Tag Archives: transition zone

Bermuda might hold the key to a whole new type of volcano

Researchers studying a volcano in Bermuda report that it is unlike anything else we’ve seen on Earth — it formed through a mechanism we knew nothing about until now.

About 30 million years ago, a disturbance in the mantle’s transition zone supplied the magma to form the now-dormant volcanic foundation on which Bermuda sits. Image credits: Wendy Kenigsberg/Clive Howard.

With its turquoise seas and pink beaches, Bermuda draws almost 1 million tourists every year. But far beneath the crystalline water, something draws a completely different crowd: scientists.

Cornell researchers had a hunch that there was something off about Bermuda’s volcanoes, so they analyzed a 2,600-foot (800-meter) core sample taken back in 1972. They were looking for isotopes, trace elements, evidence of water content, volatile materials — anything that would give some indication as to how the volcanoes were formed.

“I first suspected that Bermuda’s volcanic past was special as I sampled the core and noticed the diverse textures and mineralogy preserved in the different lava flows,” Mazza said. “We quickly confirmed extreme enrichments in trace element compositions. It was exciting going over our first results … the mysteries of Bermuda started to unfold.”

When the team analyzed the materials from the core, they found a clear signature of the “transition zone” — a layer rich in water, crystals and melted rock that lies beneath the outer and inner mantle. Before now, researchers didn’t know that volcanoes can form from the transition zone.

“We found a new way to make volcanoes. This is the first time we found a clear indication from the transition zone deep in the Earth’s mantle that volcanoes can form this way,” said senior author Esteban Gazel, associate professor in the Department of Earth and Atmospheric Sciences at Cornell University.

Cross-polarized microscopic slice of a core sample. Blue-yellow mineral is augite. Credits: Gazel lab.

Volcanoes were thought to form through one of two mechanisms: either when two tectonic plates subduct (one moves beneath the other), or when there is a deep mantle upwelling, as is the case in Hawaii. Surprisingly this wasn’t the case in Bermuda.

“We were expecting our data to show the volcano was a mantle plume formation — an upwelling from the deeper mantle — just like it is in Hawaii,” Gazel said. However, 30 million years ago, a disturbance in the transition zone caused the magma to flow towards the surface of what is now Bermuda.

Although geochemical studies of this type have been carried out in most volcanic parts of the world, Bermuda had escaped trialing until now. Now that they know what to look for, researchers say that there’s a good chance they might find these chemical signatures in other volcanic areas as well.

This suggests that the transition zone, which is located at a depth of 410-660 km (250 to 400 mi), is an important chemical reservoir for the Earth, bringing material from that depth and onto the surface.

The study has been published in Nature. DOI:10.1038/s41586-019-1183-6.

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