Analytical Sciences

Ion-Microprobe Analysis of Rare Earth Elements in Oceanic Basalt Glass

Yuji SANO*, Kentaro TERADA*, Hiroshi HIDAKA*, Yoshiro NISHIO**, Hiroshi AMAKAWA*** and Yoshiyuki NOZAKI***

*Department of Earth and Planetary Sciences, Faculty of Science, Hiroshima University, Higashi Hiroshima 739-8526, Japan
**Geological Institute, School of Science, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
***Ocean Research Institute, The University of Tokyo, Nakano, Tokyo 164-8639, Japan

The concentrations of all rare-earth elements (REEs: La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) in oceanic basalt glasses were measured by an ion-microprobe method. A resolving power of 9300 at 1% peak height was attained by an ion-source slit set at 60 mm and a collector slit at 50 mm; the accelerating voltage was 10 kV for secondary ions in order to separate heavy REEs from the oxides of light REEs with adequate flat-topped peaks. Using NIST SRM612 glass (which is doped with ~50 ppm of all REEs), the transmission of secondary ion was estimated to be about 30% and a sensitivity of 10 cps/ppm ¹⁷¹Yb/1nA O₂^- was obtained. A background level of 0.3 cps (at 3 s) coupled with the sensitivity suggests a detection limit of ~6 ppb for glass matrix samples. The secondary ion yields of REEs (A⁺/Ba⁺) are consistent with those (A⁺/Ca⁺) in the literature. The observed contents of REEs in oceanic basalt glasses agree well with those measured by ICP-MS after chemical dissolution within a 10 - 20% error, except for La, Ce and Pr. The discrepancy of the light REEs is attributable to unresolved interference into mass numbers 139, 140 and 141, which may cause an excess abundance of each element. The contents of La, Ce and Pr were corrected by an empirical relationship between the relative intensity of mass number 124 and the excess of elements. (Keywords: Ion microprobe, resolving power, ICP-MS, rare earth element, basalt glass)

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