A preliminary numerical study on water-circulation in convecting mantle with magmatism and tectonic plates

We present a preliminary self-consistent numerical model of solid-state convection in a wet mantle with magmatism and tectonic plates to understand how magmatism and tectonic plates affect circulation of water injected into deep mantle. The solidus temperature of mantle materials depends on water-content, but their water-saturation limit is assumed to be constant and water-content dependence of viscosity is neglected. When the mantle is mildly heated by heat producing elements, plate tectonics steadily occurs, and subducted basaltic crusts accumulate on the core-mantle boundary. Water injected into deep mantle is mostly transported to basaltic accumulation areas, which serve as a major storage of injected water. The characteristic time for the water-circulation is 2–6 Gyr. Water significantly enhances magmatism. When the mantle is more strongly heated and the internal heating rate exceeds a threshold, however, basaltic accumulations become convectively unstable, cause vigorous magmatism, and split the lithosphere into smaller fragments. Water transported into deep mantle by subducting lithospheric fragments distributes more uniformly, and the water storage capacity of the mantle becomes much smaller than that under the mild internal heating below the threshold. The characteristic time for water-circulation is 1–2 Gyr. The characteristic time of water-circulation in the mantle is comparable to the age of the Earth under mild internal heating, and the water-circulation depends on the regime of magmatism and mantle convection, which in turn depends on the internal heating rate; a full understanding of dynamics of magmatism and mantle convection is crucial for understanding water-circulation in the mantle and the effect of water on mantle evolution.

► Numerical models are developed for water-circulation in a convecting mantle.
► Water is assumed to reduce the solidus temperature but not to affect the viscosity.
► The characteristic time for water-circulation in the mantle is 2–6 billion years.
► Water mainly accumulates in deep mantle where subducted basaltic crusts pile up.