^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.