Water may play an important role in the inner workings of our planet; by some estimates there could be as much water dissolved within the mantle as there is on the Earth’s surface. The discovery of an ultradeep diamond last year hinted that the transition zone between the upper and lower mantle—some 500 kilometers deep—might be composed of 1% water. Furthermore, scientists have known for decades that the upper mantle contains some water, up to 500 parts per million.
In a laboratory, Panero et al. simulated the hot and harsh conditions of the Earth’s interior to create and investigate perovskite, a common mineral of our planet’s deep reaches. Curious about how much water might be locked up in the Earth’s perovskite-rich lower mantle, the authors looked at the conditions in region to determine if water could be dissolved within the mineral in the form of hydrogen monoxide.
In a laser-heated diamond anvil cell, at pressures similar to those found in the lower mantle, they synthesized perovskite. They peered into the mineral using X-ray diffraction, infrared spectroscopy, and electron microscopy to determine if and where water could bond within the rock. Additionally, the authors did theoretical calculations and thermodynamic modeling of the mineral’s structure.
The authors found that perovskite in the lower mantle can hold only a very small amount of water, less than 40 parts per million at temperatures of over 1500°C. Colder regions might be able to hold 3 or 4 times that amount. According to the authors, if the lower mantle really is less suitable for water storage, the lack of the liquid may explain why the region is an order of magnitude more viscous than the adjacent upper mantle. (Journal of Geophysical Research: Solid Earth, doi:10.1002/2014JB011397, 2015)
—Shannon Palus, Freelance Writer
Citation: Palus, S. (2015), Dry minerals in the lower mantle, Eos, 96, doi:10.1029/2015EO027817. Published on 10 April 2015.