^1,2Fang, T.,^1,3Liu, Y.
Acta Geochimica 38, 469-471 Link to Article [DOI: 10.1007/s11631-019-00344-y]
¹State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550081, China
²University of Chinese Academy of Sciences, No. 19(A) Yuquan Road, Shijingshan District, Beijing, 100049, China
³CAS Center for Excellence in Comparative Planetology, Hefei, China

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^1,2Joshua F.Snape,³Alexander A.Nemchin,¹Martin J.Whitehouse,¹Renaud E.Merle, ⁴Thomas Hopkinson,^4,5Mahesh Anand
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.07.042]
¹Department of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
²Faculty of Earth and Life Sciences, VU Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands
³Department of Applied Geology, Curtin University, Perth, WA 6845, Australia
⁴School of Physical Science, The Open University, Milton Keynes, MK7 6AA
⁵Department of Earth Sciences, The Natural History Museum, London, SW7 5BD
Copyright Elsevier

Precise crystallisation ages have been determined for a range of Apollo basalts from Pb-Pb isochrons generated using Secondary Ion Mass Spectrometry (SIMS) analyses of multiple accessory phases including K-feldspar, K-rich glass and phosphates. The samples analysed in this study include five Apollo 11 high-Ti basalts, one Apollo 14 high-Al basalt, seven Apollo 15 low-Ti basalts, and five Apollo 17 high-Ti basalts. Together with the samples analysed in two previous similar studies, Pb-Pb isochron ages have been determined for all of the major basaltic suites sampled during the Apollo missions. The accuracy of these ages has been assessed as part of a thorough review of existing age determinations for Apollo basalts, which reveals a good agreement with previous studies of the same samples, as well as with average ages that have been calculated for the emplacement of the different basaltic suites at the Apollo landing sites. Furthermore, the precision of the new age determinations helps to resolve distinctions between the ages of different basaltic suites in more detail than was previously possible. The proposed ages for the basaltic surface flows at the Apollo landing sites have been reviewed in light of these new sample ages. Finally, the data presented here have also been used to constrain the initial Pb isotopic compositions of the mare basalts, which indicate a significant degree of heterogeneity in the lunar mantle source regions, even among the basalts collected at individual landing sites.

^1,2Lisé-Pronovost, A. et al. (>10)
Scientific Drilling 25, 1-14 Link to Article [DOI: 10.5194/sd-25-1-2019]
¹School of Earth Sciences, University of Melbourne, Melbourne, Australia
²The Australian Archaeomagnetism Laboratory, Department of Archaeology and History, La Trobe University, Melbourne Campus, Bundoora, VIC 3086, Australia

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¹Lars E.Borg,¹Amy M.Gaffney,¹Thomas S.Kruijer,¹Naomi A.Marks,¹Corliss K.Sio,¹Josh Wimpenny
Earth and Planetary Science Letters 523, 115706 Link to Article [https://doi.org/10.1016/j.epsl.2019.07.008]
¹Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
Copyright Elsevier

Mare basalt sources and ferroan anorthosite suite cumulates define a linear array on a ¹⁴⁶Sm/¹⁴⁴Nd versus ¹⁴²Nd/¹⁴⁴Nd isochron plot demonstrating these materials were derived from a common reservoir at 4336⁺³¹/₋₃₂ Ma. The minimum proportion of the Moon that was in isotopic equilibrium at this time is estimated to be 1-3% of its entire volume based on the geographic extent from which the analyzed samples were collected and the calculated depths from which the samples were derived. Scenarios in which large portions of the Moon were molten to depths of many hundreds of kilometers are required to produce the observed Sm-Nd isotopic equilibrium between the mantle and crustal rocks at 4.34 Ga. This is a consequence of the fact that limited heating of a solid Moon above the blocking temperature of the Sm-Nd isotopic system is insufficient to diffusively homogenize radiogenic Nd throughout the mantle and crust. There are three scenarios that might account for global-scale isotopic equilibrium on the Moon relatively late in Solar System history including: (1) Sm-Nd re-equilibration of a solid Moon resulting from widespread melting in response to mantle overturn or a very large impact, (2) early accretion of the Moon followed by delayed cooling due to the presence of an additional heat source that kept a large portion of the Moon molten until 4.34 Ga, or (3) late accretion of the Moon followed by rapid cooling of the magma ocean late in Solar System history. Neither density-driven overturn of the mantle, nor a large impact, are likely to homogenize the mantle and crust to the extent required by the Sm-Nd isochron. Likewise, secondary heating mechanisms, such as tidal heating or radioactive decay, are not efficient enough to keep the Moon molten to the depth of the mare basalt source regions for many tens to hundreds of millions of years. Instead, the age of equilibrium between such a compositionally diverse set of rocks, produced on a global scale, likely records the time of primordial solidification of the Moon from a magma ocean. This scenario accounts for both the petrogenetic characteristics of lunar rock suites, as well as their Sm-Nd isotopic systematics. It is supported by the preponderance of ∼4.35 Ga ages obtained for other hypothetical magma ocean crystallization products, such as ferroan anorthosite suite rocks and K, REE, and P enriched cumulates that are thought to represent flotation cumulates of the magma ocean and the last vestiges of magma ocean solidification, respectively.

