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ND scientists examine earth’s mantle

Christian Myers | Monday, November 25, 2013

Notre Dame geologists have discovered an important piece of the puzzle that is the chemical composition of the earth’s mantle – the layer of semi-liquid rock directly below the earth’s crust.

Antonio Simonetti, associate professor in the department of civil and environmental engineering and Earth Sciences and Ph.D. candidate Wei Chen recently published their findings on carbonatites and the composition of the mantle in the scientific journal “Nature Communications”, which is the third-ranked journal in the field.

Carbonatites are igneous rocks the composition of which is at least 50 percent magmatic carbonate minerals, Simonetti said. These rocks are often expelled from volcanoes and are formed in melts, which are the collections of magma from the mantle that form magma chambers within the earth’s crust, such as those within volcanoes, he said.

Simonetti said studying carbonatites leads to a better understanding of the composition of the mantle from which the melts and subsequently the carbonatites themselves are formed.

“I’ve always tried to look at carbonatites as messengers of the chemical composition of the earth’s mantle,” Simonetti said.
Simonetti said the discovery helps answer a question that has “plagued” researchers for decades, which is “What is the primary composition of carbonate rich melts?”

Essentially, Simonetti and Chen have shown that carbonate rich melts contain large amounts of the alkalis potassium and sodium in addition to the large amounts of carbon dioxide and calcium already known to be in such melts.

“Geologists thought carbonatites were coming from calcium or carbon dioxide rich regions, but in reality these areas are also sodium and potassium rich,” he said.

This better understanding of the composition of melts also helps explain the discrepancy between old carbonatites, which contain little potassium or sodium, and the new carbonatites being produced by the world’s only active carbonatite volcano Ol Doinyo Lengai in Tanzania, which contain a significant amount of potassium and sodium, Simonetti said.

“It was very surprising. We didn’t know what to expect, but it was the least likely outcome,” he said.

“For me it’s a huge impact. This work is going to have a big impact on the igneous petrology community.”

The discovery came as a result of studying melt inclusions in 120 million year old carbonatites from the Oka complex in Canada, specifically southwestern Quebec, Simonetti said.

Melt inclusions are micron-sized – a micron is one millionth of a meter – pockets of melt material trapped within the crystals of carbonatites as they form in the cooling melt, Simonetti said. Melt inclusions contain everything present in the surrounding melt at the time they were formed, as opposed to carbonatites, which contain only select minerals that precipitate from the surrounding melt. Thus, melt inclusions reveal the initial composition of a carbonatite rich melt, he said.

The chemical signatures of the melt inclusions from the Oka carbonatites were analyzed and significant amounts of sodium and potassium were present in the trapped melt, even though the surrounding carbonatite was only calcium and carbon rich, Simonetti said. This was the first evidence of alkali rich carbonatite melts found outside of East Africa, he said.

“The fact that the mantle source regions that gave rise to these melts were also alkali rich means Ol Doinyo Lengai is no longer an oddball,” Simonetti said.

Simonetti said the next step is to look for this same phenomenon in carbonatites elsewhere in the world, since the evidence of potassium and sodium is no longer unique to Africa. After that, he said geologists will seek to answer the question of how carbonatites lose alkalis after forming, which will explain why older carbonatites lack these minerals that the new carbonatites at Ol Doinyo Lengai still have.

Simonetti said he has been studying carbonatites since he was a Ph.D. candidate in the early 1990s. He said one reason neither he nor any other geologist made this discovery before now is that his discovery depended on recent technological developments, such as the ability to study melt inclusions with Raman spectroscopy.

Contact Christian Myers at cmyers@nd.edu.