^1,2Qi, Y., ^1,2Zhang, Z.
Scientia Geologica Sinica 53, 714-725 Link to Article [DOI: 10.12017/dzkx.2018.039]
¹Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China
²University of Chinese Academy of Sciences, Beijing, 100049, China

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¹Łuszczek, K.
Mining Science 25, 71-83 Link to Article [DOI: 10.5277/msc182506]
¹Wrocław University of Science and Technology, Faculty of Geoengineering, Mining and Geology, Department of Geology and Mineral Waters, Poland

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^1,2,3,4Takafumi Niihara,^4,5Sky P. Beard,^4,5Timothy D. Swindle,⁶Lillian A. Schaffer,^1,2Hideaki Miyamoto, ^3,4David A. Kring
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13237]
¹Department of Systems Innovation, University of Tokyo, , Tokyo, 113‐8656 Japan
²University Museum, University of Tokyo, , Tokyo, 113‐0033 Japan
³Center for Lunar Science and Exploration, Lunar and Planetary Institute, Universities Space Research Association, , Houston, Texas, 77058 USA
⁴SSERVI, NASA Ames Research Center, , Mountain View, California, 94035 USA
⁵Lunar and Planetary Laboratory, University of Arizona, , Tucson, Arizona, 85721–0092 USA
⁶Department of Earth and Atmospheric Sciences, University of Houston, , Houston, Texas, 77204–5007 USA
Published by arrangement with John Wiley & Sons

In a histogram of lunar impact ages from the Apollo 16 site, there is a spike circa 3.9 Ga that has been interpreted to represent either a large number of nearly synchronous events or an abundance of samples that were affected slightly differently by the event that produced the Imbrium basin. To further scrutinize those age relationships, we extracted six centimeter‐sized clasts of impact melt from ancient regolith breccia 60016 and performed petrological and geochronological (40Ar‐39Ar) analyses. Three clasts have similar poikilitic textures, while others have porphyritic, aphanitic, or intergranular textures. Compositions and abundances of relict minerals are different in all six clasts and variously imply Mg‐suite and ferroan anorthosite target sequences. Estimated bulk compositions of four clasts are similar to previously defined group 1 Apollo 16 impact melt rocks, while the other two have higher Al2O3 and lower FeO+MgO compositions. All six clasts have similar K2O and P2O5 concentrations, which could have been derived from a KREEP‐bearing component among target sequences. Eighteen 40Ar/39Ar analyses of the six clasts produced an age range from 3823 ± 75 to 4000 ± 23 Ma, consistent with estimates for the proposed late heavy bombardment. Four clasts have multiple temperature steps that define plateau ages. These ages are distinct, so they cannot be explained by a single impact event, such as the one that produced the Imbrium impact basin. The conclusion that these represent distinct ages remains after considering the possibility of artifacts in defining plateaus.

¹Weijia Gao,¹Ruining Zhao,¹Jian Gao,¹Biwei Jiang,¹Jun Li
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2018.12.010]
¹Department of Astronomy, Beijing Normal University, Beijing, 100875, China

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¹Jia Liu,^1,2Liping Qina,¹Jiuxing Xia,³Richard W.Carlson,⁴Ingo Leya,⁵Nicolas Dauphas,²Yongsheng He
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.01.032]
¹CAS Key Laboratory of Crust – Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
²State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China
³Department of Terrestrial Magnetism, Carnegie Institution for Science, 5241 Broad Branch Road, NW, Washington, DC 20015, USA
⁴Space Research and Planetology, University of Berne, Sidlerstrasse 5, 3012 Berne, Switzerland
⁵Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago IL 60637, USA
Copright Elsevier

