Ureilites are a group of ultramafic achondrites commonly thought to be residues of partial melting on a carbon-rich asteroid. They show a large variation in FeO content (olivine Fo values ranging from ~74 to 95) that cannot be due to igneous fractionation and suggests instead variation in oxidation state. The presence of chromite in only a few of the most ferroan (Fo 75-76) samples appears to support such a model. MicroXANES analyses were used in this study to determine the valence states of Cr (previously unknown) in olivine cores of eleven (11) main group ureilites. The goal of this work was to use a method that is independent of Fo to determine the oxidation conditions under which ureilites formed, in order to evaluate whether the ureilite FeO-variation is correlated with oxidation state, and whether it is nebular or planetary in origin. Two of the analyzed samples, LEW 88774 (Fo 74.2) and NWA 766 (Fo 76.7) contain primary chromite; two others, LAP 03587 (Fo 74.4) and CMS 04048 (Fo 76.2) contain sub-micrometer-sized exsolutions of chromite + Ca-rich pyroxene in olivine; and one, EET 96328 (Fo 85.2) contains an unusual chromite grain of uncertain origin. No chromite has been observed in the remaining six samples (Fo 77.4-92.3).
Chromium in olivine in all eleven samples was found to be dominated by the divalent species, with valences ranging from 2.10 ± 0.02 (1σ) to 2.46 ± 0.04. The non-chromite-bearing ureilites have the most reduced Cr, with a weighted mean valence of 2.12 ± 0.01, i.e., Cr²⁺/Cr³⁺ = 7.33. All low-Fo chromite-bearing ureilites have more oxidized Cr, with valences ranging from 2.22 ± 0.03 to 2.46 ± 0.04. EET 96328, whose chromite grain we interpret as a late-crystallizing phase, yielded a reduced Cr valence of 2.15 ± 0.07, similar to the non-chromite-bearing samples. Based on the measured Cr valences, magmatic (1200-1300°C) oxygen fugacities (fO₂) of the non-chromite-bearing samples were estimated to be in the range IW-1.9 to IW-2.8 (assuming basaltic melt composition), consistent with fO₂ values obtained by assuming olivine-silica-iron metal (OSI) equilibrium. For the primary chromite-bearing-ureilites, the corresponding fO₂ were estimated (again, assuming basaltic melt composition) to be ~IW to IW+1.0, i.e., several orders of magnitude more oxidizing than the conditions estimated for the chromite-free ureilites. In terms of Fo and Cr valence properties, ureilites appear to form two groups rather than a single “Cr-valence (or fO₂) vs. Fo” trend. The chromite-bearing ureilites show little variation in Fo (~74-76) but significant variation in Cr valence, while the non-chromite-bearing ureilites show significant variation in Fo (~77-95) and little variation in Cr valence. These groups are unrelated to petrologic type (i.e., olivine-pigeonite, olivine-orthopyroxene, or augite-bearing). The chromite-bearing ureilites also have lower contents of Cr in olivine than most non-chromite-bearing ureilites, consistent with predictions based on Cr olivine/melt partitioning in spinel saturated vs. non-spinel-saturated systems.
Under the assumption that at magmatic temperatures graphite-gas equilibria controlled fO₂ at all depths on the ureilite parent body, we conclude: 1) that ureilite precursor materials having the Fo and Cr valence properties now observed in ureilites are unlikely to have been preserved during planetary processing; and 2) that the Fo and Cr valence properties now observed in ureilites are consistent with having been established by high-temperature carbon redox control over a range of depths on a plausible-sized ureilite parent body. The apparent limit on ureilite Fo values around 74-76 suggests that the precursor material(s) had bulk mg# ≥ that of LL chondrites.

Sandra Pizzarello^a,*, Stephen K. Davidowski^a, Gregory P. Holland^a and Lynda B. Williams^b

^aDepartment of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604
^bSchool of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404

Reference
Pizzarello S, Davidowski SK, Holland GP and Williams LB (2013) Processing of meteoritic organic materials as a possible analog of early molecular evolution in planetary environments. PNAS 110:15614-15619.
[doi:10.1073/pnas.1309113110]

Link to Article

Jan D. Kramers^a,∗, Marco A.G. Andreoli^b,c, Maria Atanasova^d, Georgy A. Belyanin^a, David L. Block^e, Chris Franklyn^b, Chris Harris^f, Mpho Lekgoathi^b, Charles S. Montross^g, Tshepo Ntsoane^b, Vittoria Pischedda^h, Patience Segonyane^b, K.S. (Fanus) Viljoen^a, Johan E. Westraadt^g

^aDepartment of Geology, University of Johannesburg, Auckland Park 2006, South Africa
^bNECSA, PO Box 582, Pretoria 0001, South Africa
^cSchool of Geosciences, University of the Witwatersrand, PO Box 3, Wits 2050, South Africa
^dCouncil for Geoscience, PO Box 112, Pretoria 0001, South Africa
^eAECI and AVENG Cosmic Dust Laboratory, School of Computational and Applied Mathematics, University of the Witwatersrand, PO Box 60, Wits 2050, South Africa
^fDepartment of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa
^gElement Six (Pty) Ltd, Springs 1559, South Africa
^hLPMCN, Université Lyon 1 and CNRS, UMR 5586, F-69622 Villeurbanne, France

