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
Copyright Elsevier

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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¹Haskins, J.B.,¹Stern, E.C.,¹Bauschlicher, C.W., Jr.,²Lawson, J.W.
Journal of Applied Physics 125, 235105 Link to Article [DOI: 10.1063/1.5079418]
¹Thermal Protection Materials Branch, NASA Ames Research Center, Moffett Field, CA 94035, United States
²Intelligent Systems Division, NASA Ames Research Center, Moffett Field, CA 94035, United States

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^1,2,3Alexander N. Krot
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13350]
¹Hawai’i Institute of Geophysics and Planetology, School of Ocean, Earth Science, and Technology, University of Hawai’i at Mānoa, Honolulu, Hawaii, 96822 USA
²Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Copenhagen, Denmark
³Geoscience Institute, Goethe University, Frankfurt, Frankfurt am Main, Germany
Published by arrangement with John Wiley & Sons

Chondrites consist of three major components: refractory inclusions (Ca,Al‐rich inclusions [CAIs] and amoeboid olivine aggregates), chondrules, and matrix. Here, I summarize recent results on the mineralogy, petrology, oxygen, and aluminum‐magnesium isotope systematics of the chondritic components (mainly CAIs in carbonaceous chondrites) and their significance for understanding processes in the protoplanetary disk (PPD) and on chondrite parent asteroids. CAIs are the oldest solids originated in the solar system: their U‐corrected Pb‐Pb absolute age of 4567.3 ± 0.16 Ma is considered to represent time 0 of its evolution. CAIs formed by evaporation, condensation, and aggregation in a gas of approximately solar composition in a hot (ambient temperature >1300 K) disk region exposed to irradiation by solar energetic particles, probably near the protoSun; subsequently, some CAIs were melted in and outside their formation region during transient heating events of still unknown nature. In unmetamorphosed, type 2–3.0 chondrites, CAIs show large variations in the initial 26Al/27Al ratios, from <5 × 10–6 to ~5.25 × 10–5. These variations and the inferred low initial abundance of 60Fe in the PPD suggest late injection of 26Al by a wind from a nearby Wolf–Rayet star into the protosolar molecular cloud core prior to or during its collapse. Although there are multiple generations of CAIs characterized by distinct mineralogies, textures, and isotopic (O, Mg, Ca, Ti, Mo, etc.) compositions, the 26Al heterogeneity in the CAI‐forming region(s) precludes determining the duration of CAIs formation using 26Al‐26Mg systematics. The existence of multiple generations of CAIs and the observed differences in CAI abundances in carbonaceous and noncarbonaceous chondrites may indicate that CAIs were episodically formed and ejected by a disk wind from near the Sun to the outer solar system and then spiraled inward due to gas drag. In type 2–3.0 chondrites, most CAIs surrounded by Wark–Lovering rims have uniform Δ17O (= δ17O−0.52 × δ18O) of ~ −24‰; however, there is a large range of Δ17O (from ~−40 to ~ −5‰) among them, suggesting the coexistence of 16O‐rich (low Δ17O) and 16O‐poor (high Δ17O) gaseous reservoirs at the earliest stages of the PPD evolution. The observed variations in Δ17O of CAIs may be explained if three major O‐bearing species in the solar system (CO, H2O, and silicate dust) had different O‐isotope compositions, with H2O and possibly silicate dust being 16O‐depleted relative to both the Genesis solar wind Δ17O of −28.4 ± 3.6‰ and even more 16O‐enriched CO. Oxygen isotopic compositions of CO and H2O could have resulted from CO self‐shielding in the protosolar molecular cloud (PMC) and the outer PPD. The nature of 16O‐depleted dust at the earliest stages of PPD evolution remains unclear: it could have either been inherited from the PMC or the initially 16O‐rich (solar‐like) MC dust experienced O‐isotope exchange during thermal processing in the PPD. To understand the chemical and isotopic composition of the protosolar MC material and the degree of its thermal processing in PPD, samples of the primordial silicates and ices, which may have survived in the outer solar system, are required. In metamorphosed CO3 and CV3 chondrites, most CAIs exhibit O‐isotope heterogeneity that often appears to be mineralogically controlled: anorthite, melilite, grossite, krotite, perovskite, and Zr‐ and Sc‐rich oxides and silicates are 16O‐depleted relative to corundum, hibonite, spinel, Al,Ti‐diopside, forsterite, and enstatite. In texturally fine‐grained CAIs with grain sizes of ~10–20 μm, this O‐isotope heterogeneity is most likely due to O‐isotope exchange with 16O‐poor (Δ17O ~0‰) aqueous fluids on the CO and CV chondrite parent asteroids. In CO3.1 and CV3.1 chondrites, this process did not affect Al‐Mg isotope systematics of CAIs. In some coarse‐grained igneous CV CAIs, O‐isotope heterogeneity of anorthite, melilite, and igneously zoned Al,Ti‐diopside appears to be consistent with their crystallization from melts of isotopically evolving O‐isotope compositions. These CAIs could have recorded O‐isotope exchange during incomplete melting in nebular gaseous reservoir(s) with different O‐isotope compositions and during aqueous fluid–rock interaction on the CV asteroid.

