Tag Archives: olivine

Earth’s mantle is much hotter than we thought, scientists learn

We knew the Earth’s insides are hot, but just how hot are they? A new study found that the Earth’s mantle could go up to a whopping 1410 degrees Celsius (2570 degrees Fahrenheit), significantly more than was previously estimated.

Age of oceanic crust: youngest (red) is along spreading centres, where parts of the mantle rise up to create new crust. Credits: NOAA.

For once, something’s hotter and it’s not connected to global warming. The Earth’s mantle, the thickest layer of the Earth makes up 84% of the planet’s volume, lying between the Earth’s crust and its core. Because of its inaccessibility, pretty much everything we know about it comes from indirect evidence. Indeed, it’s a testament to how much geology has progressed that we’re able to describe it in such detail, but as it so often happens with indirect evidence, it can be quite difficult to get the figures exactly right. Now, this new study found that beneath our planet’s oceans, the mantle might be significantly hotter than we thought: by almost 110 degrees F (60 degrees C). This change could help us better understand tectonic processes and help us develop better models of our planet.

“Having such a hot mantle could mean that the mantle is less viscous (flows more easily), which could explain how tectonic plates are able to move on top of the asthenosphere,” the upper layer of Earth’s mantle, said study lead researcher Emily Sarafian, a doctoral student in the Geology and Geophysics Department at a joint program run by the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution.

Technically, the mantle is solid, but for geological purposes, it usually behaves like a fluid. Sounds strange? Well, imagine a jar of honey, which is viscous. The hotter it gets, the more liquid it becomes — and the opposite stands too, cold honey is quite solid.

“If you put honey in the fridge for an hour, it will barely flow when you take it out,” Sarafian said in an email to Live Science. “If, instead, you put honey on the stovetop, it will flow very easily, because it’s hotter.”

So the mantle is so hot that in some regards, it acts like a fluid. This is important because according to our understanding, there are convection currents in the mantle that move the tectonic plates around (and contribute to numerous geological processes). In other words, some parts of the mantle get hotter and become less dense, rising towards the surface. As they move, they transfer some of the heat towards the surface, where plumes of less dense magma break apart the plates at the spreading centers, creating divergent plate boundaries. At the other end of the process, some other end of tectonic plates cools down and sinks — thus, new crust is sometimes formed, and sometimes recycled in the mantle, maintaining an equilibrium.

Simplistic depiction of convection currents. Image credits: Wiki Commons.

To continue the analogy, imagine that the tectonic plates are like pieces of biscuit laid on top of the honey. If the honey (I mean, mantle) is hotter than we thought, then it means that it acts more like a liquid, and “flows more,” so to speak. It means that the currents might be stronger than we thought and the biscuits (plates) might “float” with less resistance — it also means that our current models might need some tweaking.

Hotter than hell

We knew very well that the mantle is hot, there’s plenty of clues to indicate that. For instance, it generates the hot lava that flows out from underwater volcanoes. We also know of the gravitational heat, left over from when gravity first condensed our planet from the hot gases and particles, and perhaps most significantly, of the heat from radioactive decay. But because we can’t really probe it directly, we’ve relied on models and lab experiments to estimate its temperature. The one piece of direct evidence we have is from mantle xenoliths — rocks that were brought up from the mantle by convection currents and exposed by mid-oceanic ridge spreading, but because those rocks undergo processes that significantly change their structure and chemistry, researchers prefer to create synthetic rocks to model conditions.

After the synthetic rock is created in the lab, researchers subject it to pressures and temperatures ranging around those of the mantle and see at what temperature it melts. This is called the solidus temperature. The problem with estimating this temperature is water. Due to a chemical quirk, water greatly affects the solidus temperature of these rocks. Therefore, estimating the water content of the rocks is essential for determining the solidus temperature.

Other teams were aware of this issue, “but they were never able to quantify how much water was in their experiments because the mineral grains that grow during an experimental run at mantle pressures and temperatures are way too small to measure with current analytical techniques,” Sarafian said. She added that researchers were using some corrections, adding a bit of extra water to the synthetic rock — but this correction was unnecessary, as the rocks already accumulate the water from the atmosphere.

