Fig.1 (a) Seismic wave velocity tomography showing the big mantle wedge structure formed by the stagnation of the Western Pacific slab in the mantle transition zone (modified after [1]), and (b) cartoon model showing two modes of deep carbon cycling in the big mantle wedge in East Asia (modified after [2])

Fig.2 Plots of δ26Mg vs. MgO (a, c) and Cr (b, d) for Late Cretaceous and Cenozoic intraplate basalts from eastern China. Data are adapted from [4]. The calculated fractional crystallization lines for spinel or spinel + olivine are from this study. The solid and dashed lines with different colors represent the modeling results for fractional crystallization and accumulation with different mineral proportions, respectively. Squares and diamonds in each line represent 5% and 1% increments, respectively. The Mg and Cr partition coefficients between olivine and melt are 5.89 (after [14]) and 0.85 (after [15]), respectively, and between spinel and melt are 1.8 (after [14]) and 760 (after [14]), respectively. The Δ26Mgolivine-melt and Δ26Mgchromite-melt values are ~0‰ and 1.18‰, respectively (after [4]). Solid black circles in (a, b) are the assumed initial melt (MgO 17% and Cr 800×10-6, mass fraction) in this study, and gray circles in (c, d) are the initial basaltic melt (MgO 11% and Cr 700×10-6, mass fraction) commonly used in the literature (after [16]).

Fig.3 Three end-member mixing plots. (a) δ26Mg vs. MgO. (b) δ26Mg vs. Cr. Data are adapted from [4], and the calculated mixing lines are from this study. The MgO, Cr, δ26Mg values for subducting Mg-rich carbonates are 13%, 10×10-6, and -2.0‰, respectively (after [12]), for mantle peridotites are 45%, 2200×10-6, and -0.25‰, respectively (after [20-21]), and for silicate-rich melts from subducted oceanic crust are 2.5%, 26×10-6, and 0.25‰, respectively (after [22]).

Fig.4 Pressure-temperature (P-T) diagram showing the solidus for hydrated mantle and hydrated mantle with CO2. Modified after [30].

Fig.5 Spatial overlay. The area with large-scale light Mg isotope anomaly in the asthenospheric mantle in the big mantle wedge beneath East Asia (left) overlaps with high velocity anomaly (with stagnant plates) imaged by seismic tomography at 600 km depth in the mantle transition zone (middle, right). Modified from [1,6,32].

Fig.6 P-T diagram showing the solidus for peridotite, garnet pyroxenite (representing deep subducting oceanic crust), carbonated peridotite, and carbonated eclogite (representing deep subducting carbonated oceanic crust) and P-T conditions for the formation of CO2-bearing, strongly alkaline nephelinitic melt. The convective upper mantle mid-ocean ridge adiabatic line intersects the solidus for peridotite + CO2 at ~300-km depth, therefore, the carbonated convective upper mantle (at ~400 km depth) overlying the mantle transition zone starts to undergo partial melting only after upwelling to ~300 km depth.

Fig.7 Pressure and density depth profiles for magnesite, diamond, the Preliminary Reference Earth model (PREM), and peridotite melt. Modified from [36].

Fig.8 Mantle convection in a big mantle wedge. Modified after [37].

Fig.9 Statistical evidence. (a) A compilation of ages (<110 Ma) of basalts from eastern China (adapted from [6]). (b) A compilation of global continental rift lengths since the Mesozoic using two different statistical methods (adapted from [38]).

Fig.10 Two great oxidation events on Earth. Modified after [41].

Fig.11 Evolution of (a) seafloor water temperature and (b) atmospheric CO2 partial pressure since the Early Cretaceous. Modified after [42].

Fig.12 Estimates of carbon inputs/outputs to the mantle. Carbon output from island arc (blue box) is related to carbon inputs to subduction zones (modified from [43]). According this study, intraplate basaltic volcanoes can be divided into “big mantle wedge” and “mantle plume” types. Carbon released from volcanoes in a big mantle wedge is related to carbon inputs from the subducting plate.