Abstract
AbstractThe water‐rich mantle transition zone and dry lower mantle suggest that a dehydration melting layer can form at the 660‐km depth boundary. However, the water content of the melting layer (), which dominates its gravitational stability and melt fraction, remains poorly constrained. Here, the of hydrous silicate melt by mass balance calculations is investigated and found that significantly decreases with increasing temperature, but is relatively insensitive to chemical composition (FeO and SiO2 contents) and coexisting phases. Melt at 660‐km depth should contain ∼50 wt.% water at 1600 K (slab geotherm) or ∼20 wt.% water at 2000 K (topmost lower mantle geotherm). The density of the hydrous melt is <3.9 g/cm3, which makes it buoyant. With a melt fraction of ≳0.5 vol.%, the melting layer is expected to significantly reduce the viscosity and seismic velocity near slabs, which may cause slab stagnation and prohibit whole mantle convection.
Highlights
The water-rich mantle transition zone and dry lower mantle suggest that a dehydration melting layer can form at the 660-km depth boundary
The mantle transition zone is considered to be a water reservoir because its dominant minerals, wadsleyite, and ringwoodite, contain up to ∼1.0 wt.% based on several lines of evidence, including electrical conductivity (Kelbert et al, 2009), mineral viscosity (Fei et al, 2017), and naturally formed water-rich ringwoodite inclusion (Pearson et al, 2014)
The results indicate that systematically decreases with increasing temperature, but is relatively insensitive to
Summary
The mantle transition zone is considered to be a water reservoir because its dominant minerals, wadsleyite, and ringwoodite, contain up to ∼1.0 wt.% based on several lines of evidence, including electrical conductivity (Kelbert et al, 2009), mineral viscosity (Fei et al, 2017), and naturally formed water-rich ringwoodite inclusion (Pearson et al, 2014). Dehydration melting should occur during the phase transformation of hydrous ringwoodite to bridgmanite and ferropericlase by mass convection when crossing the 660-km boundary (Schmandt et al, 2014) Such a dehydration melting layer has been interpreted to explain the seismic velocity reduction at the topmost lower mantle (Schmandt et al, 2014). The question arises regarding whether this melting layer is stabilized at 660-km depth or gravitationally unstable This problem should be determined by the melt viscosity, wetting of mineral grain-boundaries, and most importantly density because the density contrast among the transition zone, lower mantle, and hydrous melt is the driving force for upward or downward melt migration.
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