Quebec Rocks Provide New Insights into Chemical Makeup of Earth’s Mantle

by johnsmith

According to geologists led by Dr Antonio Simonetti from the University of Notre Dame, a detailed analysis of calcite-rich minerals from the 120-million-year-old carbonatite complex of Oka in southeastern Quebec, Canada, has improved our knowledge of the chemical makeup of our planet’s mantle.

St Lawrence Columbium Mine, Oka complex, Quebec, Canada. Image credit: J.Y. Lamoureux.

St Lawrence Columbium Mine, Oka complex, Quebec, Canada. Image credit: J.Y. Lamoureux.

Dr Antonio Simonetti teamed up with U.S. and Australian scientists to learn the art of conducting chemical and mineralogical analyses of melt inclusions within crystals of the mineral magnetite. The magnetite crystals are hosted within igneous rocks – rocks resulting from the melting of the Earth’s mantle – referred to as carbonatites.

“The latter are an exceptional and intriguing type of igneous rock since they are composed primarily of calcium carbonate (calcite) rather than silicate minerals, which are the predominant minerals in the Earth’s crust and oceanic rocks,” Dr Simonetti said.

“Despite the small number of carbonatite occurrences worldwide compared to their volcanic counterparts in the past and present day, carbonatites continue to receive considerable deserved attention because of their unique enrichment, relative to crustal abundances in incompatible trace elements, such as niobium and the rare Earth elements.”

To date, most of the geological community believed that the sodium- and potassium-rich magmas being erupted at the Earth’s sole active carbonatite volcano at Ol Doinyo Lengai in Tanzania were unique, since all other carbonatite occurrences worldwide are dominated by calcium-rich carbonate or calcite.

In an attempt to resolve this question, the scientists sought to determine the initial melt composition that gave rise to the Oka carbonatite complex.

“We approached this issue by examining the nature and chemical composition of melt inclusions within individual magnetite crystals present in carbonatites. Melt inclusions are micron-sized ‘pockets’ present within minerals that represent a combination or mechanical mixture of co-trapped crystals and melt engulfed and isolated early in the crystallization history of the magma while the magnetite crystals were forming. Hence, investigating melt inclusions represents a powerful tool for determining the chemical composition of the initial carbonatite magma at the Oka complex,” said Dr Simonetti, who with co-authors reported the results in the journal Nature Communications.

The study revealed that the chemical composition of minerals trapped within the melt inclusions at the Oka complex is alkaline in nature and similar in composition to the minerals present at Ol Doinyo Lengai volcano.

The results will have a major impact in relation to deciphering and modeling chemical processes taking place in the Earth’s mantle throughout geologic time.


Bibliographic information: Wei Chen et al. 2013. Evidence for the alkaline nature of parental carbonatite melts at Oka complex in Canada. Nature Communications 4, article number: 2687; doi: 10.1038/ncomms3687

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