Ion microprobe analyses of oxygen three-isotope ratios of chondrules from the Sayh al Uhaymir 290 CH chondrite using a multiple-hole disk

Abstract

The ion microprobe is the only technique capable of determining high-precision stable isotope ratios in individual tiny extraterrestrial particles (≤100μm in diameter), but these small samples present special analytical challenges. We produced a new sample holder disk with multiple holes (three holes and seven holes), in which epoxy disks containing a single unknown sample and a standard grain are cast and polished. Performance tests for oxygen two-isotope analyses using San Carlos olivine standard grains show that the new multiple-hole disks allow accurate analysis of tiny particles if the particles are located within the 500μm and 1mm radius of the center of holes for seven-hole and three-hole disks, respectively. Using the new seven-hole disk, oxygen three-isotope ratios of eight magnesian cryptocrystalline chondrules (approximately 100μm in diameter) from the Sayh al Uhaymir (SaU) 290 CH chondrite were analyzed by ion microprobe at the University of Wisconsin. Five out of eight chondrules have nearly identical oxygen isotope ratios (Δ17O=-2.2± 0.6‰; 2SD), which is consistent with those of magnesian cryptocrystalline chondrules in CH/CB and CB chondrites, suggesting a genetic relationship, i.e., formation by a common (possibly impact) heating event. The other three chondrules have distinct oxygen isotope ratios (Δ17O values from -6.4‰ to +2.2‰). Given that similar variation in Δ17O values was observed in type I porphyritic chondrules in a CH/CB chondrite, the three chondrules may have formed in the solar nebula, similar to the type I porphyritic chondrules. © The Meteoritical Society, 2011.