¹Keith Putirka
¹Department of Earth and Environmental Sciences, California State University–Fresno, 2345 East San Ramon Avenue, MS/MH24, Fresno, California 93720, U.S.A.

Mantle potential temperatures (Tp) provide insights into mantle circulation and tests of whether Earth is the only planet to exhibit thermally bi-modal volcanism—a distinctive signature of modern plate tectonics. Planets that have a stagnant lid, for example, should exhibit volcanism that is uni-modal with Tp, since mantle plumes would have a monopoly on the genesis of volcanism. But new studies of magmatic ferric-ferrous ratios (XliqFe2O3/XliqFeO) (Cottrell and Kelley 2011) and the olivine-liquid Fe-Mg exchange coefficient, KD(Fe-Mg)Ol-liq (or KD) (Matzen et al. 2011) indicate that re-evaluations of Tp are needed. New tests and calibrations are thus presented for oxygen fugacity (fO2), XliqFe2O3/XliqFeO, potential temperature (Tp), melt fraction (F), KD, and peridotite enthalpies of fusion (ΔHfus) and heat capacities (CP). The new models for XliqFe2O3/XliqFeO and fO2 reduce error by 25–30%, and residual error for all models appears random; this last observation supports the common, but mostly untested, assumption that equilibrium is the most probable of states obtained by experiment, and perhaps in nature as well. Aggregate 1σ error on Tp is as high as ~±77 ºC, and estimates of F, and mantle olivine composition, are the greatest sources of error. Pressure and ΔHfus account for smaller, but systematic uncertainties (a constant ΔHfus can under-predict Texcess = Tpplume–Tpambient; assumptions of 1 atm can under-predict Tp). However, assumptions about whether parental magmas are incremental, accumulated, or isobaric batch melts induces no additional systematic error.

The new models show that maximum Tp estimates on the oldest samples from Earth, Mars, Moon, and Vesta, decrease as planet size decreases. This may be expected since Tp should scale with accretion energy and reflect the Clausius-Clapeyron slope for the melting of silicates and Fe-Ni alloys. This outcome, however, occurs only if shergottites (from Mars) are 4.3 Ga (e.g., Bouvier et al. 2009; Werner et al. 2014), and the highest MgO komatiites from Earth’s Archean era (27–30% MgO; Green et al. 1975) are used to estimate Tp. With these assumptions, Earth and Mars exhibit monotonic cooling, and support for Stevenson’s (2003) idea that smaller planets cool at similar rates (~90–135 ºC/Ga), but at lower absolute temperatures. Tp estimates for Mars and Earth are also important in two other ways: Mars exhibits non-linear cooling, with rates as high as 275–550 ºC/Ga in its first 0.5 Ga, and Archean volcanism on Earth was thermally bi-modal. Several hundred Archean volcanic compositions are in equilibrium with Fo92–94 olivine, and yield Tp modes at 1940 and 1720 ºC, possibly representing plume and ambient mantle, respectively. These estimates compare to modern Tp values of 1560–1670 ºC at Mauna Loa (plume) and 1330–1450 ºC at MORB (ambient). We conclude that plate tectonics was active in some manner in the Archean, and that assertions of an Archean “thermal catastrophe” are exaggerated. Our new models also show that the modern Hawaiian source, when compared at the same T, has a lower fO2 compared to MORB, which would discount a Hawaiian source rich in recycled pyroxenite.

Reference
Putirka K (2016) Rates and styles of planetary cooling on Earth, Moon, Mars, and Vesta, using new models for oxygen fugacity, ferric-ferrous ratios, olivine-liquid Fe-Mg exchange, and mantle potential temperature.
American Mineralogist 101, 819-840
Link to Article [doi:10.2138/am-2016-5402]
Copyright: The Mineralogical Society of America

¹Varsha P. Kulkarni, ²Monique C. Aller, ³Donald G. York, ³Daniel E. Welty, ⁴Giovanni Vladilo, ⁵Debopam Some
¹University of South Carolina, Dept. of Physics and Astronomy, Columbia, SC 29208
²Georgia Southern University, Dept. of Physics, Statesboro, GA 30460
³Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL 60637
⁴Osservatorio Astronomico di Trieste
⁵University of South Carolina, Dept. of Physics and Astronomy, Columbia, SC 29208
⁶Aix Marseille Université, CNRS, Laboratoire dAstrophysique de Marseille, UMR 7326, 13388, Marseille, France

