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.

In recent work, the Geophysical Laboratory's research scientist Zhenxian Liu (NSLS), former postdoctoral fellow and research scientist Steve Jacobsen (Northwestern), and colleagues from the University of New Mexico, University of Southern California, and University of Wyoming have used a unique combination of synchrotron infrared spectroscopy, TEM, numerical modeling, and seismic P-to-S conversions recorded by a dense seismic array in North America to examined the effects of downwelling from the transition zone into the lower mantle. 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 [Schmandt et al., Science 344, 1265-1268 (2014)].

Their research was also featured in an article in USA Today.

Caption: (A) Single-crystal of hydrous ringwoodite (blue crystal) containing 1 wt % H2O inside a diamond-anvil cell 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. (B) Synchrotron-FTIR spectra of the recovered sample were collected with a 10 μm by 10 μm aperture at beamline U2A of the NSLS. Spectrum 1 is an unheated spot, characteristic of hydrous ringwoodite. Spectra 2 and 3 from within the laser heated spots exhibit modified IR-absorption spectra in the OH region, with a broad and asymmetric band at 3400 cm-1 (characteristic of OH in quenched glass) and a sharp peak (3680 cm-1) associated with brucite. On conversion to perovskite plus (Mg,Fe)O, dehydration melting occurred as intergranular melt, viewed by TEM in panel C. In this study, dehydration melting was detected just beneath the mantle transition zone from P-to-S converted phases using seismic data from NSF-Earthscope, US-Array. Above:  Zhenxian Liu and Steve Jacobsen.  Feature images courtesy of Steve Jacobsen.

Michelle Scholtes, 29 July 2014

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