Mg and Zn Isotope Evidence for Two Types of Mantle Metasomatism and Deep Recycling of Magnesium Carbonates

Abstract

To test the ability of Mg and Zn isotopes in discriminating between different types of mantle metasomatism and identifying deep carbon cycling, here we present a comparative study on two types of Cenozoic lavas in SE Tibet, that is, K-rich (potassic-ultrapotassic) lavas and Na-rich alkali basalts. The contrasting bulk rock chemical compositions, Sr-Nd isotopic ratios, and olivine chemistry between them suggest distinct sources in the lithospheric mantle and asthenosphere, respectively. The K-rich lavas have mantle-like δ 26 Mg, slightly lighter δ 66 Zn relative to global oceanic basalts, high 87 Sr/ 86 Sr, and low 143 Nd/ 144 Nd, indicating source metasomatism by recycled siliciclastic sediments. By contrast, the alkali basalts possess remarkably lighter δ 26 Mg and heavier δ 66 Zn values relative to the mantle that are typically characterized by carbonates. The coupling of high δ 66 Zn with high-Zn contents and Zn/Fe ratios further suggests a pyroxenite source containing recycled Zn-rich magnesium carbonates. This is strongly corroborated by the similarity in major elements between the alkali basalts and experimental partial melts of pyroxenite + CO 2. Thus, mantle silicate and carbonate metasomatism contributed to the origin of K-rich and Na-rich lavas in SE Tibet, respectively. Notably, the occurrence of the alkali basalts is spatially consistent with a stagnant slab in the mantle transition zone (410-660 km), the latter of which is interpreted to represent the deeply subducted oceanic slab. These observations provide evidence for recycling of carbonates into the deep mantle, which represents a long-term circulation of subducted carbon compared with that of arc-trench systems and has crucial significance for global deep carbon cycling.

Identifying deep recycled carbonates through Miocene basalts in the Maguan area, SE Tibetan Plateau

“…5,6). As the basalts in Eastern China are produced by partial melting of asthenospheric mantle (e.g., , the Maguan alkali basalts are more likely derived from asthenospheric mantle, nor lithospheric mantle, which agrees well with the previous studies (e.g., Liu et al, 2020).…”

“…The fields of oceanic basalts are modified from Hofmann et al (1986). The sources of literature data: Miocene basalts in the Maguan area (Wang et al, 2001;Wei and Wang, 2004;Xia and Xu, 2005;Flower et al, 2013;Huang, 2012;Liu et al, 2020), Eocene-Oligocene mantle-derived potassic magmas in Western Yunnan (Wang et al, 2001;Xia and Xu, 2005;Guo et al, 2005;Huang et al, 2010;Liu et al, 2020), and Tengchong J o u r n a l P r e -p r o o f mafic volcanic suites (Chen et al, 2002;Li and Liu, 2012;Zhou et al, 2012;Zou et al, 2014;Tian et al, 2018). To avoid the effects of significant magma differentiation and crustal contamination, the aforementioned literature data are plotted with SiO 2 <55 wt.% and MgO >5 wt.% on an anhydrous basis.…”

“…Previous studies have argued that Miocene basalts in the Maguan area, southeastern Tibetan Plateau (the SE Plateau, Fig. 1a) may be of asthenospheric origin (e.g., Wang et al, 2001;Xia and Xu, 2005;Flower et al, 2013;Liu et al, 2020). We thus choose these basalts as a window to study the evolution of the asthenospheric mantle beneath the Tibetan Plateau.…”

“…Furthermore, Flower et al (2013) suggested that tectonic extension associated with the "tectonic escape" and anomalous mantle potential temperature both have played some roles in causing the mantle melting. Based on Mg-Zn isotopes, the latest study further suggested that the recycled Zn-rich magnesium carbonates were recorded in the source of the Maguan basalts (Liu et al, 2020). In order to help resolve the heated debate, we carried out a study and present here bulk-rock geochemical and Sr-Nd-Mg isotopic data, to delineate mantle enrichment and related evolution of asthenosphere beneath the SE Plateau.…”

Petrogenetic evolution of the Zhuopan potassic alkaline complex, western Yunnan, SW China: Implications for heterogeneous metasomatism of lithospheric mantle beneath Simao and western Yangtze block

“…Sr and Nd isotopic ratios were corrected for mass fractionation using 86 Sr/ 88 Sr = 0.1194 and 146 Nd/ 144 Nd = 0.7219. Reference standards BHVO-2 and SRM987 for Sr yielded 87 Sr/ 86 Sr = 0.703508 ± 10 (2SD) and 0.710257 ± 18 (2SD) respectively, corresponding to previously reported values (e.g., Li et al, 2017;Li and Han, 2021;Liu et al, 2020) and the recommend values suggested by Zhang and Hu (2020) (0.703480 ± 16 and 0.710248 ± 24 respectively). The average 143 Nd/ 144 Nd of standard BHVO-2 is 0.512964 ± 5 (2SD), corresponding to the recommend value of 0.512983 ± 20 (2SD, n = 94) (Zhang and Hu, 2020).…”