Distinguishing silicate and carbonatite mantle metasomatism by using lithium and its isotopes

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: 1.23-13.2ppm; Opx: 3.00-82.8ppm; Cpx: 1.39-112ppm) than those from Hannuoba samples (Ol: 1.34-5.52ppm; Opx: 0.23-16.1ppm; Cpx: 1.18-79.8ppm). 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 δ⁷Li_Ol>δ⁷Li_Opx>δ⁷Li_Cpx, whereas those from Haoti xenoliths are characterized by the opposite sequence of δ⁷Li_Cpx>δ⁷Li_Opx>δ⁷Li_Ol; 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.