The ⁵³Mn-⁵³Cr short-lived radionuclide decay system is a powerful tool to investigate the timescales of early solar system processes. A complication arises, however, from the fact that spallation and thermal/epithermal neutron capture processes induced by cosmic rays can significantly alter ⁵³Cr/⁵²Cr ratios in solar system objects that have long exposure ages and high Fe/Cr ratios. Quantifying these cosmogenic effects helps constrain the cosmic ray exposure history of extraterrestrial samples. The isotopic shifts produced by cosmic ray irradiation also need to be corrected before the Cr isotope systematics can be used as a dating tool and as a tracer of nucleosynthetic provenance. To investigate the impact of cosmogenic production on Cr, the Cr isotopic compositions of 25 samples from 16 iron meteorites belonging to nine different chemical groups were measured. The measurements show that exposure to cosmic rays can cause large coupled excesses in ε⁵³Cr (-0.04 ± 0.44 to +268.29 ± 0.14; 2SE) and ε⁵⁴Cr (+0.28 ± 0.72 to +1053.78 ± 0.72; 2SE) with a best fit line of ε⁵⁴Cr= (3.90 ± 0.03) × ε⁵³Cr. The magnitude of Cr isotope production is controlled by various factors including the exposure age, the chemical composition (i.e., Cr concentration and Ni/Fe ratio) and shielding conditions. Nevertheless, the correlation of ε⁵³Cr and ε⁵⁴Cr is independent of these factors, which provides an effective method to evaluate the cosmogenic contribution to ⁵³Cr by monitoring the cosmogenic variations in ε⁵⁴Cr in meteoritic irons. The results are compared with modeling results that yield a slightly shallower slope of 3.6 ± 0.2. Modeling results for the olivine in stony meteorites yield a higher slope (∼5.4). However, the previous estimated results for lunar samples (stony targets for comic ray irradiation) exhibit an observably shallower slope (∼2.62). The reason for the different slopes is that the production rates of different cosmogenic Cr isotopes in iron meteorites and lunar samples are in different proportions. The differences may not be completely controlled by the higher thermal and epithermal neutron fluencies in lunar samples than in iron meteorites, but instead may largely reflect different radiation geometry between the two. More studies are needed to solve this open question.

¹Jason C.Lai,²Briony Horgan,¹James F.Bell III,¹Danika F.Wellington
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.01.019]
¹School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
²Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA
Copyright Elsevier

Much of Mars’ surface is mantled by bright dust, which masks the spectral features used to interpret the mineralogy of the underlying bedrock. Despite the wealth of near-infrared (NIR) and thermal infrared (TIR) data returned from orbiting spacecraft in recent decades, the detailed bedrock composition of approximately half of the Martian surface remains relatively unknown due to dust cover. To address this issue, and to help gain a better understanding of the bedrock mineralogy in dusty regions, Dust Cover Index results from the Mars Global Surveyor Thermal Emission Spectrometer (TES) and analysis of images from the Mars Reconnaissance Orbiter Mars Color Imager (MARCI) were used to identify 63 small localized areas within the classical bright dusty regions of Arabia Terra, Elysium Planitia, and Tharsis Montes as potential “windows” through the dust; that is, areas where the dust cover is thin enough to permit infrared remote sensing of the underlying materials. The mineralogy of each candidate window was inferred using spectra from the Mars Express Observatoire pour la Mineralogie, l’Eau, les Glaces et l’Activité (OMEGA) NIR spectrometer and, where possible, TES. Twelve areas of interest returned spectra that are consistent with mineral species expected to be present at the regional scale, such as high- and low-calcium pyroxene, olivine, and iron-bearing glass. Distribution maps were created using previously defined index parameters for each species present within an area. High-quality TES spectra, if present within an area of interest, were deconvolved to estimate modal mineralogy and to support NIR interpretations. OMEGA data from Arabia Terra and Elysium Planitia are largely similar and indicate the presence of high-calcium pyroxene with significant contributions of glass and olivine, while TES data suggest an intermediate between the established compositions of the southern highlands and Syrtis Major. Limited data from Tharsis are consistent with low-calcium pyroxene mixed with lesser amounts of glass and high-calcium pyroxene. TES data from southern Tharsis correlate well with the previously inferred compositions of the Aonium and Mare Sirenum highlands immediately to the south. Of particular note is the detection of iron-bearing glass as a significant component of all three analyzed regions, especially in Tharsis. Overall, the underlying compositions of the classically dust-covered regions of Mars appear consistent with the compositions of adjacent and other low-albedo (not dust covered) regions of the planet identified in previous studies, with the noted contribution from iron-bearing glass.

