The high water storage capacity of minerals in Earth’s mantle transition zone (410- to 660-kilometer depth) implies the possibility of a deep H2O reservoir, which could cause dehydration melting of vertically flowing mantle. We examined the effects of downwelling from the transition zone into the lower mantle with high-pressure laboratory experiments, numerical modeling, and seismic P-to-S conversions recorded by a dense seismic array in North America. In experiments, the transition of hydrous ringwoodite to perovskite and (Mg,Fe)O produces intergranular melt. Detections of abrupt decreases in seismic velocity where downwelling mantle is inferred are consistent with partial melt below 660 kilometers. These results suggest hydration of a large region of the transition zone and that dehydration melting may act to trap H2O in the transition zone.
Schematic cross section of the Earth’s interior. The study by Steve Jacobsen and Brandon Schmandt used seismic waves to find magma generated at the base of the transition zone, around 410 miles deep. Dehydration melting at those conditions, also observed in the study’s high-pressure experiments, suggests the transition zone may contain oceans worth of H2O dissolved in high-pressure rock. The findings alter previous assumptions about the Earth’s composition.
EVANSTON, Ill. - Researchers from Northwestern University and the University of New Mexico report evidence for potentially oceans worth of water deep beneath the United States. Though not in the familiar liquid form -- the ingredients for water are bound up in rock deep in the Earth’s mantle -- the discovery may represent the planet’s largest water reservoir.
The presence of liquid water on the surface is what makes our “blue planet” habitable, and scientists have long been trying to figure out just how much water may be cycling between Earth’s surface and interior reservoirs through plate tectonics.
Northwestern geophysicist Steve Jacobsen and University of New Mexico seismologist Brandon Schmandt have found deep pockets of magma located about 400 miles beneath North America, a likely signature of the presence of water at these depths. The discovery suggests water from the Earth’s surface can be driven to such great depths by plate tectonics, eventually causing partial melting of the rocks found deep in the mantle.
The findings, to be published June 13 in the journal Science, will aid scientists in understanding how the Earth formed, what its current composition and inner workings are and how much water is trapped in mantle rock.
“Geological processes on the Earth’s surface, such as earthquakes or erupting volcanoes, are an expression of what is going on inside the Earth, out of our sight,” said Jacobsen, a co-author of the paper. “ 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.”
Scientists have long speculated that water is trapped in a rocky layer of the Earth’s mantle located between the lower mantle and upper mantle, at depths between 250 miles and 410 miles. Jacobsen and Schmandt are the first to provide direct evidence that there may be water in this area of the mantle, known as the “transition zone,” on a regional scale. The region extends across most of the interior of the United States.
Schmandt, an assistant professor of geophysics at the University of New Mexico, uses seismic waves from earthquakes to investigate the structure of the deep crust and mantle. Jacobsen, an associate professor of Earth and planetary sciences at Northwestern’s Weinberg College of Arts and Sciences, uses observations in the laboratory to make predictions about geophysical processes occurring far beyond our direct observation.
The study combined Jacobsen’s lab experiments in which he studies mantle rock under the simulated high pressures of 400 miles below the Earth’s surface with Schmandt’s observations using vast amounts of seismic data from the USArray, a dense network of more than 2,000 seismometers across the United States.
Jacobsen’s and Schmandt’s findings converged to produce evidence that melting may occur about 400 miles deep in the Earth. H2O stored in mantle rocks, such as those containing the mineral ringwoodite, likely is the key to the process, the researchers said.
“Melting of rock at this depth is remarkable because most melting in the mantle occurs much shallower, in the upper 50 miles,” said Schmandt, a co-author of the paper. “If there is a substantial amount of H2O in the transition zone, then some melting should take place in areas where there is flow into the lower mantle, and that is consistent with what we found.”
If just one percent of the weight of mantle rock located in the transition zone is H2O, that would be equivalent to nearly three times the amount of water in our oceans, the researchers said.
This water is not in a form familiar to us -- it is not liquid, ice or vapor. This fourth form is water trapped inside the molecular structure of the minerals in the mantle rock. The weight of 250 miles of solid rock creates such high pressure, along with temperatures above 2,000 degrees Fahrenheit, that a water molecule splits to form a hydroxyl radical (OH), which can be bound into a mineral’s crystal structure.
Schmandt and Jacobsen’s findings build on a discovery reported in March in the journal Nature in which scientists discovered a piece of the mineral ringwoodite inside a diamond brought up from a depth of 400 miles by a volcano in Brazil. That tiny piece of ringwoodite -- the only sample in existence from within the Earth -- contained a surprising amount of water bound in solid form in the mineral.
