Diamonds show depth extent of Earth’s carbon cycle

Scientists have speculated for some time that the Earth’s carbon cycle extends deep into the planet’s interior, but until now there has been no direct evidence. The mantle-Earth’s thickest layer -is largely inaccessible. A team of researchers analyzed diamonds that originated from the lower mantle at depths of 435 miles (700 kilometers) or more, and erupted to the surface in volcanic rocks called kimberlites. The diamonds contain what are impurities to the gemologist, but are known as mineral inclusions to the geologist. Analysis shows compositions consistent with the mineralogy of oceanic crust. This finding is the first direct evidence that slabs of oceanic crust sank or subducted into the lower mantle and that material, including carbon, is cycled between Earth’s surface and depths of hundreds of miles. The research is published in the September 15, 2011, online Science Express.

The mantle extends from as little as 5 to 1,800 miles (10-2,900 kilometers) beneath the Earth’s surface. Most diamonds are free from inclusions and come from depths less than 120 miles (200 km). But in a few localities researchers have found super-deep diamonds from the depths of the convecting upper and lower mantle, as well as the transition zone in between. Whereas inclusions in diamonds from the depths of the upper mantle and transition zone have been consistent with a surface-rock origin, none from the lower mantle have borne this signature until now.

The team,* which included Carnegie scientists, was led by former Carnegie postdoctoral fellow Michael Walter, now a professor at the University of Bristol, UK. The scientists analyzed minute (one to two hundredths of a millimeter) mineral grains from six diamonds from the Juina region in Brazil. The analysis showed that diamond inclusions initially crystallized as a single mineral that could form only at depths greater than 435 miles (700 km). But the inclusions recrystallized into multiple minerals as they were carried up to the surface-first probably from a mantle upwelling known as a plume, then as they erupted to the surface in kimberlites

The diamonds were analyzed for carbon at Carnegie. Four of the diamonds contained low amounts of carbon-13, a signature not found in the lower mantle and consistent with an ocean-crust origin at Earth’s surface. “The carbon identified in other super-deep, lower mantle diamonds is chiefly mantle-like in composition,” remarked co-author Steven Shirey * at Carnegie. “We looked at the variations in the isotopes of the carbon atoms in the diamonds. Carbon originating in a rock called basalt, which forms from lava at the surface, is often different from that which originates in the mantle, in containing relatively less carbon-13. These super-deep diamonds contained much less carbon-13, which is most consistent with an origin in the organic component found in altered oceanic crust.”

“I find it astonishing that we can use the tiniest of mineral grains to show some of the motions of the Earth’s mantle at the largest scales,” concluded Shirey.

Tackling mysteries about carbon, possible oil formation and more deep inside Earth

How do diamonds the size of potatoes shoot up at 40 miles per hour from their birthplace 100 miles below Earth’s surface? Does a secret realm of life exist inside the Earth? Is there more oil and natural gas than anyone dreams, with oil forming not from the remains of ancient fossilized plants and animals near the surface, but naturally deep, deep down there? Can the greenhouse gas, carbon dioxide, be transformed into a pure solid mineral?

Those are among the mysteries being tackled in a real-life version of the science fiction classic, A Journey to the Center of the Earth, that was among the topics of a presentation here today at the 242nd National Meeting & Exposition of the American Chemical Society (ACS). Russell Hemley, Ph.D., said that hundreds of scientists will work together on an international project, called the Deep Carbon Observatory (DCO), to probe the chemical element that’s in the news more often than perhaps any other. That’s carbon as in carbon dioxide.

“Concerns about climate change have made millions of people aware of carbon’s role on the surface of the Earth, in the atmosphere and in the oceans,” Hemley said. “The Deep Carbon Observatory will uncover critical information about the movement and fate of carbon hundreds and thousands of miles below Earth’s surface. We call that the deep carbon cycle.”

Hemley said this basic research could have practical implications in the future. Using laboratory equipment that reproduces pressures deep within the Earth, which are thousands to millions of times higher than on the surface, scientists in these labs have discovered a way to convert carbon dioxide into a rock-like material called polymeric carbon dioxide. With further refinements, scientists could enhance its stability closer to the Earth’s surface.

