^1,2Ananya Mallik,^1,3Tariq Ejaz,¹Svyatoslav Shchek,⁴Gordana Garapic
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.02.014]
¹Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany
²Department of Geosciences, University of Rhode Island, 9 E. Alumni Avenue, Kingston RI 02881, United States of America
³National Centre for Antarctic and Ocean Research, Headland Sada, Vasco-da-Gama, Goa 403804, India
⁴State University of New York at New Paltz, 1 Hawk Drive, New Paltz, NY 12561, United States of America
Copyright Elsevier

Lunar mantle overturn caused by gravitational instability of the Fe-Ti rich KREEP layer (formed as the last 5% of a crystallizing magma ocean, and emplaced between the overlying anorthitic crust and the underlying lunar mantle) is a process that would introduce Fe-Ti enriched bodies deep inside the lunar interior. These chemical heterogeneities in the lunar mantle may be the source of the very Fe-Ti enriched near-primary Apollo basalts. Also, the Fe-Ti and KREEP enriched layer deep inside the Moon may be responsible for the 5-30% partial melt seismically detected close to the core-mantle boundary (CMB). This is assuming that that the partial melt is neutrally buoyant at P-T conditions of the CMB. Here, we experimentally investigate the phase equilibria of the overturned Fe-Ti rich layer mixed with the mantle, at P-T conditions deep inside the lunar interior, focusing on the partial melt compositions formed. Our aim is to test (a) whether potential partial melt compositions formed near the CMB are neutrally buoyant with respect to the surrounding mantle, hence, stable; (b) if the partial melts formed within the lunar interior are positively buoyant and ascend, whether they can reproduce chemical characteristics of Apollo basalts. The densities calculated for the Fe-Ti rich partial melts from this study, using the physical parameters from previous studies, range from lower to higher values compared to that of the lunar mantle. This provides a basis for future investigations to experimentally constrain better the densities of these partial melts. Depending on the buoyancy of the partial melts, the following two scenarios are likely to happen. Firstly, if the partial melts are neutrally buoyant at the CMB, 5-30% partial melt would constrain the CMB temperatures between 1330(±1) – 1470(±19) °C. This can be used by future studies to derive the selenotherm better. Secondly, if the partial melts are positively buoyant, they should ascend and react with the mantle along their path. Upon reaching shallow depths below the crust, they may likely assimilate any Fe-Ti rich layer that was left over from the gravitational overturn, as well as undergo olivine fractionation upon pooling in a shallow magma chamber. We modeled assimilation-fractional crystallization of the partial melts using the Gibb’s-free minimization algorithm alphaMELTS. Our results show that reactive ascent of Fe-Ti rich partial melts through the lunar mantle and subsequent olivine fractionation in a shallow magma chamber is a promising way to evolve the melt compositions to converge with the lunar basalts better. Shallow level assimilation of Fe-Ti rich lithology post reactive-ascent through the mantle is also feasible, but only for low degrees of assimilation.

¹Hikari Hasegawa,^1,2Takashi Mikouchi,^3,4Akira Yamaguchi,⁵Masahiro Yasutake,⁶Richard C. Greenwood,⁶Ian A. Franchi
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13249]
¹Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, , Tokyo, 113‐0033 Japan
²The University Museum, The University of Tokyo, , Tokyo, 113‐0033 Japan
³Antarctic Meteorite Research Center, National Institute of Polar Research, Tachikawa, Tokyo, 190‐8518 Japan
⁴Department of Polar Science, School of Multidisciplinary Science, Graduate University for Advanced Sciences, Tachikawa, Tokyo, 190‐8518 Japan
⁵Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto, 606‐8502 Japan
⁶ and Space Sciences, School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA UK
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 6112, Miller Range (MIL) 090206 (plus its pairs: MIL 090340 and MIL 090405), and Divnoe are olivine‐rich ungrouped achondrites. We investigated and compared their petrography, mineralogy, and olivine fabrics. We additionally measured the oxygen isotopic compositions of NWA 6112. They show similar petrography, mineralogy, and oxygen isotopic compositions and we concluded that these five meteorites are brachinite clan meteorites. We found that NWA 6112 and Divnoe had a c axis concentration pattern of olivine fabrics using electron backscattered diffraction (EBSD). NWA 6112 and Divnoe are suggested to have been exposed to magmatic melt flows during their crystallization on their parent body. On the other hand, the three MIL meteorites have b axis concentration patterns of olivine fabrics. This indicates that the three MIL meteorites may be cumulates where compaction of olivine grains was dominant. Alternatively, they formed as residues and were exposed to olivine compaction. The presence of two different olivine fabric patterns implies that the parent body(s) of brachinite clan meteorites experienced diverse igneous processes.

