Conjoint Release of N, C, He, and AR from C Delta by Stepped Pyrolysis: Implications for the Identification of Their Carriers
Recently, on the basis of a systematic difference in release patterns of the species during stepped pyrolysis  we have argued that anomalous noble gases (Ne-A2, Xe-HL, and others) and light nitrogen (delta-15N = -350 permil) appear to be located in different parts of C-delta nanometer-sized diamond. However, nitrogen and noble gases in C-delta samples have never been measured simultaneously in the same experiment so that our arguments, of necessity, were speculative. Here, we present the results of the first experiments where C, N, He, and Ar were measured in unison in single aliquots of pure C-delta from several meteorites. The figure, which appears in the hard copy, shows results for Cold Bokkeveld, which can be considered typical. Like others previously  we observe a clear temperature dependence in the 36Ar release pattern suggesting the existence of two components: a low-temperature component defined as the planetary gas and higher-temperature component believed to be part of the HL family which, for the purposes of discussion we will call Ar-C-delta. We can now directly compare results with the Ar data. It is conclusively shown that the release pattern of N-C-delta (the light nitrogen delta-15N= -350 permil) peaks at a temperature several hundred degrees lower than Ar-C-delta and 400 degrees C above planetary Ar. Combustion of the Cold Bokkeveld sample after pyrolysis showed that about 35% of Ar-C-delta (not surprising since we only pyrolyzed to 1300 degrees C), but only 7% of total N-C-delta remains in the sample, strengthening arguments that they are decoupled; about 73% of the carbon in the sample survives to be converted to CO2 by combustion. The release pattern of N-C-delta itself also turns out to be bimodal, with the major component having a similar temperature to 4He. We note that the second nitrogen component does not begin to be evolved, like the Ar-C-delta, until the C starts to degrade significantly at 1100 degrees C and above. The data for N, He, and Ar are difficult to rationalize on the basis of diffusion effects. Nitrogen as an assumed chemically bound element in C-delta  ought to be released at a higher temperature than 36Ar, if the species reside in the same diamond carrier grains. Another interesting observation, mentioned in our earlier study  is also now confirmed for samples of Orgueil, Krymka, Murchison, Efremovka, and Cold Bokkeveld (see figure). A significant fraction of the C, 8-16 wt% of the sample, is released at temperature corresponding to the evolution of planetary Ar. The C is liberated by pyrolysis as a mixture of CO2 and CO, thus a reaction of oxygen situated at grain surfaces is implied. We interpret that planetary Ar is also located at or very near the surface of C-delta. So far attempts to measure the isotopic composition of the oxygen to identify its source has proved unsuccessful. References:  A. B. Verchovsky et al. (1993) LPSC XXIV, 1461.  G. R. Huss and R. S. Lewis (1993) Meteoritics, submitted.  R. S. Lewis et al. (1989) Nature, 339, 117.
- Pub Date:
- July 1993
- NOBLE GASES;
- PRESOLAR GRAINS