The findings also may lead to new materials for commercial and industrial products. Hemley’s laboratory, for instance, has developed a way to produce “super” diamonds, or high-quality diamonds that are bigger and better than existing ones. Natural diamonds form slowly under the high-pressure, high-temperature conditions that exist deep within the Earth, while today’s synthetic diamonds form under similar conditions in the laboratory. Using a process called chemical vapor deposition, Hemley’s research group made diamonds rapidly and at low pressure. The new diamonds have superior qualities, including extreme hardness, improved transparency and better electrical and temperature properties. The diamonds could lead to improved computer chips that run faster and generate less heat than existing silicon chips, Hemley said. They also show promise for use in advanced cutting-tools, more durable and heat-resistant windows for spacecraft and other applications, he noted.

The DCO project will probe the big mystery about the formation of natural diamonds, including their chemical composition and how they shoot up quickly from deep within the Earth. Scientists can’t directly observe that process at present, as there’s no practical way to travel down 100 miles beneath the surface of the planet. Observations are limited to laboratory simulations of this process for now, said Hemley, who is director of the Geophysical Laboratory at the Carnegie Institution of Washington in Washington, D.C. His laboratory specializes in the chemistry and physics of materials under extreme conditions. Hemley’s presentation at the ACS meeting, entitled “Chemistry of Planetary Gases, Liquids, and Ices in Extreme Environments,” focused on what happens to planetary material under conditions of extreme pressure and temperature, as well as other insights relevant to Earth.

Another area that the DCO will explore is energy. The extent to which hydrocarbons in the Earth form from inorganic processes deep within the Earth rather than only from the fossilized remains of plants and animals remains an important unanswered question. Exploring the nature of carbon deep within the Earth may provide clues on how and to what extent this abiotic process might contribute to energy reserves, Hemley said.

Finally, DCO research has implications in the search for other life forms on Earth and even outer space. Scientists have already identified microbes at about a mile or so deep within the Earth under high temperatures. They suspect that some forms may exist at even deeper levels.

Past studies suggest that bacteria and other life forms can’t survive beyond several thousand atmospheres of pressure. But new studies by scientists in Hemley’s lab show that some bacteria are capable of surviving pressures of up to 20,000 atmospheres. That supports the theory that life might exist in extreme extraterrestrial environments, Hemley noted.

Diamond impurities bonanza for geologists studying Earth’s history

This is an optical photomicrograph of a sulfide-inclusion-bearing rough diamond from Botswana. -  Steven Shirey
This is an optical photomicrograph of a sulfide-inclusion-bearing rough diamond from Botswana. – Steven Shirey

Jewelers abhor diamond impurities, but they are a bonanza for scientists.

Safely encased in super-hard diamond, impurities are unaltered, ancient minerals that tell the story of Earth’s distant past.

Researchers analyzed data from more than 4,000 of these mineral inclusions to find that continents started the cycle of breaking apart, drifting, and colliding about three billion years ago.

The research results, published in this week’s issue of the journal Science, pinpoint when this so-called Wilson cycle began.

Lead author Steven Shirey of the Carnegie Institution’s Department of Terrestrial Magnetism says that the Wilson cycle is responsible for the growth of the Earth’s continental crust, the continental structures we see today, the opening and closing of ocean basins through time, mountain building, and the distribution of ores and other materials in the crust.

“But when it all began has remained elusive until now,” Shirey says.

“We used the impurities, or inclusions, contained in diamonds, because they are perfect time capsules from great depth beneath the continents.

“They provide age and chemical information for a span of more than 3.5 billion years that includes the evolution of the atmosphere, the growth of the continental crust, and the beginning of plate tectonics.”

Co-author Stephen Richardson of the University of Cape Town says that it’s “astonishing that we can use the smallest mineral grains that can be analyzed to reveal the origin of some of Earth’s largest geological features.”

“The tiny inclusions found inside diamonds studied by this team have recorded the chemistry and evolution of the Earth over 3.5 billion years,” says Jennifer Wade, program director in the National Science Foundation (NSF)’s Division of Earth Sciences, which funded the research. “They help pinpoint when the cycle of plate tectonics first began on Earth.”