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Reference
Kulkarni VP, Aller MC, York DG, Welty DE, Vladilo G, Som D (2016) Probing the Interstellar Dust in Galaxies over >10Gyr of Cosmic History. Planetary and Space Science (in Press)
Link to Article [doi:10.1016/j.pss.2016.03.011]

^1,2Umberto Maio, ^3,4Edoardo Tescari
¹Leibniz-Institut für Astrophysik, An der Sternwarte 16, D-14482 Potsdam, Germany
²INAF – Osservatorio Astronomico di Trieste, via G. Tiepolo, 11, I-34131 Trieste, Italy
³School of Physics, University of Melbourne, Parkville, VIC 3010, Australia
⁴ARC Centre of Excellence for All-Sky Astrophysics (CAASTRO), University of Melbourne, Markville, VIC 3010, Australia

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Reference
Maio U, Tescari E (2015) Origin of cosmic chemical abundances. Monthly Notices of the Royal Astronomical Society 453, 3798-3820.
Link to Article [doi: 10.1093/mnras/stv1714]

¹Patrick N. Peplowski, ¹Rachel L. Klima, ¹David J. Lawrence, ¹Carolyn M. Ernst, ¹Brett W. Denevi, ²Elizabeth A. Frank, ¹John O. Goldsten, ¹Scott L. Murchie, ²Larry R. Nittler ^2,3Sean C. Solomon
¹The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland 20723, USA
²Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA
³Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964

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Reference
Peplowski PN, Klima RL, Lawrence DJ, Ernst CM, Denevi BW, Frank EA, Goldsten JO, Murchie SL, Nittler LR, Solomon SC (2016) Remote sensing evidence for an ancient carbon-bearing crust on Mercury. Nature Geoscience 9, 273–276
Link to Article [doi:10.1038/ngeo2669]

¹Piers Koefoed, ¹Yuri Amelin, ²Qing-Zhu Yin, ²Josh Wimpenny, ²Matthew E. Sanborn, ³Tsuyoshi Iizuka, ⁴Anthony J. Irving
¹Research School of Earth Sciences, Australian National University, Canberra, ACT 2601, Australia
²Department of Earth and Planetary Sciences, University of California at Davis, Davis, California, 95616, USA
³Department of Earth and Planetary Science, University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan
⁴Department of Earth & Space Sciences, University of Washington, Seattle, WA 98195, USA

Northwest Africa (NWA) 7325 is a unique ungrouped gabbroic achondrite which has characteristics consistent with a possible link to the planet Mercury. In order to understand the origin of this meteorite and the nature of its parent body, we have determined its crystallisation age using the long-lived U-Pb and short-lived Al-Mg chronometers. An internal Pb-Pb isochron defined by six acid leached pyroxene fractions yields an age of 4563.4 ± 2.6 Ma, assuming that the 238U/235U ratio for NWA 7325 is identical to the bulk Earth and Solar System value of 137.794. The Al-Mg isotope analyses of seven fractions (four plagioclase, one pyroxene, one olivine and one whole rock) define a regression line corresponding to 26Al/27Al0 = (3.03 ± 0.14) × 10-7 and an initial δ26Mg∗ of 0.093 ± 0.004‰. When anchored to the D’Orbigny angrite, this initial 26Al/27Al yields an age of 4563.09 ± 0.26 Ma. The Pb-Pb age of 4563.4 ± 2.6 Ma and Al-Mg age of 4563.09 ± 0.26 Ma are in complete agreement, but the low U concentrations of NWA 7325 resulted in a relatively low precision Pb-Pb age. The observed excess in initial δ26Mg∗ can be explained by 27Al/24Mg fractionation and subsequent Mg isotopic evolution after planetary differentiation. Furthermore, the parental magma of NWA 7325 most likely formed within 1.72 Ma after calcium-aluminium rich inclusion (CAI) formation. NWA 7325 formed near simultaneously with quenched angrites and a number of ungrouped achondrites at ∼4563 Ma, suggesting that a multitude of planetary bodies had formed and differentiated by ∼4-5 Myr after CAI formation. This ancient age may be interpreted as an argument against NWA 7325 originating from Mercury, however it does not completely rule it out.

