Kathryn A. Dyl^a,b, James S. Cleverley^b, Phil A. Bland^a, Chris G. Ryan^b, Louise A. Fisher^b and Robert M. Hough^b

^aDepartment of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
^bCSIRO Earth Sciences and Resource Engineering, 26 Dick Perry Avenue, Kensington, Perth, WA 6151, Australia

We present the application of a new synchrotron-based technique for rapid mapping of trace element distributions across large areas of the CV3 meteorites Allende and Vigarano. This technique utilizes the Australian Synchrotron X-ray Fluorescence Microscopy (XFM) beam line with its custom designed and built X-ray detector array called Maia. XFM with Maia allows data to be collected using a 2 μm spot size at very low dwell times (~0.1-0.5 ms), resulting in maps of entire thin sections in ~5 hours. Maia is an energy dispersive detector system with a large collection solid-angle, which allows full spectral acquisition and high sensitivity. Hence, there is no need to constrain the elements of interest a priori.
We collected whole section maps (~2 cm x 1 cm) from 3 thick sections of Allende and a single map (2 cm x 1.5 cm) from a thick section of Vigarano. Our experimental conditions provide data for elements with 20 ⩽ Z ⩽ 40 (K-shell, Ca through Zr) and the L-emissions of Os, Ir, Pt, Au, and Pb. We illustrate the unique capabilities of this technique by presenting observations across myriad length scales, from the centimeter-scale down to the detection of sub-micrometer particles within these objects. Our initial results show the potential of this technique to help decipher spatial and textural variations in trace element chemistry between CAIs, chondrules, matrix, and other chondritic components. We also illustrate how these datasets can be applied to understanding both nebular and parent-body processes within meteorites.

Reference
Dyl KA, Cleverley JS, Bland PA, Ryan CG, Fisher LA and Hough RM (in press) Quantified, whole section trace element mapping of carbonaceous chondrites by Synchrotron X-ray fluorescence microscopy: 1. CV meteorites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.02.020]
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L.J. Hallis^a,b, H.A. Ishii^c, J.P. Bradley^c and G.J. Taylor^a,b

^aNASA Astrobiology Institute, Institute for Astronomy, University of Hawai’i, 2680 Woodlawn Drive, Honolulu, Hawaii 96822-1839, United States
^bHawai’i Institute of Geophysics and Planetology, Pacific Ocean Science and Technology (POST) Building, University of Hawai’i, 1680 East-West Road, Honolulu, HI 96822, United States
^cInstitute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, L-415, Livermore, CA 94550, United States

The nakhlite group of martian meteorites found in the Antarctic contain varying abundances of both martian and terrestrial secondary alteration phases. The aim of this study was to use transmission electron microscopy (TEM) to compare martian and terrestrial alteration embodied within a single nakhlite martian meteorite find – MIL 090032. Martian alteration veins in MIL 090032 are composed of poorly ordered Fe-smectite phyllosilicate. This poorly-ordered smectite appears to be equivalent to the nanocrystalline phyllosilicate/hydrated amorphous gel phase previously described in the martian alteration veins of other nakhlites. Chemical differences in this nanocrystalline phyllosilicate between different nakhlites imply localised alteration, which occurred close to the martian surface in MIL 090032. Both structurally and compositionally the nakhlite nanocrystalline phyllosilicate shows similarities to the amorphous/poorly ordered phase recently discovered in martian soil by the Mars Curiosity Rover at Rocknest, Gale Crater.
Terrestrially derived alteration phases in MIL 090032 include jarosite and gypsum, amorphous silicates, and Fe-oxides and hydroxides. Similarities between the mineralogy and chemistry of the MIL 090032 terrestrial and martian alteration phases suggest the alteration conditions on Mars were similar to those in the Antarctic. At both sites a small amount of fluid at low temperatures infiltrated the rock and became acidic as a result of the conversion of Fe²⁺ to Fe³⁺ under oxidising conditions.

