Rocks in Earth’s mantle could hold five oceans

The zone of rocks just outside Earth’s core could hold enough water to fill the oceans five times. That’s the indication from an analysis of minerals created in the laboratory under conditions that simulate those deep within the planet.

Scientists believe that Earth’s core is primarily iron with a smattering of silicon (SN: 1/12/02, p. 22: Earth’s inner core could include silicon). The lower portion of Earth’s mantle–the layer of hot, viscous rocks between the core and the cooler crust–is thought to be made up mostly of silicate minerals, which include silicon and oxygen.

Recently, geologist Kei Hirose of the Tokyo Institute of Technology and his colleagues mixed up blends of elements that, when heated under intense pressure, transform into the three most common minerals in Earth’s lower mantle. After cooking each batch of ingredients at 1,600C for more than an hour under a pressure equal to 250,000 times that of the atmosphere at sea level, the researchers quickly cooled the rocks so that big crystals formed within them.

Using an ion beam to vaporize crystal specks and a mass spectrometer to analyze the gas, the Japanese researchers found that two of the minerals–magnesiowüstite and a magnesium-rich form of perovskite–could hold up to 0.2 percent of their weight in water. Together, these minerals make up about 95 percent of the lower mantle. The other mineral, a calcium-rich form of perovskite suspected of making up the remaining 5 percent of the mantle, could hold up to 0.4 percent of its weight in water, says Hirose.

If these percentages hold true throughout the lower mantle, the water in the three minerals would add up to about five times the amount of water in Earth’s oceans. Hirose and his colleagues report their findings in the March 8 Science.

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