¹P. Rochette, ²R. Alaç³P. Beck,²G. Brocard,⁴A. J. Cavosie,⁵V. Debaille,¹B. Devouard,⁴F. Jourdan,^6,7B. Mougel,¹F. Moustard,⁶F. Moynier,⁸S. Nomade,⁹G. R. Osinski,¹⁰B. Reynard,¹¹J. Cornec
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13244]
¹Aix‐Marseille Univ., CNRS, INRA, IRD, Coll. France, CEREGE, 13545 Aix‐en‐Provence, France
²Basin Genesis Hub, School of Geosciences, University of Sydney, Sydney, Australia
³Univ Grenoble Alpes, CNRS, IPAG, , 38041 Grenoble, France
⁴Space Science and Technology Centre and The Institute for Geoscience Research (TIGeR), School of Earth and Planetary Science, Curtin University, Perth, Western Australia, Australia
⁵Laboratoire G‐Time, Université Libre de Bruxelles, Brussels, Belgium
⁶Institut de Physique du Globe de Paris, Université Sorbonne Paris Cité, CNRS UMR 7154, Paris, France
⁷Centro de geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro
⁸LSCE, CEA, , 91190 Gif sur Yvette, France
⁹Centre for Planetary Science and Exploration and Department of Earth Science, University of Western Ontario, London, Canada
¹⁰University of Lyon, ENS de Lyon, Université Claude Bernard Lyon 1, , 69007 Lyon, France
¹¹Geologist, Denver, USA
Published by arrangement with John Wiley & Sons

The circa 14 km diameter Pantasma circular structure in Oligocene volcanic rocks in Nicaragua is here studied for the first time to understand its origin. Geomorphology, field mapping, and petrographic and geochemical investigations all are consistent with an impact origin for the Pantasma structure. Observations supporting an impact origin include outward‐dipping volcanic flows, the presence of former melt‐bearing polymict breccia, impact glass (with lechatelierite and low H₂O, <300 ppm), and also a possible ejecta layer containing Paleozoic rocks which originated from hundreds of meters below the surface. Diagnostic evidence for impact is provided by detection in impact glass of the former presence of reidite in granular zircon as well as coesite, and extraterrestrial ε⁵⁴Cr value in polymict breccia. Two ⁴⁰Ar/³⁹Ar plateau ages with a combined weighted mean age of 815 ± 11 ka (2 σ; P = 0.17) were obtained on impact glass. This age is consistent with geomorphological data and erosion modeling, which all suggest a rather young crater. Pantasma is only the fourth exposed crater >10 km found in the Americas south of N30 latitude, and provides further evidence that a significant number of impact craters may remain to be discovered in Central and South America.

¹J. Semprich,²S. P. Schwenzer,¹A.H. Treiman,¹J. Filiberto
Journal of Geophysica research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005869]
¹Lunar and Planetary Institute, USRA, Houston, TX, USA
²School of Environment, Technology, Engineering and Mathematics, The Open University, Milton Keynes, UK
Published by arrangement with John Wiley & Sons

