“The main significance of this paper is the finding of extremely low 7Li/6Li values in several mantle-derived samples.”

Elucidating the material cycle in the terrestrial mantle is one of the major goals of the field of geosciences. Radiogenic isotopic ratios such as strontium (Sr), neodymium (Nd), and lead (Pb), have been used to track recycled components in the mantle. This paper shows that 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.

Yes. This paper describes a new discovery of extremely low 7Li/6Li values in several mantle-derived samples. This new discovery is useful for the investigation of the material cycle in the terrestrial mantle. In addition, this discovery involves an investigation of eastern Asian mantle heterogeneity.

Li has two stable isotopes, 7Li and 6Li, that can be fractionated from each other during certain low temperature (near-surface) processes. The main significance of this paper 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 (Zack et al., Earth Planet. Sci. Lett. 208, 279-290, 2003). It was therefore predicted that a significantly low 7Li/6Li component could be subducted deeply into the mantle to form a distinct mantle reservoir that could be found in mantle-derived samples. However, 7Li/6Li values significantly lower than the bulk solar system value had never been observed in any mantle-derived samples before this paper. This paper also showed that extremely low 7Li/6Li values were observed in mantle-derived samples from Far East Russia and southwestern Japan, but not from northeastern Japan and southeastern Australia. Thus, the extremely low 7Li/6Li property may be related to mantle heterogeneity of the Far East Russia and southwestern Japan regions. Furthermore, Li isotopic data shed new light on the still-debated origin of the enriched mantle type1 end-member component (EM1). From the Li-Sr-Nd isotopic systematics reported in this paper, it appears that the enrichment of isotopically light Li may be general property of EM1 mantle source. In this scenario, the Li in the EM1 source mainly originates from Li in the highly altered basalt of the uppermost part of subducted oceanic crust. Whereas the Pb, Sr, and Nd isotopic signatures are dominantly influenced by the involvement of sediments in a source, the Li isotopic signature is more 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. It therefore provides information complementary to that provided by the radiogenic isotopes.

I have studied Li isotopic geochemistry since 1999 when I was a postdoctoral researcher at the Earthquake Research Institute, University of Tokyo. For the first two years, I could not measure Li isotopic compositions accurately. The experience and knowledge gained from the repeated failures, however, greatly enhanced my analytical skills, and since 2001 I have been able to make accurate sample measurements. Initially, I measured the Li isotopic composition of abyssal basalts. One day, I analyzed a mantle-derived xenolith as a test, after a discussion I had with Dr. Junji Yamamoto of the Institute for Geothermal Sciences at Kyoto University, who studies mantle-derived xenoliths from Far East Russia. The results of the test were surprising, and our Li isotopic research on mantle-derived xenoliths started from that point.