¹Alice Stephant,^1,2Mahesh Anand,¹Xuchao Zhao,¹Queenie H.S.Chan,³Magali Bonifacie,¹Ian A.Franchi
Earth and Planetary Science Letters 523, 115715 Link to Article [https://doi.org/10.1016/j.epsl.2019.115715]
¹School of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK
²Department of Earth Sciences, The Natural History Museum, London, SW7 5BD, UK
³Institut de Physique du Globe de Paris, Sorbone Paris Cité, Université Paris Diderot, UMR7154 CNRS, F-75005 Paris, France
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The Moon exhibits a heavier chlorine (Cl) isotopic composition compared to the Earth. Several hypotheses have been put forward to explain this difference, based mostly on analyses of apatite in lunar samples complemented by bulk-rock data. The earliest hypothesis argued for Cl isotope fractionation during the degassing of anhydrous basaltic magmas on the Moon. Subsequently, other hypotheses emerged linking Cl isotope fractionation on the Moon with the degassing during the crystallization of the Lunar Magma Ocean (LMO). Currently, a variant of the LMO degassing model involving mixing between two end-member components, defined by early-formed cumulates, from which mare magmas were subsequently derived, and a KREEP component, which formed towards the end of the LMO crystallization, seems to reconcile some existing Cl isotope data on lunar samples. To further ascertain the history of Cl in the Moon and to investigate any evolution of Cl during magma crystallization and emplacement events, which could help resolve the chlorine isotopic variation between the Earth and the Moon, we analysed the Cl abundance and its isotopic composition in 36 olivine- and pyroxene-hosted melt inclusions (MI) in five Apollo basalts (10020, 12004, 12040, 14072 and 15016). Olivine-hosted MI have an average of 3.3±1.4ppm
Cl. Higher Cl abundances (11.9 ppm on average) are measured for pyroxene-hosted MI, consistent with their formation at later stages in the crystallization of their parental melt compared to olivines. Chlorine isotopic composition (δ37Cl) of MI in the five Apollo basalts have weighted averages of ‰+12.8±2.4‰ and‰+10.1±3.2‰for olivine- and pyroxene-hosted MI, respectively, which are statistically indistinguishable. These isotopic compositions are also similar to those measured in apatite in these lunar basalts, with the exception of sample 14072, which is known to have a distinct petrogenetic history compared to other mare basalts. Based on our dataset, we conclude that, post-MI-entrapment, no significant Cl isotopic fractionation occurred during the crystallization and subsequent eruption of the parent magma and that Cl isotopic composition of MI and apatite primarily reflect the signature of the source region of these lunar basalts. Our findings are compatible with the hypothesis that in the majority of the cases the heavy Cl isotopic signature of the Moon was acquired during the earliest stages of LMO evolution. Interestingly, MI data from 14072 suggests that Apollo 14 lunar basalts might be an exception and may have experienced post-crystallization processes, possibly metasomatism, resulting in additional Cl isotopic fractionation recorded by apatite but not melt inclusions.

^1,2Thomas S.Kruijer,²ThorstenKleine
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.07.039]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Ger many
²Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA, 94550, USA
Copyright Elsevier

