Distinguishing silicate and carbonatite mantle metasomatism by using lithium and its isotopes Ben-Xun Su a,b, ⁎, Hong-Fu Zhang a , Etienne Deloule b , Nathalie Vigier b , Yan Hu c , Yan-Jie Tang a , Yan Xiao a , Patrick Asamoah Sakyi d a State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China b Centre National de la Recherche Scientifique, CRPG, BP20, 54501 Vandoeuvre-Les-Nancy Cedex, France c Isotope Laboratory, Department of Earth and Space Sciences, University of Washington, 38 Seattle, 4000 15th Avenue NE, Seattle, WA 98195, USA d Department of Earth Science, University of Ghana, P.O. Box LG 58, Legon, Accra, Ghana abstract article info Article history: Received 8 February 2014 Received in revised form 30 April 2014 Accepted 2 May 2014 Available online 22 May 2014 Editor: K. Mezger Keywords: Li isotopes Mantle xenolith Carbonatite metasomatism Silicate metasomatism Lithospheric mantle 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, where- as the Haoti xenoliths vary from harzburgite to wehrlite and were affected by carbonatite metasomatism. Lithi- um 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. δ 7 Li 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 δ 7 Li 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 δ 7 Li Ol N δ 7 Li Opx N δ 7 Li Cpx , whereas those from Haoti xenoliths are characterized by the opposite sequence of δ 7 Li Cpx N δ 7 Li Opx N δ 7 Li Ol ; in particular there is considerable difference in δ 7 Li 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. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Metasomatism is an important and prevalent mechanism control- ling the physical and chemical properties of the mantle through infiltra- tion and percolation of silicate or carbonatite melts and aqueous fluids within the upper mantle (Roden and Murthy, 1985; Dautria et al., 1992). Different consequences can be expected, due to the significant disparity in the physical and chemical properties between silicate and carbonatite metasomatic agents. For example, carbonatite melts have much lower viscosity and density, and greater tendency toward wetting grain boundaries than the silicate melts (Genge et al., 1995; Dobson et al., 1996; Gasparik and Litvin, 2002), hence carbonatite metasoma- tism can elevate the electrical conductivity of the mantle by 2–3 orders of magnitude compared to silicate metasomatism (Gaillard et al., 2008). Considering their significant roles in the evolutionary history of the mantle and the genesis of basaltic and carbonatitic magmas, it is therefore important to establish geochemical proxies that will aid in deciphering the metasomatic history of mantle samples (Dautria et al., 1992; Laurora et al., 2001; Ying et al., 2004; Halama et al., 2009; Zhang et al., 2009). Silicate metasomatism transforms lherzolite to websterite and orthopyroxenite through olivine (Ol)-consumption and orthopyroxene (Opx)-formation, whereas carbonatite metasomatism would consume Opx to form clinopyroxene (Cpx) and produce a rock series from harzburgite and/or lherzolite to Cpx-rich lherzolite and wehrlite (e.g., Yaxley et al., 1991; Ionov et al., 1996; Laurora et al., 2001). Silicon- Chemical Geology 381 (2014) 67–77 ⁎ Corresponding author at: State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China. Tel.: +86 10 82998514; fax: +86 10 62010846. E-mail address: subenxun@mail.igcas.ac.cn (B.-X. Su). http://dx.doi.org/10.1016/j.chemgeo.2014.05.016 0009-2541/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo