Study hints that ancient Earth made its own water — geologically

A new study is helping to answer a longstanding question that has recently moved to the forefront of earth science: Did our planet make its own water through geologic processes, or did water come to us via icy comets from the far reaches of the solar system?

The answer is likely “both,” according to researchers at The Ohio State University– and the same amount of water that currently fills the Pacific Ocean could be buried deep inside the planet right now.

At the American Geophysical Union (AGU) meeting on Wednesday, Dec. 17, they report the discovery of a previously unknown geochemical pathway by which the Earth can sequester water in its interior for billions of years and still release small amounts to the surface via plate tectonics, feeding our oceans from within.

In trying to understand the formation of the early Earth, some researchers have suggested that the planet was dry and inhospitable to life until icy comets pelted the earth and deposited water on the surface.

Wendy Panero, associate professor of earth sciences at Ohio State, and doctoral student Jeff Pigott are pursuing a different hypothesis: that Earth was formed with entire oceans of water in its interior, and has been continuously supplying water to the surface via plate tectonics ever since.

Researchers have long accepted that the mantle contains some water, but how much water is a mystery. And, if some geological mechanism has been supplying water to the surface all this time, wouldn’t the mantle have run out of water by now?

Because there’s no way to directly study deep mantle rocks, Panero and Pigott are probing the question with high-pressure physics experiments and computer calculations.

“When we look into the origins of water on Earth, what we’re really asking is, why are we so different than all the other planets?” Panero said. “In this solar system, Earth is unique because we have liquid water on the surface. We’re also the only planet with active plate tectonics. Maybe this water in the mantle is key to plate tectonics, and that’s part of what makes Earth habitable.”

Central to the study is the idea that rocks that appear dry to the human eye can actually contain water–in the form of hydrogen atoms trapped inside natural voids and crystal defects. Oxygen is plentiful in minerals, so when a mineral contains some hydrogen, certain chemical reactions can free the hydrogen to bond with the oxygen and make water.

Stray atoms of hydrogen could make up only a tiny fraction of mantle rock, the researchers explained. Given that the mantle is more than 80 percent of the planet’s total volume, however, those stray atoms add up to a lot of potential water.

In a lab at Ohio State, the researchers compress different minerals that are common to the mantle and subject them to high pressures and temperatures using a diamond anvil cell–a device that squeezes a tiny sample of material between two diamonds and heats it with a laser–to simulate conditions in the deep Earth. They examine how the minerals’ crystal structures change as they are compressed, and use that information to gauge the minerals’ relative capacities for storing hydrogen. Then, they extend their experimental results using computer calculations to uncover the geochemical processes that would enable these minerals to rise through the mantle to the surface–a necessary condition for water to escape into the oceans.

In a paper now submitted to a peer-reviewed academic journal, they reported their recent tests of the mineral bridgmanite, a high-pressure form of olivine. While bridgmanite is the most abundant mineral in the lower mantle, they found that it contains too little hydrogen to play an important role in Earth’s water supply.

Another research group recently found that ringwoodite, another form of olivine, does contain enough hydrogen to make it a good candidate for deep-earth water storage. So Panero and Pigott focused their study on the depth where ringwoodite is found–a place 325-500 miles below the surface that researchers call the “transition zone”–as the most likely region that can hold a planet’s worth of water. From there, the same convection of mantle rock that produces plate tectonics could carry the water to the surface.

One problem: If all the water in ringwoodite is continually drained to the surface via plate tectonics, how could the planet hold any in reserve?

For the research presented at AGU, Panero and Pigott performed new computer calculations of the geochemistry in the lowest portion of the mantle, some 500 miles deep and more. There, another mineral, garnet, emerged as a likely water-carrier–a go-between that could deliver some of the water from ringwoodite down into the otherwise dry lower mantle.

If this scenario is accurate, the Earth may today hold half as much water in its depths as is currently flowing in oceans on the surface, Panero said–an amount that would approximately equal the volume of the Pacific Ocean. This water is continuously cycled through the transition zone as a result of plate tectonics.

“One way to look at this research is that we’re putting constraints on the amount of water that could be down there,” Pigott added.

Panero called the complex relationship between plate tectonics and surface water “one of the great mysteries in the geosciences.” But this new study supports researchers’ growing suspicion that mantle convection somehow regulates the amount of water in the oceans. It also vastly expands the timeline for Earth’s water cycle.

