Earth is considered a watery planet, simply by virtue of the fact that 71% of its surface is covered by oceans. But researchers have discovered that, in the massive volume of material in Earth’s interior, minerals can serve as an important water reservoir, providing a new perspective on our planet’s water budget.
Minerals in the transition zone between the upper and lower mantles (between 400 and 670 km deep) can store up to one percent of their weight in water. Given the volume of this layer, that could add up to an ocean’s worth of water. However, the amount of water that might be stored in the lower mantle—representing 55% of the Earth’s volume—is unknown.
Despite some controversy, the current consensus is that the two most abundant minerals in the lower mantle have little capacity for water storage. In this work, researchers studied the third most abundant mineral, CaSiO3-perovskite (Ca-Pv).
At the Advanced Light Source (ALS), the researchers were able to conduct both infrared and x-ray experiments on the same samples at the same high pressures. Infrared spectroscopy at Beamline 1.4 revealed whether Ca-Pv can incorporate water in its crystal structure, and x-ray diffraction at Beamline 12.2.2 elucidated how water might alter the crystal structure.
The results showed that Ca-Pv can store up to 1% of its weight in water—representing about half an ocean of water. Furthermore, since Ca-Pv is the dominant mineral in the subducting oceanic crust, the study opens up the possibility that Ca-Pv could be an important factor in water circulation between the surface and deep interior of Earth.
H. Chen, K. Leinenweber, V. Prakapenka, C. Prescher, Y. Meng, H. Bechtel, M. Kunz, and S.-H. Shim, “Possible H2O storage in the crystal structure of CaSiO3 perovskite,” Phys. Earth Planet. Inter. 299, 106412 (2019), doi:10.1016/j.pepi.2019.106412.