Paul H. Warrena, Alan E. Rubina, Junko Isaa, Nicholas Gesslerb, Insu Ahnc, Byeon-Gak Choic

^aInstitute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA
^bInformation Science & Information Studies Program, Duke University, Durham, NC 27708 USA
^cEarth Science Education, Seoul National University, Seoul 151-748, Korea

The Northwest Africa 5738 eucrite contains a record of unprecedented geochemical complexity for a sample from the HED asteroid. It originated with a uniquely evolved (Stannern Trend) primary igneous composition, combining ultra-high bulk incompatible element and Na₂O concentrations with a relatively low mg. Its bulk oxygen-isotopic composition (Δ’¹⁷O = – 0.27 ‰), as well as its trace element composition (e.g., Ga/Al), confirm other evidence for classification as a eucrite. Pyroxene mg equilibration, exsolution and “cloudy” inclusions, all reflect a typical eucritic degree of thermal metamorphism. The rock contains an unprecedented array of microscopic fluid-metasomatic vein deposits. Most common are curvy microveins within pyroxene, which consist dominantly of Ca-plagioclase (typically An₉₅, in stark contrast with the rock’s An_68-78 primary-igneous plagioclase), with Fe-olivine (Fo₁₄) and Cr-spinel as additional major constituents. Likely related to these microveins are small masses of intergrown Ca-plagioclase (again roughly An₉₅) and silica (or high-Si glass). Analyses of the microvein Cr-spinels show stoichiometry implying a significant Fe³⁺ content (Fe₂O₃0.7-2.3 wt%), and fO₂ up to roughly IW+3; clearly elevated in comparison to the normal HED fO₂ of about IW-1. The fO₂ results show an anticorrelation with equilibration T (and with Mg/Fe), which suggests the parent fluid system became more oxidizing as it cooled. NWA 5738 also contains apparent secondary iron metal. The Fe-metals are very pure, with Ni consistently below an EPMA detection limit of ∼0.01 wt%. The vein-like shapes of roughly 1/3 of the largest Fe-metals suggest origin by deposition from a fluid. The role of pyroxene exsolution as template for a denticular (sawtooth) Fe-metal edge shape, and the survival of Fo₁₄ olivine in a rock with abundant silica and a far higher bulk mg, suggest that the most intense thermal metamorphism occurred no later than the secondary alteration. Near-complete lack of spatial association suggests that the Fe-metals formed during a distinct time period from the curvy microveins. The immediate cause of Fe-metal deposition was most plausibly (or anyway, least implausibly) an abrupt downshift in the fluid fO₂. Considering the extremely evolved bulk composition, the fluid(s) may have been largely deuteric. However, more likely the main source of fluid was a nearby buried mass of volatile-rich impactor matter, such as carbonaceous chondrite, that hit the asteroid at low enough velocity to remain mostly intact. We further speculate that the abrupt drop in fluid fO₂ may have been caused by a process of carbon-fueled “smelting” (cf. ureilites), triggered by an impact-effected shift of the carbonaceous material to a changed environment, with higher Tand/or lower P. These and other recent eucrite results point to a need for greater scrutiny regarding the absence of comparable alteration-veining in rocks from the lunar highland crust, a mysterious lack in view of recent evidence for abundant lunar water.

Reference
Warren PH, Rubin AE, Isa J, Gessler N, Ahn I and Choi B-G (in press) Northwest Africa 5738: Multistage fluid-driven secondary alteration in an extraordinarily evolved eucrite. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.06.008]
Copyright Elsevier

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Derek W. G. Sears

NASA Ames Research Center/Bay Area Environmental Research Institute, Space Science and Astrobiology Division, Mountain View, California, USA

