Evidence for H2O-bearing fluids in the lower mantle from diamond inclusion

In this study, we report the first direct evidence for water-bearing fluids in the uppermost lower mantle from natural ferropericlase crystal contained within a diamond from São Luíz, Brazil. The ferropericlase exhibits exsolution of magnesioferrite, which places the origin of this assemblage in the uppermost part of the lower mantle. The presence of brucite–Mg(OH)2 precipitates in the ferropericlase crystal reflects the later-stage quenching of H2O-bearing fluid likely in the transition zone, which has been trapped during the inclusion process in the lower mantle. Dehydration melting may be one of the key processes involved in transporting water across the boundary between the upper and lower mantle. •Direct evidence for water-bearing fluids in the uppermost lower mantle.•Exsolution of magnesioferrite from ferropericlase starts in the lower mantle.•Brucite precipitates reflect the later-stage quenching of H2O-bearing fluids trapped in the lower mantle.•Dehydration melting is a key process in transport of water across the 660km discontinuity.