independent geochemical support for a giant-impact origin of the Moon and show that lunar thermal models should start with a fully molten Moon. Our results also provide quantitative constraints on the effects of high-temperature lunar differentiation on the lunar mantle geochemistry of volatile, and potentially siderophile elements Cu, Zn, Ga, Ge, Se, Sn, Cd, In, Te and Pb. At the extreme temperatures recorded by Ni and Co, many of these elements behave insufficiently siderophile to explain their depletions by core formation only, consistent with the inferred volatility-related loss of Cr, Cu, Zn, Ga and Sn during the Moon-forming event and/or subsequent magma-ocean degassing.

Potassium Isotope Compositions of Carbonaceous and Ordinary Chondrites: Implications on the Origin of Volatile Depletion in the Early Solar System

1Hannah Bloom,1Katharin Lodders,1,2Heng Chen,1,3Chen Zhao,1Zhen Tian,1Piers Koefoed, 4Mária K.Pető,5,6Yun Jiang,1Kun Wang (王昆)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.03.018]
1Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA
2Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA
3Faculty of Earth Sciences, China University of Geosciences, Wuhan, Hubei 430074, China
4Konkoly Observatory, Research Center for Astronomy and Earth Sciences, H-1121 Budapest, Hungary
5CAS Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China
6CAS Center for Excellence in Comparative Planetology, China
Copyright Elsevier

Among solar system materials there are variable degrees of depletion in moderately volatile elements (MVEs, such as Na, K, Rb, Cu, and Zn) relative to the proto-solar composition. Whether these depletions are due to nebular and/or parent-body (asteroidal or planetary) processes is still under debate. In order to help decipher the MVE abundances in early solar system materials, we conducted a systematic study of high-precision K stable isotopic compositions of a suite of whole-rock samples of well-characterized carbonaceous and ordinary chondrites. We analyzed 16 carbonaceous chondrites (CM1-2, CO3, CV3, CR2, CK4-5 and CH3) and 28 ordinary chondrites covering petrological types 3 to 6 and chemical groups H, L, and LL. We observed significant K isotope (δ41K) variations (−1.54 to 0.70 ‰) among the carbonaceous and ordinary chondrites. In general, the two major chondrite groups are distinct: The K isotope compositions of carbonaceous chondrites are largely higher than the Bulk Silicate Earth (BSE) value, whereas ordinary chondrites show K isotope compositions that are typically lower than the BSE value. Neither carbonaceous nor ordinary chondrites show clear/resolvable correlations between K isotopes and chemical groups, petrological types, shock levels, cosmic-ray exposure ages, fall/find occurrence, or terrestrial weathering. Importantly, the lack of a clear trend between K isotopes and K content among chondrites indicates that the K isotope fractionations were decoupled from the relative elemental K depletions, which is inconsistent with a single-stage partial vaporization or condensation process to account for these MVE depletion patterns among chondrites. The range of K isotope variations in the carbonaceous chondrites in this study is consistent with a four-component (chondrule, refractory inclusion, matrix and water) mixing model that is able to explain the bulk elemental and isotopic compositions of the main carbonaceous chondrite groups, but requires a fractionation in K isotopic compositions in chondrules. We propose that the major control of the isotopic compositions of group averages is condensation and/or vaporization in pre-accretional (nebular) environments that is preserved in the compositional variation of chondrules. Parent-body processes, such as aqueous alteration, thermal metamorphism, and metasomatism, can mobilize K and affect the K isotopes in individual samples. In the case of the ordinary chondrites, the full range of K isotopic variations can only be explained by the combined effects of the size and relative abundances of chondrules, parent-body aqueous and thermal alteration, and possible sampling bias.