Vagn Buchwald (Fig. 1) was born in Copenhagen where he attended school and college. Then after 18 months of military service, he assumed a position at the Technical University of Copenhagen. A few years later, he was presented with a piece of the Cape York meteorite, which led to an interest in iron meteorites. Through a campaign of informed searching, Vagn found the 20 ton Agpalilik meteorite (part of the Cape York shower) on 31st July 1963 and by September 1967 had arranged its transport to Copenhagen. After sorting and describing the Danish collection, which included application of the Fe-Ni-P phase diagram to iron meteorite mineralogy, Vagn was invited to sort and describe other iron meteorite collections. This led to a 7 yr project to write his monumental Handbook of Iron Meteorites. Vagn spent 3 yr in the United States and visited most of the world’s museums, the visit to Berlin being especially important since the war had left their iron meteorites in bad condition and without labels. During a further decade or more of iron meteorite research, he documented natural and anthropomorphic alterations experienced by iron meteorites, discovered five new minerals (roaldite, carlsbergite, akaganeite, hibbingite, and arupite); had a mineral (buchwaldite, NaCaPO₄) and asteroid (3209 Buchwald 1982 BL1) named after him; and led expeditions to Chile, Namibia, and South Africa in search of iron meteorites and information on them. Vagn then turned his attention to archeological metal artifacts. This work resulted in many papers and culminated in two major books on the subject published in 2005 and 2008, after his retirement in 1998. Vagn Buchwald has received numerous Scandinavian awards and honors, and served as president of the Meteoritical Society in 1981–1982.

Reference
Sears DWG (in press) Oral histories in meteoritics and planetary science—XXV: Vagn F. Buchwald. Meteoritics & Planetary Science
[doi:10.1111/maps.12332]
Published by arrangement with John Wiley & Sons

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Danielle Piskorz and David J. Stevenson

Division of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125

The giant impact hypothesis for the origin of the Moon points to the creation of a hot, young moon, likely with a global magma ocean. Such a magma ocean would produce a flotation crust of plagioclase crystals, or an anorthositic crust. Early calculations of the expected anorthosite content of the lunar highlands crust did not match initial measurements of Apollo samples, and more recently have not matched Clementine measurements or SELENE measurements, because they assumed interstitial melt would freeze. We consider a physical model of a magma ocean with an accumulating flotation lid, and find that significant escape of melt can take place for reasonable physical parameters and timescales of melt migration, even allowing for the inhibiting effect of plausible compaction viscosities. This model permits more nearly pure lunar anorthosites, consistent with observations. The model encounters some difficulty in explaining the expulsion of melt for the near-surface crust (up to five-kilometers’ depth) that presumably dominates the Apollo samples, suggesting that impact mixing and tidal heating are needed to explain the discrepancy.

Reference
Piskorz D and Stevenson DJ (in press) The Formation of Pure Anorthosite on the Moon. Icarus
[doi:10.1016/j.icarus.2014.06.015]
Copyright Elsevier

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M. Mommert¹, D. Farnocchia², J. L. Hora³, S. R. Chesley², D. E. Trilling¹, P. W. Chodas², M. Mueller⁴, A. W. Harris⁵, H. A. Smith³ and G. G. Fazio³

¹Department of Physics and Astronomy, Northern Arizona University, P.O. Box 6010, Flagstaff, AZ 86011, USA
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
³Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 65, Cambridge, MA 02138-1516, USA
⁴SRON Netherlands Institute for Space Research, Postbus 800, 9700-AV Groningen, The Netherlands
⁵DLR Institute of Planetary Research, Rutherfordstrasse 2, D-12489 Berlin, Germany

We report on observations of near-Earth asteroid 2011 MD with the Spitzer Space Telescope. We have spent 19.9 hr of observing time with channel 2 (4.5 μm) of the Infrared Array Camera and detected the target within the 2σ positional uncertainty ellipse. Using an asteroid thermophysical model and a model of nongravitational forces acting upon the object, we constrain the physical properties of 2011 MD, based on the measured flux density and available astrometry data. We estimate 2011 MD to be (6) m in diameter with a geometric albedo of 0.3 (uncertainties are 1σ). We find the asteroid’s most probable bulk density to be (1.1) g cm^-3, which implies a total mass of (50–350) t and a macroporosity of ≥65%, assuming a material bulk density typical of non-primitive meteorite materials. A high degree of macroporosity suggests that 2011 MD is a rubble-pile asteroid, the rotation of which is more likely to be retrograde than prograde.

Reference
Mommert M, Farnocchia D, Hora JL, Chesley SR, Trilling DE, Chodas PW, Mueller M, Harris AW, Smith HA and Fazio GG (in press) Physical properties of near-Earth asteroid 2011 MD. The Astrophysical Journal Letters 789:L22.
[doi:10.1088/2041-8205/789/1/L22]
Copyright Elsevier

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