Carbonatites, and other carbon-rich magmas that originate hundreds of kilometers below Earth’s surface and intrude into the crust, offer a window into deep carbon processes occurring within the mantle. Scientists have long used characteristics of carbonatite as a stand-in for carbon-bearing magma in the deep mantle when modeling the subsurface. A new study, however, finds that carbonatite magma undergoes extensive changes before reaching the surface and may be a poor proxy for deep carbon.
Sebastian Tappe (University of Johannesburg, South Africa) and Katie Smart (University of the Witwatersrand, South Africa), who both are members of the Deep Energy, Deep Life, Extreme Physics and Chemistry, and Reservoirs and Fluxes Communities, report these findings in a new paper in the journal Earth and Planetary Science Letters. Tappe and Smart worked with colleagues from Germany, Denmark, Norway, and Canada to examine the isotopic and geochemical fingerprints of associated carbonatite and kimberlite magmas that had penetrated the same stable section of continental crust, called a craton. The association of the two rock types enabled the researchers to explore the origin and compare the evolution of these two carbon-rich magmas.
When kimberlite and carbonatite magmas come bubbling up to the surface, they form vertical sheets called dykes that intrude into the crust. The researchers analyzed the isotopic signature and geochemical composition of 17 kimberlite dykes and 14 carbonatite sheets in the North Atlantic craton, near Tikiusaaq in West Greenland. Kimberlite is an unusual rock in Earth’s crust, which forms deep in the mantle and sometimes contains diamonds. Like carbonatite, its magma can also carry large amounts of carbon dioxide and other volatile carbon compounds.
While the kimberlite compositions closely matched the source material, the isotopic signatures of the carbonatites showed that they had become contaminated with continental crust materials during the trip from the deep mantle up to the surface. In the paper, the authors offer recommendations for ways to correct for this contamination when using carbonatite values for geochemical modeling in studies of deep carbon.
“Most people would take these carbonatites as a good proxy for deep mantle carbon,” said Tappe. “In this association of these two rock types, the kimberlites are much more primitive and give you a much better picture of the mantle than the associated carbonatites.”
The high concentrations of volatile carbon compounds in two deep mantle-derived magma types enable them to intrude sections of stiff continental lithosphere, which can ultimately lead to the break up of continents. “These mobile, carbon-rich melts infiltrate these stiff, cold rocks and create new mineral assemblages that make rocks weaker,” said Tappe, “so when exposed to some stress, they easily break.” This process initiated the large-scale continental rupture between Greenland and North America that later became the Labrador Sea ocean basin.
The ability of these melts to move so freely to the surface suggests that, at least in local conditions under West Greenland, the subsurface is much more oxidized than would be expected at depths of 200 km. If this section of the deep mantle had fewer oxygen-containing compounds, then the carbon would have solidified into graphite or diamonds. But instead, the carbon formed more volatile compounds, such as carbon dioxide.
Initially, the project began as a collaboration with the Geological Survey of Denmark and Greenland to investigate a new carbonatite intrusion, freshly exposed by melting glaciers. The Survey was searching for rare earth elements and other metals critical to high-tech industries in the U.S., Japan, Germany, and China. These deposits often occur in association with deep, carbon-rich magmas like carbonatites. The close association with the kimberlite and the pristine condition of the two rare rock types enabled Tappe and colleagues to make these discoveries.
“The project is a fantastic example of how economic geology and mineral exploration can turn into more fundamental research with all stakeholders benefitting from it,” said Tappe.