Reference
Koefoed P, Amelin Y, Yin Q-Z, Wimpenny J, Sanborn ME, Iizuka T, Irving AJ (2016)
U-Pb and Al-Mg systematics of the ungrouped achondrite Northwest Africa 7325. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.03.028]
Copyright Elsevier

^1,2Craig Hardgrove,¹A. Deanne Rogers,¹Timothy, D. Glotch,^1,3Jessica, Anne Arnold
¹Stony Brook University, Department of Geosciences, Stony Brook, NY
²Arizona State University, United States
³University of Oxford, United Kingdom

Distinguishing between micro and macrocrystalline mineral phases can help constrain the conditions under which those minerals formed or the degree of post-depositional alteration. This study demonstrates the effects of crystal size and surface roughness on thermal infrared emission spectra of micro and macrocrystalline phases of the two most common minerals on Earth, quartz and calcite. Given the characteristic depositional and environmental conditions under which microcrystalline minerals form, and the recent observations of high-silica deposits on Mars, it is important to understand how these unique materials can be identified using remote infrared spectroscopy techniques. We find that (a) microcrystalline minerals exhibit naturally rough surfaces compared to their macrocrystalline counterparts at the 10 µm scale; and that (b) this roughness causes distinct spectral differences within the Reststrahlen bands of each mineral. These spectral differences occur for surfaces that are rough on the wavelength scale, where the absorption coefficient (k) is large. Specifically, the wavelength positions of the Reststrahlen features for microcrystalline phases are narrowed and shifted compared to macrocrystalline counterparts. The spectral shape differences are small enough that the composition of the material is still recognizable, but large enough such that a roughness effect could be detected. Petrographic and topographic analyses of microcrystalline samples suggest a relationship between crystal size and surface roughness. Together, these observations suggest it may be possible to make general inferences about microcrystallinity from the thermal infrared spectral character of samples, which could aid in reconstructions of sedimentary rock diagenesis where corresponding petrographic or micro-imaging is not available.

Reference
Hardgrove C, Rogers AD, Glotch TD, Arnold JA (2016) Thermal Emission Spectroscopy of Microcrystalline Sedimentary Phases: Effects of Natural Surface Roughness on Spectral Feature Shape. Journal of Geophysical Research, Planets (in Press)
Link to Article [DOI: 10.1002/2015JE004919]
Published by arrangement with John Wiley & Sons

¹James R. Darling, ²Desmond E. Moser, ²Ivan R. Barker, ³Kim T. Tait, ⁴Kevin R. Chamberlain, ^5,6Axel K. Schmitt, ³Brendt C. Hyde
¹School of Earth and Environmental Sciences, University of Portsmouth, Portsmouth PO1 3QL, UK
²Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada
³Department of Natural History, Mineralogy, Royal Ontario Museum, Toronto, Ontario M5S 2C6, Canada
⁴Department of Geology and Geophysics, University of Wyoming, 3006, Laramie, WY 82071, USA
⁵Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095, USA
⁶Institut für Geowissenschaften, Universität Heidelberg, 69120 Heidelberg, Germany

The accurate dating of igneous and impact events is vital for the understanding of Solar System evolution, but has been hampered by limited knowledge of how shock metamorphism affects mineral and whole-rock isotopic systems used for geochronology. Baddeleyite (monoclinic ZrO2) is a refractory mineral chronometer of great potential to date these processes due to its widespread occurrence in achondrites and robust U–Pb isotopic systematics, but there is little understanding of shock-effects on this phase. Here we present new nano-structural measurements of baddeleyite grains in a thin-section of the highly-shocked basaltic shergottite Northwest Africa (NWA) 5298, using high-resolution electron backscattered diffraction (EBSD) and scanning transmission electron microscopy (STEM) techniques, to investigate shock-effects and their linkage with U–Pb isotopic disturbance that has previously been documented by in-situ U–Pb isotopic analyses.

The shock-altered state of originally igneous baddeleyite grains is highly variable across the thin-section and often within single grains. Analyzed grains range from those that preserve primary (magmatic) twinning and trace-element zonation (baddeleyite shock Group 1), to quasi-amorphous ZrO2 (Group 2) and to recrystallized micro-granular domains of baddeleyite (Group 3). These groups correlate closely with measured U–Pb isotope compositions. Primary igneous features in Group 1 baddeleyites (n=5)(n=5) are retained in high shock impedance grain environments, and an average of these grains yields a revised late-Amazonian magmatic crystallization age of 175±30 Ma175±30 Ma for this shergottite. The youngest U–Pb dates occur from Group 3 recrystallized nano- to micro-granular baddeleyite grains, indicating that it is post-shock heating and new mineral growth that drives much of the isotopic disturbance, rather than just shock deformation and phase transitions.

