Long‐Term Stability of Plate‐Like Behavior Caused by Hydrous Mantle Convection and Water Absorption in the Deep Mantle

Abstract

We investigate the cycling of water (regassing, dehydration, and degassing) in mantle convection simulations as a function of the strength of the oceanic lithosphere and its influence on the evolution of mantle water content. We also consider pseudo‐plastic yielding with a friction coefficient for simulating brittle behavior of the plates and the water‐weakening effect of mantle materials. This model can generate long‐term plate‐like behavior as a consequence of the water‐weakening effect of mantle minerals. This finding indicates that water cycling plays an essential role in generating tectonic plates. In vigorous plate motion, the mantle water content rapidly increases by up to approximately 4–5 ocean masses, which we define as the “burst” effect. A burst is related to the mantle temperature and water solubility in the mantle transition zone. When the mantle is efficiently cooled down, the mantle transition zone can store water transported by the subducted slabs that can pass through the “choke point” of water solubility. The onset of the burst effect is strongly dependent on the friction coefficient. The burst effect of the mantle water content could have significantly influenced the evolution of the surface water if the burst started early, in which case the Earth's surface cannot preserve the surface water over the age of the Earth.

Controls on the Deep‐Water Cycle Within Three‐Dimensional Mantle Convection Models

“…From the high values at the surface we see a rapid drop off in the water content caused by the major reduction of C w , max of basaltic material at 80 km depth. This continues down to 150 km where we find the choke point in the ambient mantle saturation table (as described in Nakagawa & Iwamori, ), after which, the rate at which the average water content decreases (radially) reduces as it approaches 300 km (the point where basaltic material's water carrying capacity drops off significantly). We then find that from 330 km down to the upper‐lower mantle boundary at 660 km, the average mantle water content increases due to the large storage capacity of ambient mantle material colder than 1500 K (up to 15 wt%).…”

On the evolution of the water ocean in the plate-mantle system

“…However, our previous studies have indicated that the evolution of the mantle water mass cannot be determined with a simple regassing-degassing balance, which essentially incorporates the excess water transport associated with the dehydration reaction (Nakagawa et al 2015;Nakagawa and Spiegelman 2017;Nakagawa and Iwamori 2017). Hence, the water solubility limit of each mantle mineral should be included to compute the excess water in the dehydration reaction.…”

“…By considering this effect, our previous studies have indicated that the mantle water evolution may be strongly regulated by the water solubility limits of upper mantle minerals. In addition, the rheological properties of hydrous mantle rocks may affect the behavior of surface plate motion, with hydrous mantle conditions allowing for more vigorous surface plate motion and a much larger friction coefficient than dry mantle conditions (Nakagawa and Iwamori 2017). Such vigorous surface plate motion induced by a pseudo-plastic rheology may transport large amounts of water (i.e., several ocean masses) into the deep mantle over geologic timescales (approximately 2 billion years), and the mantle transition zone can absorb this amount of water over geologic timescales (Nakagawa and Iwamori 2017).…”

“…In addition, the rheological properties of hydrous mantle rocks may affect the behavior of surface plate motion, with hydrous mantle conditions allowing for more vigorous surface plate motion and a much larger friction coefficient than dry mantle conditions (Nakagawa and Iwamori 2017). Such vigorous surface plate motion induced by a pseudo-plastic rheology may transport large amounts of water (i.e., several ocean masses) into the deep mantle over geologic timescales (approximately 2 billion years), and the mantle transition zone can absorb this amount of water over geologic timescales (Nakagawa and Iwamori 2017). Therefore, the total reservoir size of water at the surface and in the deep interior should be resolved in a plate-mantle system using numerical mantle convection simulations that include the water solubility limits of mantle minerals.…”

Geoscience for Understanding Habitability in the Solar System and Beyond

“…At the same time, the volatile content of the surface environment, particularly the presence of liquid water is thought to have a large feedback on the interior, for example by favoring plate tectonics (e.g. Nakagawa and Iwamori, 2017 i) Atmospheric escape that covers interaction between radiation by the sun and the upper atmosphere, leading to the loss of volatile into space (Tian, 2015). It is generally most efficient early in the evolution of the planet.…”