Lithium Isotope Constraints on Slab and Mantle Contribution to Arc Magmas

Dehydration of subducting oceanic lithosphere (slab) induces Li-isotope fractionation between the fluid and the slab, suggested by the δ⁷Li variation (∼10‰) in exhumed subduction complexes. Given that arc magmas represent melt of the supraslab mantle, a large δ⁷Li variation is anticipated for arc volcanic rocks. However, the δ⁷Li values in these rocks are mostly homogeneous within the range of mid-ocean ridge basalts (+1.6 to +5.6‰). The lack of a subduction-related δ⁷Li signature has been explained by (a) homogenization by mixing of different magma sources, (b) loss of Li from the slab via dehydration, or (c) homogenization by diffusive exchange of slab-derived Li and the mantle. The Chugoku district in SW Japan is an ideal place to study the process responsible for Li-isotope variation in arc magmas, since the Chugoku volcanic rocks show large δ⁷Li variation (−1.9 to +7.4‰). High δ⁷Li values (+6.3 to +7.4‰) are found in some high-Sr andesites and dacites (adakites) whereas low δ⁷Li values (−1.0 to −0.1‰) are found in high-Mg andesites. The parental magmas of these rocks have been sourced from subducted oceanic crust and sediments, respectively, with various extents of the interaction with wedge mantle. The limited extents of Li isotope modification are indicated by the similarity of the δ⁷Li values of these rocks and their supposed sources. The models for a slab dehydration and a diffusive exchange between slab-derived melt and mantle demonstrate that the δ⁷Li signatures of the sources can be preserved in the adakites if they ascent rapidly in mantle.