The isotopic composition and origins of silicon nitride from ordinary and enstatite chondrites

Abstract— The N-isotopic composition of acid-resistant residues of three low petrologic type ordinary chondrites (Adrar 003, LL3.2; Inman, L3.4; Tieschitz, H3.6) and an enstatite chondrite (Indarch, EH4) have been measured by static mass spectrometry. All of these samples have been shown by transmission electron microscopy (TEM) to contain silicon nitride (Si₃N₄), and no other nitrides were detected in any of the residues (Lee et al., 1995). Stepped combustion has demonstrated the presence of at least two components with low C/N ratios, which have been interpreted as Si₃N₄. The most abundant component, common to all the meteorites studied, released during combustion at temperatures >1150 °C, may have formed during metamorphism of the meteorite's parent body. In addition, the ordinary chondrites Tieschitz and Inman show evidence for a second component of Si₃N₄ that is less stable to combustion than the first and is enriched in ¹⁵N. The unusual N-isotope signature suggests that this second type of Si₃N₄ may constitute a new type of interstellar grain. A comparison of the isotope and microscope data suggests that the >1150 °C component can be related to nierite (α-Si₃N₄) and the less stable component to β-Si₃N₄.