Research Reveals The Scale At Which Earth’s Mantle Composition Varies

New research by Brown University geochemists provides new insights on the scale at which Earth’s mantle varies in chemical composition. The findings could help scientists better understand the mixing process of mantle convection, the slow churning that drives the movement of Earth’s tectonic plates.

“We know that the mantle is heterogeneous in composition, but it’s been difficult to figure out how large or small those heterogeneities might be,” said Boda Liu, a Ph.D. student in geology at Brown. “What we show here is that there must be heterogeneities of at least a kilometer in size to produce the chemical signature we observe in rocks derived from mantle materials.”

The research, which Liu co-authored with Yan Liang, a professor in Brown’s Department of Earth Environmental and Planetary Sciences, is published in Science Advances.

Earth’s crust is on a constantly moving conveyer belt driven by the convecting mantle. At mid-ocean ridges, the boundaries on the ocean floor where tectonic plates are pulling away from each other, new crust is created by eruption of magmas formed by the rising of the mantle materials from depth. At subductions zones, where one tectonic plate slides beneath another, old crust material, weathered by processes on the surface, is pushed back down into the mantle. This recycling can create mantle materials of different or “enriched” compositions, which geochemists refer to as “heterogeneities.” What happens to that enriched material once it’s recycled isn’t fully understood.

“This is one of the big questions in Earth science,” Liang said. “To what extent does mantle convection mix and homogenize these heterogeneities out? Or how might these heterogeneities be preserved?”

Scientists learn about the composition of the mantle by studying mid-ocean ridge basalts (MORBs), rocks formed by the solidification of magmas erupted on the seafloor. Like fingerprints, isotope compositions of MORBs can be used to trace the mantle source from which they were derived.

Another type of seafloor rock called abyssal peridotites is the leftover mantle after the formation of MORBs. These are chunks of mantle rock that once were the uppermost mantle and later uplifted to the seafloor. Abyssal peridotites have a different isotope composition than MORBs that appear to come from the same mantle region. To explain that difference in isotope compositions, scientists have concluded that the MORBs are capturing the isotope signal from pockets of enriched material—the remnants of subducted crust preserved in the mantle.

The question this new study sought to answer is how large those enriched pockets would need to be for their isotope signature to survive the trip to the surface. As magma rises toward the surface, it interacts with the ambient mantle, which would tend to dampen the signal of enriched material in the melt. For their study, Liu and Liang modeled the melting and magma transport processes. They found that in order to produce the different isotope signals between MORBs and abyssal peridotites, the pockets of enriched material at depth would need to be at least one kilometer in size.

“If the length scale of the heterogeneity is too small, the chemical exchange during magma flow would wipe the heterogeneities out,” Liang said. “So in order to produce the composition difference we see, our model shows that the heterogeneity needs to be a kilometer or more.”

The researchers hope their study will add a new perspective to the fine-scale structure of the mantle produced by mantle convection.

“Our contribution here is to give some sense of how large some of these heterogeneities might be,” Liang said. “So the question to the broader community becomes: What might be the deep mantle processes that can produce this?”

Mysterious Deep-Earth Seismic Signature Explained

New research on oxygen and iron chemistry under the extreme conditions found deep inside Earth could explain a longstanding seismic mystery called ultralow velocity zones. Published in Nature, the findings could have far-reaching implications on our understanding of Earth’s geologic history, including life-altering events such as the Great Oxygenation Event, which occurred 2.4 billion years ago.

Sitting at the boundary between the lower mantle and the core, 1,800 miles beneath Earth’s surface, ultralow velocity zones (UVZ) are known to scientists because of their unusual seismic signatures. Although this region is far too deep for researchers to ever observe directly, instruments that can measure the propagation of seismic waves caused by earthquakes allow them to visualize changes in Earth’s interior structure; similar to how ultrasound measurements let medical professionals look inside of our bodies.