She found the missing puzzle piece in olivine, a magnesium iron silicate with larger grains, which can be better measured. So she added olivine to the mix, which allowed her to measure water content more accurately.

A beautiful olivine crystal, not from this study. Image credits:
Rob Lavinsky

 

“We performed melting experiments the same way that previous scientists did, putting a synthetic rock to high pressure and temperatures, but by adding these grains to our experiments, we were giving ourselves a target that was large enough to analyze for water content,” she said.

With the new water correction, a different solidus temperature emerged, and a different temperature of the mantle.

Paul Asimow, a professor of geology and geochemistry at the California Institute of Technology who was not involved with the study, complemented the research in an accompanying commentary in the journal Science, saying that it is “an appreciable correction.”

Journal Reference: Emily SarafianGlenn A. GaetaniErik H. HauriAdam R. Sarafian — Experimental constraints on the damp peridotite solidus and oceanic mantle potential temperature. Science 03 Mar 2017: Vol. 355, Issue 6328, pp. 942-945.
DOI: 10.1126/science.aaj2165

 

Earth may have generated its own water – geologically

A new study may have finally found where Earth’s water came from. There are currently two competing theories, with one claiming that our planet generated its own water geologically, while the other suggests that water was brought by icy comets or asteroids from outside. A new study concluded that most of the water we see today likely comes from the Earth’s mantle.

Water beneath the surface

Image via Special Papers.

Until recently, the idea that water came to Earth from somewhere else in the solar system seemed to have more support, but studies conducted by the European Space Agency on the Rosetta missions showed that water almost certainly didn’t come from comets. Wendy Panero, associate professor of earth sciences at Ohio State, and doctoral student Jeff Pigott believe that when the Earth formed, it had huge bodies of water in its interior, and has been continuously supplying water to the surface via plate tectonics, circulating material upward from the mantle.

Researchers have long known that there is some water in the earth’s mantle, but nobody knows just how much. Because you can’t go 40 km deep and study the mantle, you have to rely on indirect information and computer simulations.

“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.”

The thing is, when we’re talking about water in the mantle, it’s not actually liquid water – what seems dry to the human eye may actually have significant quantities of water – in the form of hydrogen and oxygen waters. Hydrogen is typically stored in crystal voids and defects, while oxygen is usually plentiful in most minerals. Certain reactions can free up the hydrogen and oxygen, resulting in water; but could it be enough water to amount for the oceans we see today?

The key element here is ringwoodite.

High-Pressure Olivine

Olivine is a magnesium-iron silicate typically found in the mantle and igneous rocks. However, in the mantle, at very high pressures and temperatures, the olivine structure is no longer stable. Below depths of about 410 km (250 mi) olivine undergoes a transformation, transforming into ringwoodite or bridgmanite. Ringwoodite is notable for being able to contain hydroxide ions (oxygen and hydrogen atoms bound together) and previous research has already shown that the earth’s mantle holds huge quantities of water.

Now, this team has found that the mineral bridgmanite doesn’t contain enough water to play a significant role in this issue – so it’s all about the ringwoodite. But the question is – if ringwoodite is trapped in the mantle and the water is drained towards the surface in plate tectonics, how does our planet still have water reserves now, in the mantle?

But while they were creating some models and simulations of ringwoodite water behavior, another likely candidate emerged: garnet. Garnet could be a water carrier, transporting some of the water to the surface, while some of it still remains in the mantle.

“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.”

 

 

Rock with 30,000 diamonds found Russian diamond mine

Do you fancy diamonds? If the answer is ‘yes’, then you’ll absolutely love this rock extracted from a Russian mine. The rock is littered with over 30,000 diamonds, something which is extremely rare and may yield valuable information about how diamonds form in natural conditions.

What’s unlucky for gem sellers was very fortunate for researchers – because the tiny diamonds are so small, they are pretty much worthless as gems, so they donated the rock for study. Hurray for science!

The rock was extracted from the huge Udachnaya pipe, an open-pit mine located in Russia, just outside the Arctic circle. It’s one of the biggest diamond mines in Europe and in the world. The results were reported by geologist Larry Taylor from the University of Tennessee this week at the American Geophysical Union’s annual meeting.

