¹Alison C. Hunt, ¹David L. Cook, ^2,3Tim Lichtenberg, ¹Philip M. Reger,¹ Mattias Ek, ⁴Gregor J. Golabek, ¹Maria Schönbächler
Earth and Planetary Science Letters 482, 490-500 Link to Article [https://doi.org/10.1016/j.epsl.2017.11.034]
¹Institute of Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland
²Institute of Geophysics, ETH Zürich, Sonneggstrasse 5, 8092 Zürich, Switzerland
³Institute for Astronomy, ETH Zürich, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland
⁴Bayerisches Geoinstitut, University of Bayreuth, Universitätsstrasse 30, 95440 Bayreuth, Germany
Copyright Elsevier

The short-lived ¹⁸²Hf–¹⁸²W decay system is a powerful chronometer for constraining the timing of metal–silicate separation and core formation in planetesimals and planets. Neutron capture effects on W isotopes, however, significantly hamper the application of this tool. In order to correct for neutron capture effects, Pt isotopes have emerged as a reliable in-situ neutron dosimeter. This study applies this method to IAB iron meteorites, in order to constrain the timing of metal segregation on the IAB parent body.

The ε¹⁸²W values obtained for the IAB iron meteorites range from −3.61 ± 0.10 to −2.73 ± 0.09. Correlating εⁱPt with ε¹⁸²W data yields a pre-neutron capture ε¹⁸²W of −2.90 ± 0.06. This corresponds to a metal–silicate separation age of 6.0 ± 0.8 Ma after CAI for the IAB parent body, and is interpreted to represent a body-wide melting event. Later, between 10 and 14 Ma after CAI, an impact led to a catastrophic break-up and subsequent reassembly of the parent body. Thermal models of the interior evolution that are consistent with these estimates suggest that the IAB parent body underwent metal–silicate separation as a result of internal heating by short-lived radionuclides and accreted at around 1.4±0.1 Ma after CAIs with a radius of greater than 60 km.

¹Nigel M.Kelly, ^1,2Rebecca M.Flowers, ¹James R.Metcalf, ^1,2,3Stephen J.Mojzsis
Earth and Planetary Science Letters 482, 222-235 Link to Article [https://doi.org/10.1016/j.epsl.2017.11.009]
¹Department of Geological Sciences, University of Colorado, 2200 Colorado Avenue, UCB 399, Boulder, CO 80309-0399, USA
²Collaborative for Research in Origins (CRiO), The John Templeton Foundation – FfAME Origins Program, USA
³Institute for Geological and Geochemical Research, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, 45 Budaörsi Street, H-1112 Budapest, Hungary
Copyright Elsevier

We conducted zircon (U–Th)/He (ZHe) analysis of lunar impact-melt breccia 14311 with the aim of leveraging radiation damage accumulated in zircon over extended intervals to detect low-temperature or short-lived impact events that have previously eluded traditional isotopic dating techniques. Our ZHe data record a coherent date vs. effective Uranium concentration (eU) trend characterized by >3500 Ma dates from low (≤75 ppm) eU zircon grains, and ca. 110 Ma dates for high (≥100 ppm) eU grains. A progression between these date populations is apparent for intermediate (75–100 ppm) eU grains. Thermal history modeling constrains permissible temperatures and cooling rates during and following impacts. Modeling shows that the data are most simply explained by impact events at ca. 3950 Ma and ca. 110 Ma, and limits allowable temperatures of heating events between 3950–110 Ma. Modeling of solar cycling thermal effects at the lunar surface precludes this as the explanation for the ca. 110 Ma ZHe dates. We propose a sample history characterized by zircon resetting during the ca. 3950 Ma Imbrium impact event, with subsequent heating during an impact at ca. 110 Ma that ejected the sample to the vicinity of its collection site. Our data show that zircon has the potential to retain 4He over immense timescales (≥3950 Myrs), thus providing a valuable new thermochronometer for probing the impact histories of lunar samples, and martian or asteroidal meteorites.