Our data demonstrate that a systematic multi-stage microstructural evolution in baddeleyite results from a single cycle of shock-loading, heating and cooling during transit to space, and that this leads to variable disturbance of the U–Pb isotope system. Furthermore, by linking in-situ U–Pb isotopic measurements with detailed micro- to nano-structural analyses, it is possible to resolve the timing of both endogenic crustal processes and impact events in highly-shocked planetary materials using baddeleyite. This opens up new opportunities to refine the timing of major events across the Solar System.

Reference
Darling JR, Moser DE, Barker IR, Tait KT, Chamberlain KR, Schmitt AK, Hyde BC (2016)
Variable microstructural response of baddeleyite to shock metamorphism in young basaltic shergottite NWA 5298 and improved U–Pb dating of Solar System events. Earth and Planetary Science Letters 444, 1–12
Link to Article [doi:10.1016/j.epsl.2016.03.032]
Copyright Elsevier

¹J. Guignard, ²M.J. Toplis,²M. Bystricky, ²M. Monnereau
¹European Synchrotron Radiation Facility, 71 Avenue de martyrs, 38000 Grenoble, France
²IRAP, Université de Toulouse, CNRS, UPS, Toulouse, France

Grain growth experiments in the system forsterite (Fo) + nickel (Ni) have been performed on two analogue mixtures of ordinary chondrites, with volume % of Fo:Ni (95:5) and (80:20). These two mixtures have been studied at temperatures of 1390 °C and 1340 °C, at an oxygen fugacity (fO2) three orders of magnitude below the Ni–NiO buffer, for durations between 2 h and 10 days. Microstructures and grain size distributions show that grain growth is normal and that for durations >10 h the Zener relation is verified (i.e., the ratio of Fo and Ni grain size is independent of time). Comparison with results previously obtained at 1440 °C shows a similar grain growth exponent (n∼5n∼5) for both phases, consistent with growth of forsterite by grain boundary migration, limited by the growth-rate of nickel. The details of size distribution frequencies and the value of grain-growth exponent indicate that the nickel grains, which pin forsterite grain boundaries, grow by diffusion along one-dimensional paths (i.e., along forsterite triple junctions). The derived activation energies for nickel and forsterite are 235±33 kJ/mol235±33 kJ/mol and 400±48 kJ/mol400±48 kJ/mol respectively. Within the framework of the Zener relation, this unexpected difference of activation energy is shown to be related to temperature-dependent variations in the ratio of Ni and Fo grain-size that are consistent with observed variations in Fo–Ni–Fo dihedral angle. These data thus indicate that the presence of all phases should be taken into account when considering the activation energy of growth rate of individual phases. As an application, the experimentally derived growth law for metal has been used in conjunction with temperature–time paths taken from models of the thermal history of the H-chondrite parent body to estimate the grain size evolution of metal in H-chondrites. A remarkably self-consistent picture emerges from experimentally derived grain-growth laws, textural data of metal grains in well characterised H-chondrite samples, and geochemically constrained temperature–time paths, providing the potential to use textural data of metal as a window into the thermal history of chondritic samples.

Reference
Guignard J, Toplis MJ, Bystricky M, Monnereau M (2016) Temperature dependent grain growth of forsterite–nickel mixtures: Implications for grain growth in two-phase systems and applications to the H-chondrite parent body. Earth and Planetary Science Letters 443, 20–31
Link to Article [doi:10.1016/j.epsl.2016.03.007]
Copyright Elsevier

¹Jon Wade, ¹Bernard J. Wood
¹Department of Earth Sciences, South Parks Road, Oxford OX1 3AN, UK

Physical simulations of the origin of the Moon have, until recently, centred on impact, about 100 M.yr after the origin of the solar system, of a Mars-like body (10–20% Earth mass) on a near fully-accreted protoEarth. Although this model provides an explanation of the distribution of mass and moment of inertia of the Earth–Moon system it has recently been found that modification of the initial conditions greatly expands the range of permissible impactor masses. Here we take an alternative approach and consider how the oxidation state and mass of the impactor affect the chemical compositions of the product Earth and Moon. We apply the constraints that silicate Moon is richer in FeO than silicate Earth (9–13% as opposed to 8.05%), that their Hf/W ratios are both ∼25 and that they are virtually identical in isotopes of O, Ti, Si, Ni, Cr and W. We then grow protoEarth using a standard accretionary model which yields the correct mantle abundances of Ni, Co, W, Mo, Nb, V and Cr, and add to this body different masses of impactor. The impactor is assumed to be either highly oxidised (∼18% FeO), highly reduced (∼0.3% FeO) or undifferentiated and chondritic. In order to satisfy the isotopic constraints silicate Moon is assumed to be derived principally from silicate protoEarth.