“The exciting thing for me is there are 30,000 itty-bitty, perfect octahedrons, and not one big diamond,” said Taylor at the meeting. “It’s like they formed instantaneously.”

The Udachnaya pipe. Image via Wiki Commons.

Even thought the diamonds are so small, the concentration of diamonds in the ore is humongous: million times more than usually. This remarkable association of diamonds and other minerals will hopefully reveal the exact chemical reactions which lead to the formation of diamonds on Earth – which are still a mystery. Taylor said:

“The associations of minerals will tell us something about the genesis of this rock, which is a strange one indeed. The [chemical] reactions in which diamonds occur still remain an enigma,” Taylor told Live Science.

Although highly regarded as the a gem and extracted for this purpose for centuries, we still don’t know exactly how diamonds form. According to our current understanding, diamonds are formed at high temperature and pressure at depths of 140 to 190 kilometers (87 to 118 mi) in the Earth’s mantle. Carbon-containing minerals provide the carbon source, and the growth occurs over extremely long periods from 1 billion to 3.3 billion years! Diamonds are then brought close to the Earth’s surface through deep volcanic eruptions by a magma, which cools into igneous rocks known as kimberlites and lamproites. The heat destroys most of the material surrounding the diamonds, but the diamonds still resist. There are also ways of creating artificial diamonds, but the exact chemistry still eludes us.

Diamond formation. Image via GeoScienceWorld.

But while you do see several diamonds on the same rock, you almost never find a rock with so many. Working with researchers at the Russian Academy of Sciences, Taylor analysed the rock using an industrial X-ray tomography scanner to figure out how it ended up with such a staggering amount of diamonds and remained intact when it was raised to the surface.

“The clear crystals are just 0.04 inches (1 millimetre) tall and are octahedral, meaning they are shaped like two pyramids that are glued together at the base,” says Oskin. “The rest of the rock is speckled with larger crystals of red garnet, and green olivine and pyroxene. Minerals called sulphides round out the mix. A 3D model built from the X-rays revealed the diamonds formed after the garnet, olivine and pyroxene minerals.”

The minerals also had some exotic material included in their structure. These inclusions were once fluids that seeped out of the Earth’s oceanic crust when one tectonic plate crashed onto another. These fluids crystallized and became an integral part of the diamonds, much deeper in the earth and much, much later. This is either a very strange and unusual formation, or…

“[The source] could be just a really, really old formation that’s been down in the mantle for a long time,” Sami Mikhail from the Carnegie Institution for Science in the US, who was not involved in the research, told Live Science.

 

 

First ringwoodite sample confirms huge quantities of water in the Earth’s mantle

The first ever terrestrial discovery of ringwoodite seems to confirm the existence of massive amounts of water hundreds of kilometers below the Earth’s surface. Let me explain how.

Under pressure

Ringwoodite is a high-pressure polymorph of olivine; it’s basically olivine, but with a different crystal structure. The mineral is thought to exist in large quantities in the so-called transition zone, 410km to 660 km deep. Judging by its properties and lab experiments, crystallographers believe that the mineral is restricted between 525 and 660 km deep.

Ringwoodite has been found in meteorites, but until now, no terrestrial sample has ever been unearthed because, well, geologists can’t go 500 km deep underground. However, a University of Alberta diamond scientist has found the first terrestrial sample. The team led by Graham Pearson, Canada Excellence Research Chair in Arctic Resources analyzed this ringwoodite sample and reported that it contains a significant amount of water – 1.5 per cent of its weight. Since this mineral is thought to be found in enormous quantities in the transition zone, that means that the equivalent of all the surface water is found inside the minerals.

“This sample really provides extremely strong confirmation that there are local wet spots deep in the Earth in this area,” said Pearson, a professor in the Faculty of Science, whose findings were published March 13 in Nature. “That particular zone in the Earth, the transition zone, might have as much water as all the world’s oceans put together.”

Interestingly enough, the mineral is notable for being able to contain water within its structure, present not as a liquid but as hydroxide ions (oxygen and hydrogen atoms bound together) . This has huge implications because ringwoodite is thought to be the most abundant mineral phase in the lower part of Earth’s transition zone, so abundant that its properties directly affect those of the mantle – so the existence of water is quite a game changer.