^1,2Christopher J.Davies, ²Anne Pommier
Earth and Planetary Science 481, 189-200 Link to Article [https://doi.org/10.1016/j.epsl.2017.10.026]
¹School of Earth & Environment, University of Leeds, Leeds LS2 9JT, UK
²Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0225, USA
Copyright Elsevier

The decline of Mars’ global magnetic field some 3.8–4.1 billion years ago is thought to reflect the demise of the dynamo that operated in its liquid core. The dynamo was probably powered by planetary cooling and so its termination is intimately tied to the thermochemical evolution and present-day physical state of the Martian core. Bottom-up growth of a solid inner core, the crystallization regime for Earth’s core, has been found to produce a long-lived dynamo leading to the suggestion that the Martian core remains entirely liquid to this day. Motivated by the experimentally-determined increase in the Fe–S liquidus temperature with decreasing pressure at Martian core conditions, we investigate whether Mars’ core could crystallize from the top down. We focus on the “iron snow” regime, where newly-formed solid consists of pure Fe and is therefore heavier than the liquid. We derive global energy and entropy equations that describe the long-timescale thermal and magnetic history of the core from a general theory for two-phase, two-component liquid mixtures, assuming that the snow zone is in phase equilibrium and that all solid falls out of the layer and remelts at each timestep. Formation of snow zones occurs for a wide range of interior and thermal properties and depends critically on the initial sulfur concentration, ξ0. Release of gravitational energy and latent heat during growth of the snow zone do not generate sufficient entropy to restart the dynamo unless the snow zone occupies at least 400 km of the core. Snow zones can be 1.5–2 Gyrs old, though thermal stratification of the uppermost core, not included in our model, likely delays onset. Models that match the available magnetic and geodetic constraints have ξ0≈10% and snow zones that occupy approximately the top 100 km of the present-day Martian core.

^1,2Steven M. Chemtob,²Ryan D. Nickerson,³Richard V. Morris,⁴David G. Agresti,²Jeffrey G. Catalano
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2017JE005331]
¹Department of Earth and Environmental Sciences, Temple University, Philadelphia, PA, U.S.A.
²Department of Earth and Planetary Sciences, Washington University, St. Louis, MO, U.S.A.
³EIS Directorate, NASA Johnson Space Center, Houston, TX, U.S.A.
⁴Department of Physics, University of Alabama at Birmingham, Birmingham, AL, U.S.A.
Published by arrangement with John Wiley & Sons

Surface conditions on early Mars were likely anoxic, similar to early Earth, but the timing of the evolution to oxic conditions characteristic of contemporary Mars is unresolved. Ferrous trioctahedral smectites are the thermodynamically predicted products of anoxic basalt weathering, but orbital analyses of Noachian-aged terrains find primarily Fe3+-bearing clay minerals. Rover-based detection of Fe2+-bearing trioctahedral smectites at Gale Crater suggest that ferrous smectites are the unoxidized progenitors of orbitally-detected ferric smectites. To assess this pathway, we conducted ambient-temperature oxidative alteration experiments on four synthetic ferrous smectites having molar Fe/(Mg+Fe) from 1.00 to 0.33. Smectite suspension in air-saturated solutions produced incomplete oxidation (24–38% Fe3+/ΣFe). Additional smectite oxidation occurred upon re-exposure to air-saturated solutions after anoxic hydrothermal recrystallization, which accelerated cation and charge redistribution in the octahedral sheet. Oxidation was accompanied by contraction of the octahedral sheet (d(060) decreased from 1.53-1.56 Å to 1.52 Å), consistent with a shift towards dioctahedral structure. Ferrous smectite oxidation by aqueous hydrogen peroxide solutions resulted in nearly complete Fe2+ oxidation but also led to partial Fe3+ ejection from the structure, producing nanoparticulate hematite. Reflectance spectra of oxidized smectites were characterized by (Fe3+,Mg)2-OH bands at 2.28-2.30 μm, consistent with oxidative formation of dioctahedral nontronite. Accordingly, ferrous smectites are plausible precursors to observed ferric smectites on Mars, and their presence in late-Noachian sedimentary units suggests that anoxic conditions may have persisted on Mars beyond the Noachian.

