Friday, August 22, 2014

Evidence of 'oceans worth' of water in Earth's mantle detected

Schematic cross section of the Earth’s interior highlighting the transition zone layer (light blue, 410-660 km depth), which has an anomalously high water storage capacity. 

The study by Schmandt and Jacobsen used seismic waves to detect magma generated near the top of the lower mantle at about 700 km depth.

Dehydration melting at those conditions, also observed in the study’s high-pressure experiments, suggests the transition zone may be nearly saturated with H2O dissolved in high-pressure rock. 

Credit: Steve Jacobsen/Northwestern University

Researchers have found evidence of a potential "ocean's worth" of water deep beneath the United States.

Although not present in a familiar form, the building blocks of water are bound up in rock located deep in the Earth's mantle, and in quantities large enough to represent the largest water reservoir on the planet, according to the research.

For many years, scientists have attempted to establish exactly how much water may be cycling between the Earth's surface and interior reservoirs through the action of plate tectonics.

Northwestern University geophysicist Steve Jacobsen and University of New Mexico seismologist Brandon Schmandt have found deep pockets of magma around 400 miles beneath North America, a strong indicator of the presence of H₂O stored in the crystal structure of high-pressure minerals at these depths.

"The total H₂O content of the planet has long been among the most poorly constrained 'geochemical parameters' in Earth science. Our study has found evidence for widespread hydration of the mantle transition zone," says Jacobsen.

For at least 20 years geologists have known from laboratory experiments that the Earth's transition zone, a rocky layer of the Earth's mantle located between the lower mantle and upper mantle, at depths between 250 and 410 miles, can, in theory, hold about 1 percent of its total weight as H₂O, bound up in minerals called wadsleyite and ringwoodite.

However, as Schmandt explains, up until now it has been difficult to figure out whether that potential water reservoir is empty, as many have suggested, or not.

If there does turn out to be a substantial amount of H₂O in the transition zone, then recent laboratory experiments conducted by Jacobsen indicate there should be large quantities of what he calls "partial melt" in areas where mantle flows downward out of the zone.

This water-rich silicate melt is molten rock that occurs at grain boundaries between solid mineral crystals and may account for about 1 percent of the volume of rocks.

"Melting occurs because hydrated rocks are carried from the transition zone, where the rocks can hold lots of H₂O, downward into the lower mantle, where the rocks cannot hold as much H₂O."

"Melting is the way to get rid of the H₂O that won't fit in the crystal structure present in the lower mantle," says Jacobsen.

He adds:
"When a rock starts to melt, whatever H₂O is bound in the rock will go into the melt right away. So the melt would have much higher H₂O concentration than the remaining solid. We're not sure how it got there."

"Maybe it's been stuck there since early in Earth's history or maybe it's constantly being recycled by plate tectonics."

Seismic Waves
Melt strongly affects the speed of seismic waves, the acoustic-like waves of energy that travel through the Earth's layers as a result of an earthquake or explosion.

This is because stiff rocks, like the silicate-rich ones present in the mantle, propagate seismic waves very quickly.

According to Schmandt, if just a little melt, even 1 percent or less, is added between the crystal grains of such a rock it causes it to become less stiff, meaning that elastic waves propagate more slowly.

"We were able to analyse seismic waves from earthquakes to look for melt in the mantle just beneath the transition zone," says Schmandt.

Brandon Schmandt (University of New Mexico, left) and Steve Jacobsen (Northwestern University, right) combined seismic observations from the US-Array with laboratory experiments to detect dehydration melting of hydrous mantle material beneath North America at depths of 700-800 km. 

Credit: University of New Mexico/Northwestern University

"What we found beneath the U.S. is consistent with partial melt being present in areas of downward flow out of the transition zone."

"Without the presence of H₂O, it is very difficult to explain melting at these depths. This is a good hint that the transition zone H₂O reservoir is not empty, and even if it's only partially filled that could correspond to about the same mass of H₂O as in Earth's oceans," he adds.

Jacobsen and Schmandt hope that their findings, published in the June issue of the journal Science, will help other scientists to understand how the Earth formed and what its current composition and inner workings are, as well as establish how much water is trapped in mantle rock.

"I think we are finally seeing evidence for a whole-Earth water cycle, which may help explain the vast amount of liquid water on the surface of our habitable planet. Scientists have been looking for this missing deep water for decades," says Jacobsen

Schematic representation of seismometers placed in the US-Array between 2004 and 2014 and used in the study by Schmandt and Jacobsen to detect dehydration melting at the top of the lower mantle beneath North America. 

Credit: NSF-Earthscope

Crystals of laboratory-grown hydrous ringwoodite, a high-pressure polymorph of olivine that is stable from about 520-660 km depth in the Earth’s mantle. 

The ringwoodite pictured here contains around one weight percent of H2O, similar to what was inferred in the seismic observations made by Schmandt and Jacobsen. 

Credit: Steve Jacobsen/Northwestern University

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