The sample that almost wasn’t

Pearson holding the sample. Remember that the ringwoodite inclusion is a very small part of the sample.

The sample was found in 2008 in the Juina area of Mato Grosso, Brazil, where artisan miners unearthed the host diamond from shallow river gravels. Diamonds are most often associated and brought to the surface by minerals called kimberlites – the most deeply derived of all volcanic rocks. But the discovery itself was almost accidental.

Pearson’s team was looking for something entirely different when they stumbled onto a three-millimetre-wide, dirty-looking, commercially worthless brown diamond. The ringwoodite itself is invisible to the naked eye, and hidden beneath the surface, so it’s a surprise that graduate student, John McNeill, found it in 2009.

“It’s so small, this inclusion, it’s extremely difficult to find, never mind work on,” Pearson said, “so it was a bit of a piece of luck, this discovery, as are many scientific discoveries.”

Three-dimensional confocal μXRF view of two-phase inclusion within the diamond

It took years of analysis and redoing the tests over and over again before it was finally confirmed that the sample is ringwoodite; infrared spectroscopy and X-ray diffraction confirmed this, while the critical water measurements were performed at Pearson’s Arctic Resources Geochemistry Laboratory at the U of A.

A remarkable collaboration

Aside from actually finding the sample, it’s also notable how this study came to fruition. It is a remarkable example of ome of the top leaders from various fields, including the Geoscience Institute at Goethe University, University of Padova, Durham University, University of Vienna, Trigon GeoServices and Ghent University. For Pearson, one of the world’s leading authorities in the study of deep Earth diamond host rocks, this is one of the most notable discoveries in his career, apparently confirming 50 years of theories.

Geophysicists and seismologists have long theoretized that the composition of the transition zone has to feature immense quantities of water, but that was never confirmed – until now. The existence of water in the ringwoodite in the transition zone has immense implications for volcanism and plate tectonics, affecting how rock melts, cools and shifts below the crust.

“One of the reasons the Earth is such a dynamic planet is the presence of some water in its interior,” Pearson concluded. “Water changes everything about the way a planet works.”

Journal Reference:

  1. D. G. Pearson, F. E. Brenker, F. Nestola, J. McNeill, L. Nasdala, M. T. Hutchison, S. Matveev, K. Mather, G. Silversmit, S. Schmitz, B. Vekemans, L. Vincze.Hydrous mantle transition zone indicated by ringwoodite included within diamondNature, 2014; 507 (7491): 221 DOI: 10.1038/nature13080

Largest deep earthquake ever recorded still baffles seismologists

A magnitude 8.3 earthquake that struck deep beneath the Sea of Okhotsk on May 24, 2013 still poses a lot of questions to geophysicists. At a depth of about 609 kilometers (378 miles), the kind of rupture which generates an earthquake of this magnitude should just not happen.

earthquake 1

The vast majority of significant earthquakes takes place on shallow depths, usually when at the boundary of two or more tectonic plates – those of course, are the most unstable area. If you correlate a map of tectonic plates with a map of the recent earthquakes at any given time, you’ll find that the vast majority are clustered around those areas. Earthquake also occur at major faults, which are also relatively shallow (in the crust).

Intermediary earthquakes have the focus between 70 (or 40, depending on who you listen to) and 300 km; and deep earthquakes take place at over 300 km depth. Of course, there can be no tectonic boundaries and faults at that depth – we’re talking mantle here.

The cause of deep focus earthquakes is still not entirely understood since subducted lithosphere at that pressure and temperature regime should not exhibit brittle behavior. Probably the most discussed possibility is a mineral transition, like for example olivine undergoing a phase transition into a spinel structure. Still, they may still be influenced by crustal tectonics, and most specifically by what is called the Wadati–Benioff zone.

earthquake

But at these depths, with huge temperatures and pressures, you wouldn’t typically expect such big earthquakes.

“It’s a mystery how these earthquakes happen. How can rock slide against rock so fast while squeezed by the pressure from 610 kilometers of overlying rock?” said Thorne Lay, professor of Earth and planetary sciences at the University of California, Santa Cruz.

