Abstract
A 28 g sample of the Qingzhen enstatite (EH3) chondrite was subjected to chemical and physical separation procedures to yield several grain-size residues. Ion mapping of isotopes of Si, O, and C in the ion microprobe of two size fractions (QZR4: 0.4-0.8 μm; QZR5: 0.8-2 μm) identified 55 30Si-depleted candidates out of 37,917 Si-rich grains and six 18O-depleted grains out of 54,410 oxides. Subsequent isotopic analyses of C, N, and Si of 48 grains of the 30Si-depleted candidates and additional randomly selected SiC and Si3N4 grains confirmed 36 of X-type SiC, nine of X-type Si3N4, and one of A+B-type SiC. The isotopic compositions of most X grains overlap those of previously measured X grains from the Murchison carbonaceous chondrite, but ~25% show more pronounced 29Si deficits, suggestive of multiple stellar origins of X grains. Presolar Si3N4 grains have isotopic compositions similar to those of X SiC grains, except that their C isotopic ratios are close to solar. The relative abundances of various presolar grain types in Qingzhen are different from those in Murchison, suggestive of heterogeneity and/or size sorting in the primitive solar nebula.