¹T. Suhasaria, ¹J. D. Thrower, ¹H. Zacharias
¹Physikalisches Institut, Westfälische Wilhelms-Universität, D-48149 Münster, Germany

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Reference
Suhasaria T, Thrower JD, Zacharias H (2015) Thermal desorption of ammonia from crystalline forsterite surfaces. Monthly Notices of the Royal Astronomical Society 454, 3317-3327.
Link to Article [doi: 10.1093/mnras/stv2197]

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