2006 年 40 巻 2 号 p. 83-96
The non-traditional lithium (Li) isotopic tracer has a great potential to provide a major breakthrough in the investigation of the material cycle in the terrestrial mantle. Using a developed multiple-collector ICP mass spectrometry method, we revealed Li isotopic systematics of mantle-derived samples. The main significance in our results is the finding of extremely low 7Li/6Li values in several mantle-derived samples. Based on earlier results for eclogites, it had been proposed that subducted highly altered oceanic crust would have extremely low 7Li/6Li values. The significantly low 7Li/6Li values, however, had never been observed in any mantle-derived samples before our finding. We have also proposed that the enrichment of isotopically light Li may be general property of the enriched mantle type 1 end-member component (EM 1). In this scenario, the Li in the EM 1 source mainly originates from Li in the highly altered basalt of the uppermost part of subducted oceanic crust. Thus, the Li isotopic signature is sensitive to the degree of alteration experienced by the basaltic crust and can thus be used to distinguish what part of the basaltic crust was recycled.
