Magnesium Isotopic Evidence for Ancient Subducted Oceanic Crust in LOMU‐Like Potassium‐Rich Volcanic Rocks

Abstract

To evaluate the role of subducted oceanic crust in the genesis of potassium‐rich magmas, we report high‐precision Mg isotopic data for a set of Cenozoic volcanic rocks from Northeast China. These rocks overall are lighter in Mg isotopic composition than the normal mantle and display considerable Mg isotopic variations, with δ26Mg ranging from −0.61 to −0.23. The covariation of δ26Mg with TiO2 in these rocks suggests that their light Mg isotopic compositions were derived from recycled oceanic crust in the form of carbonated eclogite in the source region. The strong correlations between δ26Mg and (Gd/Yb)N ratio as well as Sr‐Pb isotopes further indicate a multicomponent and multistage origin of these rocks. Magnesium isotopes may thus be used as a novel tracer of recycled oceanic crust in the source region of mantle‐derived magmas.

Light Mg Isotopic Composition in the Mantle Beyond the Big Mantle Wedge Beneath eastern Asia

“…We therefore further examine if light Mg isotopic and unradiogenic Sr isotopic source compositions can be derived from dolomite-rich components that have undergone reaction during subduction. Mg sodic basalts from eastern China (gray diamonds) and Chaihe basalts are shown for comparison Huang & Xiao, 2016;Li et al, 2017;Qian et al, 2015;Sun, Teng, et al, 2017;Tang et al, 2006;Wei, 2014;Xue et al, 2019;Yang et al, 2011;Yang et al, 2012). Yellow dots represent new measured δ 26 Mg values of Chaihe samples in this study.…”

“…Journal of Geophysical Research: Solid Earth basalts in previous studies; Li et al, 2017;Sun, Teng, et al, 2017). As carbonation lowers the solidus of mantle peridotite/eclogite (Dasgupta et al, 2005;, the carbonated part in a heterogeneous mantle melts first, thus contributing a higher proportion to melts at low-degree melting.…”

“…Fractional crystallization and crustal contamination also have the potential to affect Mg isotopic compositions. Crustal contamination may be particularly important in the evolution of continental intraplate Basu et al, 1991;Chu et al, 2013;Kuritani et al, 2009;Liu, Chen, et al, 2017;Rasskazov et al, 2011;Sun, Teng, et al, 2017;Sun et al, 2014;Wang et al, 2017;Yan & Zhao, 2008;Yan et al, 2007;Zhang et al, 2016;Zhang et al, 1995;Zhao et al, 2014;Zou et al, 2008;Zou et al, 2003. North China: Basu et al, 1991Chen et al, 2009;Choi et al, 2008;Guo et al, 2016;Han et al, 1999;Ho et al, 2011;Li, Ma, et al, 2016;Wang et al, 2011;Xu et al, 2012;Yu et al, 2015;Zeng et al, 2013;Zhang et al, 2009;Zhang et al, 2012;Zou et al, 2000.…”

“…To further our understanding, we have targeted Cenozoic volcanic rocks erupted west of the DTGL in the Huihe volcanic field (Figure 1), for which we report whole-rock Mg, Sr, Nd, Pb, and Hf isotopic data, as well as major-and trace-element compositions. For comparison, we compiled geochemical data for the Cenozoic Chaihe basalts Ho et al, 2013;Li et al, 2017;Sun, Teng, et al, 2017;Xue et al, 2019;Zhao & Fan, 2012) and for Cenozoic basalts (sensu lato) from areas above the BMW, including data for Shandong nephelinites that have erupted in the eastern part of the North China Craton (Figure 1a). (Huang & Zhao, 2006).…”

“…The region above this stagnant slab has been termed the "Big Mantle Wedge" (BMW; Zhao et al, 2009), which extends to over 1,000 km from the trench, where the BMW and the crustal section above are considered as an intraplate setting (Xu et al, 2018). Cenozoic basalts occurring to the east of the DTGL in eastern China have consistently light Mg isotopic compositions and define a large low δ 26 Mg anomaly within the BMW (δ 26 Mg = −0.60‰ to −0.34‰; Tian et al, 2016;Wang et al, 2017;Yang et al, 2012;Sun, Teng, et al, 2017;Li et al, 2017). Basalts erupted at Chaihe in the nearby DTGL area (Figure 1a), in contrast, show terrestrial, mantle-like Mg isotopic composition (δ 26 Mg = −0.34‰ to −0.19‰; Li et al, 2017;Sun, Teng, et al, 2017), which may suggest a spatial relationship between the large-scale Mg isotopic anomaly and the stagnant Pacific slab or the BMW beneath eastern Asia (Li et al, 2017).…”