Blue mineral offers peek inside the mantle
Steve Jacobsen/Northwestern Univ
An ocean’s worth of water is locked deep within the Earth and may influence all sorts of geological processes, including the grinding of tectonic plates, the formation of volcanoes and the movement of Earth’s elements.
Researchers examined the rumbling of seismic waves and performed lab experiments that mimicked the crushing pressures and extreme temperatures of Earth’s mantle, the thick layer of the planet between the crust and core. The results make a strong case that a huge amount of water resides within the planet, says Yale University geophysicist Jennifer Girard, who was not involved with the study. The findings appear in the June 13 Science.
Some of this water may have been trapped during the early days of Earth’s history, before the oceans formed. Water also travels into the mantle during subduction, when tectonic plates collide and ocean-soaked crust dives deep into the mantle. The water probably influences the flow of material in Earth’s innards. But exactly where the water goes hasn’t been clear.
Many geologists think the water gets trapped about 410 to 660 kilometers beneath Earth’s surface, where the upper mantle transitions into the partially molten lower mantle. But evidence has been scant. Holes dug from the surface merely penetrate Earth’s crust, so scientists have relied on indirect evidence, including bits of rock ejected by volcanoes to get a sense of what’s happening deeper down.
A recent analysis of a battered diamond that made the 400-kilometer trek from mantle to surface in an estimated 10 hours revealed a tiny bit of the sapphire-blue mineral ringwoodite — a mineral that’s especially good at holding water in the form of hydroxide ions. Ringwoodite is a version of the mineral olivine, which is common in shallow parts of the mantle but is thought to exist as ringwoodite in the higher-pressure environment of the mantle’s transition zone.
The ringwoodite was about 1.5 percent water by weight, providing tantalizing evidence of a watery reservoir in the mantle. Yet scientists didn’t know whether the diamond came from a particularly water-rich area. The alternative is that the mantle’s water is widespread, says seismologist Brandon Schmandt of the University of New Mexico in Albuquerque.
To investigate, Schmandt and colleagues first figured out what might happen to the water in ringwoodite if the mineral traveled down into the lower mantle, where temperatures begin at 2,000°Celsius. Crushing ringwoodite in the lab with an anvil and blasting it with high-temperature lasers resulted in what scientists call dehydration melting. The ringwoodite shed its water, leaving a thin layer of “melt” around the crystal. That behavior suggested that water can’t make the move to the lower mantle and gets trapped in the transition zone.
If the water bound in ringwoodite can’t travel to the lower mantle, then there should be evidence of this melting in areas of the transition zone where the mantle is flowing downward, Schmandt says. So he and colleagues looked at data from seismic waves traveling deep within Earth, gathered from a network of seismometers planted across the United States. The team compared these data with computer simulations of the mantle flowing up or down in the same areas.
Sure enough, in spots when the mantle is believed to flow downward, the researchers saw a shift in the type of seismic waves. That kind of shift indicates that the waves were hitting the proposed melting region between the transition zone and the lower mantle. The seismic data suggest that the mantle’s water is widespread, Schmandt says, perhaps containing as much water as all of Earth’s oceans or even more.
B. Schmandt et al. Dehydration melting at the top of the lower mantle. Science. Vol. 344, June 13, 2014, p. 1265. doi:10.1126/science.1253358.
D.G. Pearson et al. Hydrous mantle transition zone indicated by ringwoodite included within diamond. Nature. Vol. 507, March 13, 2014, p. 221. doi:10.1038/nature13080.
N. Lubick. Tiny minerals may have shaped Earth's first plate boundaries. Science News. Vol. 185, May 17, 2014, p. 14.
G. Popkin. Earth's plate boundaries may nurture diamond formation. Science News. Vol. 185, January 11, 2014, p. 13.