Light element geochemistry of the Tagish Lake CI2 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), 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 (delta-13C is approximately equal to +24.4 ppm) of the chip was dominated by 13C-enriched carbon from the decomposition of carbonates (between 1.29 and 2.69 wt%; delta-13C is approximately equal to +67 ppm and delta-18O is approximately equal to +35 ppm, in the proportions ~4:1 dolomite to calcite). In addition to carbonates, Tagish Lake contains organic carbon (~2.6 wt%; delta-13C is approximately equal to -9 ppm; 1033 ppm N, delta-15N is approximately equal to +77 ppm), 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 (~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.