The largest diamonds come from cratons, the most ancient formations within continental interiors that have deep mantle roots or keels around which younger continental material gathered.

Cratons contain the oldest rocks on the planet, and their keels extend into the mantle more than 125 miles where pressures are sufficiently high, but temperatures sufficiently low, for diamonds to form and be stored for billions of years.

Over time, diamonds have arrived at the surface as accidental passengers during volcanic eruptions of deep magma that solidified into rocks called kimberlites.

The inclusions in diamonds come in two major varieties: peridotitic and eclogitic.

Peridotite is the most abundant rock type in the upper mantle, whereas eclogite is generally thought to be the remnant of oceanic crust recycled into the mantle by the subduction or sinking of tectonic plates.

Shirey and Richardson reviewed the data from more than 4,000 inclusions of silicate–the Earth’s most abundant material–and more than 100 inclusions of sulfide from five ancient continents.

The most crucial aspects, they say, looked at when the inclusions were encapsulated and the associated compositional trends.

Compositions vary and depend on the geochemical processing that precursor components underwent before they were encapsulated.

Two systems used to date inclusions were compared. Both rely on natural isotopes that decay at exceedingly slow but predictable rates–about one disintegration every ten years on the scale of an inclusion–making them excellent atomic clocks for determining absolute ages.

The researchers found that before 3.2 billion years ago, only diamonds with peridotitic compositions formed, whereas after three billion years ago, eclogitic diamonds dominated.

“The simplest explanation,” says Shirey, “is that this change came from the initial subduction of one tectonic plate under the deep mantle keel of another as continents began to collide on a scale similar to that of the supercontinent cycle today.

“The sequence of underthrusting and collision led to the capture of eclogite in the subcontinental mantle keel along with the fluids that are needed to make diamond.”

Concludes Richardson, “This transition marks the onset of the Wilson cycle of plate tectonics.”

Diamonds pinpoint start of colliding continents

Jewelers abhor diamond impurities, but they are a bonanza for scientists. Safely encased in the super-hard diamond, impurities are unaltered, ancient minerals that can tell the story of Earth’s distant past. Researchers analyzed data from the literature of over 4,000 of these mineral inclusions to find that continents started the cycle of breaking apart, drifting, and colliding about 3 billion years ago. The research, published in the July 22, 2011, issue of Science, pinpoints when this so-called Wilson cycle began.

Lead author Steven Shirey at the Carnegie Institution’s Department of Terrestrial Magnetism explained: “The Wilson cycle is responsible for the growth of the Earth’s continental crust, the continental structures we see today, the opening and closing of ocean basins through time, mountain building, and the distribution of ores and other materials in the crust. But when it all began has remained elusive until now. We used the impurities, or inclusions, contained in diamonds, because they are perfect time capsules from great depth beneath the continents. They provide age and chemical information for a span of more than 3.5 billion years that includes the evolution of the atmosphere, the growth of the continental crust, and the beginning of plate tectonics.”

Coauthor and longtime colleague Stephen Richardson of the University of Cape Town added: “It is astonishing that we can use the smallest mineral grains that can be analyzed to reveal the origin of some of Earth’s largest geological features.”

The largest diamonds come from cratons, the most ancient formations within continental interiors that have deep mantle roots or keels around which younger continental material gathered. Cratons contain the oldest rocks on the planet, and their keels extend into the mantle more than 125 miles (200 km) where pressures are sufficiently high, but temperatures sufficiently low, for diamonds to form and be stored for billions of years. The diamonds arrived at the surface as accidental passengers during volcanic eruptions of deep magma that solidified into rocks called kimberlites. The inclusions in diamonds come in two major varieties: peridotitic and eclogitic. Peridotite is the most abundant rock type in the upper mantle, whereas eclogite is generally thought to be the remnant of oceanic crust recycled into the mantle by the subduction or sinking of tectonic plates.

Shirey and Richardson, using their own work with other coinvestigators published in more than 20 papers over a 25-year period, reviewed the data from more than 4,000 inclusions of silicate-the Earth’s most abundant material-and more than 100 inclusions of sulfide from five ancient continents. The most crucial aspects were to look at when the inclusions were encapsulated and the associated compositional trends. Compositions vary and depend on the geochemical processing that precursor components underwent before they were encapsulated.

