Substantial water retained early in Earth’s deep mantle

Abstract Water in Earth’s deep mantle may have been acquired early 1−7 , when deep magma oceans that extend to Earth’s lower mantle, possibly to the core-mantle boundary, were prevalent in the aftermath of energetic impacts 8,9 . However, how early water was retained and distributed during crystallization of magma oceans remains unclear. This is primarily determined by partitioning of water between mantle mineral and silicate melt. Bridgmanite is the first and dominant mineral crystallized from deep magma oceans 10 . Yet early attempts are restricted to bridgmanite at the topmost lower mantle condition or minerals in the upper mantle and transition zone 11 . Here we use laser-heated diamond anvil cell to simulate relevant magma ocean conditions and investigate partitioning of water between bridgmanite and melt. Our data show that partitioning of water in bridgmanite over silicate melt is significantly higher than previous estimates and strongly dependent on temperature. Given plausible magma ocean crystallization scenarios, appreciable amounts of water may be retained in the lower mantle following the crystallization of a deep magma ocean. Circulation of such early, deep, and likely primordial water in Earth’s interior could modulate mantle dynamics and facilitate the transition of early Earth to a habitable planet.