Complex Analysis Yields Simple Estimate of Carbon Dioxide Trapped in the Lithospheric Mantle

Carbon dioxide leaks that seep upward from Earth’s mantle into hydrocarbon reserves pose a problem for oil and gas companies because contaminated reserves are more expensive to harvest and process. The carbon dioxide initially enters the mantle through carbonates held in Earth’s crust, which sinks into the mantle at the edges of tectonic plates, due to the force of their collision. While some of the gas returns to the surface through volcanic eruptions, an unknown amount of carbon dioxide stays trapped in the lithosphere, Earth’s rocky crust and upper mantle.

A new study combines existing analytic techniques to estimate the amount of carbon dioxide stored in the lithosphere of the Pannonian Basin in Central Europe. Laura Créon (Universidad Nacional Autónoma de México), a member of DCO’s Reservoirs and Fluxes Community, and Virgile Rouchon (IFP Energies nouvelles (IFPEN), France), who belongs to the Deep Energy and Reservoirs and Fluxes Communities, analyzed several rocks brought from the mantle to the surface through volcanic activity, called xenoliths. The analyses suggest that the lithosphere directly below the basin is supersaturated with carbon dioxide, containing at least 0.2%. The researchers report their findings in a new paper in the journal Lithos.

“It’s the first three-dimensional quantification of carbon dioxide trapped in mantle rocks,” said first author Créon. “With this method, we can determine the real concentration of the carbon dioxide in the lithospheric mantle.”

Reconstructing the carbon dioxide within a xenolith is a challenging task due to the information obliterated during the xenolith’s ascent and its prolonged residence time at the surface. “Non-destructive three-dimensional investigations, such as what is made possible by X-ray tomography, allow us to compute the proportion of textural features which can be attributed to fluids once held within the rock, without any doubts regarding their indigenous nature,” said Rouchon. Two-dimensional analyses typically require invasive sample preparation, which makes it difficult to guarantee the absence of textural artifacts, or chemical contamination. Three-dimensional observations also offer more straightforward volume measurements.

This 3D animation of sample SZB51 shows the rock’s composition and structure, complete with veins and vesicles. White is olivine, yellow is clinopyroxene, brown is orthopyroxene, blue is glass (melt) and, pink is vesicles. Credit: Video courtesy of Laura Créon.

Créon and her international team of collaborators employed several complex analytical methods to answer a simple question: What is the mass of carbon dioxide within a volume of rock? She visited the European Synchrotron Radiation Facility in Grenoble, France, to perform synchrotron X-ray microtomography on xenoliths collected from a volcanic field of the Pannonian basin, an area of central Europe known for large-scale mantle carbon dioxide degassing. Much like a CAT scan, the technique creates three-dimensional density images at very high resolution. Créon and her colleagues observed tiny bubbles called fluid inclusions and larger bubbles called vesicles, which were interconnected with veins running through the rocks. Using these images, they could calculate the volume of the carbon dioxide once contained within the bubbles and veins.

Next, Créon and collaborators determined the concentration of carbon dioxide in the rocks using NanoSIMS, a technique available through the Museum National d'histoire Naturelle in Paris. The instrument blasts off bits of rock with an ion beam and measures the mass of the resulting particles to determine their composition.

Finally, by collaborating with DCO members Paul Asimow and Paula Antoshechkina (both at the California Institute of Technology, USA), Créon used the Rhyolite-MELTS model, developed by DCO member Mark Ghiorso (University of Washington, USA), to calculate the pressure that existed when the gas became trapped. With these analyses complete, Créon could easily calculate the mass of carbon dioxide inside the rocks.

Individual rocks varied, but ultimately the researchers estimated that carbon dioxide composes 0.2% of the lithospheric mantle at this location. The estimate matches previous calculations of carbon dioxide in the lithosphere, based on different approaches. These findings suggest that the mantle beneath the basin is supersaturated with gas and likely acts as a carbon reservoir. Researchers believe that the gas exits the reservoir either through volcanic activity or through shifting tectonic plates that may open a release valve.

The study is also a first step toward developing better oil and gas exploration guidelines. “It’s a challenging proposition to improve the tools that we have to explore where and why carbon dioxide is associated with hydrocarbons,” said Rouchon.

In future work, Rouchon and Créon are independently interested in applying this methodology to other locations to understand how carbon dioxide content varies with geography and how the gas migrates through rocks.

“These rocks have been there forever and the techniques also have existed for a while,” said Rouchon. “What is exciting is to look at an object that everybody knows, but with different eyes.”

Laura Créon performing synchrotron experiments on xenolith samples. Credit: Photo courtesy of Laura Créon

A collection of some of the xenolith samples analyzed in the study. Credit: Photo by Laura Créon.

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