Continued Investigation of the Light Element Geochemistry of Tagish Lake

Introduction: Preliminary analysis of the light element stable isotope geochemistry of the Tagish Lake primitive carbonaceous chondrite showed that it was a carbonate-rich meteorite containing abundant presolar grains [1, 2]. Tagish Lake seemed to be more carbon-rich than CI chondrites, with over 5 wt. % carbon distributed almost equally between organic and inorganic (mostly carbonate) components. We have continued our investigation of this unusual meteorite, turning attention to (i) the abundant carbonates; (ii) the distribution of nitrogen between organic and inorganic components and (iii) pre-solar grains. Isotope analyses were carried out at the Open University. The abundance and isotopic composition of carbonates were obtained by sequential dissolution of two 30 mg aliquots taken from a powdered 300 mg sample of pristine Tagish Lake material. The carbon and oxygen isotopic composition of the carbonates were analysed by twostage orthophosphoric acid dissolution [3, 4]. The first extraction, performed at 25°C primarily releases calcite, whilst the second, at 75°C, attacks dolomite/siderite. After purification, the CO2 liberated was analysed on a PDZ Europa Geo 20-20 mass spectrometer; δC and δO were measured to precisions of ± 0.5‰. Nitrogen and argon results were obtained simultaneously by stepped combustion (room temperature to 1400°C in steps of 25°C) of a single chip of Tagish Lake weighing 4.40 mg. Isotopic compositions were measured on the Finesse noble-gas type static vacuum mass spectrometer to precisions of ± 1‰. Carbonates: petrographic analysis of Tagish Lake has shown that areas of the chondrite are rich in fine-grained carbonates, both Caand Fe-rich [2]. Initial measurement of the carbonates by thermal decomposition indicated that they were enriched in C, around 3.2 wt.% carbon as carbonate, with δC between +45‰ and +65‰, and Fe, Mg carbonate approximately twice as abundant as calcite [1]. Sequential acid dissolution of whole rock Tagish Lake has allowed better distinction to be drawn between the different carbonates present (see table). The overall carbonate yield is lower, by a factor of three, than that determined from thermal decomposition, and carbon isotopic composition higher. Reasons for the difference between the two sets of results are probably due to sample heterogenity and failure of the stepped combustion method to resolve adequately the organic and carbonate components. Acid dissolution results show that Fe,Mg carbonates dominate over Ca carbonates, confirming petrographic and combustion observations. However, the difference in δC between the two generations of carbonate, as seen from stepped combustion, is not mirrored by the dissolution experiments.