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Our current knowledge regarding the distribution patterns of ancient and recent recycled materials, as well as the origin of intraplate basalts in the eastern Asia Big Mantle Wedge (BMW), is limited. To address this, we conducted a detailed geochemical analysis of olivine melt inclusions (OLMI) in nephelinite samples. The normal OLMIs detected in our investigation exhibit geochemical features that closely resemble those of the hosting nephelinite, indicating a consistent association with a carbonated mantle source. Additionally, we identified a distinct group of anomalous OLMIs that displayed different geochemical characteristics from the hosting nephelinite but showed similarities to regional alkali basalts sourced from pyroxenite. The observed geochemical diversity in the nephelinite OLMIs suggests an interaction between deeply derived carbonated melts originating from the flattened Pacific slab in the mantle transition zone and pyroxenite in the lithospheric mantle. Carbonated fluid‐fluxed melting is key to basalt formation in the BMW. Plain Language Summary: The Big Mantle Wedge (BMW) refers to the mantle regions situated between the flattened subducted slab in the mantle transition zone (MTZ) and the overlying plates. In order to better understand the distribution patterns of recycled materials and the origin of intraplate basalts within the Eastern Asia BMW, we have focused on studying the Cenozoic basalts from the eastern North China region. Initially, it was believed that these basalts were formed through decompression melting of the mantle caused by back‐arc extension associated with the subduction of the Pacific plate. However, recent studies have revealed unique characteristics of these basalts, such as high water content and isotopic values that differ from those of the depleted mantle. This suggests a possible connection between the basalts and decarbonation and dehydration processes occurring within the stagnant subducted slab in the MTZ. According to the proposed model, carbonated melts/fluids released from the stagnant slab interacted with the lithospheric mantle, resulting in the transformation of the melts and the involvement of pyroxenite in the lithospheric mantle. The pyroxenite forms through ancient melt metasomatism events. This interaction ultimately leads to the generation of basalts with a wide range of SiO2 content. Key Points: The olivine melt inclusions (OLMI) originate from either a carbonated mantle source or ancient pyroxenite in the lithosphereThe geochemical diversity in the OLMI reflects the interaction between deeply derived carbonated melts and pyroxeniteCarbonated fluid‐fluxed melting stands as a crucial mechanism in the formation of basalt within the Big Mantle Wedge [ABSTRACT FROM AUTHOR]

The Cascadia margin is an unusual subduction zone characterized by the downdip movement of young and thin oceanic plates, where mantle flow and intraslab deformation are still unclear. Here we present new anisotropic tomography of the Cascadia subduction zone, in which the hexagonal symmetry axis of anisotropy is tilting rather than horizontal or vertical as assumed in previous studies of seismic anisotropy. Subduction‐induced entrained and toroidal flows under the Cascadia margin are discriminated well by the spatial relationship between tilting‐axis anisotropy and slab geometry. The obliquely entrained flow is trapped in a narrow zone (<100 km wide) above and below the subducting slab and reaches ∼200 km depth, which is surrounded by large‐scale sub‐horizontal toroidal flow. The intraslab anisotropy is trench‐normal above 80 km depth but changes to trench‐parallel at 100–400 km depths, which may reflect fossil anisotropy overprinted by deep deformation beneath the arc, or joint effect of serpentinization and hydrous faulting. Plain Language Summary: The slab deformation and mantle flow under the Cascadia margin are controversial. We obtain new anisotropic tomography of the Cascadia subduction zone, which can reveal tilting 3‐D fast velocity directions (FVDs) and planes. Because the observed trench‐normal FVD in the Cascadia margin can be explained by both 3‐D toroidal and 2‐D entrained flows, we discriminate the two types of mantle flow with 3‐D FVDs that contain dip angle information. The 2‐D entrained flow above and below the subducting slab is trapped in a narrow zone of ∼100 km wide and is surrounded by large‐scale sub‐horizontal toroidal flow. The slab anisotropy above 80 km depth is consistent with that in the Juan de Fuca plate before subduction, but it is reshaped beneath the arc due to the slab deformation or phase transition. Key Points: Tilting‐axis anisotropy can reconcile contradictory assumptions of azimuthal and radial anisotropiesEntrained flow occurs within ∼100 km depth above and below the subducting slab and is surrounded by toroidal flowFossil anisotropy in the slab is overprinted beneath the arc due to intraslab deformation or phase transition [ABSTRACT FROM AUTHOR]