I. Strashnov^1,2 and J. D. Gilmour¹

¹School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester, M13 9PL, UK
²School of Physics and Astronomy, The University of Manchester, Manchester, UK

⁸¹Kr-Kr cosmic ray exposure (CRE) ages of individual chondrules (6–10 mg) and adjacent matrix samples (5–10 mg) from the Allegan H5 chondrite have been measured using a new highly sensitive resonance ionization mass spectrometer. No conclusive evidence of variations among the CRE ages of individual chondrules or between chondrules and matrix has been observed—average CRE ages of 5.90 ± 0.42 Ma (⁸¹Kr-⁷⁸Kr) and 5.04 ± 0.37 Ma (⁸¹Kr-⁸⁰⁺⁸²Kr) are identical within error to those determined for the matrix (7.42 ± 1.27 Myr, ⁸¹Kr-⁸⁰⁺⁸²Kr) and agree well with the literature value for bulk Allegan. If any accumulation of cosmogenic krypton in the early solar system took place, either it was below our detection limit in these samples (<100 atoms), or any such gas was lost during parent body metamorphism. However, this demonstration that useful ⁸¹Kr-Kr ages can be obtained from few milligram samples of chondritic material has clear relevance to the analysis of samples returned by planned missions to asteroids and to the search for a signature of pre-exposure in other, less processed meteorites.

Reference
Strashnov I and Gilmour JD (in press) 81Kr-Kr cosmic ray exposure ages of individual chondrules from Allegan. Meteoritics & Planetary Science
[doi:10.1111/maps.12228]
Published by arrangement with John Wiley & Sons

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N. Tosi^1,*, M. Grott², A.-C. Plesa^2,3 and D. Breuer²

¹Department of Planetary Geodesy, Technische Universität Berlin, Berlin, Germany
²Department of Planetary Physics, German Aerospace Center, Berlin, Germany
³Department of Planetology, Westfälische Universität Münster, Münster, Germany

A number of observations performed by the MESSENGER spacecraft can now be employed to better understand the evolution of Mercury’s interior. Using recent constraints on interior structure, surface composition, volcanic and tectonic histories, we modeled the thermal and magmatic evolution of the planet. We ran a large set of Monte Carlo simulations based on one-dimensional parametrized models, spanning a wide range of parameters. We complemented these simulations with selected calculations in 2-D cylindrical and 3-D spherical geometry, which confirmed the validity of the parametrized approach and allowed us to gain additional insight into the spatiotemporal evolution of mantle convection. Core radii of 1940 km, 2040 km, and 2140 km have been considered, and while in the first two cases several models satisfy the observational constraints, no admissible models were found for a radius of 2140 km. A typical thermal evolution scenario consists of an initial phase of mantle heating accompanied by planetary expansion and the production of a substantial amount of partial melt. The evolution subsequent to 2 Gyr is characterized by secular cooling that proceeds approximately at a constant rate and implies that planetary contraction should be ongoing today. Most of the models predict mantle convection to cease after 3–4 Gyr, indicating that Mercury may be no longer dynamically active. Finally, assuming the observed surface abundance of radiogenic elements to be representative for the entire crust, we determined bulk silicate concentrations of 35–62 ppb Th, 20–36 ppb U, and 290–515 ppm K, similar to those of other terrestrial planets.

Reference
Tosi N, Grott M, Plesa A-C and Breuer D (in press) Thermochemical evolution of Mercury’s interior. Journal of Geophysical Research – Planets
[doi:10.1002/jgre.20168]
Published by arrangement with John Wiley & Sons

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J.J. Papike¹, P.V. Burger^1,*, A.S. Bell¹, L. Le², C.K. Shearer¹, S.R. Sutton³, J. Jones⁴ and M. Newville³

¹Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque New Mexico 87131, U.S.A.
²JSC Engineering, Technology and Science (JETS), NASA Johnson Space Center, Mail Code JE-23, Building 31, Houston, Texas 77058, U.S.A.
³Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, U.S.A.
⁴NASA/Johnson Space Center, Houston, Texas 77058, U.S.A.