These seismic measurements enabled scientists to visualize these ultralow velocity zones in some regions along the core-mantle boundary, by observing the slowing down of seismic waves passing through them. But knowing UVZs exist didn’t explain what caused them.

However, recent findings about iron and oxygen chemistry under deep-Earth conditions provide an answer to this longstanding mystery.

It turns out that water contained in some minerals that get pulled down into Earth due to plate tectonic activity could, under extreme pressures and temperatures, split up — liberating hydrogen and enabling the residual oxygen to combine with iron metal from the core to create a novel high-pressure mineral, iron peroxide.

Led by Carnegie’s Ho-kwang “Dave” Mao, the research team believes that as much as 300 million tons of water could be carried down into Earth’s interior every year and generate deep, massive reservoirs of iron dioxide, which could be the source of the ultralow velocity zones that slow down seismic waves at the core-mantle boundary.

To test this idea, the team used sophisticated tools at Argonne National Laboratory to examine the propagation of seismic waves through samples of iron peroxide that were created under deep-Earth-mimicking pressure and temperature conditions employing a laser-heated diamond anvil cell. They found that a mixture of normal mantle rock with 40 to 50 percent iron peroxide had the same seismic signature as the enigmatic ultralow velocity zones.

For the research team, one of the most-exciting aspects of this finding is the potential of a reservoir of oxygen deep in the planet’s interior, which if periodically released to Earth’s surface could significantly alter Earth’s early atmosphere, potentially explaining the dramatic increase in atmospheric oxygen that occurred about 2.4 billion years ago according to the geologic record.

“Finding the existence of a giant internal oxygen reservoir has many far-reaching implications,” Mao explained. “Now we should reconsider the consequences of sporadic oxygen outbursts and their correlations to other major events in Earth’s history, such as the banded-iron formation, snowball Earth, mass extinctions, flood basalts, and supercontinent rifts.”

How The Earth Stops High-Energy Neutrinos In Their Tracks

For the first time, a science experiment has measured Earth’s ability to absorb neutrinos — the smaller-than-an-atom particles that zoom throughout space and through us by the trillions every second at nearly the speed of light. The experiment was achieved with the IceCube detector, an array of 5,160 basketball-sized sensors frozen deep within a cubic kilometer of very clear ice near the South Pole. The results of this experiment by the IceCube collaboration, which includes Penn State physicists, will be published in the online edition of the journal Nature on November 22, 2017.

“This achievement is important because it shows, for the first time, that very-high-energy neutrinos can be absorbed by something — in this case, the Earth,” said Doug Cowen, professor of physics and astronomy & astrophysics at Penn State. The first detections of extremely-high-energy neutrinos were made by IceCube in 2013, but a mystery remained about whether any kind of matter could truly stop a neutrino’s journey through space. “We knew that lower-energy neutrinos pass through just about anything,” Cowen said, “but although we had expected higher-energy neutrinos to be different, no previous experiments had been able to demonstrate convincingly that higher-energy neutrinos could be stopped by anything.”

The results in the Nature paper are based on one year of data from about 10,800 neutrino-related interactions. Cowen and Tyler Anderson, an assistant research professor of physics at Penn State, are members of the IceCube collaboration. They are coauthors of the Nature paper who helped to build the IceCube detector and are contributing to its maintenance and management.

This new discovery with IceCube is an exciting addition to our deepening understanding of how the universe works. It also is a little bit of a disappointment for those who hope for an experiment that will reveal something that cannot be explained by the current Standard Model of Particle Physics. “The results of this Ice Cube study are fully consistent with the Standard Model of Particle Physics — the reigning theory that for the past half century has described all the physical forces in the universe except gravity,” Cowen said.

Neutrinos first were formed at the beginning of the universe, and they continue to be produced by stars throughout space and by nuclear reactors on Earth. “Understanding how neutrinos interact is key to the operation of IceCube,” explained Francis Halzen, principal investigator for the IceCube Neutrino Observatory and a University of Wisconsin-Madison professor of physics. “We were of course hoping for some new physics to appear, but we unfortunately find that the Standard Model, as usual, withstands the test,” Halzen said.

