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.

¹State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China
²State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550002, China

In this study, a high-precision method for the determination of Sm and Nd concentrations and Nd isotopic composition in highly depleted ultramafic rocks without a preconcentration step is presented. The samples were first digested using the conventional HF + HNO₃ + HClO₄ method, followed by the complete digestion of chromite in the samples using HClO₄ at 190–200 °C and then complete dissolution of fluoride formed during the HF decomposition step using H₃BO₃. These steps ensured the complete digestion of the ultramafic rocks. The rare earth elements (REEs) were separated from the sample matrix using conventional cation-exchange chromatography; subsequently, Sm and Nd were separated using the LN columns. Neodymium isotopes were determined as NdO⁺, whereas Sm isotopes were measured as Sm⁺, both with very high sensitivity using single W filaments with TaF₅ as an ion emitter. Several highly depleted ultramafic rock reference materials including USGS DTS-1, DTS-2, DTS-2b, PCC-1 and GSJ JP-1, which contain extremely low amounts of Sm and Nd (down to sub ng g^-1 level), were analysed, and high-precision Sm and Nd concentration and Nd isotope data were obtained. This is the first report of the Sm-Nd isotopic compositions of these ultramafic rock reference materials except for PCC-1.

Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, BP 10448, F-63000 Clermont-Ferrand, France CNRS, UMR 6524, LMV, F-63038 Clermont-Ferrand, France
IRD, R 163, LMV, F-63038 Clermont-Ferrand, France

We carried out stepwise dissolutions of four primitive enstatite chondrites (EC) belonging to the EH subgroup. Large Nd isotope anomalies are found in the most refractory phases, dissolved using strong acids. Residues are characterized by excesses in ¹⁴²Nd and deficits in ¹⁴⁵Nd, ¹⁴⁸Nd and ¹⁵⁰Nd isotopes. The Nd anomalies measured in the ALHA77295 residue are even greater than those measured in the Murchison carbonaceous chondrite (CC) using a similar analytical technique (Qin et al., 2011). Once corrected for a common Sm/Nd evolution, the ¹⁴²Nd excess in the ALHA77295 residue is equal to 700 ppm relative to the terrestrial standard value. The Nd isotope patterns measured in EC and CC residues can be adjusted to coincide by adding a small amount of an s-process-rich carrier phase such as SiC and 0.075% is required to fit the ALHA7795 residue. Small isotope differences still persist between these residues even if they can be considered similar within error. In enstatite chondrites, residues have a deficit in ¹⁵⁰Nd similar to or smaller than that measured in ¹⁴⁸Nd, whereas in SiC extracted from carbonaceous chondrites or in whole rock, the deficit in ¹⁵⁰Nd is always greater than that in ¹⁴⁸Nd. Moreover in a binary ¹⁴²Nd–¹⁴⁸Nd diagram, the best-fit lines obtained for leachates and residues from carbonaceous chondrites and enstatite chondrites have slightly different slopes. For the same ¹⁴⁸Nd/¹⁴⁴Nd ratio, the anomalous component in an enstatite chondrite has a higher ¹⁴²Nd/¹⁴⁴Nd ratio compared to carbonaceous chondrites, a feature already observed at the whole rock scale. Our results suggest that different chondrite groups sample different reservoirs of presolar grains formed in different environments. Assuming that the carrier of this anomalous component measured in residues of enstatite chondrites are SiC, our results may suggest that different meteorite parent bodies sample reservoirs of presolar SiC formed in different stellar environments. This could explain why ALHA77295, the sample which is the most enriched in presolar grains, has a bulk ¹⁴²Nd isotope composition similar to the terrestrial value. Further investigation of enstatite chondrites is needed to test whether the isotope composition of the most refractory phases is similar to that measured in carbonaceous chondrites and in particular the ¹⁴⁴Sm that is a p-process isotope only. Finally this study highlights the difficulty of interpreting the ¹⁴²Nd excess in terrestrial samples relative to chondrites since incomplete mixing of nucleosynthetic material in the solar nebula creates significant ¹⁴²Nd variation, as shown by ALHA77295.