Two systems used to date inclusions-the rhenium-osmium and samarium-neodymium techniques-were compared. Both rely on natural isotopes that decay at exceedingly slow but predictable rates- around one disintegration every ten years on the scale of an inclusion-making them excellent atomic clocks for determining absolute ages.

The researchers found that before 3.2 billion years ago, only diamonds with peridotitic compositions formed-whereas subsequent to 3 billion years ago, eclogitic diamonds dominated. “The simplest explanation is that this change came from the initial subduction of one tectonic plate under the deep mantle keel of another as continents began to collide on a scale similar to that of the supercontinent cycle today. The sequence of underthrusting and collision led to the capture of eclogite in the subcontinental mantle keel along with the fluids that are needed to make diamonds.” remarked Shirey. “This transition marks the onset of the Wilson cycle of plate tectonics,” concluded Richardson.

Scientists’ work improves odds of finding diamonds

While prospectors and geologists have been successful in finding diamonds through diligent searching, one University of Houston professor and his team’s work could help improve the odds by focusing future searches in particular areas. Kevin Burke, professor of geology and tectonics at UH, and his fellow researchers describe these findings in a paper titled “Diamonds Sampled by Plumes from the Core-Mantle Boundary,” appearing July 15 in Nature, the weekly scientific research journal.

While prospectors and geologists have been successful in finding diamonds through diligent searching, one University of Houston professor and his team’s work could help improve the odds by focusing future searches in particular areas.

Kevin Burke, professor of geology and tectonics at UH, and his fellow researchers describe these findings in a paper titled “Diamonds Sampled by Plumes from the Core-Mantle Boundary,” appearing July 15 in Nature, the weekly scientific research journal.

Burke’s team found that kimberlites, which are rare volcanic rocks that include diamonds, owe their origin to occasional pulses of hot mantle rock – called mantle plumes – that have risen through the entire thickness of the Earth’s mantle from deep down next to the core, or innermost part, of the planet. This core/mantle boundary lies at a depth of about 2,000 miles. While the idea there might be mantle plumes rising from the core/mantle boundary was first suggested about 40 years ago, it is only within the past few years that evidence of plumes coming all the way from this boundary to the Earth’s surface has been clearly demonstrated by Burke’s group.

“Our approach is new, because it combines observations of the Earth’s deep interior from seismology with evidence of how tectonic plates have moved about on the Earth’s surface during the past 500 million years,” Burke said. “I have been interested in mantle plumes from the core/mantle boundary since they were first hypothesized in 1971. About 10 years ago, I realized there might be a link between the seismically defined structure at the core/mantle boundary and volcanic rocks at the Earth’s surface that had been suggested to be linked to mantle plumes. I immediately realized how the existence of that link could be tested, and it was then that I came in contact with Trond Torsvik in Norway, who proved to be uniquely qualified to carry out the required tests.”

Torsvik, a professor at the University of Oslo in Norway, and Burke developed the conceptual ideas for this research. Additional members of the team were Bernhard Steinberger at the Helmholtz Centre Potsdam in Germany, and Lew Ashwal and Sue Webb from the University of the Witwatersrand in South Africa. The research consisted of applying and interpreting the results of mathematical analysis, much of it applying spherical geometry to the Earth’s surface, to publicly available data-sets put together mainly by Ashwal, Webb and Torsvik.

The present structure of the Earth’s mantle has been increasingly understood by researchers in seismology during the past 25 years, and Burke and his colleagues’ work has helped confirm the seismologists’ results. The work of the Burke group, however, also describes the structure as it was in the past, revealing the history of deep mantle structure over the geologically long period of 500 million years. That, Burke said, is new.

“Establishing the history of deep mantle structure has shown, unexpectedly, that two large volumes lying just above the core/mantle boundary have been stable in their present positions for the past 500 million years,” he said. “The reason this result was not expected is that those of us who study the Earth’s deep interior have assumed that, although the deep mantle is solid, the material making it up would all be in motion all the time, because the deep mantle is so hot and under such high pressure from the weight of rock above it.”