We find that an oxidised or chondritic impactor of ∼0.15 ME∼0.15 ME can satisfy the isotopic constraints (most importantly ε182W), FeO contents and Nb/Ta of Earth and Moon, but leads to implausibly low Hf/W of ∼12–16∼12–16 in silicate Earth and ∼4–6∼4–6 in silicate Moon. This is because the Moon requires more impactor mantle, with low Hf/W, than Earth to reach its higher FeO content. In contrast, impact of a similar mass (10–20% MEME) of highly reduced, Mercury-like impactor on an oxidised protoEarth (∼10.7% FeO in mantle) satisfies the isotopic constraints, FeO contents, Nb/Ta and Hf/W of silicate Earth and Moon given a small amount of post-impact re-equilibration of terrestrial mantle with impactor core. The presence of a small S-rich lunar core is consistent with this reduced impactor scenario. We conclude that the geochemical properties of Earth and Moon strongly favour a reduced impactor of 10–20% MEME.

Reference
Wade J, Wood BJ (2016) The oxidation state and mass of the Moon-forming impactor. Earth and Planetary Science Letters 442, 186–193
Link to Article [doi:10.1016/j.epsl.2016.02.053]
Copyright Elsevier

¹Gianrico Filacchione et al. (>10)*
¹INAF-IAPS, Istituto di Astrofisica e Planetologia Spaziali, Area di Ricerca di Tor Vergata, via del Fosso del Cavaliere, 100, 00133 Rome, Italy
*Find the extensive, full author and affiliation list on the publishers website

From August to November 2014 the Rosetta orbiter has performed an extensive observation campaign aimed at the characterization of 67P/CG nucleus properties and to the selection of the Philae landing site. The campaign led to the production of a global map of the illuminated portion of 67P/CG nucleus. During this prelanding phase the comet’s heliocentric distance decreased from 3.62 to 2.93 AU while Rosetta was orbiting around the nucleus at distances between 100 to 10 km. VIRTIS-M, the Visible and InfraRed Thermal Imaging Spectrometer – Mapping channel (Coradini et al., [2007] Space Sci. Rev., 128, 529–559) onboard the orbiter, has acquired 0.25–5.1 µm hyperspectral data of the entire illuminated surface, e.g. the north hemisphere and the equatorial regions, with spatial resolution between 2.5 and 25 m/pixel. I/F spectra have been corrected for thermal emission removal in the 3.5–5.1 µm range and for surface’s photometric response. The resulting reflectance spectra have been used to compute several Cometary Spectral Indicators (CSI): single scattering albedo at 0.55 µm, 0.5–0.8 µm and 1.0–2.5 µm spectral slopes, 3.2 µm organic material and 2.0 µm water ice band parameters (center, depth) with the aim to map their spatial distribution on the surface and to study their temporal variability as the nucleus moved towards the Sun. Indeed, throughout the investigated period, the nucleus surface shows a significant increase of the single scattering albedo along with a decrease of the 0.5–0.8 and 1.0–2.5 µm spectral slopes, indicating a flattening of the reflectance. We attribute the origin of this effect to the partial removal of the dust layer caused by the increased contribution of water sublimation to the gaseous activity as comet crossed the frost-line. The regions more active at the time of these observations, like Hapi in the neck/north pole area, appear brighter, bluer and richer in organic material than the rest of the large and small lobe of the nucleus. The parallel coordinates method (Inselberg [1985] Vis. Comput., 1, 69–91) has been used to identify associations between average values of the spectral indicators and the properties of the geomorphological units as defined by (Thomas et al., [2015] Science, 347, 6220) and (El-Maarr et al., [2015] Astron. Astrophys., 583, A26). Three classes have been identified (smooth/active areas, dust covered areas and depressions), which can be clustered on the basis of the 3.2 µm organic material’s band depth, while consolidated terrains show a high variability of the spectral properties resulting being distributed across all three classes. These results show how the spectral variability of the nucleus surface is more variegated than the morphological classes and that 67P/CG surface properties are dynamical, changing with the heliocentric distance and with activity processes.

Reference
Filacchione G et al. (2016) The global surface composition of 67P/CG nucleus by Rosetta/VIRTIS. (I) Prelanding mission phase. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.02.055]
Copyright Elsevier