“If all of the Earth’s water is on the surface, that gives us one interpretation of the water cycle, where we can think of water cycling from oceans into the atmosphere and into the groundwater over millions of years,” she said. “But if mantle circulation is also part of the water cycle, the total cycle time for our planet’s water has to be billions of years.”

Study hints that ancient Earth made its own water — geologically

A new study is helping to answer a longstanding question that has recently moved to the forefront of earth science: Did our planet make its own water through geologic processes, or did water come to us via icy comets from the far reaches of the solar system?

The answer is likely “both,” according to researchers at The Ohio State University– and the same amount of water that currently fills the Pacific Ocean could be buried deep inside the planet right now.

At the American Geophysical Union (AGU) meeting on Wednesday, Dec. 17, they report the discovery of a previously unknown geochemical pathway by which the Earth can sequester water in its interior for billions of years and still release small amounts to the surface via plate tectonics, feeding our oceans from within.

In trying to understand the formation of the early Earth, some researchers have suggested that the planet was dry and inhospitable to life until icy comets pelted the earth and deposited water on the surface.

Wendy Panero, associate professor of earth sciences at Ohio State, and doctoral student Jeff Pigott are pursuing a different hypothesis: that Earth was formed with entire oceans of water in its interior, and has been continuously supplying water to the surface via plate tectonics ever since.

Researchers have long accepted that the mantle contains some water, but how much water is a mystery. And, if some geological mechanism has been supplying water to the surface all this time, wouldn’t the mantle have run out of water by now?

Because there’s no way to directly study deep mantle rocks, Panero and Pigott are probing the question with high-pressure physics experiments and computer calculations.

“When we look into the origins of water on Earth, what we’re really asking is, why are we so different than all the other planets?” Panero said. “In this solar system, Earth is unique because we have liquid water on the surface. We’re also the only planet with active plate tectonics. Maybe this water in the mantle is key to plate tectonics, and that’s part of what makes Earth habitable.”

Central to the study is the idea that rocks that appear dry to the human eye can actually contain water–in the form of hydrogen atoms trapped inside natural voids and crystal defects. Oxygen is plentiful in minerals, so when a mineral contains some hydrogen, certain chemical reactions can free the hydrogen to bond with the oxygen and make water.

Stray atoms of hydrogen could make up only a tiny fraction of mantle rock, the researchers explained. Given that the mantle is more than 80 percent of the planet’s total volume, however, those stray atoms add up to a lot of potential water.

In a lab at Ohio State, the researchers compress different minerals that are common to the mantle and subject them to high pressures and temperatures using a diamond anvil cell–a device that squeezes a tiny sample of material between two diamonds and heats it with a laser–to simulate conditions in the deep Earth. They examine how the minerals’ crystal structures change as they are compressed, and use that information to gauge the minerals’ relative capacities for storing hydrogen. Then, they extend their experimental results using computer calculations to uncover the geochemical processes that would enable these minerals to rise through the mantle to the surface–a necessary condition for water to escape into the oceans.

In a paper now submitted to a peer-reviewed academic journal, they reported their recent tests of the mineral bridgmanite, a high-pressure form of olivine. While bridgmanite is the most abundant mineral in the lower mantle, they found that it contains too little hydrogen to play an important role in Earth’s water supply.

Another research group recently found that ringwoodite, another form of olivine, does contain enough hydrogen to make it a good candidate for deep-earth water storage. So Panero and Pigott focused their study on the depth where ringwoodite is found–a place 325-500 miles below the surface that researchers call the “transition zone”–as the most likely region that can hold a planet’s worth of water. From there, the same convection of mantle rock that produces plate tectonics could carry the water to the surface.

One problem: If all the water in ringwoodite is continually drained to the surface via plate tectonics, how could the planet hold any in reserve?

For the research presented at AGU, Panero and Pigott performed new computer calculations of the geochemistry in the lowest portion of the mantle, some 500 miles deep and more. There, another mineral, garnet, emerged as a likely water-carrier–a go-between that could deliver some of the water from ringwoodite down into the otherwise dry lower mantle.

If this scenario is accurate, the Earth may today hold half as much water in its depths as is currently flowing in oceans on the surface, Panero said–an amount that would approximately equal the volume of the Pacific Ocean. This water is continuously cycled through the transition zone as a result of plate tectonics.