A spiked (with REE, V, Sc) martian basalt Yamato 980459 (Y98) composition was used to synthesize olivine, spinel, and pyroxene at 1200 °C at five oxygen fugacities: IW−1, IW, IW+1, IW+2, and QFM. These run products were analyzed by electron microprobe, ion microprobe, and X-ray absorption near-edge spectroscopy to establish four oxybarometers based on vanadium partitioning behavior between the following pairs of phases: V spinel-melt, V/(Cr+Al) spinel-melt, olivine-melt, and spinel-olivine. The results for the spinel-melt, olivine-melt, and V/(Cr+Al) spinel-melt are applicable for the entire oxygen fugacity range while the spinel-olivine oxybarometer is only applicable between IW−1 and IW+1. The oxybarometer based on V partitioning between spinel-olivine is restricted to basalts that crystallized under low oxygen fugacities, some martian, all lunar, as well as samples from 4 Vesta. The true potential and power of the new spinel-olivine oxybarometer is that it does not require samples representative of a melt composition or samples with some remnant of quenched melt present. It just requires that the spinel-olivine pairs were in equilibrium when the partitioning of V occurred. We have applied the V spinel-olivine oxybarometer to the Y98 meteorite as a test of the method.

Reference
Papike JJ, Burger PV, Bell AS, Le L, Shearer CK, Sutton SR, Jones J and Newville M (2013) Developing vanadium valence state oxybarometers (spinel-melt, olivine-melt, spinel-olivine) and V/(Cr+Al) partitioning (spinel-melt) for martian olivine-phyric basalts. American Mineralogist 98:2193-2196.
[doi:10.2138/am.2013.4622]
Copyright: The Mineralogical Society of America

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Pavithra Sekhar and Scott D. King

Department of Geosciences, Virginia Tech, Blacksburg, VA 24061, United States

While most of Tharsis rise was in place by end of the Noachian period, at least one volcano on Tharsis swell (Arsia Mons) has been active within the last 2 Ma. This places an important constraint on mantle convection and on the thermal evolution of Mars. The existence of recent volcanism on Mars implies that adiabatic decompression melting and, hence, upwelling convective flow in the mantle remains important on Mars at present. The thermal history on Mars can be constrained by the history of melt production, specifically generating sufficient melt in the first billion years of the planets history to produce Tharsis rise as well as present day melt to explain recent volcanism. In this work, mantle convection simulations were performed using finite element code CitcomS in a 3D sphere starting from a uniformly hot mantle and integrating forward in time for the age of the solar system. We implement constant and decaying radioactive heat sources; and vary the partitioning of heat sources between the crust and mantle, and consider decreasing core–mantle boundary temperature and latent heat of melting. The constant heat source calculations produce sufficient melt to create Tharsis early in Martian history and continue to produce significant melt to the present. Calculations with decaying radioactive heat sources generate excessive melt in the past, except when all the radiogenic elements are in the crust, and none produce melt after 2 Gyr. Producing a degree-1 or degree-2 structure may not be pivotal to explain the Tharsis rise: we present multi-plume models where not every plume produces melt. The Rayleigh number controls the timing of the first peak of volcanism while late-stage volcanism is controlled more by internal mantle heating. Decreasing the Rayleigh number increases the lithosphere thickness (i.e., depth), and increasing lithosphere thickness increases the mean mantle temperature. Increasing pressure reduces melt production while increasing temperature increases melt production; hence predicting melt production from convection parameters is not straightforward. Generating enough melt in the mantle to create Tharsis early on and also to explain recent volcanism may require other mechanisms such as small-scale convection or lowering the thermal conductivity of the crust.

Reference
Sekhar P and King SD (2013) 3D spherical models of Martian mantle convection constrained by melting history. Earth and Planetary Science Letters 388:27–37.
[doi:10.1016/j.epsl.2013.11.047]
Copyright Elsevier

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Richard A. Kerr

Recent results from the Curiosity Mars rover have helped scientists formulate a plan for the next phase of its mission: looking for possible “molecular fossils” left by ancient martian microbes. Analyses of rocks show that Curiosity landed near a former lake that at least intermittently held enough water to have supported life. Now, papers published online in Science show that rocks that once formed the lakebed and bottom mud layer are high in organic carbon molecules. Researchers can’t tell yet whether the molecules came from ancient life or rained down from space. But future analyses—especially of recently eroded rocks that spent most of their history shielded from the cosmic rays thought to sterilize the top meter or so nearest the martian surface—should help researchers determine whether Mars ever harbored life.