Hydrous phases have been identified to be a significant component of martian mineralogy. Particularly prehnite, zeolites, and serpentine are evidence for low‐grade metamorphic reactions at elevated temperatures in mafic and ultramafic protoliths. Their presence suggests that at least part of the martian crust is sufficiently hydrated for low‐grade metamorphic reactions to occur. A detailed analysis of changes in mineralogy with variations in fluid content and composition along possible martian geotherms can contribute to determine the conditions required for subsurface hydrous alteration, fluid availability and rock properties in the martian crust. In this study, we use phase equilibria models to explore low‐grade metamorphic reactions covering a pressure‐temperature range of 0‐0.5 GPa and 150‐450 °C for several martian protolith compositions and varying fluid content. Our models replicate the detected low‐grade metamorphic/hydrothermal mineral phases like prehnite, chlorite, analcime, unspecified zeolites, and serpentine. Our results also suggest that actinolite should be a part of lower‐grade metamorphic assemblages, but actinolite may not be detected in reflectance spectra for several reasons. By gradually increasing the water content in the modeled whole rock composition, we can estimate the amount of water required to precipitate low‐grade metamorphic phases. Mineralogical constraints do not necessarily require an elevated geothermal gradient for the formation of prehnite. However, restricted crater excavation depths even for large impact craters are not likely sampling prehnite along colder gradients, either suggesting a geotherm of ~ 20 °C/km in the Noachian or an additional heat source such as hydrothermal or magmatic activity.

¹Hibiya, Y.,¹Iizuka, T.,²Yamashita, K.,³Yoneda, S.,⁴Yamakawa, A.
Geostandards and Geoanalytical Research (in Press) Link to Article [DOI: 10.1111/ggr.12249]
¹Department of Earth and Planetary Science, University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan
²Graduate School of Natural Science and Technology, Okayama University, Okayama, 700-8530, Japan
³Department of Science and Engineering, National Museum of Nature and Science, Tsukuba, 305-0005, Japan
⁴National Institute for Environmental Studies (NIES), 16-2 Onogawa, Tsukuba, 305-8506, Japan

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^1,2Einsle, J.F.,³Eggeman, A.S.,²Martineau, B.H.,⁴Saghi, Z.,²Collins, S.M.,¹Blukis, R.,⁵Bagot, P.A.J.,²Midgley, P.A.,¹Harrison, R.J.
Proceedings of the National Academy of Sciences of the United States of America 115, E11436-E11445 Link to Article [DOI: 10.1073/pnas.1809378115]
¹Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, United Kingdom
²Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, CB3 0FS, United Kingdom
³School of Materials, University of Manchester, Manchester, M13 9PL, United Kingdom
⁴Commissariat à l’Energie Atomique et aux Energies Alternatives, Laboratoire d’électronique des Technologies de l’Information, MINATEC Campus, Grenoble, F-38054, France
⁵Department of Materials, University of Oxford, Oxford, OX1 3PH, United Kingdom

Meteorites contain a record of their thermal and magnetic history, written in the intergrowths of iron-rich and nickel-rich phases that formed during slow cooling. Of intense interest from a magnetic perspective is the “cloudy zone,” a nanoscale intergrowth containing tetrataenite—a naturally occurring hard ferromagnetic mineral that has potential applications as a sustainable alternative to rare-earth permanent magnets. Here we use a combination of high-resolution electron diffraction, electron tomography, atom probe tomography (APT), and micromagnetic simulations to reveal the 3D architecture of the cloudy zone with subnanometer spatial resolution and model the mechanism of remanence acquisition during slow cooling on the meteorite parent body. Isolated islands of tetrataenite are embedded in a matrix of an ordered superstructure. The islands are arranged in clusters of three crystallographic variants, which control how magnetic information is encoded into the nanostructure. The cloudy zone acquires paleomagnetic remanence via a sequence of magnetic domain state transformations (vortex to two domain to single domain), driven by Fe–Ni ordering at 320^◦C. Rather than remanence being recorded at different times at different positions throughout the cloudy zone, each subregion of the cloudy zone records a coherent snapshot of the magnetic field that was present at 320^◦C. Only the coarse and intermediate regions of the cloudy zone are found to be suitable for paleomagnetic applications. The fine regions, on the other hand, have properties similar to those of rare-earth permanent magnets, providing potential routes to synthetic tetrataenite-based magnetic materials.