“Whether or not this unique sample is representative of the Earth’s interior composition is not known, however,” Jacobsen said. “Now we have found evidence for extensive melting beneath North America at the same depths corresponding to the dehydration of ringwoodite, which is exactly what has been happening in my experiments.”
For years, Jacobsen has been synthesizing ringwoodite, colored sapphire-like blue, in his Northwestern lab by reacting the green mineral olivine with water at high-pressure conditions. (The Earth’s upper mantle is rich in olivine.) He found that more than one percent of the weight of the ringwoodite’s crystal structure can consist of water -- roughly the same amount of water as was found in the sample reported in the Nature paper.
“The ringwoodite is like a sponge, soaking up water,” Jacobsen said. “There is something very special about the crystal structure of ringwoodite that allows it to attract hydrogen and trap water. This mineral can contain a lot of water under conditions of the deep mantle.”
For the study reported in Science, Jacobsen subjected his synthesized ringwoodite to conditions around 400 miles below the Earth’s surface and found it forms small amounts of partial melt when pushed to these conditions. He detected the melt in experiments conducted at the Advanced Photon Source of Argonne National Laboratory and at the National Synchrotron Light Source of Brookhaven National Laboratory.
Jacobsen uses small gem diamonds as hard anvils to compress minerals to deep-Earth conditions. “Because the diamond windows are transparent, we can look into the high-pressure device and watch reactions occurring at conditions of the deep mantle,” he said. “We used intense beams of X-rays, electrons and infrared light to study the chemical reactions taking place in the diamond cell.”
Jacobsen’s findings produced the same evidence of partial melt, or magma, that Schmandt detected beneath North America using seismic waves. Because the deep mantle is beyond the direct observation of scientists, they use seismic waves -- sound waves at different speeds -- to image the interior of the Earth.
“Seismic data from the USArray are giving us a clearer picture than ever before of the Earth's internal structure beneath North America,” Schmandt said. “The melting we see appears to be driven by subduction -- the downwelling of mantle material from the surface.”
The melting the researchers have detected is called dehydration melting. Rocks in the transition zone can hold a lot of H2O, but rocks in the top of the lower mantle can hold almost none. The water contained within ringwoodite in the transition zone is forced out when it goes deeper (into the lower mantle) and forms a higher-pressure mineral called silicate perovskite, which cannot absorb the water. This causes the rock at the boundary between the transition zone and lower mantle to partially melt.
“When a rock with a lot of H2O moves from the transition zone to the lower mantle it needs to get rid of the H2O somehow, so it melts a little bit,” Schmandt said. “This is called dehydration melting.”
“Once the water is released, much of it may become trapped there in the transition zone,” Jacobsen added.
Just a little bit of melt, about one percent, is detectible with the new array of seismometers aimed at this region of the mantle because the melt slows the speed of seismic waves, Schmandt said.
The USArray is part of EarthScope, a program of the National Science Foundation that deploys thousands of seismic, GPS and other geophysical instruments to study the structure and evolution of the North American continent and the processes the cause earthquakes and volcanic eruptions.
The National Science Foundation (grants EAR-0748797 and EAR-1215720) and the David and Lucile Packard Foundation supported the research.
The paper is titled “Dehydration melting at the top of the lower mantle.” In addition to Jacobsen and Schmandt, other authors of the paper are Thorsten W. Becker, University of California, Los Angeles; Zhenxian Liu, Carnegie Institution of Washington; and Kenneth G. Dueker, the University of Wyoming.
Fragments of the blue-colored mineral ringwoodite synthesized in the laboratory.
Deep within the Earth's rocky mantle lies oceans' worth of water locked up in a type of mineral called ringwoodite, new research shows.
The results of the study will help scientists understand Earth's water cycle, and how plate tectonics moves water between the surface of the planet and interior reservoirs, researchers say.
The Earth's mantle is the hot, rocky layer between the planet's core and crust. Scientists have long suspected that the mantle's so-called transition zone, which sits between the upper and lower mantle layers 255 to 410 miles (410 to 660 kilometers) below Earth's surface, could contain water trapped in rare minerals. However, direct evidence for this water has been lacking, until now. [See Images of Water-Rich Ringwoodite and Earth's Layers]
To see if the transition zone really is a deep reservoir for water, researchers conducted experiments on water-rich ringwoodite, analyzed seismic waves travelling through the mantle beneath the United States, and studied numerical models. They discovered that downward-flowing mantle material is melting as it crosses the boundary between the transition zone and the lower mantle layer.
