Hiding Deep Hydrous Melts at the Core-Mantle Boundary
Silicate melts containing H2O in the lowermost mantle are surprisingly dense and may stagnate there, trapping primordial volatiles and potentially causing some of the ultra-low velocity zones.
SOURCE:Geophysical Research Letters
Calculated structure of hydrous eutectic silicate melts at 4000 K and at pressures of 9.4 GPa (left) and at 146.1 GPa (right). The coordination polyhedra of Si-O evolve from four-fold (cyan) and five-fold (blue) to more six-fold (magenta) as pressure increases. The speciation of water in the melt evolves from having more hydroxyls (red-red sphere pairs) and molecular water (red-red-red sphere triplets) to having more bridging hydrogen bonds (red-yellow-red sphere triplets) with pressure. Based on the structure at conditions of the core-mantle boundary (right), hydrous partial melts are denser than coexisting solids when melt fractions are less than about 15%. Credit:Du et al. [2019], Figure S1 (left and right panels)
H2O is notorious for having a solid phase (ice) that is less dense than the melt (water), making ice float in water. But what happens when H2O is added to silicate melts in the Earth’s interior at pressures near the core-mantle boundary? Would such melts sink, or float?
Du et al. [2019]used sophisticated molecular dynamics simulations to determine the atomic-scale structures and density of iron-rich, hydrous silicate melts at pressure-temperature conditions mimicking those near the base of the Earth’s mantle. They found that partial melts of mantle rock containing several weight percent of water can be denser than the bulk solid left behind in the lowermost mantle and therefore should sink, instead of float.
The results can potentially explain seismologically observed slow and dense structures on the core-mantle boundary with further implications for the cycling of water and other volatiles in the Earth’s deep interior throughout its evolution.
Citation: Du, Z., Deng, J., Miyazaki, Y., Mao, H.‐k., Karki, B. B., & Lee, K. K. M. [2019]. Fate of hydrous Fe‐rich silicate melt in Earth’s deep mantle.Geophysical Research Letters, 46. https://doi.org/10.1029/2019GL083633
—Steven D. Jacobsen, Editor,Geophysical Research Letters
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