IceCube’s sensors do not directly observe neutrinos, but instead measure flashes of blue light, known as Cherenkov radiation, emitted after a series of interactions involving fast-moving charged particles that are created when neutrinos interact with the ice. By measuring the light patterns from these interactions in or near the detector array, IceCube can estimate the neutrinos’ energies and directions of travel. The scientists found that the neutrinos that had to travel the farthest through Earth were less likely to reach the detector.

Most of the neutrinos selected for this study were more than a million times more energetic than the neutrinos produced by more familiar sources, like the Sun or nuclear power plants. The analysis also included a small number of astrophysical neutrinos, which are produced outside the Earth’s atmosphere, from cosmic accelerators unidentified to date, perhaps associated with supermassive black holes.

“Neutrinos have quite a well-earned reputation of surprising us with their behavior,” says Darren Grant, spokesperson for the IceCube Collaboration, a professor of physics at the University of Alberta in Canada, and a former postdoctoral scholar at Penn State. “It is incredibly exciting to see this first measurement and the potential it holds for future precision tests.”

In addition to providing the first measurement of the Earth’s absorption of neutrinos, the analysis shows that IceCube’s scientific reach extends beyond its core focus on particle physics discoveries and the emerging field of neutrino astronomy into the fields of planetary science and nuclear physics. This analysis also is of interest to geophysicists who would like to use neutrinos to image the Earth’s interior in order to explore the boundary between the Earth’s inner solid core and its liquid outer core.

“IceCube was built to both explore the frontiers of physics and, in doing so, possibly challenge existing perceptions of the nature of universe. This new finding and others yet to come are in that spirt of scientific discovery,” said James Whitmore, program director in the National Science Foundation’s physics division. Physicists now hope to repeat the study using an expanded, multiyear analysis of data from the full 86-string IceCube array, and to look at higher ranges of neutrino energies for any hints of new physics beyond the Standard Model.

Minerals In Volcanic Rock Offer New Insights Into The First 1.5 Billion Years Of Earth’s Evolution

The first 1.5 billion years of Earth’s evolution is subject to considerable uncertainty due to the lack of any significant rock record prior to four billion years ago and a very limited record until about three billion years ago. Rocks of this age are usually extensively altered making comparisons to modern rock quite difficult. In new research conducted at LSU, scientists have found evidence showing that komatiites, three-billion-year old volcanic rock found within the Earth’s mantle, had a different composition than modern ones. Their discovery may offer new information about the first one billion years of Earth’s development and early origins of life.

Results of the team’s work has been published in the October 2017 edition of Nature Geoscience.

The basic research came from more than three decades of LSU scientists studying and mapping the Barberton Mountains of South Africa. The research team, including LSU geology professors Gary Byerly and Huiming Bao, geology PhD graduate Keena Kareem, and LSU researcher Benjamin Byerly, conducted chemical analyses of hundreds of komatiite rocks sampled from about 10 lava flows.

“Early workers had mapped large areas incorrectly by assuming they were correlatives to the much more famous Komati Formation in the southern part of the mountains. We recognized this error and began a detailed study of the rocks to prove our mapping-based interpretations,” said Gary Byerly.

Within the rocks, they discovered original minerals called fresh olivine, which had been preserved in remarkable detail. Though the mineral is rarely found in rocks subjected to metamorphism and surface weathering, olivine is the major constituent of Earth’s upper mantle and controls the nature of volcanism and tectonism of the planet. Using compositions of these fresh minerals, the researchers had previously concluded that these were the hottest lavas to ever erupt on Earth’s surface with temperatures near 1600 degrees centigrade, which is roughly 400 degrees hotter than modern eruptions in Hawaii.

“Discovering fresh unaltered olivine in these ancient lavas was a remarkable find. The field work was wonderfully productive and we were eager to return to the lab to use the chemistry of these preserved olivine crystals to reveal clues of the Archean Mantle,” said Kareem.

