Li isotopes in lavas track mantle, not slab, effects in subduction zones

A now substantial body of Li isotope data on arc volcanic lavas has demonstrated a remarkable lack of variability: the vast majority of lavas fall in a narrow range of δ7Li, between +3‰ and +6‰, comparable to the observed ranges for MORBs and intraplate lavas. Direct sampling of slab- modified mantle wedge materials, as represented by the seamount serpentinite suites from the Mariana forearc, indicate relatively limited Li mobility, and a "slab signature" δ7Li of +6-+7‰, only slightly higher than most arc lavas. Thus, it appears that Li isotope signatures in lavas reflect mantle signatures, not slab influences, and that overall the upper mantle has a uniform δ7Li signature. While mantle xenoliths and high-grade metamorphic rocks can preserve extremely high or low δ7Li, such extreme values are not reflected at the ~km3 sampling scale for the mantle that arc magmas necessarily represent. Interestingly, subduction-related lavas in a few localities do present significant Li isotopic variation. Pliocene to Recent arc lavas of all varieties from the Panama segment of the Central American arc (a hot, slow subducting arc segment) range in δ7Li from +1‰ to +11‰ (Tomascak et al 2000), and Miocene-Recent lavas and shallow intrusives from western Anatolia (where subduction has transitioned into collision) range from -6‰ to +8‰, closely paralleling B isotopic variations that are interpreted to reflect diminishing slab inputs and changing mantle sources (Agostini et al 2008). The lack of similar Li isotopic variability in the Trans-Mexican Volcanic Belt, the Cascades or the New Georgia/Woodlark subduction systems (all of which show "normal" δ7Li) demonstrates that hot slabs on their own cannot result in changeable Li isotope signatures. It appears that only in settings where convective overturn of the the mantle wedge has ceased, and it can then become Li depleted via ongoing melt extraction, can anomalous δ7Li signatures be observed through the mantle "buffer". Variable δ7Li signals over time may thus be an indicator of tectonic transition, pointing to changes in mantle dynamics associated with the cessation of subduction.