“One way to look at this research is that we’re putting constraints on the amount of water that could be down there,” Pigott added.

Panero called the complex relationship between plate tectonics and surface water “one of the great mysteries in the geosciences.” But this new study supports researchers’ growing suspicion that mantle convection somehow regulates the amount of water in the oceans. It also vastly expands the timeline for Earth’s water cycle.

“If all of the Earth’s water is on the surface, that gives us one interpretation of the water cycle, where we can think of water cycling from oceans into the atmosphere and into the groundwater over millions of years,” she said. “But if mantle circulation is also part of the water cycle, the total cycle time for our planet’s water has to be billions of years.”

Earth’s most abundant mineral finally has a name

An ancient meteorite and high-energy X-rays have helped scientists conclude a half century of effort to find, identify and characterize a mineral that makes up 38 percent of the Earth.

And in doing so, a team of scientists led by Oliver Tschauner, a mineralogist at the University of Las Vegas, clarified the definition of the Earth’s most abundant mineral – a high-density form of magnesium iron silicate, now called Bridgmanite – and defined estimated constraint ranges for its formation. Their research was performed at the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility located at DOE’s Argonne National Laboratory.

The mineral was named after 1964 Nobel laureate and pioneer of high-pressure research Percy Bridgman. The naming does more than fix a vexing gap in scientific lingo; it also will aid our understanding of the deep Earth.

To determine the makeup of the inner layers of the Earth, scientists need to test materials under extreme pressure and temperatures. For decades, scientists have believed a dense perovskite structure makes up 38 percent of the Earth’s volume, and that the chemical and physical properties of Bridgmanite have a large influence on how elements and heat flow through the Earth’s mantle. But since the mineral failed to survive the trip to the surface, no one has been able to test and prove its existence – a requirement for getting a name by the International Mineralogical Association.

Shock-compression that occurs in collisions of asteroid bodies in the solar system create the same hostile conditions of the deep Earth – roughly 2,100 degrees Celsius (3,800 degrees Farenheit) and pressures of about 240,000 times greater than sea-level air pressure. The shock occurs fast enough to inhibit the Bridgmanite breakdown that takes place when it comes under lower pressure, such as the Earth’s surface. Part of the debris from these collisions falls on Earth as meteorites, with the Bridgmanite “frozen” within a shock-melt vein. Previous tests on meteorites using transmission electron microscopy caused radiation damage to the samples and incomplete results.

So the team decided to try a new tactic: non-destructive micro-focused X-rays for diffraction analysis and novel fast-readout area-detector techniques. Tschauner and his colleagues from Caltech and the GeoSoilEnviroCARS, a University of Chicago-operated X-ray beamline at the APS at Argonne National Laboratory, took advantage of the X-rays’ high energy, which gives them the ability to penetrate the meteorite, and their intense brilliance, which leaves little of the radiation behind to cause damage.

The team examined a section of the highly shocked L-chondrite meteorite Tenham, which crashed in Australia in 1879. The GSECARS beamline was optimal for the study because it is one of the nation’s leading locations for conducting high-pressure research.

Bridgmanite grains are rare in the Tenhma meteorite, and they are smaller than 1 micrometer in diameter. Thus the team had to use a strongly focused beam and conduct highly spatially resolved diffraction mapping until an aggregate of Bridgmanite was identified and characterized by structural and compositional analysis.

This first natural specimen of Bridgmanite came with some surprises: It contains an unexpectedly high amount of ferric iron, beyond that of synthetic samples. Natural Bridgmanite also contains much more sodium than most synthetic samples. Thus the crystal chemistry of natural Bridgmanite provides novel crystal chemical insights. This natural sample of Bridgmanite may serve as a complement to experimental studies of deep mantle rocks in the future.

Prior to this study, knowledge about Bridgmanite’s properties has only been based on synthetic samples because it only remains stable below 660 kilometers (410 miles) depth at pressures of above 230 kbar (23 GPa). When it is brought out of the inner Earth, the lower pressures transform it back into less dense minerals. Some scientists believe that some inclusions on diamonds are the marks left by Bridgmanite that changed as the diamonds were unearthed.