¹Sarah E.Roberts,¹Clive R.Neal
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.01.023]
¹Department of Civil & Env. Eng. & Earth Sciences, University of Notre Dame, Notre Dame, IN 46556, USA
Copyright Elsevier

Very high potassium or VHK basalts have been described from the Apollo 14 landing site at Fra Mauro on the lunar near side. Many of the known samples are clasts in polymict breccias with a few as <1 cm rocklets in the regolith samples. VHK basalts are distinct from the high-Al basalts that are more common at the Apollo 14 site in that they are enriched in the alkali/alkaline earth elements (e.g., K, Rb, Ba, etc.). The source of this enrichment was initially proposed to be through assimilation of granite by a crystallizing high-Al basaltic magma. However, with the discovery of more VHK basalt clasts from three Apollo 14 polymict breccias, the number of different AFC episodes and granite assimilants had to increase in order to explain the whole rock compositional diversity within the VHK basalt suite.
New work completed on fourteen VHK basalts include Crystal Size Distributions (CSDs) and in-situ chemical analyses and element maps collected by electron probe microanalysis (EPMA). CSDs completed on three VHK basalt samples indicate that these basalts are not impact melts and represent endogenous melts of the lunar interior. Potassium Kα element maps reveal the spatial relationship between the K-rich material and the basalt clasts with attached breccia matrix if present. In-situ EPMA analyses have identified two distinct types of K-rich material: K-feldspar and K-rich glass. With these new data, an additional mechanism for an exogenic petrogenesis of VHK basalts is proposed that involves the impact process. VHK basalts can be divided into two groups based on density and viscosity. VHK-1 basalts formed when a hot impact ejecta covered the granite/felsite-rich matrix material containing high-Al basalt clasts. Heat from the impact ejecta partially melted and possibly evaporated nearby K-bearing materials, which then infiltrated and contaminated the high-Al clasts. VHK-2 basalts are consistent with the hypothesis of granite assimilation by a high-Al magma, as seen in the higher abundances of network-forming elements.

¹J.J.Bellucci,^1,2A.A.Nemchin,²M.Grange,^3,4K.L.Robinson,⁵G.Collins,¹M.J.Whitehouse,^1,6J.F.Snape,⁷M.D.Norman,^3,4D.A.Kring
Earth and Planetary Science Letters 510, 173-185 Link to Article [https://doi.org/10.1016/j.epsl.2019.01.010]
¹Department of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
²Department of Applied Geology, Curtin University, Perth, WA 6845, Australia
³Lunar and Planetary Institute, Universities Space Research Association, 3600 Bay Area Blvd., Houston, TX 77058, United States of America
⁴Center for Lunar Science and Exploration, NASA Solar System Exploration Research Virtual Institute, United States of America
⁵Department of Earth Science & Engineering, Imperial College London, Kensington, London SW7 2AZ, UK
⁶Department of Earth Sciences, Faculty of Science, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, the Netherlands
⁷Research School of Earth Sciences, The Australian National University, 142 Mills Road, Acton ACT, 2601, Australia
Copyright Elsevier

A felsite clast in lunar breccia Apollo sample 14321, which has been interpreted as Imbrium ejecta, has petrographic and chemical features that are consistent with formation conditions commonly assigned to both lunar and terrestrial environments. A simple model of Imbrium impact ejecta presented here indicates a pre-impact depth of 30–70 km, i.e. near the base of the lunar crust. Results from Secondary Ion Mass Spectrometry trace element analyses indicate that zircon grains recovered from this clast have positive Ce/Ce^⁎ anomalies corresponding to an oxygen fugacity +2 to +4 log units higher than that of the lunar mantle, with crystallization temperatures of 771±88 to 810 ± 37 °C (2σ) that are unusually low for lunar magmas. Additionally, Ti-in-quartz and zircon calculations indicate a pressure of crystallization of 6.9±1.2 kbar, corresponding to a depth of crystallization of 167±27 km on the Moon, contradicting ejecta modelling results. Such low-T, high-fO₂, and high-P have not been observed for any other lunar clasts, are not known to exist on the Moon, and are broadly similar to those found in terrestrial magmas.

The terrestrial-like redox conditions inferred for the parental magma of these zircon grains and other accessory minerals in the felsite contrasts with the presence of Fe-metal, bulk clast geochemistry, and the Pb isotope composition of K-feldspar grains within the clast, all of which are consistent with a lunar origin. The dichotomy between redox conditions and the depth of origin inferred from the zircon compositions compared to the ejecta modelling necessitates a multi-stage petrogenesis. Two, currently unresolvable hypotheses for the origin and history of the clast are allowed by these data. The first postulates that the relatively oxidizing conditions were developed in a lunar magma, possibly by fractional crystallization and enrichment of incompatible elements in a fluid-rich, phosphate-saturated magma, at the base of the lunar crust to form the zircon grains and their host felsite. Subsequent excavation by the Imbrium impact introduced more typical lunar features to the clast but preserved primary chemical characteristics in zircon and some other accessory minerals. However, this hypothesis fails to explain the high P of crystallization. Alternatively, the felsite and its zircon crystallized on Earth at a modest depth of 19±3 km in the continental crust where oxidizing, low-T, fluid-rich conditions are common. Subsequently, the clast was ejected from the Earth during a large impact, entrained in the lunar regolith as a terrestrial meteorite with the evidence of reducing conditions introduced during its incorporation into the Imbrium ejecta and host breccia.

