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.

Laboratoire de Minéralogie et de Cosmochimie du Muséum, Muséum National d’Histoire Naturelle, 57 rue Cuvier, Paris, 75005, France

The isotopic ratio of hydrogen was measured in an iron meteorite and terrestrial native iron using an IMS 3f ion microprobe. The extraterrestrial D/H ratio (93 ± 9 × 10^-6) was close to the terrestrial value (105 ± 6 × 10^-6), and both samples had low H concentrations (7 ± 4 and 33 ± 11 ng g^-1 for the iron meteorite and the terrestrial sample, respectively). Experiments on artificially D-enriched samples showed that the measured hydrogen signal is a combination of indigenous H and terrestrial atmospheric contamination. This contamination comes from the isotope exchange reaction between water adsorbed on the sample surface and atmospheric water, and would be continuously added to the indigenous H in the ion crater by the adsorbed water sinking into the crater during sputtering. Experiments showed that this contamination represents up to 20% of the signal but was within the uncertainty of the measured D/H ratio.