Study finds Earth's deep water reservoirs

Recent studies have found the existence of vast, hitherto unknown reservoirs of primordial water thousands of kilometers below the Earth's surface, which could offer clues to understanding the process of the planet's evolution.

While simulating the extreme high-temperature and high-pressure conditions that exist 660 kilometers underground, researchers discovered that the main mineral of the Earth's mantle — bridgmanite — possessed significant water-retention capacity even at temperatures of up to 4,100 C.

The Earth's mantle is the thick, semi-solid layer of hot, dense rock that lies between the crust and the outer core, representing roughly 84 percent of the planet's volume and 67 percent of its mass. The hottest parts of this semi-molten mass circulate slowly, like thick plastic.

The findings, published in the academic journal Science on Friday, reshape human understanding of how water is stored and distributed deep in the Earth, indicating that early-retained water may have been crucial in transforming the planet from a fiery inferno into a habitable world.

Du Zhixue, a professor at the Guangzhou Institute of Geochemistry, who led the research, said that appreciable amounts of water could have been "locked away" deep within the mantle as molten rock cooled and crystallized into a more solid state.

The Earth 4.6 billion years ago was not the gentle blue planet it is today. Frequent violent celestial impacts churned its surface and interiors into a seething ocean of magma, Du said. It was so hot that water could not have existed in liquid form.

"But in that fiery youth, substantial amounts of water did get captured deep inside the Earth," he said.

As the early magma ocean cooled, it crystallized to form solid minerals, gradually creating the thick mantle layer. Bridgmanite, the first and most abundant mineral crystallized in the mantle, may have acted like a sponge at the microscopic level, he said. Its water-locking capacity directly determined how much water could be retained from the magma.

By modeling the crystallization process of the magma ocean, the research team found that thanks to bridgmanite's strong water-locking ability, the lower mantle became the largest water reservoir in the overall mantle once the magma ocean began to solidify.

Du said that previous studies, based on relatively low temperatures, suggested that bridgmanite's water storage capacity was limited. However, researchers successfully increased the temperature to 4,100 C with an ultra high-pressure experimental simulation device they invented. This revealed that bridgmanite's water-locking capacity increases with heat and could be five to 100 times greater than earlier estimates.

Researchers designed an experiment to produce heat with lasers, as well as high-temperature imaging similar to the conditions of the early deep mantle. They were able to determine the temperatures at which water could be stored, providing a solid foundation for uncovering the key role of temperature in water partitioning.

In collaboration with Long Tao, a professor at the Institute of Geology under the Chinese Academy of Geological Sciences, Du's team also incorporated atom probe tomography, which collectively enabled the development of a series of innovative methods for water analysis down to the nanometer scale. It was akin to equipping the micro world with an ultra high-resolution chemical CT scan, which allowed the researchers to clearly visualize the distribution of water within tiny samples and to confirm that water was indeed carried within the structure of bridgmanite.

The amount of water retained in the early solid mantle may have been equivalent to 0.08 to 1 times the volume of all modern oceans, Du said.

Water locked in stone deep underground is not a static stockpile. Instead, it acts as a kind of lubricant for the Earth's colossal geological engine. It lowers the melting point and viscosity of molten rock in the mantle, he said, promoting the slow circulation of what could be described as a thick, heavy soup of hot rock. This in turn animates tectonic plates at the surface — the continental and oceanic plates — and supplies the planet with lasting evolutionary vitality.

Over time, the deeply sequestered water gradually made its way back to the surface through the slow movement of magma. This contributed to the formation of the primordial atmosphere and oceans, he said. Water sealed within Earth's early structure likely served as the crucial force that ultimately drove the planet's transformation from a molten inferno into the blue, livable world today.

The research was supported by the Chinese Academy of Sciences, the National Natural Science Foundation of China, the Ministry of Science and Technology, the Guangdong Basic and Applied Basic Research Foundation and the China Postdoctoral Science Foundation.

zhengcaixiong@chinadaily.com.cn