Extremely high Li and low δ⁷Li signatures in the lithospheric mantle

Geochemical behavior of lithium (Li) in mantle processes is generally explained by: (1) Li diffuses into minerals from melts, faster in clinopyroxene than in olivine, creating isotopically lighter-Li clinopyroxene and heavier-Li olivine; (2) Partitioning of Li changes with temperature, and as cooling proceeds Li diffuses into clinopyroxene from olivine, also resulting in low-δ⁷Li clinopyroxene and high-δ⁷Li olivine, similar to the result of process (1). Our results obtained from highly carbonatite-metasomatized peridotite xenoliths from the Western Qinling reveal that the minerals of these xenoliths contain extremely high Li in a range of 2-16ppm for olivine, 2-43ppm for orthopyroxene, and up to 75ppm for clinopyroxene. Secondly, olivines have much lower δ⁷Li (down to -42‰) relative to co-existent pyroxenes. Above observations provide important insights into the behavior of Li during mantle processes. One explanation is that the fractionation of Li strongly depends on the metasomatic medium. Alkaline melts, particularly Na-rich carbonatitic melts, which have high Li contents (~200ppm) but mantle level δ⁷Li (3-5‰), tend to effectively transfer ⁶Li into olivine and ⁷Li into pyroxene, while silicate melts behave on the contrary. Alternative might be the existence of a reservoir with a high Li content but low δ⁷Li, like eclogite, in the mantle since the ⁷Li released into the overlying mantle wedge during dehydration. This study reveals the distinctive behavior of Li between the carbonatite and silicate metasomatism in the mantle.