Rhian H. Jones^a, Francis M. McCubbin^a,b, Linda Dreeland^a, Yunbin Guan^c, Paul V. Burger^a,b Charles K. Shearer^a,b

^aDepartment of Earth and Planetary Sciences, MSC03 2040, University of New Mexico, Albuquerque, NM 87131, U.S.A.
^bInstitute of Meteoritics, MSC03 2050, University of New Mexico, Albuquerque, NM 87131, U.S.A.
^cDivision of Geological and Planetary Sciences, California Institute of Technology, MC 170-25, Pasadena, CA 91125, U. S. A.

Ordinary chondrites contain two phosphate minerals, merrillite and chlorapatite, both of which are secondary minerals that developed in response to metamorphism on the chondrite parent bodies. We have studied the phosphate mineralogy of four LL chondrites, of petrologic types 3.9 to 6, in order to determine the petrogenesis of the two minerals and interpret the conditions under which they formed. Characterization of merrillite and apatite includes textural observations, mineral compositions determined by electron probe microanalysis, and ion microprobe analyses of trace element and volatile anion elemental abundances. Initial formation of phosphate minerals during mild metamorphism, to petrologic type 4 conditions, resulted in oxidation of P that was originally incorporated in metal, and growth of merrillite as inclusions within metal grains. Subsequent development of both phosphate minerals occurred in response to diffusional equilibration, possible precipitation from fluids as well as replacement reactions resulting from interactions with fluids. Porosity and vein-filling textures in both merrillite and chlorapatite, as well as textures indicating replacement of merrillite by chlorapatite, support a model in which fluid played a significant role and suggest an interface-coupled dissolution-reprecipitation mechanism during metasomatism. Some associations of phosphate minerals with chromite-plagioclase assemblages suggest that phosphate minerals could also be related to impact processes, either as precipitation from an impact melt or as a result of interactions with a fluid or vapor derived from an impact melt. Fluid compositions may have been water-bearing initially, at low temperatures of metamorphism, but later evolved to become halogen-rich and very dry. Late-stage halogen-rich fluids that dominated during cooling of LL chondrite material may have been derived from vaporization of partial melts in the interior of the parent body. Overall, the LL chondrite parent body underwent a complex chemical evolution, in which metasomatism played a significant role.

Reference
Jones RH, McCubbin FM, Dreeland L, Guan Y, Burger PV and Shearer CK (in press) Phosphate Minerals in LL Chondrites: A Record of the Action of Fluids During Metamorphism on Ordinary Chondrite Parent Bodies. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.01.027]
Copyright Elsevier

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T. Kneissl^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aFreie Universitaet Berlin, Berlin, Germany

A variety of geologic landforms and features are observed within Quadrangle Av-13 Tuccia in the southern hemisphere of Vesta. The quadrangle covers parts of the highland Vestalia Terra as well as the floors of the large Rheasilvia and Veneneia impact basins, which results in a substantial elevation difference of more than 40 km between the northern and the southern portions of the quadrangle. Measurements of crater size-frequency distributions within and surrounding the Rheasilvia basin indicate that gravity-driven mass wasting in the interior of the basin has been important, and that the basin has a more ancient formation age than would be expected from the crater density on the basin floor alone. Subsequent to its formation, Rheasilvia was superimposed by several mid-sized impact craters. The most prominent craters are Tuccia, Eusebia, Vibidia, Galeria, and Antonia, whose geology and formation ages are investigated in detail in this work. These impact structures provide a variety of morphologies indicating different sorts of subsequent impact-related or gravity-driven mass wasting processes. Understanding the geologic history of the relatively young craters in the Rheasilvia basin is important in order to understand the even more degraded craters in other regions of Vesta.

Reference
Kneissl et al. (in press) Morphology and Formation Ages of Mid-Sized Post-Rheasilvia Craters – Geology of Quadrangle Tuccia, Vesta. Icarus
[doi:10.1016/j.icarus.2014.02.012]
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B. W. Grefenstette et al. (>10)*
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Cahill Center for Astrophysics, 1216 East California Boulevard, California Institute of Technology, Pasadena, California 91125, USA

We do not have a copyright agreement with Nature, and therefore the abstract is currently only available from the link below.

Reference
Grefenstette et al. (2014) Asymmetries in core-collapse supernovae from maps of radioactive 44Ti in Cassiopeia A. Nature 506, 339–342.
[doi:10.1038/nature12997]

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J. Martin Laming

Space Science Division, Naval Research Laboratory, Washington DC 20375, USA.

We do not have a copyright agreement with Nature, and therefore the abstract is currently only available from the link below.

Reference
Laming JM (2014) Astrophysics: Lopsided stellar death. Nature 506, 298–299.
[doi:10.1038/506298a]

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Alexandra Witze

We do not have a copyright agreement with Nature, and therefore the abstract is currently only available from the link below.

Reference
Witze A (2014) Astronomy: Death of a comet. Nature 506, 281–283.
[doi:10.1038/506281a]

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