Water plays an important role in the Earth's evolution and dynamics, and it can be stored and delivered into the Earth's interior by subducting slabs from the lithosphere. Even trace amounts of water incorporation could significantly affect the chemical and physical properties of minerals and thereby the deep mantle. The capacity of minerals within subducting slabs to store water under high-pressure and high-temperature (HPHT) conditions of the mantle is crucial for these dynamic processes. Pyrope, a member of the garnet mineral group existing in basaltic slabs at depths down to the topmost lower mantle, is a prime candidate for transporting water into the deep mantle, but its water solubility is poorly constrained. The role of pyrope garnet in water transport and storage is a long-standing scientific problem with great implications for the evolution and dynamics of the Earth.
The research team first developed high-pressure large-volume press techniques and then synthesized large-size single crystals of pyrope garnets up to 100 mm at pressure and temperature conditions of 150-700 km mantle depths. "It was indeed challenging but exciting to get these gem-quality pyrope garnets for water analysis using our developed high-pressure techniques (Figure 1a)", says Prof. Zhaodong Liu from Jilin University in Changchun, one of the leaders of this study.
The team found that the pyrope garnet can retain water up to 0.2 wt.% and potentially dominate the water transport via basaltic slabs into the topmost lower mantle. In addition, water solubility in pyrope garnet has a strong dependence on pressure and temperature. (Figure 1b). These pyrope garnets can carry 0.2-0.4 wt.% water from the mantle transition zone to the lower mantle (Figure 2). "These results highlight the important role of pyrope garnet in the deep mantle hydration. It will help understand the water storage and transport from the Earth's surface to the topmost lower mantle." Says Prof. Wenliang Xu from Jilin University, another leader for this study.
Ms. Luyao Chen and Mr. Xinyu Zhao, two PhD candidates at Jilin University, did these challenging high-pressure experiments and water analysis. The study lasted over 5 years from 2020, interrupted several times by the Covid pandemic. "It was a difficult time to conduct such experiments and get the results, but I enjoyed the research process. It taught me how to be a good research scientist in future" says Ms. Luyao Chen.
Dr. Chaowen Xu and Xin Li also synthesized pyrope garnets. Ms. Xinran Zhang, Mr. Bingtao Feng, Prof. Bingbing Liu, and Dr. Kuo Hu analyzed the experimental data for these pyrope garnets. All the authors contributed to the writing of the manuscript.
