Contributed by Ho-Kwang Mao, November 20, 2019 (sent for review August 19, 2019; reviewed by Arianna E. Gleason and Jin S. Zhang)
The great abundance of water on Earth’s surface makes it different from other terrestrial planets, and it may be linked to a deep interior water cycle. How water is transported into Earth’s deep interior and in which phases it is held are still a matter of much debate. Stishovite is known as a major component of subducted oceanic basalt, and recent experiments have reported that it can incorporate significant amounts of water in its crystal structure. Here, we show that, at the pressure–temperature conditions of a mantle geotherm, stishovite may be a key phase for transporting water into the deep mantle, providing important information on the water cycle in the deep earth.
The distribution and transportation of water in Earth’s interior depends on the stability of water-bearing phases. The transition zone in Earth’s mantle is generally accepted as an important potential water reservoir because its main constituents, wadsleyite and ringwoodite, can incorporate weight percent levels of H2O in their structures at mantle temperatures. The extent to which water can be transported beyond the transition zone deeper into the mantle depends on the water carrying capacity of minerals stable in subducted lithosphere. Stishovite is one of the major mineral components in subducting oceanic crust, yet the capacity of stishovite to incorporate water beyond at lower mantle conditions remains speculative. In this study, we combine in situ laser heating with synchrotron X-ray diffraction to show that the unit cell volume of stishovite synthesized under hydrous conditions is ∼2.3 to 5.0% greater than that of anhydrous stishovite at pressures of ∼27 to 58 GPa and temperatures of 1,240 to 1,835 K. Our results indicate that stishovite, even at temperatures along a mantle geotherm, can potentially incorporate weight percent levels of H2O in its crystal structure and has the potential to be a key phase for transporting and storing water in the lower mantle.
Author contributions: Y.L., Q.H., M.W., and H.-K.M. designed research; Y.L., Q.H., Y.M., M.W., and H.-K.M. performed research; Y.L., Q.H., Y.M., and M.W. analyzed data; and Y.L., Q.H., M.W., and H.-K.M. wrote the paper.
Reviewers: A.E.G., SLAC National Accelerator Laboratory; and J.S.Z., University of New Mexico.
The authors declare no competing interest.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1914295117/-/DCSupplemental.
Published under the PNAS license.