^1,2Joana S. Oliveira,³Mark A. Wieczorek,^4,5,6Gunther Kletetschka
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2017JE005397]
¹Institut de Physique du Globe de Paris, Université Paris Diderot, Paris, France
²CITEUC, Geophysical and Astronomical Observatory, University of Coimbra, Coimbra, Portugal
³Observatoire de la Côte d’Azur, Laboratoire Lagrange, Nice, France
⁴Charles University in Prague, Faculty of Science, Czech Republic
⁵Institute of Geology of the CAS, Prague, Czech Republic
⁶University of Alaska-Fairbanks, Geophysical Institute, USA
Published by arrangement with John Wiley & Sons

Magnetic field data acquired from orbit shows that the Moon possesses many magnetic anomalies. Though most of these are not associated with known geologic structures, some are found within large impact basins within the interior peak ring. The primary magnetic carrier in lunar rocks is metallic iron, but indigenous lunar rocks are metal poor and can not account easily for the observed field strengths. The projectiles that formed the largest impact basins must have contained a significant quantity of metallic iron, and a portion of this iron would have been retained on the Moon’s surface within the impact melt sheet. Here, we use orbital magnetic field data to invert for the magnetization within large impact basins using the assumption that the crust is unidirectionally magnetized. We develop a technique based on laboratory thermoremanent magnetization acquisition to quantify the relationship between the strength of the magnetic field at the time the rock cooled and the abundance of metal in the rock. If we assume that the magnetized portion of the impact melt sheet is 1 km thick, we find average abundances of metallic iron ranging from 0.11% to 0.45 wt.%, with an uncertainty of a factor of about three. This abundance is consistent with the metallic iron abundances in sampled lunar impact melts and the abundance of projectile contamination in terrestrial impact melts. These results help constrain the composition of the projectile, the impact process, and the time evolution of the lunar dynamo.

Pia Friend^a, Dominik C.Hezel^a,b, Herbert Palme^c, Addi Bischoff^d, Marko Gellissen^e
Earth and Planetary Science Letters 482, 105-114 Link to Article [https://doi.org/10.1016/j.epsl.2017.09.049]
^aUniversity of Cologne, Department of Geology and Mineralogy, Zülpicher Str. 49b, 50674 Köln, Germany
^bNatural History Museum, Department of Mineralogy, Cromwell Road, SW7 5BD London, UK
^cForschungsinstitut und Naturmuseum Senckenberg, Senckenberganlage 25, D-60325 Frankfurt am Main, Germany
^dInstitut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
^eDepartment of Geosciences, Christian-Albrecht-Universität zu Kiel, Germany
Copyright Elsevier

The complimentary chemical composition of chondrules and matrix has so far only been studied in carbonaceous chondrites. We have extended these studies to the matrix-rich Rumurutis We have determined the chemical composition of 27 bulk chondrules and 100 matrix spots in unequilibrated fragments of three different Rumuruti (R) chondrites (NWA 2446, NWA 753, Hughes 030). Also, the bulk chemical composition of NWA 753 was determined. Bulk R chondrites have about CI chondritic (= solar) ratios of Fe/Mg (1.89), Al/Ti (18.62), and Al/Ca (0.94), while chondrules and matrix are complementary to each other. Mean Fe/Mg ratios in chondrules are 0.43 (NWA 2446), 0.36 (NWA 753), and 0.34 (Hughes). Chondrules are depleted in Fe, while matrices are enriched in Fe with respective values of 2.59, 2.45, and 2.39. Bulk Fe/Mg ratio of NWA 753, the only sample which is unaffected by terrestrial weathering, is reproduced by careful mass balance calculations using compositions and abundances of chondrules, matrix and sulphides from the same thin section. Refractory element ratios are also complementary: Al/Ti and Al/Ca are sub-chondritic in chondrules (Al/Ti: 10.43 in NWA 753, 12.24 in NWA 2446 and 11.47 in Hughes 030; Al/Ca: 0.66 in NWA 2446, 0.53 in NWA 753, and 0.46 in Hughes 030), while matrices generally have super-chondritic Al/Ca and Al/Ti ratios (Al/Ti: 28.00 in NWA 2446, 23.20 in NWA 753 and 19.00 in Hughes 030; Al/Ca: 2.30 in NWA 2446, 1.90 in NWA 753, and 0.41 in Hughes 030). Calcium is enriched in the Hughes 030 matrix due to terrestrial weathering. These complementary element ratios and the CI chondritic bulk strongly suggest a common reservoir from which all R chondrite components formed, similar to carbonaceous chondrites. Further, super-chondritic Si/Mg, and CI chondritic Fe/Mg ratios in bulk R chondrites require addition of Si to their reservoir, most probably before chondrule formation.

