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.

Conel M.O’D. Alexander^a,*, Kieren T. Howard^b, Roxane Bowden^c and Marilyn L. Fogel^caDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington, DC 20015, USA
^bKingsborough Community College of the City University of New York (CUNY), 2001 Oriental Blvd., Brooklyn, NY 11235, USA
^cGeophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington, DC 20015, USA

Yangting Lin^1*, Sen Hu¹, Bingkui Miao², Lin Xu³, Yu Liu¹, Liewen Xie¹, Lu Feng¹, and Jing Yang¹

¹Key Laboratory of the Earth’s Deep Interior, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
²Department of Resources & Environmental Engineering, Guilin University of Technology, Guilin 541004, China
³National Astronomical Observatories, Chinese Academy of Sciences, Beijing, China

^aGéosciences Environnement Toulouse, CNRS Université de Toulouse – IRD, 14 avenue Edouard Belin, 31400 Toulouse, France
^bDepartment of Geology – Natural History Building, University of Illinois at Urbana-Champaign, 1301 W. Green Street, 61801 Urbana, IL, USA
^cInstitut de Recherche en Astrophysique et Planétologie, CNRS – Université de Toulouse, 14 avenue Edouard Belin, 31400 Toulouse, France
^dInstituto de Geociencias, Universidad Austral de Chile, Casilla 567, Valdivia, Chile
^eLaboratoire de Géologie de Lyon: Terre, Planètes, Environnement, CNRS, ENS de Lyon, Université Lyon 1, 46 allée d’Italie, 69364 Lyon, France
^fDepartment of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK
^gDepartment of Mineralogy, The Natural History Museum, London SW7 5BD, UK

^aUniversity of Texas at San Antonio, San Antonio, TX 78249, United States
^bSouthwest Research Institute, San Antonio, TX 78238, United States

The contribution of the meteoroid population to the generation of Mercury’s exosphere is analyzed to determine which segment contributes most greatly to exospheric refilling via the process of meteoritic impact vaporization. For the meteoroid data, a differential mass distribution based on work by Grün et al. [1985] and a differential velocity distribution based on the work of Zook [1975] is used. These distributions are then evaluated using the method employed by Cintala [1992] to determine impact rates for selected mass and velocity segments of the meteoroid population.
The amount of vapor created by a single meteor impact is determined by using the framework created by Berezhnoy & Klumov [2008]. By combining the impact rate of meteoroids with the amount of vapor a single such impact creates, we derive the total vapor production rate which that meteoroid mass segment contributes to the Herman exosphere. It is shown that meteoroids with a mass of 2.1 × 10⁻⁴ g release the largest amount of vapor into Mercury’s exosphere. For meteoroids in the mass range of 10⁻¹⁸ g to 10 g, 90% of all the vapor produced is due to impacts by meteoroids in the mass range 4.2 × 10⁻⁷ g ≤ m ≤ 8.3×10⁻² g.

^aSchool of Geographical and Earth Sciences, University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow G12 8QQ, UK
^bPlanetary & Space Sciences, The Open University, Milton Keynes MK7 6AA, UK

The CM2 carbonaceous chondrite LON 94101 contains aragonite and two generations of calcite that provide snapshots of the chemical and isotopic evolution of aqueous solutions during parent body alteration. Aragonite was the first carbonate to crystallize. It is rare, heterogeneously distributed within the meteorite matrix, and its mean oxygen isotope values are δ¹⁸O 39.9 ± 0.6‰, Δ¹⁷O -0.3 ± 1.0‰ (1σ). Calcite precipitated soon afterwards, and following a fall in solution Mg/Ca ratios, to produce small equant grains with a mean oxygen isotope value of δ¹⁸O 37.5 ± 0.7‰, Δ¹⁷O 1.4 ± 1.1‰ (1σ). These grains were partially or completely replaced by serpentine and tochilinite prior to precipitation of the second generation of calcite, which occluded an open fracture to form a millimetre-sized vein, and replaced anhydrous silicates within chondrules and the matrix. The vein calcite has a mean composition of δ¹⁸O 18.4 ± 0.3‰, Δ¹⁷O -0.5 ± 0.5‰ (1σ). Petrographic and isotopic results therefore reveal two discrete episodes of mineralisation that produced calcite generations with contrasting δ¹⁸O, and mean Δ¹⁷O values. The aragonite and equant calcite crystallized over a relatively brief period early in the aqueous alteration history of the parent body, and from static fluids that were evolving chemically in response to mineral dissolution and precipitation. The second calcite generation crystallized from solutions of a lower Δ¹⁷O, and a lower δ¹⁸O and/or higher temperature. As two generations of calcite whose petrographic characteristics and oxygen isotopic compositions are similar to those in LON 94101 occur in at least one other CM2, multiphase carbonate mineralisation could be the typical outcome of the sequence of chemical reactions during parent body aqueous alteration. It is equally possible however that the second generation of calcite formed in response to an event such as impact fracturing and concomitant fluid mobilisation that affected a large region of the common parent body of several CM2 meteorites. These findings show that integrated petrographic, chemical and isotopic studies can provide new insights into the mechanisms of parent body alteration including the spatial and temporal dynamics of the aqueous system.