Written by: Lisa Stelzner
Primary Source: Daily Dose of Science Blog
I’m catching up on posting news from the past few weeks . . . Scientists recently found the first direct evidence that the transition zone between the Earth’s upper and lower mantle layers (below the crust) contains water in a rare mineral called ringwoodite. They discovered this by analyzing seismic waves in the mantle below the United States, lab experiments on ringwoodite, and numerical models. Ringwoodite forms from olivine under high pressure and temperature in the transition zone, and contains water in its molecular structure. The mantle’s ringwoodite flows downward and melts as it travels through the transition zone. Water is unstable in the lower mantle, so melting allows the ringwoodite to change its molecular structure and get rid of its water as it moves downward, changing into silicate perovskite. The transition zone may have as much water as the world’s oceans from this melting! However, the water doesn’t sit and remain in this layer as a liquid water reservoir – the water flows upward and is incorporated into other minerals that can incorporate water. This means for our water cycle, liquid water at the Earth’s surface came from molten rock deeper down that degassed its water.
The lab experiments that scientists conducted to discover this used lasers to heat synthetic ringwoodite and diamonds to compress it to simulate the high temperatures and pressures found in the transition zone. The ringwoodite changed to silicon perovskite and produced melt. Seismometers across the U.S. measured seismic waves as they moved from the transition zone to the lower mantle, and the waves’ velocity decreased as they moved from the transition zone to the lower mantle, indicating that water melt was present and slowing them down. Next, scientists want to see if this mantle melting is occurring worldwide and not just under North America to better understand water’s role in plate tectonics and mantle subduction.
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