LIGHT ELEMENT GEOCHEMISTRY OF THE TAGISH LAKE C12 CHONDRITE: COMPARISON WITH CI1 AND CM2 METEORITES

Abstract

We have studied the carbon and nitrogen stable isotope geochemistry of a small pristine sample of the Tagish Lake carbonaceous chondrite by high-resolution stepped-combustion mass spectrometry, and compared the results with data from the Orgueil (CI1), Elephant Moraine (EET) 83334 (CM1) and Murchison (CM2) chondrites. The small chip of Tagish Lake analysed herein had a higher carbon abundance (5.81 wt%) than any other chondrite, and a nitrogen content (˜1220 ppm) between that of CI1 and CM2 chondrites. Owing to the heterogeneous nature of the meteorite, the measured carbon abundance might be artificially high: the carbon inventory and whole-rock carbon isotopic composition (δ13C ≅ +24.4%o) of the chip was dominated by 13C-enriched carbon from the decomposition of carbonates (between 1.29 and 2.69 wt%; δ13C ≅ +67%o and δ18O ≅ +35%o, in the proportions ˜4:1 dolomite to calcite). In addition to carbonates, Tagish Lake contains organic carbon (˜2.6 wt%, δ13C ≅ −9%o; 1033 ppm N, δ15N ≅ +77%o), a level intermediate between CI and CM chondrites. Around 2% of the organic material is thermally labile and solvent soluble. A further −18% of the organic species are liberated by acid hydrolysis. Tagish Lake also contains a complement of presolar grains. It has a higher nanodiamond abundance (approximately 3650–4330 ppm) than other carbonaceous chondrites, along with ˜8 ppm silicon carbide. Whilst carbon and nitrogen isotope geochemistry is not diagnostic, the data are consistent with classification of Tagish Lake as a CI2 chondrite.