The researchers suggest that maybe a chunk of early-Earth magma ocean is preserved in the approximately 3.2 billion year-old minerals.

“The modern Earth shows little or no evidence of this early magma ocean because convection of the mantle has largely homogenized the layering produced in the magma ocean. Oxygen isotopes in these fresh olivines support the existence of ancient chunks of the frozen magma ocean. Rocks like this are very rare and scientifically valuable. An obvious next step was to do oxygen isotopes,” said Byerly.

This study grew out of work taking place in LSU’s laboratory for the study of oxygen isotopes, a world-class facility that attracts scientists from the U.S. and international institutions for collaborative work. The results of the study were so unusual that it required extra care to be certain of the results. Huiming Bao, who is also the head of LSU’s oxygen isotopes lab, said that the team triple and quadruple checked the data by running with different reference minerals and by calibrating with other independent labs.

“We attempted to reconcile the findings with some of the conventional explanations for lavas with oxygen isotope compositions like these, but nothing could fully explain all of the observations. It became apparent that these rocks preserve signatures of processes that occurred over four billion years ago and that are still not completely understood,” said Benjamin Byerly.

Oxygen isotopes are measured by the conversion of rock or minerals into a gas and measuring the ratios of oxygen with the different masses of 16, 17, and 18. A variety of processes fractionate oxygen on Earth and in the Solar System, including atmospheric, hydrospheric, biological, and high temperature and pressure.

“Different planets in our solar system have different oxygen isotope ratios. On Earth this is modified by surface atmosphere and hydrosphere, so variations could be due either to heterogeneous mantle (original accumulation of planetary debris or remnants of magma ocean) or surface processes,” said Byerly. “Either might be interesting to study. The latter because it would also provide information about the early surface temperature of Earth and early origins of life.”

New Greenland Maps Show More Glaciers At Risk

New maps of Greenland’s coastal seafloor and bedrock beneath its massive ice sheet show that two to four times as many coastal glaciers are at risk of accelerated melting as had previously been thought.

Researchers at the University of California, Irvine, NASA and 30 other institutions have published the most comprehensive, accurate and high-resolution relief maps ever made of Greenland’s bedrock and coastal seafloor. Among the many data sources incorporated into the new maps is data from NASA’s Ocean Melting Greenland campaign.

Lead author Mathieu Morlighem of UCI had demonstrated in an earlier study that data from OMG’s survey of the shape and depth, or bathymetry, of the seafloor in Greenland’s fjords improved scientists’ understanding of both the coastline and the inland bedrock beneath glaciers that flow into the ocean. That’s because the bathymetry at a glacier’s front limits the possibilities for the shape of bedrock farther upstream.

The nearer to the shoreline, the more valuable the bathymetry data are for understanding on-shore topography, Morlighem said. “What made OMG unique compared to other campaigns is that they got right into the fjords, as close as possible to the glacier fronts. That’s a big help for bedrock mapping,” he added.

Additionally, the OMG campaign surveyed large sections of the Greenland coast for the first time ever. In fjords for which there are no data, it’s difficult to estimate how deep the glaciers extend below sea level.

The OMG data are only one of many datasets Morlighem and his team used in the ice sheet mapper, which is named BedMachine. Another comprehensive source is NASA’s Operation IceBridge airborne surveys. IceBridge measures the ice sheet thickness directly along a plane’s flight path. This creates a set of long, narrow strips of data rather than a complete map of the ice sheet.

Besides NASA, almost 40 other international collaborators also contributed various types of survey data on different parts of Greenland.

No survey, not even OMG, covers every glacier on Greenland’s long, convoluted coastline. To infer the bed topography in sparsely studied areas, BedMachine averages between existing data points using physical principles such as the conservation of mass.

The new maps reveal that two to four times more oceanfront glaciers extend deeper than 600 feet (200 meters) below sea level than earlier maps showed. That’s bad news, because the top 600 feet of water around Greenland comes from the Arctic and is relatively cold. The water below it comes from farther south and is 6 to 8 eight degrees Fahrenheit (3 to 4 degrees Celsius) warmer than the water above. Deeper-seated glaciers are exposed to this warmer water, which melts them more rapidly.

