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Figure 3. Cartoon illustrating fluid distribution from surface to the central core of Earth. C-O-H-S fluids may constitute up to 10% in the metallic core. The recycled mid-ocean-ridge basalt (MORB) starts from the trench through mantle transition zone and finally down to the core-mantle boundary (CMB). It is then heated up by the outer core, leading to partial melting. The dense iron-rich melts accumulate on the bottom of the D'' layer. The remaining restite MORB with dominant andesitic composition rises upward to form a superplume. The subducted slab from the trench is hydrous, and it is heated up by the surrounding mantle, which releases the water in the mantle wedge and enhances the viscosity. The vertically rising superplume enters into the upper mantle, transforms to horizontal, and branches out into several hot spots. These hot spots cause the rifting of the continent and deliver the mantle fluid to the surface. Surface CO2 was selectively transported into the mantle in the Hadean to the Archean. After the Neoproterozoic, surface water started to be transported into the mantle transition zones (410–660 km). For the major part of Earth's fluid history, the fluid transport was mostly one way—from the outer core to the surface. The return flow of water started probably only after 750 Ma, although it has not yet entered into the lower mantle.
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