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Water Content of the Dehydration Melting Layer in the Topmost Lower Mantle
Author(s) -
Fei Hongzhan
Publication year - 2021
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2020gl090973
Subject(s) - mantle (geology) , geology , slab , dehydration , geothermal gradient , transition zone , mantle wedge , silicate , partial melting , convection , chemical composition , mineralogy , geophysics , geochemistry , petrology , thermodynamics , subduction , chemistry , paleontology , biochemistry , physics , organic chemistry , tectonics
The 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 (C H 2 Omelt ), which dominates its gravitational stability and melt fraction, remains poorly constrained. Here, theC H 2 Omeltof hydrous silicate melt by mass balance calculations is investigated and found thatC H 2 Omeltsignificantly decreases with increasing temperature, but is relatively insensitive to chemical composition (FeO and SiO 2 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/cm 3 , 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.