The team’s results were published in the November 28 issue of the journal Science as “Discovery of bridgmanite, the most abundant mineral in Earth, in a shocked meteorite,” by O. Tschauner at University of Nevada in Las Vegas, N.V.; C. Ma; J.R. Beckett; G.R. Rossman at California Institute of Technology in Pasadena, Calif.; C. Prescher; V.B. Prakapenka at University of Chicago in Chicago, IL.

This research was funded by the U.S. Department of Energy, NASA, and NSF.

Earth’s most abundant mineral finally has a name

An ancient meteorite and high-energy X-rays have helped scientists conclude a half century of effort to find, identify and characterize a mineral that makes up 38 percent of the Earth.

And in doing so, a team of scientists led by Oliver Tschauner, a mineralogist at the University of Las Vegas, clarified the definition of the Earth’s most abundant mineral – a high-density form of magnesium iron silicate, now called Bridgmanite – and defined estimated constraint ranges for its formation. Their research was performed at the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility located at DOE’s Argonne National Laboratory.

The mineral was named after 1964 Nobel laureate and pioneer of high-pressure research Percy Bridgman. The naming does more than fix a vexing gap in scientific lingo; it also will aid our understanding of the deep Earth.

To determine the makeup of the inner layers of the Earth, scientists need to test materials under extreme pressure and temperatures. For decades, scientists have believed a dense perovskite structure makes up 38 percent of the Earth’s volume, and that the chemical and physical properties of Bridgmanite have a large influence on how elements and heat flow through the Earth’s mantle. But since the mineral failed to survive the trip to the surface, no one has been able to test and prove its existence – a requirement for getting a name by the International Mineralogical Association.

Shock-compression that occurs in collisions of asteroid bodies in the solar system create the same hostile conditions of the deep Earth – roughly 2,100 degrees Celsius (3,800 degrees Farenheit) and pressures of about 240,000 times greater than sea-level air pressure. The shock occurs fast enough to inhibit the Bridgmanite breakdown that takes place when it comes under lower pressure, such as the Earth’s surface. Part of the debris from these collisions falls on Earth as meteorites, with the Bridgmanite “frozen” within a shock-melt vein. Previous tests on meteorites using transmission electron microscopy caused radiation damage to the samples and incomplete results.

So the team decided to try a new tactic: non-destructive micro-focused X-rays for diffraction analysis and novel fast-readout area-detector techniques. Tschauner and his colleagues from Caltech and the GeoSoilEnviroCARS, a University of Chicago-operated X-ray beamline at the APS at Argonne National Laboratory, took advantage of the X-rays’ high energy, which gives them the ability to penetrate the meteorite, and their intense brilliance, which leaves little of the radiation behind to cause damage.

The team examined a section of the highly shocked L-chondrite meteorite Tenham, which crashed in Australia in 1879. The GSECARS beamline was optimal for the study because it is one of the nation’s leading locations for conducting high-pressure research.

Bridgmanite grains are rare in the Tenhma meteorite, and they are smaller than 1 micrometer in diameter. Thus the team had to use a strongly focused beam and conduct highly spatially resolved diffraction mapping until an aggregate of Bridgmanite was identified and characterized by structural and compositional analysis.

This first natural specimen of Bridgmanite came with some surprises: It contains an unexpectedly high amount of ferric iron, beyond that of synthetic samples. Natural Bridgmanite also contains much more sodium than most synthetic samples. Thus the crystal chemistry of natural Bridgmanite provides novel crystal chemical insights. This natural sample of Bridgmanite may serve as a complement to experimental studies of deep mantle rocks in the future.

Prior to this study, knowledge about Bridgmanite’s properties has only been based on synthetic samples because it only remains stable below 660 kilometers (410 miles) depth at pressures of above 230 kbar (23 GPa). When it is brought out of the inner Earth, the lower pressures transform it back into less dense minerals. Some scientists believe that some inclusions on diamonds are the marks left by Bridgmanite that changed as the diamonds were unearthed.

The team’s results were published in the November 28 issue of the journal Science as “Discovery of bridgmanite, the most abundant mineral in Earth, in a shocked meteorite,” by O. Tschauner at University of Nevada in Las Vegas, N.V.; C. Ma; J.R. Beckett; G.R. Rossman at California Institute of Technology in Pasadena, Calif.; C. Prescher; V.B. Prakapenka at University of Chicago in Chicago, IL.

