Mantle Rain Towards the Earth's Surface

Abstract:

Regardless of the origin of water on Earth, the internal cycle of water determines how much of the accreted water resides at the surface or is trapped into the planet interior. With subducted slabs supplying water to the deep mantle continuously for the last 2-3 billion years, some mantle regions should become saturated and turn into a deep source of water. We argue that the low-velocity layer sitting above the 410-km mantle discontinuity is such a source. Its seismic properties are compatible with the presence of 0.5 to 1% of melt, which is produced by dehydration melting after the upwelling of water-rich material from the transition zone just below. Water-bearing silicate melt can be gravitationally stable in the low-velocity layer at low degree of mantle partial melting. However, any further supply of water or a local increase of the mantle temperature can make this melt buoyant. The melt can then percolate rapidly into the mantle toward the Earth’s surface. Due to the ubiquitous presence of hydrous-melt, the olivine-bearing mantle could be almost saturated in water. Based on this model, there should be ~0.9 times the mass of the oceans stored in the upper mantle today.