Reference
Hallis LJ, Ishii HA, Bradley JP and Taylor GJ (in press) Transmission Electron Microscope Analyses of Alteration Phases in Martian Meteorite MIL090032. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.02.007]
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Tsuyoshi Iizuka^a,b, Yuri Amelin^b, Angela Kaltenbach^c, Piers Koefoed^b and Claudine H. Stirling^c

^aDepartment of Earth and Planetary Science, University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan
^bResearch School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
^cCentre for Trace Element Analysis and Department of Chemistry, University of Otago, PO Box 56, Union Place, Dunedin, New Zealand

Ibitira is an unbrecciated, equilibrated vesicular basaltic achondrite that is considered to have originated on a parent body distinct from all other known meteorites. We present the first combined high-precision U and Pb isotopic data for this unique meteorite. The ²³⁸U/²³⁵U value of 137.777 ± 0.013 determined for the whole rock is comparable to values determined for bulk chondrites and other basaltic achondrites. This value results in corrections of -1.1 Ma for Pb-Pb dates calculated using the previously assumed invariant ²³⁸U/²³⁵U value of 137.88. Using the determined ²³⁸U/²³⁵U value, the 7 most radiogenic Pb isotopic analyses for acid-leached pyroxene-rich and whole rock fractions yield an isochron Pb-Pb age of 4556.75 ± 0.57 Ma, in excellent agreement with the results of Mn-Cr chronology which give the ages of 4557.4 ± 2.5 Ma and 4555.9 ± 3.2 Ma using the U-corrected Pb-Pb age of D’Orbigny as a time anchor. Along with the previously proposed thermal history of Ibitira and our closure temperature estimates for Pb diffusion, the Pb-Pb age is interpreted as the timing of the last chemical equilibration and coarse pyroxene exsolution that occurred during high temperature metamorphism. The metamorphism may have been caused by burial of Ibitira lava under successive lava flows and, if so, the Pb-Pb age should post-date the crystallization by a short time interval. The Pb isotopic data for acid leachates suggest partial re-equilibration of Pb between plagioclase and phosphate, perhaps during an impact event at 4.49 Ga, as recorded by K-Ar systematics. The whole rock²³⁸U/²⁰⁴Pb indicates that compared to CI chondrites, Ibitira is less depleted in Pb than in some alkali elements despite a lower condensation temperature of Pb than the alkali elements. The restricted Pb depletion may reflect preferential concentration of metals with high fluid/melt partition coefficients including Pb and Zn as a result of fluid exsolution and migration within the parent magma. We discuss the implications of the U-Pb systematics for the origin and differentiation of the parent body.

Reference
Iizuka T, Amelin Y, Kaltenbach A, Koefoed P and Stirling CH (in press) U-Pb systematics of the unique achondrite Ibitira: Precise age determination and petrogenetic implications. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.02.017]
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Tetsuo Yamamoto¹, Toshihiko Kadono², and Koji Wada³

¹Center for Planetary Science, Integrated Research Center of Kobe University, Minatojima, Chuo-ku, Kobe 650-0047, Japan
²School of Medicine, University of Occupational and Environmental Health, Yahatanishi-ku, Kitakyushu, Fukuoka 807-8555, Japan
³Planetary Exploration Research Center, Chiba Institute of Technology, Tsudanuma 2-17-1, Chiba 275-0016, Japan

N-body simulations of collisions of dust aggregates in protoplanetary disks performed so far have revealed that silicate aggregates suffer from catastrophic disruption if the collision velocities are higher than about 10 m s^-1, which is much lower than those expected in the disks. This is mainly due to the low surface energy of the quartz used in the simulations. We find a simple relation between the surface energy and melting temperature for various materials including those of astrophysical interest, and show that the surface energy of the quartz used in the previous simulations is much lower than the present estimate. This result may provide a way out of the difficulty of growing silicate dust inside the snowline in disks. We show that silicate dust can evade catastrophic disruption and grow even at high-velocity collisions expected in the disks if one takes the present estimate of the surface energy into account.

Reference
Yamamoto T, Kadono T and Wada K (2014) An examination of collisional growth of silicate dust in protoplanetary disks. The Astrophysical Journal – Letters 784:L36.
[doi:10.1088/2041-8205/783/2/L36]

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