"If we are seeing this melting, then there has to be this water in the transition zone," said Brandon Schmandt, a seismologist at the University of New Mexico and co-author of the new study published today (June 12) in the journal Science. "The transition zone can hold a lot of water, and could potentially have the same amount of H2O [water] as all the world's oceans." (Melting is a way of getting rid of water, which is unstable under conditions in Earth's lower mantle, the researchers said.)
A water-rich mineral
Ringwoodite is a rare type of mineral that forms from olivine under very high pressures and temperatures, such as those present in the mantle's transition zone. Laboratory studies have shown that the mineral can contain water, which isn't present as liquid, ice or vapor; instead, it is trapped in the ringwoodite's molecular structure as hydroxide ions (bonded oxygen and hydrogen atoms).
In March, another research group discovered an unusual diamond from the mantle that encased hydrous ringwoodite. Though the find suggested the transition zone could contain a lot of water, it was the first and only ringwoodite specimen from the mantle scientists have ever analyzed (all other samples were produced in the lab or found in meteorites), and may not be representative of other mantle ringwoodite. [Shine On: Photos of Dazzling Mineral Specimens]
"Right now, we're one-for-one, because that ringwoodite had some H2O in it, but we didn't know if it was normal," Schmandt told Live Science. So Schmandt and geophysicist Steven Jacobsen of Northwestern University in Illinois set out to observationally test if other mantle ringwoodite also contains water.
The researchers knew the crystal structure of ringwoodite allows the transition zone to hold water, but that structure changes if the material moves across the boundary to the lower mantle (due to increasing pressures and temperatures). Because the structure of minerals in the lower mantle can't trap water the way ringwoodite can, Schmandt and Jacobsen reasoned the rocks would melt as they flowed from the transition zone to the lower mantle. "Melting is just a mechanism of getting rid of the water," Schmandt said.
To test this hypothesis, Jacobsen and his colleagues conducted lab experiments to simulate what would happen to transition zone ringwoodite as it travels deeper into the Earth. They synthesized hydrous ringwoodite and recreated the temperatures and pressures it would experience in the transition zone by heating it with lasers and compressing it between hard, anvil-like diamonds.
Using their setup, they then slowly increased the temperature and pressure to mimic the conditions in the lower mantle. The ringwoodite transformed into another mineral called silicate perovskite, and transmission electron microscopy showed that the mineral contained silicate melt around single crystals of perovskite.
"What that tells us is if there is similarly hydrated ringwoodite in the transition zone that's dragged down, we would expect it to produce melt," Schmandt said. "Because melt changes how seismic waves propagate, that's a target I can hunt for [with seismometers]."
Finding the melt
Using the Earthscope USArray, a network of portable seismometers across the United States, Schmandt analyzed seismic waves as they passed from the transition zone to the lower mantle. He found the waves slowed as they crossed into the lower mantle, suggesting that melt was present in the boundary. Importantly, the decrease in seismic velocity didn't happen everywhere — models showed the wave velocity decreased only where material was flowing downward from the transition zone to the lower mantle, as the researchers predicted. [Infographic: Earth's Tallest Mountain to Its Deepest Ocean Trench]
The melt produced in the boundary likely then flows back upward, returning to minerals that can hold the water, Schmandt said, adding that this mechanism allows the transition zone to be a stable water reservoir.
"The study] provides critical experimental support for the important role that the transition zone plays in controlling the melting behavior and flux of hydrogen in the deep Earth," Graham Pearson, a mantle geochemist at the University of Alberta, who wasn't involved in the work, told Live Science in an email.
Anna Kelbert, a geophysicist at Oregon State University who also wasn’t involved in the study, notes that scientists have previously used numerous approaches to look for evidence of Earth's interior water reservoir, but this is the first time researchers have searched for clues of the reservoir by focusing on the potential water-induced melting at the bottom of the transition zone. "It provides an important multidisciplinary perspective on this problem," Kelbert said. "It has important implications on our understanding of the behavior of subducting slabs deep in the mantle, and on our understanding of [the] overall water budget/distribution in the Earth."
Schmandt hopes to now analyze seismic data from other areas across the globe and see how common mantle melting is. This would allow researchers to see if there's something special about the subduction history of the mantle beneath North America, or how the Earth's plates have shifted beneath one another over time.