Morlighem’s team used the maps to refine their estimate of Greenland’s total volume of ice and its potential to add to global sea level rise if the ice were to melt completely, which is not expected to occur within the next few hundred years. The new estimate is higher by 2.76 inches (7 centimeters) for a total of 24.34 feet (7.42 meters).

OMG principal investigator Josh Willis of JPL, who was not involved in producing the maps, said, “These results suggest that Greenland’s ice is more threatened by changing climate than we had anticipated.”

On Oct. 23, the five-year OMG campaign completed its second annual set of airborne surveys to measure for the first time the amount that warm water around the island is contributing to the loss of the Greenland ice sheet. Besides the one-time bathymetry survey, OMG is collecting annual measurements of the changing height of the ice sheet and the ocean temperature and salinity in more than 200 fjord locations. Morlighem looks forward to improving BedMachine’s maps with data from the airborne surveys.

The maps and related research are in a paper titled “BedMachine v3: Complete bed topography and ocean bathymetry mapping of Greenland from multi-beam echo sounding combined with mass conservation” in Geophysical Research Letters. This project received support from NASA’s Cryospheric Sciences Program and the National Science Foundation’s ARCSS program.

Frozen Earth: The Planet Got Warm After Frequent Volcano Eruptions Melted The Last Ice Age

Fire melts ice, but so does ash: Dark particles settling onto white ice make the surface trap more heat, the same way wearing a black shirt on a sunny day is hotter than wearing a white shirt. And scientists have seen the connection play out in real time across Earth’s surface as volcanic eruptions have scattered ash on snow and made it melt faster. But for the first time, a team of researchers has pinpointed the phenomenon in the distant past, as they report in a new article published in the journal Nature Communications.

“The paper is the first to document that this phenomenon likely also occurred during the last deglaciation, and raises interesting questions regarding the role of volcanism on deglaciation,” James Baldini, an Earth scientist at Durham University in the U.K. not affiliated with the study wrote Newsweek in an email.

He notes that traditionally, scientists thinking about the impact of volcanoes on climate focus on tiny particles called aerosols, which are released during eruptions, form clouds that block sunlight and keep the Earth cooler. This paper, on the other hand, suggests that effect might have been balanced out by melting ice—leaving the planet no cooler than it was before.

The team used an unusual form of evidence: glacial varves, or the layers of dirt and mud deposited each year beneath a glacier. Just like the rings of new wood trees grow every year in a light-dark pattern, glaciers annually deposit first a wide lighter layer of sandier soil during the summer, then a narrower layer of darker clay during the winter. The thickness of each layer lets scientists calculate how much the glacier in question melted, since the more a glacier melts the more sediment it carries away.

He notes that traditionally, scientists thinking about the impact of volcanoes on climate focus on tiny particles called aerosols, which are released during eruptions, form clouds that block sunlight and keep the Earth cooler. This paper, on the other hand, suggests that effect might have been balanced out by melting ice—leaving the planet no cooler than it was before.

The team used an unusual form of evidence: glacial varves, or the layers of dirt and mud deposited each year beneath a glacier. Just like the rings of new wood trees grow every year in a light-dark pattern, glaciers annually deposit first a wide lighter layer of sandier soil during the summer, then a narrower layer of darker clay during the winter. The thickness of each layer lets scientists calculate how much the glacier in question melted, since the more a glacier melts the more sediment it carries away.

New Study Explains How Continents Leave Their Roots Behind

In some areas of the seafloor, a tectonic mystery lies buried deep underground.

The ocean floor contains some of the newest rock on Earth, but underneath these young oceanic plates are large swatches of much older continents that have been dislocated from their continental plates and overtaken by the younger, denser oceanic plate.

Researchers have been puzzled by this phenomenon for some time: how does a continental plate leave some of itself behind?