Reference
Kerr RA (2013) New Results Send Mars Rover on a Quest for Ancient Life. Science 342:1300-1301.
[doi:10.1126/science.342.6164.1300]
Reprinted with permission from AAAS

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Stuart J. Mills^1,*, Fabrizio Nestola², Volker Kahlenberg³, Andrew G. Christy⁴, Clivia Hejny³ and Günther J. Redhammer⁵

¹Geosciences, Museum Victoria, GPO Box 666, Melbourne 3001, Victoria, Australia
²Dipartimento di Geoscienze, Università di Padova, Via Gradenigo 6, Padova I-35131, Italy
³Institut für Mineralogie und Petrographie der Universität Innsbruck, Innrain 52, 6020 Innsbruck, Austria
⁴Centre for Advanced Microscopy, Australian Natioanl University, Canberra, ACT 0200, Australia
⁵Department of Materials Engineering and Physics, University of Salzburg, Hellbrunnerstr. 34, A-5020 Salzburg, Austria

Single-crystal diffraction of jarosite, KFe₃³⁺(SO₄)₂(OH)₆, has been undertaken at low temperatures that proxy for martian surface conditions. Room-temperature data are consistent with literature data [a = 7.2913(5), c = 17.1744(17), and V = 790.72(11) in R3̄m], while the first low-temperature data for the mineral is presented (at 253, 213, 173, and 133 K). Data collections between 297 and 133 K show strongly anisotropic thermal expansion, with the c axis much more expandable than the a axis. Much of the anisotropy is due to strong distortion of the KO₁₂ polyhedron, which increases by 8% between 297 and 133 K. The data sets can aid in the identification of jarosite by X-ray diffraction of martian soils using the Curiosity Rover’s CheMin instrument.

Reference
Mills SJ, Nestola F, Kahlenberg V, Christy AG, Hejny C and Redhammer GJ (2013) Looking for jarosite on Mars: The low-temperature crystal structure of jarosite. American Mineralogist 98:1966-1971.
[doi:10.2138/am.2013.4587]
Copyright: The Mineralogical Society of America

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Sourav Chatterjee¹ and Jonathan C. Tan²

¹Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
²Departments of Astronomy & Physics, University of Florida, Gainesville, FL 32611, USA

The compact multi-transiting planet systems discovered by Kepler challenge planet formation theories. Formation in situ from disks with radial mass surface density, Σ, profiles similar to the minimum mass solar nebula but boosted in normalization by factors  10 has been suggested. We propose that a more natural way to create these planets in the inner disk is formation sequentially from the inside-out via creation of successive gravitationally unstable rings fed from a continuous stream of small (~cm-m size) “pebbles,” drifting inward via gas drag. Pebbles collect at the pressure maximum associated with the transition from a magnetorotational instability (MRI)-inactive (“dead zone”) region to an inner MRI-active zone. A pebble ring builds up until it either becomes gravitationally unstable to form an ~1 M ⊕ planet directly or induces gradual planet formation via core accretion. The planet may undergo Type I migration into the active region, allowing a new pebble ring and planet to form behind it. Alternatively, if migration is inefficient, the planet may continue to accrete from the disk until it becomes massive enough to isolate itself from the accretion flow. A variety of densities may result depending on the relative importance of residual gas accretion as the planet approaches its isolation mass. The process can repeat with a new pebble ring gathering at the new pressure maximum associated with the retreating dead-zone boundary. Our simple analytical model for this scenario of inside-out planet formation yields planetary masses, relative mass scalings with orbital radius, and minimum orbital separations consistent with those seen by Kepler. It provides an explanation of how massive planets can form with tightly packed and well-aligned system architectures, starting from typical protoplanetary disk properties.

Reference
Chatterjee S and Tan JC (2014) Inside-out Planet Formation. The Astrophysical Journal 780:53.
[doi:10.1088/0004-637X/780/1/53]

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E. Sellentin, J. P. Ramsey, F. Windmark and C. P. Dullemond

Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Überle-Str. 2, Heidelberg 69120, Germany

Context. The growth process of dust particles in protoplanetary disks can be modeled via numerical dust coagulation codes. In this approach, physical effects that dominate the dust growth process often must be implemented in a parameterized form. Due to a lack of these parameterizations, existing studies of dust coagulation have ignored the effects a hydrodynamical gas flow can have on grain growth, even though it is often argued that the flow could significantly contribute either positively or negatively to the growth process.
Aims. We intend to qualitatively describe the factors affecting small particle sweep-up under hydrodynamical effects, followed by a quantification of these effects on the growth of dust particles, such that they can be parameterized and implemented in a dust coagulation code.
Methods. Using a simple model for the flow, we numerically integrate the trajectories of small dust particles in disk gas around a proto-planetesimal, sampling a large parameter space in proto-planetesimal radii, headwind velocities, and dust stopping times.
Results. The gas flow deflects most particles away from the proto-planetesimal, such that its effective collisional cross section, and therefore the mass accretion rate, is reduced. The gas flow however also reduces the impact velocity of small dust particles onto a proto-planetesimal. This can be beneficial for its growth, since large impact velocities are known to lead to erosion. We also demonstrate why such a gas flow does not return collisional debris to the surface of a proto-planetesimal.
Conclusions. We predict that a laminar hydrodynamical flow around a proto-planetesimal will have a significant effect on its growth. However, we cannot easily predict which result, the reduction of the impact velocity or the sweep-up cross section, will be more important. Therefore, we provide parameterizations ready for implementation into a dust coagulation code.

