A bigger volume of the world's oceans is seeping deeper into Earth's mantle than expected: That is the result of a study investigating a water-bearing mineral abundant in the oceanic crust. High-pressure experiments at DESY's X-ray source PETRA III show that the mineral glaucophane is surprisingly stable up to 240 kilometres underground, which means it also carries water down to this depth. Scientists attribute this to the gradual cooling of Earth's interior over geological timescales. The cooler temperatures let glaucophane and possibly other water-bearing minerals survive to greater pressures, as the team headed by Yongjae Lee from Yonsei University in South Korea reports in the journal Nature Communications. The scientists estimate that in about 200 million years, an additional volume equal to the Arctic Ocean could seep deep into Earth's mantle this way.

“Under the ocean, there are so-called subduction zones with a total length of approximately 55,000 kilometres, much longer than the circumference of Earth, where slabs from the crust and upper mantle dive into the interior of our planet,” explains Lee. “Every year, about a trillion litres of ocean water are carried along with these slabs in form of water-bearing minerals, such as amphiboles. However, these minerals usually succumb to the high pressures and temperatures at depths of no more than about 100 kilometres.” When the amphiboles break down, their water is set free and drives earthquakes in the subduction slab and volcanism through the overlying mantle. These phenomena eventually deliver the water back to Earth's surface. Glaucophane is an important member of the group of amphiboles.

However, Earth's interior is slowly cooling by about 50 to 100 degrees Celsius per one billion years, and this cooling is getting faster. As a consequence, “cold” subduction zones have developed in many places on the globe in recent geological history. The temperatures in these zones are still sizzling hot compared to everyday standards, but significantly cooler than in “warm” subduction zones, as well as cooler than in the geologic past. “Using high-pressure high-temperature equipment, we have simulated today's conditions in cold subduction zones in the lab and studied the behaviour of glaucophane under these conditions,” explains co-author Hanns-Peter Liermann, head of the Extreme Conditions Beamline P02.2 at DESY's X-ray source PETRA III. “To our surprise, glaucophane remains stable at conditions corresponding to much greater depths of up to 240 kilometres in cold subduction zones.”

Previously, scientists had estimated that about a third of all water carried underground in subduction zones reaches these depths in the mantle, from where it is not clear if and how it can return. “If we assume that all subduction zones eventually become 'cold', an additional volume equivalent to the Arctic Ocean could be stored in the mantle in about 200 million years,” calculates Yoonah Bang from Yonsei University, the first author of this study. “However, it would take five billion years until all oceans would be dried up assuming such a one-way process.” Since our sun will be slowly getting hotter and hotter, other scientists have previously estimated that Earth's oceans might already evaporate in about a billion years. “It appears that Earth may retain its surface water by storing it in the interior and thus preventing it from escaping into space”, adds Lee.

The investigation has also other implications for the evolution of Earth: Since underground water in the shallower depths is a major driver for volcanism and earthquakes, these phenomena are getting less frequent on geological timescales, as Lee points out. “As Earth continuous to cool down, the transport of water to its interior is expected to be extended to greater depths, thereby suppressing earthquakes and volcanism.”

Scientists from Yonsei University and Seoul National University in South Korea, Lawrence Livermore National Laboratory, Argonne National Laboratory and University of Chicago in the U.S., the Center for High Pressure Science & Technology Advanced Research in China, Ehime University in Japan and DESY have contributed to this research. The study is part of the Early Science Program of the Centre for Molecular Water Science (CMWS) that is currently being set up at DESY, and associated to a long-term project of Yongjae Lee at the Extreme Conditions Beamline P02.2 at PETRA III.

 

Reference:
The stability of subducted glaucophane with the Earth’s secular cooling; Yoonah Bang, Huijeong Hwang, Taehyun Kim, Hyunchae Cynn, Yong Park, Haemyeong Jung, Changyong Park, Dmitry Popov, Vitali B. Prakapenka, Lin Wang, Hanns-Peter Liermann, Tetsuo Irifune, Ho-Kwang Mao and Yongjae Lee; Nature Communications, 2021; DOI: 10.1038/s41467-021-21746-8