Ion Microprobe Studies of Iodine Contents in Silicate Glasses and in Semarkona Chondrules

Details Ion Microprobe Studies of Iodine Contents in Silicate Glasses and in Semarkona Chondrules Ion Microprobe Studies of Iodine Contents in Silicate Glasses and in Semarkona Chondrules

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.355g&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 355

Computer Science

2

Chondrules, Iodine, Ion Probe, Silicates, Sulphides

Scientific paper

Isotopic studies of electronegative elements (e.g. H, C, O, S, I, etc.) by the ion microprobe is best done in the negative secondary mode as the negative ion yields for these elements are much higher compared to their positive ion yields. However, analysis of non-conducting solids (e.g., silicates) in the negative secondary mode is beset with the problem of sample charging. In addition, for heavy elements like iodine, the problem of molecular interferences is also difficult to resolve. We have used a normal incidence electron gun, that generates a cloud of low energy electron near the sample surface, to overcome the problem of sample charging. The problem of molecular interferences was effectively removed by the energy filtering technique, commonly used for trace element studies [1]. Since the normal energy filtering procedure that involves introduction of appropriate offset to the sample high voltage cannot be followed in the negative secondary mode, we have introduced offset to the electrostatic analyzer (ESA) voltage, to achieve the required energy filtering. A silicon sample was analyzed to calibrate ESA voltage offset with sample voltage offset. We have initially analyzed silicon samples and a set of silicate glasses doped with iodine (0.1 to 1.5% by weight) to check for optimum conditions for measurement of low iodine concentration (= 50V should be sufficient to suppress most of the molecular interferences excluding the hydrides. Analysis of iodine doped silicate glasses showed that even for an energy filter of 40V, the ion signals at masses 111 and 126 are sufficiently small for any oxide or hydride to contribute effectively to the iodine signal at mass 127. The count rate at mass 127 for the silicate glasses showed a linear relation with the iodine content for both 40V and 50V energy filter. Following the optimization of the instrument parameters and calibration with silicate glasses, we have analyzed iodine content in several well defined phases (FeS, glass, silicates) in several individual chondrules from the Semarkona(LL3) meteorite. These chondrules have been analyzed earlier for their trace- element content and I-Xe systematics. Because of the small size of the analyzed phases we have used a primary beam of 2nA and an energy filter of 40V for analyzing the silicate phases and both 40 and 50V energy filter for the sulphide. The results show that the iodine signal at mass 127 is maximum for the sulphide followed by glass and silicates. If we assume that the iodine ion yield for all the phases (FeS, glass, and silicates) to be similar to that for the iodine doped silicate glasses, the estimated iodine concentration varies from ~2.5 ppm in FeS to <20 ppb in olivines. References: [1] Zinner E. and Crozaz G.(1986) Int. J. Mass Spec. Ion Proc., 69, 17-38.

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