Earth and Planetary Science Letters 522 (2019) 79–86 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Light Mg isotopes in mantle-derived lavas caused by chromite crystallization, instead of carbonatite metasomatism Ben-Xun Su a,b,c,∗ , Yan Hu b , Fang-Zhen Teng b,∗∗ , Yan Xiao d , Hong-Fu Zhang c,d,e , Yang Sun b,d , Yang Bai a,b,c , Bin Zhu d , Xin-Hua Zhou c,d , Ji-Feng Ying c,d a Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China b Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA c University of Chinese Academy of Sciences, Beijing 100049, China d State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China e State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China a r t i c l e i n f o a b s t r a c t Article history: Received 25 January 2019 Received in revised form 19 May 2019 Accepted 15 June 2019 Available online xxxx Editor: F. Moynier Keywords: carbonatite metasomatism lithospheric mantle Mg isotopes peridotite xenoliths chromite crystallization Carbonatite metasomatism plays an important role in modifying the composition of Earth’s mantle, however, its effect on mantle Mg isotopic composition is poorly constrained. Here, we report high- precision mineral Mg isotope data for three suites of mantle peridotite xenoliths that experienced variable degrees of carbonatite metasomatism. The δ 26 Mg values of minerals in these xenoliths are variable and range from −0.32 to −0.11❤ in olivine, from −0.28 to −0.09❤ in orthopyroxene, from −0.27 to −0.05❤ in clinopyroxene, from 0.06 to 0.44❤ in spinel and from −0.61 to −0.37❤ in garnet. Calculated bulk-rock δ 26 Mg values of the peridotites vary from −0.27 to −0.10❤, falling within and slightly higher than the normal mantle range (−0.25 ± 0.07❤). The coexisting minerals are in isotopic equilibrium, with clinopyroxene δ 26 Mg values correlated with the carbonatite metasomatic indices such as MgO and Na 2 O in orthopyroxene. These results suggest that carbonatite metasomatism does not produce light Mg isotopic signature in mantle peridotites as previously suggested, instead it might slightly elevate their δ 26 Mg values. Therefore, carbonatite-metasomatized peridotites in the mantle cannot be the primary source rocks of low-δ 26 Mg mantle-derived magmas. Instead, fractional crystallization and accumulation of chromite during ascent of the basaltic magmas may explain the isotopically light basalts, as supported by the covariations of δ 26 Mg with chemical indices of chromite crystallization (e.g., Cr, V, Fe and Ti). Consequently, chromite crystallization may significantly influence the physiochemical processes on the genesis of basalts, which would require comprehensive evaluation in future studies.  2019 Elsevier B.V. All rights reserved. 1. Introduction Mantle heterogeneity caused by carbonatite metasomatism has been well documented through studies of petrology and geochem- istry. Carbonatite melts originate from partial melting of carbon- atite metasomatized-peridotite mantle (e.g., Wallace and Green, 1988; Harmer and Gittins, 1998; Ying et al., 2004), or fractional crystallization and separation from silicate melts by liquid immis- cibility (e.g., Gittins, 1988; Brooker and Hamilton, 1990; Chen et al., 2013; Guzmics et al., 2015). Their interaction with lithospheric mantle during ascent generates some unique petrological and geo- * Corresponding author at: Key Laboratory of Mineral Resources, Institute of Ge- ology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China. ** Corresponding author. E-mail addresses: subenxun@mail.igcas.ac.cn (B.-X. Su), fteng@uw.edu (F.-Z. Teng). chemical signatures in mantle xenoliths (e.g., Rudnick et al., 1993; Coltorti et al., 1999; Halama et al., 2008; Su et al., 2010). Car- bonatite metasomatism typically transforms orthopyroxene in peri- dotite to clinopyroxene due to the Si-undersaturation nature of carbonatite, leading to the formation of wehrlite (e.g., Yaxley et al., 1991, 1998; Ionov et al., 1996; Laurora et al., 2001; Xiao et al., 2010, 2013). It can also slightly elevate forsterite (Fo) con- tent in olivine because of the high Mg/Fe ratio of carbonatite (Fig. 1a; Su et al., 2010). In addition, carbonatite metasomatism displays high La/Yb and low Ti/Eu ratios compared to silicate metasomatism (Fig. 1b) because carbonatites are extremely rich in rare earth elements (particularly light ones) and relatively de- pleted in high field strength elements (e.g., Rudnick et al., 1993; Coltorti et al., 1999). Recently, Li isotopic studies also indicate that carbonatite metasomatism leads to the preferential Li enrichment and significant low-δ 7 Li in olivine compared to silicate metaso- https://doi.org/10.1016/j.epsl.2019.06.016 0012-821X/ 2019 Elsevier B.V. All rights reserved.