In situ determination of iodine content and iodine-xenon systematics in silicates and troilite phases in chondrules from the LL3 chondrite Semarkona

Abstract

Iodine concentrations in small domains (~10 µm) of silicates and troilite (FeS) phases in three chondrules from the Semarkona (LL3) meteorite were determined by an ion microprobe. Independent determination of I content in some of these phases was accomplished by in situ laser probe mass spectrometric analysis of I-derived ¹²⁸Xe in one of these neutron-irradiated chondrules. The ion microprobe data suggest low I content for olivines (20-45 ppb) and relatively higher values for pyroxene and glass (mesostasis) (40-160 ppb). The broad similarity in the measured I contents in pyroxenes in a porphyritic pyroxene chondrule by ion microprobe (42-138 ppb) and by laser probe (37-76 ppb) demonstrate the feasibility of in situ determination of I content in silicate phases via ion microprobe. The I contents in troilite measured by ion microprobe, however, are prone to uncertainty because of the lack of a sulfide standard. The ion microprobe data suggest I content of 1 ppm in troilite, if the calibration from our silicate standard is used. However, the noble gas data suggest that the I content in troilite is comparable to that in silicates. We attribute this apparent discrepancy to an enhanced sputter ion yield of I from sulfides. Iodine-derived ¹²⁹Xe excesses were observed in both pyroxene and troilite within this chondrule. The I-Xe model ages of these selected phases are consistent with the I-Xe studies of the bulk chondrule. The individual data points fall on or near the isochron obtained from the bulk chondrule, although all except the most radiogenic data point contain evidence of low-temperature uncorrelated iodine.