As for how this improves the odds of finding these precious gems, Burke explained that geologists interested in diamonds have known for more than 50 years that rare diamond-bearing kimberlite volcanic rocks are highly concentrated in ancient cratons within areas of the Earth’s continents. This has concentrated the search for diamond-bearing rocks within an area amounting to no more than about 10 percent of the entire area of the world’s continents. The new work has shown that most of the kimberlites have been erupted into one or the other of those old cratons only under certain conditions. These findings will enable the search for diamonds to be further concentrated.

Ultimately aiming for a better integrated understanding of how the solid Earth of the crust and mantle works, the group hopes to obtain further results within months. They hope to better establish how plate motions at the Earth’s surface have evolved over the last 500 million years and how to work out just how those movements have related to both the stable and the moving parts of the Earth’s mantle during the same interval.

Tiny diamonds on Santa Rosa Island give evidence of cosmic impact

This is James Kennett (left) and Douglas J. Kennett. -  UCSB
This is James Kennett (left) and Douglas J. Kennett. – UCSB

Nanosized diamonds found just a few meters below the surface of Santa Rosa Island off the coast of Santa Barbara provide strong evidence of a cosmic impact event in North America approximately 12,900 years ago, according to a new study by scientists. Their hypothesis holds that fragments of a comet struck across North America at that time.

The research, published this week in the Proceedings of the National Academy of Sciences (PNAS), was led by James Kennett, professor emeritus at UC Santa Barbara, and Douglas J. Kennett, first author, of the University of Oregon. The two are a father-son team. They were joined by 15 other researchers.

“The pygmy mammoth, the tiny island version of the North American mammoth, died off at this time,” said James Kennett. “Since it coincides with this event, we suggest it is related.” He explained that this site, with its layer containing hexagonal diamonds, is also associated with other types of diamonds and with dramatic environmental changes and wildfires. They are part of a sedimentary layer known as the Younger Dryas Boundary.

“There was a major event 12,900 years ago,” said James Kennett. “It is hard to explain this assemblage of materials without a cosmic impact event and associated extensive wildfires. This hypothesis fits with the abrupt climatic cooling as recorded in ocean-drilled sediments beneath the Santa Barbara Channel. The cooling resulted when dust from the high-pressure, high-temperature, multiple impacts was lofted into the atmosphere, causing a dramatic drop in solar radiation.”

The tiny diamonds were buried below four meters of sediment and they correspond with the disappearance of the Clovis culture — the first well-established and distributed North American peoples. An estimated 35 types of mammals and 19 types of birds also became extinct in North America about this time.

“The type of diamond we have found — lonsdaleite — is a shock-synthesized mineral defined by its hexagonal crystalline structure,” said Douglas Kennett, associate professor of anthropology at the University of Oregon. “It forms under very high temperatures and pressures consistent with a cosmic impact. These diamonds have only been found thus far in meteorites and impact craters on earth, and appear to be the strongest indicator yet of a significant cosmic impact [during Clovis].”

The diamonds were found in association with soot, which forms in extremely hot fires, and they suggest associated regional wildfires, based on nearby environmental records. Such soot and diamonds are rare in the geological record. They were found in sediment dating to massive asteroid impacts 65 million years ago in a layer widely known as the K-T Boundary, known to be associated with the extinction of dinosaurs and many other types of organisms.

6 North American sites hold 12,900-year-old nanodiamond-rich soil

Abundant tiny particles of diamond dust exist in sediments dating to 12,900 years ago at six North American sites, adding strong evidence for Earth’s impact with a rare swarm of carbon-and-water-rich comets or carbonaceous chondrites, reports a nine-member scientific team.

These nanodiamonds, which are produced under high-temperature, high-pressure conditions created by cosmic impacts and have been found in meteorites, are concentrated in similarly aged sediments at Murray Springs, Ariz., Bull Creek, Okla., Gainey, Mich., and Topper, S.C., as well as Lake Hind, Manitoba, and Chobot, Alberta, in Canada. Nanodiamonds can be produced on Earth, but only through high-explosive detonations or chemical vaporization.