This research was funded by the U.S. Department of Energy, NASA, and NSF.

Asteroid impacts on Earth make structurally bizarre diamonds

Diamond grains from the Canyon Diablo meteorite are shown. The tick marks are spaced one-fifth of a millimeter (200 microns) apart. -  Arizona State University/Laurence Garvie
Diamond grains from the Canyon Diablo meteorite are shown. The tick marks are spaced one-fifth of a millimeter (200 microns) apart. – Arizona State University/Laurence Garvie

Scientists have argued for half a century about the existence of a form of diamond called lonsdaleite, which is associated with impacts by meteorites and asteroids. A group of scientists based mostly at Arizona State University now show that what has been called lonsdaleite is in fact a structurally disordered form of ordinary diamond.

The scientists’ report is published in Nature Communications, Nov. 20, by Péter Németh, a former ASU visiting researcher (now with the Research Centre of Natural Sciences of the Hungarian Academy of Sciences), together with ASU’s Laurence Garvie, Toshihiro Aoki and Peter Buseck, plus Natalia Dubrovinskaia and Leonid Dubrovinsky from the University of Bayreuth in Germany. Buseck and Garvie are with ASU’s School of Earth and Space Exploration, while Aoki is with ASU’s LeRoy Eyring Center for Solid State Science.

“So-called lonsdaleite is actually the long-familiar cubic form of diamond, but it’s full of defects,” says Péter Németh. These can occur, he explains, due to shock metamorphism, plastic deformation or unequilibrated crystal growth.

The lonsdaleite story began almost 50 years ago. Scientists reported that a large meteorite, called Canyon Diablo after the crater it formed on impact in northern Arizona, contained a new form of diamond with a hexagonal structure. They described it as an impact-related mineral and called it lonsdaleite, after Dame Kathleen Lonsdale, a famous crystallographer.

Since then, “lonsdaleite” has been widely used by scientists as an indicator of ancient asteroidal impacts on Earth, including those linked to mass extinctions. In addition, it has been thought to have mechanical properties superior to ordinary diamond, giving it high potential industrial significance. All this focused much interest on the mineral, although pure crystals of it, even tiny ones, have never been found or synthesized. That posed a long-standing puzzle.

The ASU scientists approached the question by re-examining Canyon Diablo diamonds and investigating laboratory samples prepared under conditions in which lonsdaleite has been reported.

Using the advanced electron microscopes in ASU’s Center for Solid State Science, the team discovered, both in the Canyon Diablo and the synthetic samples, new types of diamond twins and nanometer-scale structural complexity. These give rise to features attributed to lonsdaleite.

“Most crystals have regular repeating structures, much like the bricks in a well-built wall,” says Peter Buseck. However, interruptions can occur in the regularity, and these are called defects. “Defects are intermixed with the normal diamond structure, just as if the wall had an occasional half-brick or longer brick or row of bricks that’s slightly displaced to one side or another.”

The outcome of the new work is that so-called lonsdaleite is the same as the regular cubic form of diamond, but it has been subjected to shock or pressure that caused defects within the crystal structure.

One consequence of the new work is that many scientific studies based on the presumption that lonsdaleite is a separate type of diamond need to be re-examined. The study implies that both shock and static compression can produce an intensely defective diamond structure.

The new discovery also suggests that the observed structural complexity of the Canyon Diablo diamond results in interesting mechanical properties. It could be a candidate for a product with exceptional hardness.

The School of Earth and Space Exploration is an academic unit of ASU’s College of Liberal Arts and Sciences.

Asteroid impacts on Earth make structurally bizarre diamonds

Diamond grains from the Canyon Diablo meteorite are shown. The tick marks are spaced one-fifth of a millimeter (200 microns) apart. -  Arizona State University/Laurence Garvie
Diamond grains from the Canyon Diablo meteorite are shown. The tick marks are spaced one-fifth of a millimeter (200 microns) apart. – Arizona State University/Laurence Garvie

Scientists have argued for half a century about the existence of a form of diamond called lonsdaleite, which is associated with impacts by meteorites and asteroids. A group of scientists based mostly at Arizona State University now show that what has been called lonsdaleite is in fact a structurally disordered form of ordinary diamond.