Non-magmatic iron meteorites, including the IIE group, can provide important insights into the history of metal-silicate differentiation and collisions on planetesimals. To better constrain the evolution of metal segregation and impacts on the IIE parent body, W isotopic data are reported for 10 IIE iron meteorites and a metal vein from the Portales Valley H6 chondrite. In addition, Pt isotopic data were obtained to quantify cosmic ray-induced neutron capture effects on W isotopes. After correction for these effects, the IIE iron meteorites exhibit variable pre-exposure 182W compositions, translating into Hf-W model age clusters of ∼4-5 million years (Ma), ∼10 Ma, ∼15 Ma, and ∼27 Ma after CAI formation. These distinct 182W clusters likely represent samples from several discrete metallic melt pools on the IIE parent asteroid. The earliest metal segregation event at ∼4-5 Ma was likely facilitated by 26Al decay as an internal heat source. By contrast, the younger Hf-W model ages may not be chronologically meaningful, and probably reflect the effects of secondary mixing and re-equilibration of metal and silicates, likely facilitated by impacts on the IIE parent body. Thus, contrary to prior work, the Hf-W systematics of IIE iron meteorites do not require a protracted history of metal-silicate separation on the IIE parent body. Instead the results of this study are fully consistent with a single partial metal-silicate differentiation event driven by endogenic heating at ∼4-5 Ma, followed by one or multiple impact events causing mixing and re-equilibration of metal and silicates at a later stage. The exact timing of these impact event(s) remains poorly constrained, but they most likely occurred in the first few tens of Ma of Solar System history.

¹Rachel R.Rahib,¹Arya Udry,²Geoffrey H.Howarth,³Juliane Gross,⁴Marine Paquet,¹Logan M.Combs,⁵Dara L.Laczniak,⁴James M.D.Day
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.07.034]
¹Department of Geoscience, University of Nevada Las Vegas, Las Vegas NV 89154, USA
²Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa
³Department of Earth and Planetary Sciences, Rutgers University, Piscataway NJ 08854, USA
⁴Scripps Institution of Oceanography, University of California San Diego, La Jolla CA 92093, USA
⁵Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN 47907, USA
Copyright Elsevier

Poikilitic shergottites make up >20% of the current martian meteorite collection, with a total of 27 samples. These meteorites are intrusive gabbroic to lherzolitic rocks and represent igneous materials recording important processes in the martian crust. To further constrain petrogenetic relationships amongst enriched and intermediate poikilitic shergottites, we studied a comprehensive suite of poikilitic shergottites — including four newly recovered samples (Northwest Africa [NWA] 11065, NWA 11043, NWA 10961, NWA 10618) — using bulk rock major- and trace-element compositions, mineral major-element compositions, oxygen fugacity (ƒO₂) values, crystallization temperatures, phosphorus maps of olivine grains, and quantitative textural analyses. The characteristic bimodal textures (poikilitic and non-poikilitic textures) of poikilitic shergottites record evolving magmatic conditions at different stages of crystallization. Higher temperatures and more reducing conditions during early-stage crystallization are recorded in the poikilitic textures, while lower temperature and more oxidizing conditions are recorded in the non-poikilitic textures during late-stage crystallization. Oxygen fugacity estimates relative to the quartz-fayalite-magnetite (QFM) buffer for early-stage olivine-pyroxene-spinel assemblages of enriched and intermediate poikilitic shergottites suggest decoupling of ƒO₂ and the degree of light rare earth element (LREE)-enrichment (i.e., [La/Yb]_CI). An increase in ƒO₂ exceeding 1 log unit from poikilitic to non-poikilitic textures implies degassing, with possible auto-oxidation, and/or crustal contamination. Quantitative textural analyses support the emplacement of both enriched and intermediate poikilitic shergottites as various shallow intrusive bodies, as well as a potentially widespread emplacement mechanism responsible for a major lithology of the martian crust. In addition, early assemblages (i.e., pyroxene oikocrysts) of all the poikilitic shergottites likely formed close to the crust-mantle boundary, implying a possible widespread presence of magma staging chambers at these depths. Fractional crystallization and magma storage in these chambers could have possibly resulted in all of the different enriched and intermediate shergottites that have been analyzed from Mars.

¹Martin R.Lee,¹Benjamin E.Cohen,²Ashley J.King,³Richard C.Greenwood
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.07.027]
¹School of Geographical and Earth Sciences, University of Glasgow, G12 8QQ, U.K
²Department of Earth Science, Natural History Museum (London), Cromwell Road, London SW7 5BD, U.K
³Planetary and Space Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, U.K
Copyright Elsevier