¹Ryan C. Ogliore,²Donald E. Brownlee,³Kazuhide Nagashima,²David J. Joswiak,¹Josiah B. Lewis,³Alexander N. Krot,¹Kainen L. Utt,³Gary R. Huss
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13364]
¹Department of Physics, Washington University in St. Louis, St. Louis, Missouri, 63130 USA
²Department of Astronomy, University of Washington, Seattle, Washington, 98195 USA
³Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Mānoa, Honolulu, Hawaii, 96822 USA
Published by arrangement with John Wiley & Sons

Filamentary enstatite crystals are found in interplanetary dust particles (IDPs) of likely cometary origin but are very rare or absent in meteorites. Crystallographic characteristics of filamentary enstatites indicate that they condensed directly from vapor. We measured the O isotopic composition of an enstatite ribbon from a giant cluster IDP to be δ¹⁸O = 25 ± 55, δ¹⁷O = −19 ± 129, ∆¹⁷O = −32 ± 134 (2σ errors), which is inconsistent at the 2σ level with the composition of the Sun inferred from the Genesis solar wind measurements. The particle’s O isotopic composition, consistent with the terrestrial composition, implies that it condensed from a gas of nonsolar O isotopic composition, possibly as a result of vaporization of disk region enriched in ¹⁶O‐depleted solids. The relative scarcity of filamentary enstatite in asteroids compared to comets implies either that this crystal condensed from dust vaporized in situ in the outer solar system where comets formed or it condensed in the inner solar system and was subsequently transported outward to the comet‐forming region.

¹Cosette M. Gilmour,¹Christopher D. K. Herd,²Pierre Beck
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13362]
¹Department of Earth and Atmospheric Sciences, University of Alberta, 1‐26 Earth Sciences Building, Edmonton, Alberta, T6G 2E3 Canada
²UJF‐Grenoble 1, CNRS‐INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), UMR 5274, Grenoble, F‐38041 France
Published by arrangement with John Wiley & Sons

Here, we evaluate the extent of aqueous alteration among five pristine specimens of the ungrouped Tagish Lake carbonaceous chondrite (TL5b, TL11h, TL11i, TL4, and TL10a) using thermogravimetric analysis (TGA) and infrared (IR) transmission spectroscopy. Both TGA and IR spectroscopy have proven to be reliable methods for determining the extent of aqueous alteration among different carbonaceous chondrites, in particular the CM chondrites (e.g., Garenne et al. 2014), with which Tagish Lake shares some affinities. Using these two methods, our goal is to incorporate TL4 and TL10a into the known alteration sequence of TL5b < TL11h < TL11i (Herd et al. 2011; Blinova et al. 2014a). This study highlights the compositional variability of the Tagish Lake specimens, which we ascribe to its brecciated nature. Our TGA and IR spectroscopy results are congruent with the reported alteration sequence, allowing us to introduce the TL4 and TL10a specimens in the following order: TL4 < TL5b ≤ TL10a < TL 11h < TL11i. Notably, these two specimens appear to be similar to the least altered lithologies previously reported, and the alteration of Tagish Lake is similar to that experienced by lesser altered members of the CM chondrites (>CM1.6). Based on these findings, Tagish Lake could be considered a 1.6–2.0 ungrouped carbonaceous chondrite. Visible and near‐IR reflectance measurements of Tagish Lake were also acquired in this study to revisit the Tagish Lake parent body connection. While other studies have paired Tagish Lake with D‐ and T‐type asteroid parent bodies, the reflectance spectra acquired in this study are variable among the different Tagish Lake specimens in relation to their alteration sequences; results match with spectra characteristic of C‐, X‐, Xc‐, and D‐type asteroids. The heterogeneity of Tagish Lake coupled with its low albedo makes the parent body connection a challenge.