Discussion on Fractionation Mechanism of Lithium Isotopes

As a burgeoning tracer of stable isotopes, lithium geochemistry has attracted increasing attention from the international geological community recently.Its application field covers the interactions between fluids and minerals from the surface to the mantle.During weathering, the lighter isotope ⁶Li is preferentially retained in the solid phase while ⁷Li goes into solution.Thus river waters have heavy Li isotopic compositions compared to the original bedrock.The heavy Li isotopic compositions of river water feed the oceans.Low-temperature alteration of oceanic crust then makes seawater heavier than river water.In a subduction zone, heavy Li of fluids escaping the slab at low temperature likely enrich the forearc mantle wedge and hydrate the adjacent mantle and make them isotopically heavier than before.Meanwhile, an isotopically light component of slab dehydrated is subducted deeply and may form a locally light-Li mantle reservoir.There are three main factors that can result Li isotopic fractionation in mantle peridotites.They are temperature, diffusion-driven fractionation and mixing with exotic melts.At high temperatures of magmatic processes, Li isotopes do not show per mil-level mass fractionation, and simple diffusive process can not account for the Li isotopic fractionation between the minerals in peridotitic xenoliths from the Hannuoba area in the North China Craton, thus the reaction between peridotites and melts with light Li isotopic compositions maybe a reasonable explanation.It should point out that diffusion of Li isotopes during peridotite-melt reaction makes some contributions to the isotopic fractionation between the mantle minerals.