A Carbon and Nitrogen Isotope Study of Chondritic Diamond and Silicon Carbide

Russell, Sara (1993). A Carbon and Nitrogen Isotope Study of Chondritic Diamond and Silicon Carbide. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000ff32

Abstract

A suite of identically prepared, acid-resistant residues from the three chondrite classes have been analysed for carbon and nitrogen by stepped combustion. Diamond and silicon carbide were found to be ubiquitous components of the residues along with some other minor carbonaceous phases. The diamond and silicon carbide were associated with isotope anomalies suggesting that they are presolar circumstellar condensates.

The diamond nitrogen content ranges from 2000 to 13000 ppm, and the δ13C from -32 to -38‰. These two variables are systematically related to petrologic type, with the high petrologic types containing diamond with a heavier carbon isotopic composition and lower nitrogen content. However, the δ15N, at -351±8‰, was found to be indistinguishable between meteorite samples, suggesting that the variation in nitrogen content cannot be explained by metamorphic degassing of the diamond. A more likely explanation is that the more nitrogen rich crystals are more prone to metamorphic destruction that the nitrogen poor grains, perhaps implying that the grains are a mixture of nitrogen rich and nitrogen poor grains that may originate in different sources.

Silicon carbide was found to have a similar 12C/13C ratio in all the meteorites in which its abundance was >10ppm of the whole rock, with 12C/13C =36.6±0.5; this is interpreted as evidence that a similar mixture of grains were incorporated into each parent body. SiC poor, higher petrologic type meteorites have lower δ13C values. The combustion dynamics of the grains differ from meteorite to meteorite, suggesting that processing since accretion has caused the SiC grains of separate meteorite classes to have different characteristics.

The enstatite chondrite Abee (EH4) appears to be free of carbonaceous phases containing isotopic anomalies but contains isotopically “normal” diamond and silicon carbide. Diamond and sihcon carbide may have been stable condensates in some regions of the early solar nebula.

Overall the isotope data suggest that presolar grains, from a mixture of sources, were well mixed in the meteorite parent body region of the early solar nebula before accretion. Subsequent metamorphism has however caused the grains isolated from different meteorite classes to be distinguishable.

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