Reference
Sellentin E, Ramsey JP, Windmark F and Dullemond CP (2013) A quantification of hydrodynamical effects on protoplanetary dust growth. Astronomy & Astrophysics 560:A96.
[doi:10.1051/0004-6361/201321587]
Reproduced with permission © ESO

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J.A. Barrat^a, B. Zanda^b, A. Jambon^c, C. Bollinger^d

^aU.B.O.-I.U.E.M., CNRS UMR 66538 (Domaines Océaniques), Place Nicolas Copernic, 29280 Plouzané Cedex, France
^bMuséum National d’Histoire Naturelle, Laboratoire de Minéralogie et de Cosmochimie du Muséum, CNRS UMR7202, 61 rue Buffon, 75005 Paris, France
^cUniversité Pierre et Marie Curie-Paris 6 ISTeP, CNRS UMR 7193, case 110, 4 place Jussieu, 75252 Paris cedex 05, France
^dCNRS UMS 3113, I.U.E.M., Place Nicolas Copernic, 29280 Plouzané Cedex, France

We report on the abundances of a selected set of lithophile trace elements (namely REEs, Y, Rb, Ba, Sr, Zr, Hf, Nb, Th, U) in a comprehensive suite of enstatite chondrites (EC – 13 EH and 11 EL). EH3 and EL3 display only minor deviations from chondritic distributions for these elements. In most metamorphosed EC, a wide range of compositions is observed and suggests a mobility of many of the elements studied during the history of these rocks. For example, EL6 chondrites exhibit light-REE and Nb depletions, negative Eu anomalies, and positive Y anomalies. More important trace element fractionations are observed in metamorphosed EH like St Marks (Rb depletion), LAP 02225 (Rb, Nb, Zr, Eu, light REE depletions) and Galim (b), which displays large Ba, Sr, Eu, Nb and light REE depletions.
Leaching experiments were undertaken to investigate the contributions of sulfides in the whole rock budgets. These phases control not only the REE budget, but also important fractions of the other elements we studied. These fractions strongly depend on the type of the rock (EH or EL, and metamorphic grade). For many elements, the sulfide contributions increase with the metamorphic grades. The trace element abundances of silicate residues are extremely variable. Negative Sm and Yb anomalies are observed in EL3 and EH3 residues, and are certainly the results of early nebular processes. Such anomalies are lacking in residues obtained with most metamorphosed EC, underlining the importance of trace element redistributions during metamorphism. In addition, EL6 residues display distinctive positive Y anomalies that could be potentially ascribed to a less chalcophile behavior than Ho in the conditions that prevailed during EL metamorphism.

Reference
Barrat JA, Zanda B, Jambon A and Bollinger C (in press) The lithophile trace elements in enstatite chondrites. Geochimica et Cosmochimica Acta 98:1966-1971.
[doi:10.1016/j.gca.2013.11.042]
Copyright Elsevier

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Arnold Gucsik¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Department of Geology, University of Johannesburg, Johannesburg, South Africa

Highly forsteritic olivine (Fo: 99.2–99.7) in the Kaba meteorite emits bright cathodoluminescence (CL). CL spectra of red luminescent forsterite grains have two broad emission bands at approximately 630 nm (impurity center of divalent Mn ions) in the red region and above 700 nm (trivalent Cr ions) in the red–IR region. The cores of the grains show CL blue luminescence giving a characteristic broad band emission at 400 nm, also associated with minor red emissions related to Mn and Cr ions. CL color variation of Kaba forsterite is attributed to structural defects. Electron probe microanalyzer (EPMA) analysis shows concentrations of Ca, Al, and Ti in the center of the forsterite grain. The migration of diffusible ions of Mn, Cr, and Fe to the rim of the Kaba meteoritic forsterite was controlled by the hydrothermal alteration at relatively low temperature (estimated at about 250 °C), while Ca and Al ions might still lie in the core. A very unusual phase of FeO (wüstite) was also observed, which may be a terrestrial alteration product of FeNi-metal.

Reference
Gucsik et al. (in press) Cathodoluminescence microscopy and spectroscopy of forsterite from Kaba meteorite: An application to the study of hydrothermal alteration of parent body. Meteoritics & Planetary Science 
[doi:10.1111/maps.12238]
Published by arrangement with John Wiley & Sons

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