The new findings will also help scientists better understand Earth's water cycle. "The surface water we have now came from degassing of molten rock. It came from the original rock ingredients of Earth," Schmandt said. "How much water is still inside the Earth today relative to the surface?"
Copyright 2014 LiveScience, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.
The ultimate origin of water in the Earth’s hydrosphere is in the deep Earth—the mantle. Theory1and experiments2, 3, 4 have shown that although the water storage capacity of olivine-dominated shallow mantle is limited, the Earth’s transition zone, at depths between 410 and 660kilometres, could be a major repository for water, owing to the ability of the higher-pressure polymorphs of olivine—wadsleyite and ringwoodite—to host enough water to comprise up to around 2.5 per cent of their weight. A hydrous transition zone may have a key role in terrestrial magmatism and plate tectonics5, 6, 7, yet despite experimental demonstration of the water-bearing capacity of these phases, geophysical probes such as electrical conductivity have provided conflicting results8, 9, 10, and the issue of whether the transition zone contains abundant water remains highly controversial11. Here we report X-ray diffraction, Raman and infrared spectroscopic data that provide, to our knowledge, the first evidence for the terrestrial occurrence of any higher-pressure polymorph of olivine: we find ringwoodite included in a diamond from Juína, Brazil. The water-rich nature of this inclusion, indicated by infrared absorption, along with the preservation of the ringwoodite, is direct evidence that, at least locally, the transition zone is hydrous, to about 1 weight per cent. The finding also indicates that some kimberlites must have their primary sources in this deep mantle region.
The discovery of the rare mineral Ringwoodite points towards the possibility of hidden oceans buried beneath the Earth's surface. Source: University of Alberta Source: Supplied
THE discovery of an elusive mineral, named after an Australian geologist, has led scientists to surmise there is a vast reservoir of water deep in Earth's mantle - as visualised by Jules Verne.
Writing in the journal Nature, scientists said they had found ringwoodite, pointing to the existence of water deep in Earth's mantle, 400km to 600km beneath our feet.
Ringwoodite is named after Australian geologist Ted Ringwood, who theorised that a special mineral was bound to be created in the so-called transition zone sandwiched between the upper and lower layers of Earth's mantle because of the ultra-high pressures and temperatures there.
The find backs once-contested theories that the transition zone, or at least significant parts of it, was water-rich, the investigators said.
"This sample really provides extremely strong confirmation that there are local wet spots deep in the Earth in this area," said Graham Pearson of Canada's University of Alberta, who led the research.
"That particular zone in the Earth, the transition zone, might have as much water as all the world's oceans put together." A piece of ringwoodite has been a long-sought goal. It would resolve a long-running debate about whether the poorly understood transition zone is dry or water-rich.
But until now, it has only ever been found in meteorites.
Geologists had simply been unable to delve deep enough to find any sample on Earth.
All this changed in 2008 when amateur gem hunters digging in shallow river gravel in the Juina area of Mato Grosso, Brazil, found a tiny, grubby stone called a brown diamond.
Measuring just three millimetres across and commercially worthless, the stone was acquired by the scientists when they were on a quest for other minerals. But this turned out to be a bonanza. In its interior, they found a microscopic trace of ringwoodite - the first terrestrial evidence of the ultra-rare rock.
"It's so small, this inclusion, it's extremely difficult to find, never mind work on," Pearson said in a press release, paying tribute to the diligent work of grad student John McNeill.
The team theorise the brown diamond rocketed to the surface during a volcanic eruption, hitchhiking in a stream of kimberlite, the deepest of all volcanic rocks.
Years of analysis, using spectroscopy and X-ray diffraction, were needed in specialised labs to confirm the find officially as ringwoodite.
Hans Keppler, a geologist at the University of Bayreuth in Germany, cautioned against extrapolating the size of the subterranean water find from a single sample of ringwoodite. "In some ways it is an ocean in Earth's interior, as visualised by Jules Verne ... although not in the form of liquid water," Keppler said in a commentary also published by Nature.
又：对于此文的归类，我不知标为原创还是转载，说原创吧，是剪辑而成；说转载吧，我搜索并组合了多篇文献，并且穿插了我的一些点评。好在博文正文已经标得很清楚，标题就姑且写成“原创”吧。副标题中的“举一反三”之一为“Nature：研究发现地幔蕴含水量相当于全部海洋” （"That particular zone in the Earth, the transition zone, might have as much water as all the world's oceans put together."，摘自《Vast ocean hiding beneath Earth's surface》一文）。