Lewis Cliff (LEW) 85311 is classified as a Mighei-like (CM) carbonaceous chondrite, yet it has some unusual properties that highlight an unrealised diversity within the CMs, and also questions how many parent bodies are sampled by the group. This meteorite is composed of rimmed chondrules, chondrule fragments and refractory inclusions that are set in a fine-grained phyllosilicate-rich matrix. The chondrules are of a similar size to those in the CMs, and have narrow fine-grained rims. LEW 85311 has been mildly aqueously altered, as evidenced by the preservation of melilite and kamacite, and X-ray diffraction results showing a low phyllosilicate fraction and a high ratio of cronstedtite to Fe,Mg serpentine. The chemical composition of LEW 85311 matrix, fine-grained rims, tochilinite and P-rich sulphides is similar to mildly aqueously altered CMs. LEW 85311 is enriched in refractory elements and REEs such that its CI-normalised profile falls between the CMs and CVs, and its oxygen isotopic composition plots in the CV-CK-CO field. Other distinctive properties of this meteorite include the presence of abundant refractory inclusions, and hundreds of micrometer size objects composed of needle-fibre calcite. LEW 85311 could come from part of a single CM parent body that was unusually rich in refractory inclusions, but more likely samples a different parent body to most other members of the group that accreted a subtly different mixture of materials. The mineralogical and geochemical evolution of LEW 85311 during subsequent aqueous alteration was similar to other CMs and was arrested at an early stage, corresponding to a petrologic subtype of CM2.7, probably due to an unusually low proportion of accreted ice. The CM carbonaceous chondrites sample multiple parent bodies whose similar size and inventory of accreted materials, including radiogenic isotopes, led to a comparable post-accretionary evolution.

¹Z.Ren,¹P.M.Vasconcelos
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.07.014]
¹The University of Queensland, Brisbane, Qld 4072 Australia
Copyright Elsevier

Argon release mechanisms and diffusivity were quantified for coarsely crystalline hypogene alunite [KAl₃(SO₄)₂(OH)₆] from Marysvale, Utah, and microcrystalline supergene alunite aggregates from Coober Pedy, South Australia. Prior to diffusivity studies, all alunite samples, sorted in the 500-200, 200-100, 100-50, 50-10, and < 10 µm sieve-size ranges, were vacuum encapsulated to quantify ³⁹Ar recoil losses. Incremental-heating of single capsules inserted into Ta-crucibles and heated by a projector-lamp in a specially designed diffusion cell permits quantifying ³⁹Ar released at precisely measured temperature steps (± 1-3 °C). Incremental-heating ⁴⁰Ar/³⁹Ar analyses of the various sieve-size ranges yield reproducible ages that are indistinguishable from single grain (1-2mm) laser-heating ⁴⁰Ar/³⁹Ar dating of the same samples. The results, cast in Arrhenius plots, yield activation energies (E_a) ranging from 248.0 ± 18.5 to 281.7 ± 15.2 kJ/mol and ln(D_o/a²) from 26.2 ± 2.0 to 27.8 ± 3.2 ln(s^-1) for hypogene alunite; supergene alunite yield E_a ranging between 233.3 ± 5.4 and 293.8 ± 13.7 kJ/mol and ln(D_o/a²) between 27.3 ±1.0 and 36.8 ± 2.6 ln(s^-1). These diffusion parameters correspond to closure temperatures of 264 ± 22 °C and 246 ± 19 °C for hypogene and supergene alunite, respectively, assuming a cooling rate 100 °C·Ma^-1. In-situ TEM experiments on aliquots of alunite crystals from the same samples indicate that alunite single crystals undergo transformation to nanocrystalline aggregates at 430-460 °C, showing that alunite releases Ar by volume diffusion below ∼ 430 °C, retains a significant amount of Ar during phase transformation, and proceeds to release Ar by volume or multipath diffusion from a modified polycrystalline structure at T > 460 °C. Isothermal holding time and AGESME modelling using our calculated diffusion parameters indicate that alunite should preserve Ar quantitatively for long periods (4.0 Ga) at Earth and Mars surface conditions, and both hypogene and supergene alunite should preserve original formation ages, independently of precipitation mechanism.

^1,2Drouard, A. et al. (>10)
Geology 47, 673-676 Link to Article [DOI: 10.1130/G45831.1]
¹Aix-Marseille Université, Centre National de la Recherche Scientifique (CNRS), Centre National des Etudes Spatiales (CNES), Laboratoire d’Astrophysique de Marseille (LAM), Marseille, 13013, France
²Aix-Marseille Université, CNRS, Institut de Recherche pour le Développement (IRD), Collège de France, Institut de la Recherche Agronomique (INRA), Centre Européen de Recherche et d’Enseignement en Géosciences de l’Environnement (CEREGE), Aix-en-Provence, 13545, France

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