In a new study published in Geophysical Research Letters, a journal of the American Geophysical Union, researchers have linked the displaced pieces of continental plates to a weak link in the plate’s layers called a mid-lithospheric discontinuity.

The crust and the upper mantle make up the lithosphere, the rigid, outer part of the Earth. A mid-lithospheric discontinuity can occur in this layer, running horizontally through the middle of the lithosphere. It is at this place where the lower layer of a continent’s lithosphere can break away from itself and dislocate, leaving behind large pieces of the lower lithosphere, called a root, which can become embedded in the oceanic plate on the trailing side of the continental plate.

The new study finds thicker and weaker mid-lithospheric discontinuity layers are more likely to leave behind roots farther from their continental origins, while thinner layers have more strength to hold onto their roots as the continental plates move, according to the new study.

“This is the first mechanism to explain the large-scale displacement of continental lithosphere being left behind under oceanic lithosphere,” said Timothy Kusky, director for the Center for Global Tectonics at the China University of Geosciences in Wuhan, China, and co-author of the new study.

Kusky likens the process to a peanut butter and jelly sandwich on a table: the sandwich is the Earth’s lithosphere, and the table is the asthenosphere, the weak layer in the upper mantle that accommodates most plate displacements. The peanut butter and jelly is the mid-lithospheric discontinuity that is binding the two halves of the lithosphere together.

If someone pushed the sandwich across the table, the force from the top would move the top layer of bread, but the friction from the table pulls on the bottom slice of bread. As the sandwich moves, the two pieces of bread may get offset, and the sandwich becomes uneven, Kusky said.

Just like the sandwich, as the continental plate slowly moves, the velocity of the upper lithosphere may be faster than the lower lithosphere. If the “peanut butter and jelly” is weak, the top part of the lithosphere begins to surpass its lower half, leaving the lower lithosphere behind to be overtaken by the denser oceanic plate.

The question Kusky and his colleagues attempt to answer in the new study is: Can we model the peanut butter and jelly sandwich?

The study’s authors created a numerical model of the largest documented continental lithosphere offset, a continental root under the southern Atlantic Ocean that was left 1,300 kilometers (more than 800 miles) behind by the African continent from which it originated.

The study’s authors modeled how the minerals in the lithosphere flow and how fast the continental plate would have been moving at the time, approximately 130 million years ago. The researchers ran 225 models of the continental plate, using different thicknesses for the mid-lithospheric discontinuity between 10 and 50 kilometers wide (5 to 31 miles wide) to investigate the layer’s strength holding the two lateral halves together. The models also incorporated a range of plate velocities and viscosity, or stickiness due to friction, of the mid-lithospheric discontinuity.

The model revealed that the thicker the mid-lithospheric discontinuity zone, the larger the plate offset would be. A thinner “jelly” with a high viscosity was less likely to experience shearing from the upper lithosphere, or at least only trail slightly behind. But a thick mid-lithospheric discontinuity layer, more than 25 kilometers thick (about 15 miles thick), can lead to large offsets. Over 100 million years, some roots can wind up 10,000 kilometers (6,200 miles) away from the continent they originated from, according to the models.

In the case of the 1,300-kilometer (800-mile) African offset, the scientists estimate the mid-lithospheric discontinuity was about 40 kilometers thick and traveled apart at a rate of 1 to 3.25 centimeters a year (about 0.39 to 1.28 inches).

Understanding these plate offsets can help researchers understand how the continental pieces of lithosphere can affect oceanic plates and their composition, said Zhensheng Wang, a geoscientist at the China University of Geosciences in Wuhan, China, and co-author of the new study.

One example for further study under this new model would be the Ontong Java Plateau in the Pacific Ocean, the single largest oceanic plateau on Earth.

“Really it represents a new step in plate tectonics,” Kusky said of the new study. “If we can explain the mid-lithospheric discontinuity then we can explain a lot of the enigmatic things in oceanography and plate tectonics in general.”