¹A. Khan,²C. Liebske,¹A. Rozel,³A. Rivoldini,⁴F. Nimmo,²J. A. D. Connolly,⁵A.-C. Plesa,¹D. Giardini
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2017JE005371]
¹Institute of Geophysics, ETH Zürich, Switzerland
²Institute of Geochemistry and Petrology, ETH Zürich, Switzerland
³Royal Observatory of Belgium, Brussels, Belgium
⁴Department of Earth and Planetary Sciences, UC Santa Cruz, California, USA
⁵German Aerospace Center (DLR), Berlin, Germany
Published by arrangement with John Wiley & Sons

We invert the Martian tidal response and mean mass and moment of inertia for chemical composition, thermal state, and interior structure. The inversion combines phase equilibrium computations with a laboratory-based viscoelastic dissipation model. The rheological model, which is based on measurements of anhydrous and melt-free olivine, is both temperature and grain size sensitive and imposes strong constraints on interior structure. The bottom of the lithosphere, defined as the location where the conductive geotherm meets the mantle adiabat, occurs deep within the upper mantle (∼250–500 km depth) resulting in apparent upper mantle low-velocity zones. Assuming an Fe-FeS core, our results indicate: 1) a Mantle with a Mg# (molar Mg/Mg+Fe) of ∼0.75 in agreement with earlier geochemical estimates based on analysis of Martian meteorites; 2) absence of bridgmanite- and ferropericlase-dominated basal layer; 3) core compositions (13.5–16 wt% S), core radii (1640–1740 km), and core-mantle-boundary temperatures (1560–1660 ∘ C) that, together with the eutectic-like core compositions, suggest the core is liquid; and 4) bulk Martian compositions that are overall chondritic with a Fe/Si (wt ratio) of 1.63–1.68. We show that the inversion results can be used in tandem with geodynamic simulations to identify plausible geodynamic scenarios and parameters. Specifically, we find that the inversion results are reproduced by stagnant lid convection models for a range of initial viscosities (∼1019–1020 Pa·s) and radioactive element partitioning between crust and mantle around 0.001. The geodynamic models predict a mean surface heat flow between 15–25 mW/m2.

¹Jean-Christophe Viennet,¹Benjamin Bultel,²Lucie Riu,¹Stephanie C. Werner
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2017JE005343]
¹Centre for Earth Evolution and Dynamics, Department for Geosciences, University of Oslo, Norway
²Institut d’Astrophysique Spatiale, Université Paris-Sud, Orsay, France
Published by arrangement with John Wiley & Sons

Widespread occurrence of Fe,Mg-phyllosilicates have been observed on Noachian Martian terrains. Therefore, the study of Fe,Mg-phyllosilicates formation, in order to characterize early Martian environmental conditions, is of particular interest to the Martian community. Previous studies have shown that the investigation of Fe,Mg-smectite formation alone helps to describe early Mars environmental conditions, but there are still large uncertainties in terms of pH range, oxic/anoxic conditions, etc… Interestingly, carbonates and/or zeolites have also been observed on Noachian surfaces in association with the Fe,Mg-phyllosilicates.

