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To investigate the effects of silicate and carbonatite metasomatism on mantle heterogeneity, we report lithium (Li) concentrations and isotopic compositions for olivine (Ol), orthopyroxene (Opx) and clinopyroxene (Cpx) from two suites of mantle xenoliths (Hannuoba, the North China Craton, and Haoti, the Western Qinling Orogen). The Hannuoba xenoliths range from lherzolite to pyroxenite and were affected by silicate metasomatism, whereas the Haoti xenoliths vary from harzburgite to wehrlite and were affected by carbonatite metasomatism. Lithium concentrations and isotopic compositions display a dichotomy between Hannuoba and Haoti xenoliths, and the overall variation exceeds what was previously reported. The minerals from Haoti xenoliths are more enriched in Li (Ol: 1.23-13.2 ppm; Opx: 3.00-82.8 ppm; Cpx: 1.39-112 ppm) than those from Hannuoba samples (Ol: 1.34-5.52 ppm; Opx: 0.23-16.1 ppm; Cpx: 1.18-79.8 ppm). Lithium isotopic compositions of these samples are highly variable in both suites of samples. δ7Li ranges from + 3.0‰ to + 41.9‰ in Ol, from -21.0‰ to + 20.2‰ in Opx and from -17.4‰ to + 18.9‰ in Cpx for Hannuoba samples. Haoti minerals display a similar degree of variation with δ7Li ranging from -29.1‰ to + 19.9‰ in Ol, -16.9‰ to + 18.0‰ in Opx and -45.1‰ to + 19.6‰ in Cpx. On average, Li isotopic compositions of minerals from Hannuoba xenoliths follow the sequence of δ7LiOl > δ7LiOpx > δ7LiCpx, whereas those from Haoti xenoliths are characterized by the opposite sequence of δ7LiCpx > δ7LiOpx > δ7LiOl; in particular there is considerable difference in δ7Li values of Ol. The Li elemental and isotopic data suggest that mantle metasomatism by distinct agents is an important process for generating the large heterogeneity of Li abundances and isotopic distribution in the lithospheric mantle. The distinct geochemical characteristics of Li isotopes in silicate and carbonatite metasomatism are closely related to the preferential incorporation of Li into minerals from distinct melts. These findings further demonstrate that the Li isotopic systematics may in turn help to discriminate between silicate and carbonatite metasomatism. [ABSTRACT FROM AUTHOR]

Abstract: 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-δ 7Li clinopyroxene and high-δ 7Li 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 δ 7Li (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 δ 7Li (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 δ 7Li, 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. [Copyright &y& Elsevier]