Across-arc variation of Li isotopes in lavas and implications for crust/mantle recycling at subduction zones

Li isotope was analyzed in island arc volcanics from the Izu arc, Japan, to investigate geochemical processes in subduction zones. Li isotope ratios (δ7Li) and Li/Y of the arc lavas show clear across-arc variations, decreasing (δ7Li: +7.6 to +1.1‰, Li/Y: 0.36 to 0.25) with increasing depth to the Wadati–Benioff zone (150 to 210 km). This suggests that the amount of subduction component as a fluid added to the source region decreases with depth. δ7Li–Y/Li systematics of the arc lavas clearly indicate a simple mixing between two distinctive chemically homogenous endmembers, a slab-derived fluid and the mantle wedge. Furthermore, Li–B–Pb isotope systematics allow clear discrimination between the relative contribution of altered oceanic crust (AOC), oceanic sediment and mantle wedge to arc lavas, and suggests that AOC is the dominant subduction component, whereas the contribution of oceanic sediment is extremely small (AOC/oceanic sediment = 97/3). The contrasting physicochemical properties for Li and B in mineral structures imply that Li may be less likely to migrate from the slab into the overlying mantle wedge than B. Thus the Li isotopic composition in the Earthʹs surface material evolved under near-surface condition, could be more efficiently introduced into the deep mantle through subduction zones than the B isotopic signature, making Li isotopes a powerful geochemical tracer for better understanding of crust/mantle recycling.