Last year a 26-member team from 16 institutions proposed that a cosmic impact event, possibly by multiple airbursts of comets, set off a 1,300-year-long cold spell known as the Younger Dryas, fragmented the prehistoric Clovis culture and led to the extinction of a large range of animals, including mammoths, across North America. The team’s paper was published in the Oct. 9, 2007, issue of the Proceedings of the National Academy of Sciences. (News release on the 2007 paper is available at: http://tinyurl.com/82988t, with link to a copy of that paper.)

Now, reporting in the Jan. 2 issue of the journal Science, a team led by the University of Oregon’s Douglas J. Kennett, a member of the original research team, report finding billions of nanometer-sized diamonds concentrated in sediments — weighing from about 10 to 2,700 parts per billion — in the six locations during digs funded by the National Science Foundation.

“The nanodiamonds that we found at all six locations exist only in sediments associated with the Younger Dryas Boundary layers, not above it or below it,” said Kennett, a UO archaeologist. “These discoveries provide strong evidence for a cosmic impact event at approximately 12,900 years ago that would have had enormous environmental consequences for plants, animals and humans across North America.”

The Clovis culture of hunters and gatherers was named after hunting tools referred to as Clovis points, first discovered in a mammoth’s skeleton in 1926 near Clovis, N.M. Clovis sites later were identified across the United States, Mexico and Central America. Clovis people possibly entered North America across a land bridge from Siberia. The peak of the Clovis era is generally considered to have run from 13,200 to 12,900 years ago. One of the diamond-rich sediment layers reported sits directly on top of Clovis materials at the Murray Springs site.

X-rays use diamonds as a window to the center of the Earth


Diamonds from Brazil have provided the answers to a question that Earth scientists have been trying to understand for many years: how is oceanic crust that has been subducted deep into the Earth recycled back into volcanic rocks? A team of researchers, led by the University of Bristol, working alongside colleagues at the STFC Daresbury Laboratory, have gained a deeper insight into how the Earth recycles itself in the deep earth tectonic cycle way beyond the depths that can be accessed by drilling. The full paper on this research has been published (31 July) in the scientific journal, Nature.



The Earth’s oceanic crust is constantly renewed in a cycle which has been occurring for billions of years. This crust is constantly being renewed from below by magma from the Earth’s mantle that has been forced up at mid-ocean ridges. This crust is eventually returned to the mantle, sinking down at subduction zones that extend deep beneath the continents. Seismic imaging suggests that the oceanic crust can be subducted to depths of almost 3000km below the Earth’s surface where it can remain for billions of years, during which time the crust material develops its own unique ‘flavour’ in comparison with the surrounding magmas. Exactly how this happens is a question that has baffled Earth scientists for years.



The Earth’s oceanic crust lies under seawater for millions of years, and over time reacts with the seawater to form carbonate minerals, such as limestone, When subducted, these carbonate minerals have the effect of lowering the melting point of the crust material compared to that of the surrounding magma. It is thought that this melt is loaded with elements that carry the crustal ‘flavour’.



This team of researchers have now proven this theory by looking at diamonds from the Juina area of Brazil. As the carbonate-rich magma rises through the mantle, diamonds crystallise, trapping minute quantities of minerals in the process. They form at great depths and pressures and therefore can provide clues as to what is happening at the Earth’s deep interior, down to several hundred kilometres – way beyond the depths that can be physically accessed by drilling. Diamonds from the Juina area are particularly renowned for these mineral inclusions.


At the Synchrotron Radiation Source (SRS) at the STFC Daresbury Laboratory, the team used an intense beam of x-rays to look at the conditions of formation for the mineral perovskite which occurs in these diamonds but does not occur naturally near the Earth’s surface. With a focused synchrotron X-ray beam less than half the width of a human hair, they used X-ray diffraction techniques to establish the conditions at which perovskite is stable, concluding that these mineral inclusions were formed up to 700km into the Earth in the mantle transition zone.



These results, backed up by further experiments carried out at the University of Edinburgh, the University of Bayreuth in Germany, and the Advanced Light Source in the USA, enabled the research team to show that the diamonds and their perovskite inclusions had indeed crystallised from very small-degree melts in the Earth’s mantle. Upon heating, oceanic crust forms carbonatite melts, super-concentrated in trace elements with the ‘flavour’ of the Earth’s oceanic crust. Furthermore, such melts may be widespread throughout the mantle and may have been ‘flavouring’ the mantle rocks for a very long time.