The scientists’ report is published in Nature Communications, Nov. 20, by Péter Németh, a former ASU visiting researcher (now with the Research Centre of Natural Sciences of the Hungarian Academy of Sciences), together with ASU’s Laurence Garvie, Toshihiro Aoki and Peter Buseck, plus Natalia Dubrovinskaia and Leonid Dubrovinsky from the University of Bayreuth in Germany. Buseck and Garvie are with ASU’s School of Earth and Space Exploration, while Aoki is with ASU’s LeRoy Eyring Center for Solid State Science.

“So-called lonsdaleite is actually the long-familiar cubic form of diamond, but it’s full of defects,” says Péter Németh. These can occur, he explains, due to shock metamorphism, plastic deformation or unequilibrated crystal growth.

The lonsdaleite story began almost 50 years ago. Scientists reported that a large meteorite, called Canyon Diablo after the crater it formed on impact in northern Arizona, contained a new form of diamond with a hexagonal structure. They described it as an impact-related mineral and called it lonsdaleite, after Dame Kathleen Lonsdale, a famous crystallographer.

Since then, “lonsdaleite” has been widely used by scientists as an indicator of ancient asteroidal impacts on Earth, including those linked to mass extinctions. In addition, it has been thought to have mechanical properties superior to ordinary diamond, giving it high potential industrial significance. All this focused much interest on the mineral, although pure crystals of it, even tiny ones, have never been found or synthesized. That posed a long-standing puzzle.

The ASU scientists approached the question by re-examining Canyon Diablo diamonds and investigating laboratory samples prepared under conditions in which lonsdaleite has been reported.

Using the advanced electron microscopes in ASU’s Center for Solid State Science, the team discovered, both in the Canyon Diablo and the synthetic samples, new types of diamond twins and nanometer-scale structural complexity. These give rise to features attributed to lonsdaleite.

“Most crystals have regular repeating structures, much like the bricks in a well-built wall,” says Peter Buseck. However, interruptions can occur in the regularity, and these are called defects. “Defects are intermixed with the normal diamond structure, just as if the wall had an occasional half-brick or longer brick or row of bricks that’s slightly displaced to one side or another.”

The outcome of the new work is that so-called lonsdaleite is the same as the regular cubic form of diamond, but it has been subjected to shock or pressure that caused defects within the crystal structure.

One consequence of the new work is that many scientific studies based on the presumption that lonsdaleite is a separate type of diamond need to be re-examined. The study implies that both shock and static compression can produce an intensely defective diamond structure.

The new discovery also suggests that the observed structural complexity of the Canyon Diablo diamond results in interesting mechanical properties. It could be a candidate for a product with exceptional hardness.

The School of Earth and Space Exploration is an academic unit of ASU’s College of Liberal Arts and Sciences.

New evidence for oceans of water deep in the Earth

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.

New evidence for oceans of water deep in the Earth

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.

Diamonds in Earth’s oldest zircons are nothing but laboratory contamination

This image explains how synthetic diamond can be distinguished from natural diamond. -  Dobrzhinetskaya Lab, UC Riverside.
This image explains how synthetic diamond can be distinguished from natural diamond. – Dobrzhinetskaya Lab, UC Riverside.

As is well known, the Earth is about 4.6 billion years old. No rocks exist, however, that are older than about 3.8 billion years. A sedimentary rock section in the Jack Hills of western Australia, more than 3 billion years old, contains within it zircons that were eroded from rocks as old as about 4.3 billion years, making these zircons, called Jack Hills zircons, the oldest recorded geological material on the planet.

In 2007 and 2008, two research papers reported in the journal Nature that a suite of zircons from the Jack Hills included diamonds, requiring a radical revision of early Earth history. The papers posited that the diamonds formed, somehow, before the oldest zircons – that is, before 4.3 billion years ago – and then were recycled repeatedly over a period of 1.2 billion years during which they were periodically incorporated into the zircons by an unidentified process.

Now a team of three researchers, two of whom are at the University of California, Riverside, has discovered using electron microscopy that the diamonds in question are not diamonds at all but broken fragments of a diamond-polishing compound that got embedded when the zircon specimen was prepared for analysis by the authors of the Nature papers.

“The diamonds are not indigenous to the zircons,” said Harry Green, a research geophysicist and a distinguished professor of the Graduate Division at UC Riverside, who was involved in the research. “They are contamination. This, combined with the lack of diamonds in any other samples of Jack Hills zircons, strongly suggests that there are no indigenous diamonds in the Jack Hills zircons.”

