The high water storage capacity of minerals in Earth’s mantle transition zone (410- to 660-kilometer depth) implies the possibility of a deep H2O reservoir, which could cause dehydration melting of vertically flowing mantle. 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 a dense seismic array in North America. In experiments, the transition of hydrous ringwoodite to perovskite and (Mg,Fe)O produces intergranular melt. Detections of abrupt decreases in seismic velocity where downwelling mantle is inferred are consistent with partial melt below 660 kilometers. These results suggest hydration of a large region of the transition zone and that dehydration melting may act to trap H2O in the transition zone. Portions of this work were conducted at GSECARS, Sector 13 IDD, The Advanced Photon Source.
Laboratory experiments on hydrous ringwoodite.
- Single-crystal of hydrous ringwoodite (blue crystal) containing 1 wt % H2O inside a DAC
at 30 GPa. The sample was laser heated to 1600°C in several spots (orange circles) to perform direct transformation to perovskite and (Mg,Fe)O.
- Synchrotron-FTIR spectra of the recovered sample in three locations: an unheated part of the crystal (spectrum 1) and two locations within laser-heated spots (spectra 2 and 3). FTIR spectra were collected with a 10 mmby 10mmaperture, illustrated and numbered by white boxes in (A).
- TEM within a laserheated spot (position 2) shows crystals of perovskite and intergranular amorphous quench (melt).
Brandon Schmandt, Steve D. Jacobsen, Thorsten W. Becker, Zhenxian Liu, Kenneth G,. Dueker. Dehydration melting at the top of the lower mantle. Science, 13 June 2014: 344 (6189), 1265-1268. [DOI:10.1126/science.1253358]10.1126/science.1259369
Steve Jacobsen holds a crystal of hydrous ringwoodite in a DAC. (Photo courtesy of S. Jacobsen)
Hydrous ringwoodite synthesized from olivine in Steve Jacobsen’s laboratory at Northwestern University. (Photo courtesy of S. Jacobsen)