Deep within the hot interior of the planet, ice lurks. Now, a form of super-compact ice, found embedded in diamonds, offers the first direct clue that there is abundant water more than 610 kilometers deep in the mantle.
This ice, identified by its crystal structure and called ice-VII, doesn’t exist at Earth’s surface. It forms only at pressures greater than about 24 gigapascals — corresponding to depths between 610 and 800 kilometers, researchers report March 8 in Science. Its presence in diamonds suggests that there is water-rich fluid in the transition zone between the upper and lower mantle, and even into the top of the lower mantle.
“This is really the first time that we see water at such depths,” says Oded Navon, a mantle petrologist at the Hebrew University of Jerusalem who was not involved in the new study.
When slabs of Earth’s crust sink into the mantle layer below, they drag ocean water with them. How deep the slabs sink has been a long-standing question. Researchers have suspected that abundant aqueous fluid exists in the deep mantle, ferried there by slabs bearing water-rich minerals that shed their water when they reach the transition zone (SN: 7/12/14, p. 9). But scientists have not previously found direct evidence of that water.
That’s where diamonds come in. Diamonds form at high temperatures and pressures, crystallizing in pockets rich in the mineral carbonate before being carried to the surface with erupting magma (SN: 4/30/16, p. 8). As the diamond crystals form, they can enclose tiny amounts of fluid or rock from their surroundings. These impurities represent tiny capsules of mantle. Diamond inclusions are the only direct window scientists have into the fabric of Earth more than a kilometer beneath the surface.
...