Consequently, the present study focuses on the di/trioctahedral phyllosilicate/carbonate/zeolite formation as a function of various CO2 contents (100% N2, 10% CO2 / 90% N2, 100% CO2), from a combined approach including closed system laboratory experiments for 3 weeks at 120°C and geochemical simulations. The experimental results show that as the CO2 content decreases, the amount of dioctahedral clay minerals decreases in favour of trioctahedral minerals. Carbonates and dioctahedral clay minerals are formed during the experiments with CO2. When Ca-zeolites are formed, no carbonates and dioctahedral minerals are observed. Geochemical simulation aided in establishing pH as a key parameter in determining mineral formation patterns. Indeed, under acidic conditions dioctahedral clay minerals and carbonate minerals are formed, while trioctahedral clay minerals are formed in basic conditions with a neutral pH value of 5.98 at 120°C. Zeolites are favoured from pH >~7.2. The results obtained shed new light on the importance of dioctahedral clay minerals versus zeolites and carbonates versus zeolites competitions, to better define the aqueous alteration processes throughout early Mars history.

¹Elizabeth C. Sklute, ²A. Deanne Rogers, ²Jason C. Gregerson, ²Heidi B. Jensen, ²Richard J. Reeder, ¹M. Darby Dyar
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.11.018]
¹Department of Astronomy, Mount Holyoke College, 50 College St., South Hadley, Massachusetts, 01075, U.S.A.
²Department of Geoscience, Stony Brook University, 255 Earth and Space Science Building, Stony Brook, NY 11794-2100, U.S.A.
Copyright Elsevier

¹L. Folco, ²B.P. Glass, ¹M. D’Orazio, ³P. Rochette,
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.11.017]
¹Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy
²Department of Geosciences, University of Delaware 19716, USA
³CEREGE, Aix-Marseille Université CNRS, France
Copyright Elsevier

Impactor identification is one of the challenges of large-scale impact cratering studies due to the dilution of meteoritic material in impactites (typically < 1 wt%). The nature of the impactor that generated the Australasian tektite/microtektite strewn field, i.e., the largest Cenozoic strewn field (∼15% of the Earth’s surface), the youngest (∼0.78 Myr old) on Earth, and the only one without an associated impact crater so far, is an outstanding issue. We identify a chondritic impactor signature in 77 Australasian microtektites (size range: ∼200 to 700 µm) from within 3000 km from the hypothetical impact location in Indochina (∼17°N, 107°E) based on variations of Cr, Co and Ni interelement ratios in a Co/Ni vs Cr/Ni space (46 microtektites analyzed in this work by Laser Ablation-Inductively Coupled Plasma -Mass Spectrometry and 31 from literature by means of Instrumental Neutron Activation Analyses with Cr, Co and Ni concentrations up to ∼370, 50 and 680 µg/g, respectively). Despite substantial overlap in Cr/Ni versus Co/Ni composition for several meteorite types with chondritic composition (chondrites and primitive achondrites), regression calculation based on ∼85% of the studied microtektites best fit a mixing line between crustal compositions and an LL chondrite. However, due to some scatter mainly in the Cr versus Ni ratios in the considered dataset, an LL chondrite may not be the best fit to the data amongst impactors of primitive compositions. Eight high Ni/Cr and five low Ni/Cr outlier microtektites (∼15% in total) deviate from the above mixing trend, perhaps resulting from incomplete homogenization of heterogeneous impactor and target precursor materials at the microtektite scale, respectively.

Together with previous evidence from the ∼35 Myr old Popigai impact spherules and the ∼1 Myr old Ivory Coast microtektites, our finding suggests that at least three of the five known Cenozoic distal impact ejecta were generated by the impacts of large stony asteroids of chondritic composition, and possibly of ordinary chondritic composition. The impactor signature found in Australasian microtektites documents mixing of target and impactor melts upon impact cratering. This requires target-impactor mixing in both the two competing microtektite formation models in literature proposed for the Australasian: the impact cratering and low-altitude airburst plume models.