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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