We have studied a small, very unusual stone, here named “Hypatia”, found in the area of southwest Egypt where an extreme surface heating event produced the Libyan Desert Glass 28.5 million years ago. It is angular, black, shiny, extremely hard and intensely fractured. We report on exploratory work including X-ray diffraction, Raman spectroscopy, transmission electron microscopy, scanning electron microscopy with EDS analysis, deuteron nuclear reaction analysis, C-isotope and noble gas analyses. Carbon is the dominant element in Hypatia, with heterogeneous O/C and N/C ratios ranging from 0.3 to 0.5 and from 0.007 to 0.02, respectively. The major cations of silicates add up to less than 5%. The stone consists chiefly of apparently amorphous, but very hard carbonaceous matter, in which patches of sub-μm diamonds occur. δ¹³C values (ca. 0‰) exclude an origin from shocked terrestrial coal or any variety of terrestrial diamond. They are also higher than the values for carbonaceous chondrites but fall within the wide range for interplanetary dust particles and comet 81P/Wild2 dust. In step heating, ⁴⁰Ar/³⁶Ar ratios vary from 40 to the air value (298), interpreted as a variable mixture of extraterrestrial and atmospheric Ar. Isotope data of Ne, Kr and Xe reveal the exotic noble gas components G and P3 that are normally hosted in presolar SiC and nanodiamonds, while the most common trapped noble gas component of chondritic meteorites, Q, appears to be absent. An origin remote from the asteroid belt can account for these features.
We propose that the Hypatia stone is a remnant of a cometary nucleus fragment that impacted after incorporating gases from the atmosphere. Its co-occurrence with Libyan Desert Glass suggests that this fragment could have been part of a bolide that broke up and exploded in the airburst that formed the Glass. Its extraordinary preservation would be due to its shock-transformation into a weathering-resistant assemblage.

Reference
Kramers JD, Andreoli MAG, Atanasova M, Belyanin GA, Block DL, Franklyn C, Harris C, Lekgoathi M, Montross CS Ntsoane T, Pischedda V, Segonyane P, Viljoen KS and Westraadt JE (2013) Unique chemistry of a diamond-bearing pebble from the Libyan Desert Glass strewnfield, SW Egypt: Evidence for a shocked comet fragment. Earth and Planetary Science Letters 382:21-31.
[doi:10.1016/j.epsl.2013.09.003]
Copyright Elsevier

Link to Article

A. Collette^a,∗, Z. Sternovsky^a,b, M. Horanyi^a,c

^aColorado Center for Lunar Dust and Atmospheric Studies, LASP, University of Colorado at Boulder, Boulder, Colorado, USA
^bAerospace Engineering Sciences, University of Colorado at Boulder, Boulder, Colorado, USA
^cDepartment of Physics, University of Colorado at Boulder, Boulder, Colorado, USA

^aDepartment of the Geophysical Sciences, The University of Chicago, Chicago, IL, United States
^bChicago Center for Cosmochemistry, The University of Chicago, Chicago, IL, United States
^cEnrico Fermi Institute, The University of Chicago, Chicago, IL, United States
^dTrent University, Peterborough, ON, Canada
^eSchool of Earth and Space Exploration, Arizona State University, Tempe, AZ, United States
^fDepartment of Geoscience, University of Wisconsin, Madison, WI, United States
¹Present address: Department of Geological Sciences, University of Cape Town, South Africa.

Variable excesses of ³⁶S have previously been reported in sodalite in the Allende and Ningqiang meteorites and used to infer the presence of ³⁶Cl in the early solar system. Until now no unambiguous evidence of the major decay product, ³⁶Ar (98%), has been found. Using low fluence fast neutron activation we have measured small amounts of ³⁶Ar in the Allende sodalite Pink Angel, corresponding to ³⁶Cl/³⁵Cl = (1.9 ± 0.5) × 10^-8. This is a factor of 200 lower than the highest value inferred from ³⁶S excesses in sodalite. High resolution I–Xe analyses confirm that the sodalite formed between 4561 and 4558 Ma ago. The core of Pink Angel sodalite yielded a precise formation age of 4559.4 ± 0.6 Ma. Deposition of sodalite containing live ³⁶Cl, seven million years or so after the formation of the CAI, appears to require a local production mechanism involving intense neutron irradiation within the solar nebula. The constraint imposed by the near absence of neutron induced ¹²⁸Xe is most easily satisfied if the ³⁶Cl were produced in a fluid precursor of the sodalite. The low level of ³⁶Ar could be accounted for as a result of residual in-situ ³⁶Cl decay, up to 1–2 Ma after formation of the sodalite, and/or later diffusive loss, in line with the low activation energy for Ar diffusion in sodalite.