The Mantle Transition Zone Hosts the Missing HIMU Reservoir Beneath Eastern China

Abstract

The Earth's mantle shows large-scale geochemical heterogeneities bounded by several identifiable end-member compositions. However, the origin of extreme mantle reservoirs, for example, high 238 U/ 204 Pb (HIMU), and their location in the Earth's interior remains poorly constrained. Clinopyroxenes from late Cenozoic Wudi lavas in eastern China have heterogeneous Pb isotopic compositions, even though the erupted lavas are isotopically homogeneous. Our new results provide, to our knowledge, the first evidence for the existence of a HIMU-like component in the sources of late Cenozoic lavas. Analyses of olivines and their host lavas indicate that the basaltic melts are derived from partial melting of carbonated peridotite. The HIMU source is likely to be younger than 700 Ma old and related to subduction events of the Paleo-Asian ocean. In addition to deep and ancient recycling, storage of a metasomatically produced mantle reservoir in the mantle transition zone may be an important mechanism for developing extreme isotopic signatures.

Evidence of Volatile‐Induced Melting in the Northeast Asian Upper Mantle

“…A considerable number of geochemical studies on Cenozoic basalts and inclusions in diamond also support the subducted slab as the transmission band of volatiles from the Earth's surface to the deep upper mantle (Bulanova et al, 2010;S. Li et al, 2017;Qian et al, 2020;Walter et al, 2008). Stacey and Kramers (1975).…”

“…Figure 8b shows the modeled Pb isotopic evolution of the metasomatized mantle by addition of 1% of the low‐degree melt produced from processed MORB for time periods of 100, 500, and 1,000 Ma. Also shown are the Pb isotopic compositions of clinopyroxene xenocrysts in Wudi lavas that have recorded equilibration with a HIMU like melt beneath eastern China (Qian et al., 2020). Although these results are model dependent and are not meant to be dispositive, we note that HIMU‐type lavas can be generated from the model melt‐metasomatized peridotitic source in 500 Ma.…”

“…2 CO E are returned to the surface through subduction zone devolatilization and volcanism (Kelemen & Manning, 2015;Plank & Manning, 2019;Plank et al, 2013), some amount survives beyond the volcanic front (Gorman et al, 2006; van Keken et al, 2011) behave as incompatible elements during melting (Dasgupta & Dixon, 2009;Hirschmann, 2006) such that substantial amounts of these volatiles can be stored in the small degree melts observed in the LVL.…”

“…Partial melting in the LVL bears significant implications for global volatile cycles. Global estimates suggest that between 80-176 Mt/yr (Dasgupta & Hirschmann, 2010) and 300 E  50 Mt/yr (Plank & Manning, 2019) 2 CO E is transmitted to the mantle by subduction. Earth's mantle can be divided at minimum into a depleted MORB source and an enriched OIB source.…”

The Origin of Late Cenozoic Magmatism in the South China Sea and Southeast Asia

“…The distribution of crustal materials in the mantle, the location of these reservoirs, and the timescales of recycling are important parameters necessary for the understanding of global mantle circulation and possible upwelling mechanisms from different boundary layers (Blichert‐Toft et al., 1999; Chauvel et al., 1992; Dale et al., 2009; Konter & Becker, 2012; Mather et al., 2020; Mazza et al., 2019; Qian, Nichols, et al., 2020; Sobolev et al., 2011; Stracke et al., 2003). For example, deep and ancient recycling near the core‐mantle boundary has been conventionally proposed for the generation of extreme isotopic mantle end members (e.g., Blichert‐Toft et al., 1999; Hofmann, 1997).…”