Dr Alistair Lennie, a research scientist at STFC Daresbury Laboratory, said: “Using X-rays to find solutions to Earth science questions is an area that has been highly active on the SRS at Daresbury Laboratory for some time. We are very excited that the SRS has contributed to answering such long standing questions about the Earth in this way.”



Dr. Michael Walter, Department of Earth Sciences, University of Bristol, said: “The resources available at Daresbury’s SRS for high-pressure research have been crucial in helping us determine the origin of these diamonds and their inclusions.”

Diamonds show how Earth is recycled





A subduction zone
A subduction zone

Tiny minerals found inside diamonds have provided us with a rare glimpse of the Earth’s deepest secrets. This exciting new research by a team of scientists, led by the University of Bristol, is reported today (30 July) in Nature.



The Earth’s crust that underlies our oceans is constantly being made at mid-oceanic ridges which run down the centre of our oceans. There, magma derived from the mantle (the layer beneath the crust) is injected between diverging tectonic plates, pushing them apart. On the far side of each plate, old oceanic crust is eventually recycled by returning it to the mantle at subduction zones, huge trenches that dive deep beneath the continents.



Dr Michael Walter, from the Department of Earth Sciences at the University of Bristol, and lead author on the paper, said: “Exactly what happens to subducted oceanic crust is a long-standing question in Earth Sciences. Seismic imaging of subducted slabs has provided strong evidence that it can be taken to great depths, possibly even to the core-mantle boundary some 2,900 km below the Earth’s surface. There it can remain for billions of years in a kind of crustal graveyard. Its ultimate fate, however, remains uncertain.”



Dr Walter added: “There is also strong geochemical evidence that after stewing in the mantle for a very long time, say a billion years or so, oceanic crust acquires an isotopic ‘flavouring’ that is very different from the surrounding mantle. If this crust somehow makes its way into regions of the mantle that undergo melting, the new magmas will betray the ‘scent’ of ancient oceanic crust.”



But many questions remain as to exactly how oceanic crust yields its unique signature to magmas. Does solid crust waft around in the mantle forming a sort of marble cake that can then be melted? Or does the crust melt at great depth within the mantle and react with mantle rocks to form some sort of hybrid source rock?


Given the depths these rocks are taken to, neither of these possibilities seemed likely, since the pressures would be too high for rocks to melt at those depths. However, the team has found evidence from tiny mineral inclusions in diamonds that suggests oceanic crust can melt deep in the mantle and in this way imbue its flavouring into surrounding mantle rocks.



The rock that is erupted on the ocean floor (basalt) spends most of its life (hundreds of millions of years) exposed to seawater. Consequently, some portion of it reacts with the seawater to form carbonate minerals. The team speculated that the presence of these minerals in oceanic crust has the effect of lowering its melting point to temperatures much lower than that of the surrounding mantle.



Although there is not much carbonate in oceanic crust so only a little melt can be formed, this small-degree melt will be loaded with elements that carry a chemical signature of the crust. Subsequent melting of mantle rocks that contain these small carbonate melts (carbonatites) would then yield magmas that also carry the crustal signature. But how to prove this?



Diamonds require high pressures to form. As such, they provide clues to the Earth’s deep interior, well beyond the depths that can be directly accessed by drilling. As the carbonate-rich liquids ascend through the mantle, diamonds crystallise en route, trapping other minerals (inclusions) as they form. The team therefore studied diamonds from the Juina area in Brazil, a location famous for yielding diamonds with inclusions derived from the deep mantle.



After performing a large number of experiments, measurements and calculations, the researchers were able to show that the diamonds and their inclusions had indeed crystallized from very small-degree, carbonatite melts in the mantle. Furthermore, they speculate, such melts may be pervasive throughout the mantle and may have been imparting a crustal ‘stain’ on mantle rocks for a very long time.

Exploding Asteroid Theory Strengthened by New Evidence Located in Ohio, Indiana





Ken Tankersley
Ken Tankersley

Was the course of life on the planet altered 12,900 years ago by a giant comet exploding over Canada? New evidence found by UC Assistant Professor of Anthropology Ken Tankersley and colleagues suggests the answer is affirmative.