¹Fang Huang,¹Chen Zhou,²Wenzhong Wang, ¹Jinting Kang,²Zhongqing Wu
Earth and Planetary Science Letters 510, 153-160 Link to Article [https://doi.org/10.1016/j.epsl.2018.12.034]
¹CAS Key Laboratory of Crust–Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China
²Laboratory of Seismology and Physics of Earth’s Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China
Copyright Elsevier

Calcium is a major element of the Earth, the Moon, terrestrial planets, and rocky meteorites. Here we present equilibrium Ca isotope fractionation factors of Ca-bearing minerals using the first-principles calculations based on density functional theory (DFT). The sequence of minerals from the isotopically heaviest to the lightest in Ca is forsterite > orthopyroxene (opx) > grossular ∼ pigeonite > diopside > anorthite > oldhamite. Overall, the equilibrium fractionation of Ca isotopes is mainly controlled by the average bond lengths. Although oldhamite is enriched in light Ca isotopes relative to silicate minerals in equilibrium, natural oldhamite of enstatite chondrites are isotopically heavier than coexisting silicate materials. This implies that enstatite chondrites oldhamites should have been formed during solar nebular gas condensation instead than during parent body processing.
Following previous models for crystallization of the Lunar Magma Ocean (LMO), we simulated Ca isotopic fractionation of the LMO based on our calculated equilibrium Ca isotope fractionation factors. It shows that the δ44/40Ca of the lunar anorthositic crust should be lower than the average of the bulk Moon by 0.09–0.11‰. Considering that the lunar mantle might have overturned and mixed after solidification of the LMO, we further predict that the lunar mantle should be isotopically heavier than the bulk Moon by 0.17–0.26‰ if the mantle was fully overturned, or only by 0.06–0.08‰ for the case of fully mixing. Therefore, we predict that the potential offset of Ca isotopic composition between the anorthositic crust and the lunar mantle can be used to test LMO evolution models.

¹Romain Tartèse,^2,3Mahesh Anand,²Ian A.Franchi
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.01.024]
¹School of Earth and Environmental Sciences, The University of Manchester, Manchester, M13 9PL, UK
²School of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK
³Department of Earth Sciences, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK
Copyright Elsevier

The paired achondrites Graves Nunataks (GRA) 06128 and 06129 are samples of an asteroid that underwent partial melting within a few million years after the start of Solar System formation. In order to better constrain the origin and processing of volatiles in the early Solar System, we have investigated the abundance of H, F and Cl and the isotopic composition of H and Cl in phosphates in GRA 06128 using secondary ion mass spectrometry. Indigenous H in GRA 06128, as recorded in magmatic merrillite, is characterised by an average δD of ca. -152 ± 330‰, which is broadly similar to estimates of the H isotope composition of indigenous H in other differentiated asteroidal and planetary bodies such as Mars, the Moon and the angrite and eucrite meteorite parent bodies. The merrillite data thus suggest that early accretion of locally-derived volatiles was widespread for the bodies currently populating the asteroid belt. Apatite formed at the expense of merrillite around 100 million years after the differentiation of the GRA 06128/9 parent body, during hydrothermal alteration, which was probably triggered by an impact event. Apatite in GRA 06128 contains 5.4-5.7 wt.% Cl, 0.6-0.8 wt.% F, and ∼20 to 60 ppm H₂O, which is similar to the H₂O abundance in merrillite from which apatite formed. The apatite δD values range between around +100‰ and +2000‰ and are inversely correlated with apatite H₂O contents. The Cl isotope composition of apatite appears to be homogeneous across various grains, with an average δ³⁷Cl value of 3.2 ± 0.7‰. A possible scenario to account for the apatite chemical and isotopic characteristics involves interaction of GRA 06128/9 with fumarole-like fluids derived from D- and HCl-rich ices delivered to the GRA 06128/9 parent-body by an ice-rich impactor.