Theoretical studies on the occurrence mechanism of structural water in the lattice of Bridgmanite minerals at the core-mantle boundary of the Earth

Bridgmanite (Mg-Pv), as the most abundant mineral in the lower mantle, has been discussed by many scholars in recent years regarding to the different substitution mechanisms of structural water in Bridgmanite and their important scientific significance for the content of structural water in the lower mantle. The core–mantle boundary of the Earth is a key area bearing the processes of plate subduction and Mantle convection. To explore the relative stabilities among different occurrence mechanisms of structural water in Bridgmanite in this area will help us to understand the content, distribution, migration and circulation rules of water in the deep Earth. In this study, we have systematically studied the relative stabilities of three different substitution mechanisms of structural water in Bridgmanite including the VMg2H(V′′Mg+2OH·), VSi4H(V′′′′Si+4OH·) and AlSiH(Al′si+OH·) under temperature and pressure conditions at the core–mantle boundary of the Earth through the first principles molecular dynamics simulation. The research results indicate that among the three substitution mechanisms under high pressure, the stability of the AlSiH(Al′si+OH·) substitution mechanism was significantly affected by the temperature, and it was increased with the increase of temperature. On the other hand, the researches on the relative stabilities of the VSi4H(V′′′′Si+4OH·) and VMg2H(V′′Mg+2OH·) mechanisms show that under the condition of core–mantle boundary of the Earth, structural water may be more favourable to enter the lattice of Bridgmanite with the VSi4H(V′′′′Si+4OH·) substitution mechanism, that is, to occupy the Si site in the lattice of Bridgmanite. However, if the concentration of Si vacancy in Bridgmanite at the core–mantle boundary of the Earth is very low, the capability of structural water entering into the lattice of Bridgmanite through the dissolution of cation vacancy defects will also be very limited. In contrast, the relatively high stability of the AlSiH(Al′si+OH·) substitution mechanism under high temperature and high pressure suggests that the Al rich region at the core–mantle boundary of the Earth could have considerable water content, possibly as high as about 0.5 wt.%. In addition, the molecular hydrogen in the interstitial sites of the lattice of Bridgmanite may also have a certain degree of stability under high temperature and high pressure conditions at the bottom of the lower mantle. These provide some new insights into the occurrence state and source of structural water within the interior of the Earth.