Dehydration melting in the top of the lower mantle

Abstract:

The high water storage capacity of minerals in Earth’s mantle transition zone (410-660 km) implies the possibility of a deep H₂O reservoir, which could cause dehydration melting of mantle that flows vertically across the boundaries of the transition zone. We examined the effects of downwelling from the transition zone into the lower mantle with high-pressure laboratory experiments, numerical modeling, and seismic P-to-S conversions recorded by EarthScope’s USArray. In experiments, the transition of hydrous ringwoodite to perovskite and (Mg,Fe)O produces intergranular melt as a result of the large decrease in water storage capacity. Beneath USArray we detect abrupt decreases in seismic velocity near the top of the lower mantle. Detections are widespread beneath the Great Plains and northern Cordillera, and detections are absent beneath the southwestern U.S. Mantle circulation models show that most of the areas (>95%) with abrupt velocity decreases in the top of the lower mantle are collocated with downward flow across the 660 km discontinuity (660). Mantle circulation models predict upwelling across the 660 beneath the southwestern U.S where abrupt velocity decreases are not detected. The strong correlation between downwelling across the 660 and velocity decreases in the top of the lower mantle is consistent with a small amount (~0.5-1%) of dehydration melting, similar to that observed in laboratory experiments on hydrous ringwoodite. Our results suggest hydration of a large region of the transition zone, and that dehydration melting may act to trap H₂O in the transition zone.