IN-SITU DETERMINATION OF IODINE CONTENT AND IODINE-XENON SYSTEMATICS IN SILICATES AND TROILITE PHASES IN CHONDRULES FROM THE LL3 CHONDRITE SEMARKONA

Iodine concentrations in small domains (similar to 10 mu m) of silicat es and troilite (FeS) phases in three chondrules from the Semarkona (L L3) meteorite were determined by an ion microprobe. Independent determ ination of I content in some of these phases was accomplished by in si tu laser probe mass spectrometric analysis of I-derived Xe-128 in one of these neutron-irradiated chondrules. The ion microprobe data sugges t 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 cho ndrule by ion microprobe (42-138 ppb) and by laser probe (37-76 ppb) d emonstrate the feasibility of in situ determination of I content in si licate phases via ion microprobe. The I contents in troilite measured by ion microprobe, however, are prone to uncertainty because of the la ck 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 troili te is comparable to that in silicates. We attribute this apparent disc repancy to an enhanced sputter ion yield of I from sulfides. Iodine-de rived Xe-129, excesses were observed in both pyroxene and troilite wit hin this chondrule. The I-Xe model ages of these selected phases are c onsistent with the I-Xe studies of the bulk chondrule. The individual data points fall on or near the isochron obtained from the bulk chondr ule, although all except the most radiogenic data point contain eviden ce of low-temperature uncorrelated iodine.