Tag Archives: kimberlite

Canadian diamonds give researchers a glimpse into ancient continents

A chance discovery by diamond-prospecting geologists provides a glimpse into the Earth’s past.

Diamond embedded in a piece of kimberlite.
Image credits James St. John / Flickr

Diamond exploration samples from Baffin Island, Canada point to a never-before-seen part of the North Atlantic craton—an ancient part of Earth’s continental crust that stretches from Scotland to Labrador. The finding will allow researchers to better map the lost continents of Earth’s distant past.

A diamond in you

“For researchers, kimberlites are subterranean rockets that pick up passengers on their way to the surface,” explains University of British Columbia geologist Maya Kopylova. “The passengers are solid chunks of wall rocks that carry a wealth of details on conditions far beneath the surface of our planet over time.”

Kimberlite rocks form very, very deep in the Earth’s crust — similarly to diamonds — and are thus a staple of diamond prospectors around the world. After forming for millions of years at depths of between 150 to 400 kilometers, these rocks are sometimes pushed up to the surface by various geological processes, snatching diamonds along for the ride.

Kopylova and colleagues were analyzing samples from De Beers’ Kimberlite Province Chidliak in southern Baffin Island, when they noticed that the samples were peculiar. Their mineral makeup matched that in other portions of the North Atlantic craton, which is “so unique there was no mistaking it”. The mineral compositions of adjacent ancient cratons have “completely different mineralogies,” Kopylova explains.

Cratons are pieces of continents billions of years old that have remained stable over time and act as the kernel of today’s continents — think of them as anchors that today’s landmasses hold on to. Most cratons have been broken up and moved around by tectonics over time, but some still form the bedrock of modern tectonic plates like the North American plate. Knowing where the pieces of these cratons are today allows researchers to understand how they evolved and moved over time, in essence allowing them to map the evolution of our planet’s surface.

“Finding these ‘lost’ pieces is like finding a missing piece of a puzzle,” says Kopylova. “The scientific puzzle of the ancient Earth can’t be complete without all of the pieces.”

To the best of our knowledge so far, the continental plate of the North Atlantic craton broke apart some 150 million years ago, and the fragments spread from northern Scotland to southern Greenland and Labrador. the newly-discovered fragments would increase its known expanse by roughly 10%.

“With these samples we’re able to reconstruct the shapes of ancient continents based on deeper, mantle rocks,” says Kopylova. “We can now understand and map not only the uppermost skinny layer of Earth that makes up one percent of the planet’s volume, but our knowledge is literally and symbolically deeper.”

“We can put together 200-kilometer deep fragments and contrast them based on the details of the deep mineralogy.”

The paper “The metasomatized mantle beneath the North Atlantic Craton: Insights from peridotite xenoliths of the Chidliak kimberlite province (NE Canada)” has been published in the Journal of Petrology.

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