JMSSJ On-line

JMSSJ On-line, Vol. 48 (2000) No. 4, pp. 275-283

Isotopic Composition of Lithium in Carbonaceous Chondrites Measured by a Modified Type Surface Ionization Mass Spectrometer		>> Full Text PDF >> References
*Tomoe HATAKEYAMA,^a) Keiji HISAYOSHI,^a) Mamoru MURATA,^a) Hiroaki OZAWA,^a) Takeshi KOZAI,^a) and Hiroshi NISHIMURA^a)** ^a) Department of Geological Science, Faculty of Science, Naruto University of Education (748 Takashima, Naruto, Tokushima 772-8502, Japan)*

The isotopic ratios of lithium, ⁷Li/⁶Li, have been measured by a modified type surface ionization mass spectrometer whose ion source was newly designed. It needs no chemical preparation prior to the ionization, and makes an analysis of small portions of sample possible. We mainly analyzed the Murchison meteorite and compared with terrestrial laboratory standard samples, spodumene and lepidolite. Many kinds of isotope anomalies for Murchison have been reported, and these anomalies are considered to be originated from presolar events. Spallation reactions are thought to be one of the processes that produced lithium isotopes, and to synthesize ⁷Li and ⁶Li isotopes as almost the same quantities. If lithium isotopes produced by such reactions in the presolar stage have been maintained in meteorites, there may be an isotopic anomaly of lithium whose ratio, ⁷Li/⁶Li, is extremely lower than that for terrestrial standards. The mean isotopic ratio, ⁷Li/⁶Li, for one laboratory standard, spodumene, is 10.519±0.0004, and that for another one, lepidolite, is 11.329±0.0009. The discrepancy between these lithium isotopic ratios is estimated to be due to the different fractionation processes of these minerals. The isotopic ratios for Murchison are within a range from 10.056 to 11.356. This range of the scattering for Murchison is within those for previous works. The lithium isotopic ratios for Murchison analyzed so far gave no significant difference from those for terrestrial standards.

Key words: Lithium isotopic ratio, Murchison meteorite, Allende meteorite, Direct sample loading surface ionization mass spectrometer

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