Geological evidence found in Ohio and Indiana in recent weeks is strengthening the case to attribute what happened 12,900 years ago in North America — when the end of the last Ice Age unexpectedly turned into a phase of extinction for animals and humans – to a cataclysmic comet or asteroid explosion over top of Canada.



A comet/asteroid theory advanced by Arizona-based geophysicist Allen West in the past two years says that an object from space exploded just above the earth’s surface at that time over modern-day Canada, sparking a massive shock wave and heat-generating event that set large parts of the northern hemisphere ablaze, setting the stage for the extinctions.



Now University of Cincinnati Assistant Professor of Anthropology Ken Tankersley, working in conjunction with Allen West and Indiana Geological Society Research Scientist Nelson R. Schaffer, has verified evidence from sites in Ohio and Indiana – including, locally, Hamilton and Clermont counties in Ohio and Brown County in Indiana – that offers the strongest support yet for the exploding comet/asteroid theory.



Samples of diamonds, gold and silver that have been found in the region have been conclusively sourced through X-ray diffractometry in the lab of UC Professor of Geology Warren Huff back to the diamond fields region of Canada.



The only plausible scenario available now for explaining their presence this far south is the kind of cataclysmic explosive event described by West’s theory. “We believe this is the strongest evidence yet indicating a comet impact in that time period,” says Tankersley.



Ironically, Tankersley had gone into the field with West believing he might be able to disprove West’s theory.



Tankersley was familiar through years of work in this area with the diamonds, gold and silver deposits, which at one point could be found in such abundance in this region that the Hopewell Indians who lived here about 2,000 years ago engaged in trade in these items.



Prevailing thought said that these deposits, which are found at a soil depth consistent with the time frame of the comet/asteroid event, had been brought south from the Great Lakes region by glaciers.



“My smoking gun to disprove (West) was going to be the gold, silver and diamonds,” Tankersley says. “But what I didn’t know at that point was a conclusion he had reached that he had not yet made public – that the likely point of impact for the comet wasn’t just anywhere over Canada, but located over Canada’s diamond-bearing fields. Instead of becoming the basis for rejecting his hypothesis, these items became the very best evidence to support it.”


Additional sourcing work is being done at the sites looking for iridium, micro-meteorites and nano-diamonds that bear the markers of the diamond-field region, which also should have been blasted by the impact into this region.



Much of the work is being done in Sheriden Cave in north-central Ohio’s Wyandot County, a rich repository of material dating back to the Ice Age.



Tankersley first came into contact with West and Schaffer when they were invited guests for interdisciplinary colloquia presented by UC’s Department of Geology this spring.



West presented on his theory that a large comet or asteroid, believed to be more than a mile in diameter, exploded just above the earth at a time when the last Ice Age appeared to be drawing to a close.



The timing attached to this theory of about 12,900 years ago is consistent with the known disappearances in North America of the wooly mammoth population and the first distinct human society to inhabit the continent, known as the Clovis civilization. At that time, climatic history suggests the Ice Age should have been drawing to a close, but a rapid change known as the Younger Dryas event, instead ushered in another 1,300 years of glacial conditions. A cataclysmic explosion consistent with West’s theory would have the potential to create the kind of atmospheric turmoil necessary to produce such conditions.



“The kind of evidence we are finding does suggest that climate change at the end of the last Ice Age was the result of a catastrophic event,” Tankersley says.



Currently, Tankersley can be seen in a new documentary airing on the National Geographic channel. The film “Asteroids” is part of that network’s “Naked Science” series.



The new discoveries made working with West and Schaffer will be incorporated into two more specials that Tankersley is currently involved with – one for the PBS series “Nova” and a second for the History Channel that will be filming Tankersley and his UC students in the field this summer. Another documentary, this one being produced by the Discovery Channel and the British public television network Channel 4, will also be following Tankersley and his students later this summer.



As more data continues to be compiled, Tankersley, West and Schaffer will be publishing about this newest twist in the search to explain the history of our planet and its climate.



Climate change is a favorite topic for Tankersley. “The ultimate importance of this kind of work is showing that we can’t control everything,” he says. “Our planet has been hit by asteroids many times throughout its history, and when that happens, it does produce climate change.”