Deep earthquakes occur in the transition zone between the upper mantle and lower mantle and are not usually dangerous for humans, but yield very valuable scientific information. As for the Sea of Ohotsk earthquake, it has some very strange characteristics.

“It looks very similar to a shallow event, whereas the Bolivia earthquake ruptured very slowly and appears to have involved a different type of faulting, with deformation rather than rapid breaking and slippage of the rock,” Lay said.

The precise mechanism for initiating shear fracture under the huge pressure at that depth remains unclear, and unlikely to be solved in the nearby future.

“If the fault slips just a little, the friction could melt the rock and that could provide the fluid, so you would get a runaway thermal effect. But you still have to get it to start sliding,” Lay said. “Some transformation of mineral forms might give the initial kick, but we can’t directly detect that. We can only say that it looks a lot like a shallow event.”

Journal Reference:
L. Ye, T. Lay, H. Kanamori, K. D. Koper. Energy Release of the 2013 Mw 8.3 Sea of Okhotsk Earthquake and Deep Slab Stress Heterogeneity. Science, 2013; 341 (6152): 1380 DOI: 10.1126/science.1242032

Asteroid Vesta is a lot like Earth, study shows

The cold, lifeless Vesta asteroid might be a lot more like our planet than astronomers believed – having a very active life in the early stages of the solar system evolution, a study of a Saharan meteorite shows.

The planet that wasn’t

asteorid

Vesta might host a magmatic layer under its rocky exterior, allowing minerals to travel between softer and harder layers of material, according to a study published online Sunday by the journal Nature Geoscience. If this were true, then Vesta is a lot more Earth like than previously believed.

“People think asteroids are big, gray, cold, almost potato-shaped lumps of rock that sometimes crash into the Earth and threaten us,” said study leader Beverley Tkalcec, a planetary geologist at Goethe University in Frankfurt, Germany. Instead, she said, “it has a dynamic interior similar to what might have been at the beginning of the Earth.”

Hot or not, Vesta is just big enough to have experienced melting inside. When this happens, the thicker, heavier material sinks towards the center and the lighter stuff gets pushed towards the crust. In this way, Vesta (much like its “cousin” Ceres) are planetary embryos that never really came to life, and since there are no tectonics to recirculate the rocks, the rocks are probably as old as the solar system.

The crystal and the electron

The study was conducted on a meteorite which is believed to have carved out Vesta’s mantle by impact; they made the connection between the meteorite by analysing its chemical and isotopical composition. However, unlike other studies which focus on the composition, this one focused on how the matter is distributed; if Vesta were indeed active beneath the surface and have a magmatic layer, then some clues should pop out.

The researchers used a technique called electron backscatter diffraction, in which basically electrons are bounced off crystals to determine their structure. They focused their research on a mineral called olivine (we’ve occasionally written about this mineral, see here) and found that instead of a regular pile of crystals with one sitting on top of each other, the crystal lattice was severely deformed.

olivine

Olivine crystals

They then tried to find something equivalent to this, and they found that the only rocks which resemble this type of structure is with igneous rocks formed by forces in Earth’s mantle – something which led to the natural conclusion that the meteorite is probably a result of the same process on Vesta, with the heavier elements sinking in.

They then plugged this data into a computer model of Vesta and found that, given specific conditions, the asteroid could host a magma ocean.

“When you have dense solid material over partially molten material, then it’s unstable,” said Harry McSween, a planetary geoscientist at the University of Tennessee in Knoxville and co-investigator for the Dawn mission. “The top’s trying to become the bottom and the bottom’s trying to become the top.”

Among other things, Vesta is believer to host water and have a mountain 3 times bigger than the Everest.

Shorties: Stunning pictures of Fukang Pallasite

Photo by Captmondo.

You’re probably wondering what a Pallasite is; well Pallasites are is a type of iron meteorite, quite rare, made out of large olivine crystals in an iron-nickel matrix – and they look just fabulous. Olivine is a a magnesium iron silicate quite common in our planet’s subsurface, but which weathers fast when exposed to the surface.

Photo by Wolfgang Sauber.

What you are looking at here is a man holding a three foot slab of Pallasite in the Sun – it’s not really glowing by itself. I just can’t stop wondering where he got it though… probably just found it on the street, or on some field.