Fate of Hydrous Fe‐Rich Silicate Melt in Earth's Deep Mantle

Abstract

Density of silicate melt dictates melt migration and establishes the gross structure of Earth's interior. However, due to technical challenges, the melt density of relevant compositions is poorly known at deep mantle conditions. Particularly, water may be dissolved in such melts in large amounts and can potentially affect their density at extreme pressure and temperature conditions. Here we perform first‐principles molecular dynamics simulations to evaluate the density of Fe‐rich, eutectic‐like silicate melt (E melt) with varying water content up to about 12 wt %. Our results show that water mixes nearly ideally with the nonvolatile component in silicate melt and can decrease the melt density significantly. They also suggest that hydrous melts can be gravitationally stable in the lowermost mantle given its likely high iron content, providing a mechanism to explain seismically slow and dense layers near the core‐mantle boundary.

Behavior and properties of water in silicate melts under deep mantle conditions

“…6). The predicted pressure evolution of the hydrous speciation is generally consistent with previous computational studies of hydrous silicate melts 6,7,[24][25][26] . -O-H-O-H-O-, -O-H-O-H-O-H-O-, HO3) at 85.1 GPa and 3000 K (bottom-left).…”

“…7). Our model differs from the previous model 24 which assumed that the density contrast only depends on water content in terms of weight percent. We find that the amount of water in terms of mol percent determines the magnitude of density contrast more accurately than that in terms of weight percent.…”

“…Density change by hydration. The water-induced reduction in the melt density tends to be nearly uniform with respect to pressure and temperature 6,7,24 . The calculated density contrasts (Fig.…”

“…On the other hand, a relatively few computational studies of hydrous silicate melts have so far been reported using first-principles approach 6,7,[24][25][26] . These studies have covered relatively high temperatures and chemically simple compositions.…”

“…For example, the use of the local density approximiation (LDA) for the exchange-correlation functional tends to overbind the structure leading to significant underestimation of the volume 6 . While the calculated partial molar volume of water appears to be mostly insensitive to water concentration 26 and melt composition 6,24,25 , the issue is yet to be fully resolved. Similarly, it is not clear how hydrous contributions to electrical conduction are sensitive to pressure, temperature, and composition.…”