^1,2Laura E. Jenkins, ^1,2Roberta L. Flemming, ^1,2Phil J. A. McCausland
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13245]
¹Department of Earth Sciences, The University of Western Ontario, London, Ontario, N6A 5B7 Canada
²Centre for Planetary Science and Exploration (CPSX), The University of Western Ontario, London, Ontario, N6A 5B7 Canada
Published by arrangement with John Wiley & Sons

All Martian meteorites have experienced shock metamorphism to some degree. We quantitatively determined shock‐related strain in olivine crystals to measure shock level and peak shock pressure experienced by five Martian meteorites. Two independent methods employing nondestructive in situ micro X‐ray diffraction (μXRD) are applied, i.e., (1) the lattice strain method, in which the lattice strain value (ε) for each olivine grain is derived from a Williamson–Hall plot using its diffraction pattern (peak width variation with diffraction angle) with reference to a best fit calibration curve of ε values obtained from experimentally shocked olivine grains; (2) the strain‐related mosaicity method, allowing shock stage to be estimated by measuring the streaking along the Debye rings of olivine grain diffraction spots to define their strain‐related mosaic spread, which can then be compared with olivine mosaicity in ordinary chondrites of known shock stage. In this study, both the calculated peak shock pressures and the estimated shock stages for Dar al Gani 476 (45.6 ± 0.6 GPa), Sayh al Uhaymir 005/8 (46.1 ± 2.2 GPa), and Nakhla (18.0 ± 0.6 GPa) compare well with literature values. Formal shock assessments for North West Africa 1068/1110 (53.9 ± 2.1 GPa) and North West Africa 6234 (44.6 ± 3.1 GPa) have not been reported within the literature; however, their calculated peak shock pressures fall within the range of peak shock pressures defining their estimated shock stages. The availability of nondestructive and quantitative μXRD methods to determine shock stage and peak shock pressure from olivine crystals provides a key tool for shock metamorphism analysis.

¹E.Vos,^1,2O.Aharonson,²N.Schorghofer
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.01.018]
¹Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot 76100, Israel
²Planetary Science Institute, Tucson, AZ 85719, USA
Copyright Elsevier

The layered polar caps of Mars have long been thought to be related to variations in orbit and axial tilt. We dynamically link Mars’s past climate variations with the stratigraphy and isotopic composition of its ice by modeling the exchange of H2O and HDO among three reservoirs. The model shows that the interplay among equatorial, mid-latitude, and north-polar layered deposits (NPLD) induces significant isotopic changes in the cap. The diffusive properties of the sublimation lags and dust content in our model result in a cap size consistent with current Mars. The layer thicknesses are mostly controlled by obliquity variations, but the precession period of 50 kyr dominates the variations in the isotopic composition during epochs of relatively low and nearly constant obliquity such as at present. Isotopic sampling of the top 100 m may reveal climate oscillations unseen in the layer thicknesses and would thus probe recent precession-driven climate cycles.

¹Josefine A.M.Nanne,²Francis Nimmo,³Jeffrey N.Cuzzi,¹Thorsten Kleine
Earth and Planetary Science Letters 511, 44-54 Link to Article [https://doi.org/10.1016/j.epsl.2019.01.027]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany
²Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, USA
³Space Science Division, Ames Research Center, Moffett Field, CA 94035, USA
Copyright Elsevier

