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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M. Darby Dyar¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Department of Astronomy, Mount Holyoke College, South Hadley, Massachusetts 01075, U.S.A.

Although Fe-sulfate minerals occur only rarely on Earth as alteration products of sulfidic basalts or in hydrothermal systems, multiple lines of evidence point to the importance of Fe- (and other) sulfate minerals on the surface of Mars. One such martian data set comes from the MIMOS II Mössbauer spectrometers on the Mars Exploration Rovers, which acquired hundreds of spectra from the martian surface at two locations. Interpretation of those spectra has been limited by the lack of a comprehensive set of laboratory analog spectra of the broad range of naturally occurring sulfate minerals. Accordingly, this study reports Mössbauer data of 98 samples representing 47 different sulfate mineral species, all containing six- or higher-coordinated Fe. The resultant Mössbauer parameters are related to the local polyhedral environment around the Fe cation in each mineral to explain variations in spectral characteristics. Results show that the size of the coordination polyhedron is the best predictor of quadrupole splitting, which increases with both octahedral volume and mean bond length. Species within groups of structurally similar minerals are shown to have comparable spectral peaks that generally fall within small ranges. Although coordination polyhedron geometry is not necessarily unique to any particular mineral species or group, Mössbauer data can be used to help constrain mineral identifications from martian spectra. The number of mineral species is large, but the range of crystal structures and hyperfine parameters may be small, so that in many cases, individual minerals cannot be uniquely fingerprinted. Examples would include quenstedtite, coquimbite, kornelite, and lausenite, which have indistinguishable spectra, as do apjohnite, bilinite, dietrichite, and römerite. Overlap of Mössbauer parameters is a particular complication for identification of Fe³⁺-rich phases because the range of Mössbauer parameters for Fe³⁺ in any coordination number is so small. Previous analyses of martian Mössbauer spectra reported the presence of jarosite (Klingelhöfer et al. 2004; Morris et al. 2004) and an unspecific ferric sulfate (Morris et al. 2008). New data presented here indicate that botryogen, metasideronatrite, and slavikite exhibit Mössbauer spectra similar to those attributed to jarosite at Meridiani Planum. Fibroferrite and rhomboclase have parameters similar to those observed at Arad Samra, and copiapite and parabutlerite could be present at Tyrone Mount Darwin and Berkner Island. Unique mineral identifications are generally not possible from Mössbauer data alone, particularly for paramagnetic phases, although combining Mössbauer results with other data sets enables a greater level of confidence in constraining mineralogy. This study provides a new expansive data set for future interpretation of iron sulfates on Mars.

Reference
Dyar et al. (2013) Mössbauer parameters of iron in sulfate minerals. American Mineralogist 98:1943-1965.
[doi:10.2138/am.2013.4604]
Copyright: The Mineralogical Society of America

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Yoram Lithwick

Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA and Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Evanston, IL 60208, USA

When planetesimals begin to grow by coagulation, they first enter an epoch of runaway, during which the biggest bodies grow faster than all the others. The questions of how runaway ends and what comes next have not been answered satisfactorily. We show that runaway is followed by a new stage—the “trans-Hill stage”—that commences when the bodies that dominate viscous stirring (“big bodies”) become trans-Hill, i.e., when their Hill velocity matches the random speed of the small bodies they accrete. Subsequently, the small bodies’ random speed grows in lockstep with the big bodies’ sizes, such that the system remains in the trans-Hill state. Trans-Hill growth is crucial for determining the efficiency of growing big bodies, as well as their growth timescale and size spectrum. Trans-Hill growth has two sub-stages. In the earlier one, which occurs while the stirring bodies remain sufficiently small, the evolution is collisionless, i.e., collisional cooling among all bodies is irrelevant. The efficiency of forming big bodies in this collisionless sub-stage is very low, ~10α  1, where α ~ 0.005(a/AU)^–1 is the ratio between the physical size of a body and its Hill radius. Furthermore, the size spectrum is flat (equal mass per size decade, i.e., q = 4). This collisionless trans-Hill solution explains results from previous coagulation simulations for both the Kuiper Belt and the asteroid belt. The second trans-Hill sub-stage commences once the stirring bodies grow big enough (>α^–1 × the size of the accreted small bodies). After that time, collisional cooling among small bodies controls the evolution. The efficiency of forming big bodies rises and the size spectrum becomes more top heavy. Trans-Hill growth can terminate in one of two ways, depending on the sizes of the small bodies. First, mutual accretion of big bodies can become significant and conglomeration proceeds until half of the total mass is converted into big bodies. This mode of growth may explain the observed size distributions of small bodies in the solar system and is explored in our subsequent work. Second, if the big bodies’ orbits become separated by their Hill radius, oligarchy commences. This mode likely precedes the formation of fully fledged planets.

Reference
Lithwick Y (in press) After Runaway: The Trans-Hill Stage of Planetesimal Growth. The Astrophysical Journal 780:22.
[doi:10.1088/0004-637X/780/1/22]

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