Study results appear online this week in the journal Earth and Planetary Science Letters.

“It occurred to us that a long-term history of diamond recycling with intermittent trapping into zircons would likely leave some sort of microstructural record at the interface between the diamonds and zircon,” said Larissa Dobrzhinetskaya, a professional researcher in the Department of Earth Sciences at UCR and the first author of the research paper. “We reasoned that high-resolution electron microscopy of the material should be able to distinguish whether the diamonds are indeed what they have been believed to be.”

Using an intensive search with high-resolution secondary-electron imaging and transmission electron microscopy, the research team confirmed the presence of diamonds in the Jack Hills zircon samples they examined but could readily identify them as broken fragments of diamond paste that the original authors had used to polish the zircons for examination. They also observed quartz, graphite, apatite, rutile, iron oxides, feldspars and other low-pressure minerals commonly included into zircon in granitic rocks.

“In other words, they are contamination from polishing with diamond paste that was mechanically injected into silicate inclusions during polishing” Green said.

The research was supported by a grant from the National Science Foundation.

Green and Dobrzhinetskaya were joined in the research by Richard Wirth at the Helmholtz Centre Potsdam, Germany.

Dobrzhinetskaya and Green planned the research project; Dobrzhinetskaya led the project; she and Wirth did the electron microscopy.

Diamonds in Earth’s oldest zircons are nothing but laboratory contamination

This image explains how synthetic diamond can be distinguished from natural diamond. -  Dobrzhinetskaya Lab, UC Riverside.
This image explains how synthetic diamond can be distinguished from natural diamond. – Dobrzhinetskaya Lab, UC Riverside.

As is well known, the Earth is about 4.6 billion years old. No rocks exist, however, that are older than about 3.8 billion years. A sedimentary rock section in the Jack Hills of western Australia, more than 3 billion years old, contains within it zircons that were eroded from rocks as old as about 4.3 billion years, making these zircons, called Jack Hills zircons, the oldest recorded geological material on the planet.

In 2007 and 2008, two research papers reported in the journal Nature that a suite of zircons from the Jack Hills included diamonds, requiring a radical revision of early Earth history. The papers posited that the diamonds formed, somehow, before the oldest zircons – that is, before 4.3 billion years ago – and then were recycled repeatedly over a period of 1.2 billion years during which they were periodically incorporated into the zircons by an unidentified process.

Now a team of three researchers, two of whom are at the University of California, Riverside, has discovered using electron microscopy that the diamonds in question are not diamonds at all but broken fragments of a diamond-polishing compound that got embedded when the zircon specimen was prepared for analysis by the authors of the Nature papers.

“The diamonds are not indigenous to the zircons,” said Harry Green, a research geophysicist and a distinguished professor of the Graduate Division at UC Riverside, who was involved in the research. “They are contamination. This, combined with the lack of diamonds in any other samples of Jack Hills zircons, strongly suggests that there are no indigenous diamonds in the Jack Hills zircons.”

Study results appear online this week in the journal Earth and Planetary Science Letters.

“It occurred to us that a long-term history of diamond recycling with intermittent trapping into zircons would likely leave some sort of microstructural record at the interface between the diamonds and zircon,” said Larissa Dobrzhinetskaya, a professional researcher in the Department of Earth Sciences at UCR and the first author of the research paper. “We reasoned that high-resolution electron microscopy of the material should be able to distinguish whether the diamonds are indeed what they have been believed to be.”

Using an intensive search with high-resolution secondary-electron imaging and transmission electron microscopy, the research team confirmed the presence of diamonds in the Jack Hills zircon samples they examined but could readily identify them as broken fragments of diamond paste that the original authors had used to polish the zircons for examination. They also observed quartz, graphite, apatite, rutile, iron oxides, feldspars and other low-pressure minerals commonly included into zircon in granitic rocks.

“In other words, they are contamination from polishing with diamond paste that was mechanically injected into silicate inclusions during polishing” Green said.

The research was supported by a grant from the National Science Foundation.

Green and Dobrzhinetskaya were joined in the research by Richard Wirth at the Helmholtz Centre Potsdam, Germany.

Dobrzhinetskaya and Green planned the research project; Dobrzhinetskaya led the project; she and Wirth did the electron microscopy.