D.J. Cherniak^a and J.A. Van Orman^b

^aDepartment of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
^bDepartment of Earth, Environmental and Planetary Sciences, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, USA

Diffusion of tungsten has been characterized in synthetic forsterite and natural olivine (Fo₉₀) under dry conditions. The source of diffusant was a mixture of magnesium tungstate and olivine powders. Experiments were prepared by sealing the source material and polished olivine under vacuum in silica glass ampoules with solid buffers to buffer at NNO or IW. Prepared capsules were annealed in 1 atm furnaces for times ranging from 45 minutes to several weeks, at temperatures from 1050 to 1450°C. Tungsten distributions in the olivine were profiled by Rutherford Backscattering Spectrometry (RBS).
The following Arrhenius relation is obtained for W diffusion in forsterite:
D_W=1.0×10-8exp(-365±28kJ mol^-1/RT)m²sec^-1
Diffusivities for the synthetic forsterite and natural Fe-bearing olivine are similar, and tungsten diffusion in olivine shows little dependence on crystallographic orientation or oxygen fugacity.
The slow diffusivities measured for W in olivine indicate that Hf-W ages in olivine-metal systems will close to diffusive exchange at higher temperatures than other chronometers commonly used in cosmochronology, and that tungsten isotopic signatures will be less likely to be reset by subsequent thermal events.

Reference
Cherniak DJ and Van Orman JA (in press) Tungsten Diffusion in Olivine. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.12.020]
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E. Bowell¹, D. A. Oszkiewicz^2,3,*, L. H. Wasserman¹, K. Muinonen^2,4, A. Penttilä², D. E. Trilling⁵

¹Lowell Observatory, Flagstaff, Arizona, USA
²Department of Physics, University of Helsinki, Helsinki, Finland
³Institute Astronomical Observatory, Faculty of Physics, Adam Mickiewicz University, Poznań, Poland
⁴Finnish Geodetic Institute, Masala, Finland

By analyzing brightness variation with ecliptic longitude and using the Lowell Observatory photometric database, we estimate spin-axis longitudes for more than 350,000 asteroids. Hitherto, spin-axis longitude estimates have been made for fewer than 200 asteroids. We investigate longitude distributions in different dynamical groups and asteroid families. We show that asteroid spin-axis longitudes are not isotropically distributed as previously considered. We find that the spin-axis longitude distribution for Main Belt asteroids is clearly nonrandom, with an excess of longitudes from the interval 30°–110° and a paucity between 120° and 180°. The explanation of the nonisotropic distribution is unknown at this point. Further studies have to be conducted to determine if the shape of the distribution can be explained by observational bias, selection effects, a real physical process, or other mechanism.

Reference
Bowell E, Oszkiewicz DA, Wasserman LH, Muinonen K, Penttilä A and Trilling DE (in press) Asteroid spin-axis longitudes from the Lowell Observatory database. Meteoritics & Planetary Science
[doi:10.1111/maps.12230]
Published by arrangement with John Wiley & Sons

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Joshua L. Bandfield^a, Eugenie Song^b, Paul O. Hayne^c, Brittany D. Brand^d, Rebecca R. Ghent^e, Ashwin R. Vasavada^c, David A. Paige^f

^aSpace Science Institute
^bHawai’i Institute of Geophysics and Planetology, University of Hawai’i
^cJet Propulsion Laboratory, California Institute of Technology
^dDepartment of Geosciences, Boise State University
^eDepartment of Geology, University of Toronto
^fEarth and Space Sciences, UCLA

Systematic temperature mapping and high resolution images reveal a previously unrecognized class of small, fresh lunar craters. These craters are distinguished by near-crater deposits with evidence for lateral, ground-hugging transport. More distal, highly insulating surfaces surround these craters and do not show evidence of either significant deposition of new material or erosion of the substrate. The near-crater deposits can be explained by a laterally propagating granular flow created by impact in the lunar vacuum environment. Further from the source crater, at distances of ∼10–100 crater radii, the upper few to 10’s of centimeters of regolith appear to have been “fluffed-up” without the accumulation of significant ejecta material. These properties appear to be common to all impacts, but quickly degrade in the lunar space weathering environment. Cratering in the vacuum environment involves a previously unrecognized set of processes that leave prominent, but ephemeral, features on the lunar surface.

Reference
Bandfield JL, Song E, Hayne PO, Brand BD, Ghent RR, Vasavad AR and Paige DA (in press) Lunar cold spots: Granular flow features and extensive insulating materials surrounding young craters. Icarus
[doi:10.1016/j.icarus.2013.12.017]
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James F.J. Bryson^a, Nathan S. Church^a, Takeshi Kasama^b, Richard J. Harrison^a

^aDepartment of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
^bCenter for Electron Nanoscopy, Technical University of Denmark, Kongens Lyngby, Denmark

Nanoscale intergrowths unique to the cloudy zones (CZs) of meteoritic metal display novel magnetic behaviour with the potential to reveal new insight into the early development of magnetic fields on protoplanetary bodies. The nanomagnetic state of the CZ within the Tazewell IIICD iron meteorite has been imaged using off-axis electron holography. The CZ is revealed to be a natural nanocomposite of magnetically hard islands of tetrataenite (ordered FeNi) embedded in a magnetically soft matrix of ordered Fe₃Ni. In the remanent state, each tetrataenite island acts as a uniaxial single domain particle with its [001] magnetic easy axis oriented along one of three 〈100〉 crystallographic directions of the parent taenite phase. Micromagnetic simulations demonstrate that switching occurs via the nucleation and propagation of domain walls through individual tetrataenite particles. The switching field (H_s) varies with the length scale of the matrix phase (L_m), with H_s > 1 T for L_m ∼10 nm (approaching the intrinsic switching field for isolated single domain tetrataenite) and 0.2<H_s<0.6 T for L_m ∼30 nm. The reduction in H_s with increasing L_c is caused by exchange coupling between the hard tetrataenite islands and the soft magnetic matrix, which lowers the critical field for domain wall nucleation, providing an explanation for previously observed coercivity variations throughout the CZ. Non-random distributions of the tetrataenite easy axes are observed locally throughout the CZ, suggesting a magnetic field could have been present during nanostructure formation. This observation demonstrates the potential for stable chemical transformation remanent magnetisation to be encoded by the nanostructure, with variations in the proportions of the six possible magnetisation states reflecting the intensity and relative direction of the magnetic fields present during cooling. According to recent cooling models, the cooling rate of meteoritic metal originating near the surface of differentiated planetesimals was such that the magnetic signal across the CZ could potentially record dynamo field intensity and direction variations over time (10–100 Ma), which would enable events such as magnetic reversals and the decay of an asteroid dynamo to be observed.

Reference
Bryson JFJ, Church NS, Kasama T and Harrison RJ (2014) Nanomagnetic intergrowths in Fe–Ni meteoritic metal: The potential for time-resolved records of planetesimal dynamo fields. Earth and Planetary Science Letters 388:237–248.
[doi:10.1016/j.epsl.2013.12.004]
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