The isotopic composition of meteorites reveals a fundamental dichotomy between non-carbonaceous (NC) and carbonaceous (CC) meteorites. However, the origin of this dichotomy—whether it results from processes within the solar protoplanetary disk or is an inherited heterogeneity from the solar system’s parental molecular cloud—is not known. To evaluate the origin of the NC–CC dichotomy, we report Ni isotopic data for a comprehensive set of iron meteorites, with a special focus on groups that have not been analyzed before and belong to the CC group. The new Ni isotopic data demonstrate that the NC–CC dichotomy extends to Ni isotopes, and that CC meteorites are characterized by a ubiquitous ⁵⁸Ni excess over NC meteorites. These data combined with prior observations reveal that, in general, the CC reservoir is characterized by an excess in nuclides produced in neutron-rich stellar environments, such as ⁵⁰Ti, ⁵⁴Cr, ⁵⁸Ni, and r-process Mo isotopes. Because the NC–CC dichotomy exists for refractory (Ti, Mo) and non-refractory (Ni, Cr) elements, and is only evident for nuclides produced in specific, neutron-rich stellar environments, it neither reflects thermal processing of presolar carriers in the disk, nor the heterogeneous distribution of isotopically anomalous Ca–Al-rich inclusions (CAI). Instead, the NC–CC dichotomy reflects the distinct isotopic composition of later infalling material from the solar system’s parental molecular cloud, which affected the inner and outer regions of the disk differently. Simple models of the infall process by themselves can support either infall of increasingly NC-like material onto an initially CC-like disk, or infall of increasingly CC-like material in the absence of disk evolution by spreading. However, provided that CAIs formed close to the Sun, followed by rapid outward transport, their isotopic composition likely reflects that of the earliest infalling material, implying that the composition of the inner disk (i.e., the NC reservoir) is dominated by later infalling material, whereas the outer disk (i.e., the CC reservoir) preserved a compositional signature of the earliest disk. The isotopic difference between the inner and outer disk was likely maintained through the rapid formation of Jupiter, which prevented complete homogenization between material from inside (NC reservoir) and outside (CC reservoir) its orbit.

^1,5Noriyuki Kawasaki,²Changkun Park,³Naoya Sakamoto,²Sun Young Park,⁴Hyun Na Kim,⁵Minami Kuroda,^5,3,1Hisayoshi Yurimoto
Earth and Planetary Science Letters 511, 25-35 Link to Article [https://doi.org/10.1016/j.epsl.2019.01.026]
¹Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara 252-5210, Japan
²Division of Earth-System Sciences, Korea Polar Research Institute, Incheon 21990, Republic of Korea
³Isotope Imaging Laboratory, Creative Research Institution, Hokkaido University, Sapporo 001-0021, Japan
⁴Department of Geoenvironmental Sciences, Kongju National University, Chungcheongnam-do 32588, Republic of Korea
⁵Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
Copyright Elsevier

Al–Mg mineral isochrons of three Ca–Al-rich inclusions (CAIs) that formed primarily by condensation, one fine-grained, spinel-rich inclusion and two fluffy Type A CAIs, from the reduced CV chondrites Efremovka and Vigarano were obtained by in situ Al–Mg isotope measurements using secondary ion mass spectrometry. The slope of the isochron obtained for the fine-grained, spinel-rich inclusion gives an initial ²⁶Al/²⁷Al value, (²⁶Al/²⁷Al)₀, of (5.19 ± 0.17) × 10⁻⁵. This is essentially identical to the Solar System initial ²⁶Al/²⁷Al determined by whole-rock Al–Mg isochron studies for CAIs in CV chondrites. In contrast, the isochron slopes for the two fluffy Type A CAIs from their Al–Mg mineral isochrons, (4.703 ± 0.082) × 10⁻⁵ and (4.393 ± 0.084) × 10⁻⁵, are significantly lower than the Solar System initial value. The range of (²⁶Al/²⁷Al)₀ values of the three CAIs, from (5.19 ± 0.17) to (4.393 ± 0.084) × 10⁻⁵, corresponds to a formation age spread of 0.17 ± 0.04 Myr. This formation age spread is similar to that of igneous CAIs from CV chondrites. The data suggest that condensation and melting of minerals occurred in the hot nebular gas contemporaneously for ∼0.2 Myr at the very beginning of our Solar System, if ²⁶Al was distributed homogeneously in the CAI forming region. Alternatively, the observed variations in (²⁶Al/²⁷Al)₀ among fluffy Type A CAIs would also raise a possibility of heterogeneous distributions of ²⁶Al in the forming region.