Degassing from Mid-Ocean Ridges Refuses to Follow the Rules of Equilibrium

When gases escape at mid-ocean ridges, carbon dioxide and heavier noble gases don’t have time to stabilize between the bubbles and the magma. A portion of these gases “miss the bus” and stay behind while bubbles leave the magma. By taking into account the disequilibrium inherent in volcanic degassing, scientists can make better estimates of the carbon flux through mid-ocean ridges.

A big part of understanding how much carbon Earth holds beneath its surface relies on measuring how much carbon escapes through volcanoes on land, and under the water along mid-ocean ridges. Many scientists have tried to make estimates of degassing, often assuming that the gases in the escaping bubbles are in equilibrium with the melted rock, meaning that the magma and bubbles both hold predictable concentrations of the gases. But new research finds that not all gases follow the rules of equilibrium. 

DCO Reservoirs and Fluxes Community member Jonathan Tucker (Carnegie Institution for Science, USA), along with Sujoy Mukhopadhyay (University of California, Davis, USA), and Helge Gonnermann (Rice University, USA), discovered that carbon dioxide and the heavier noble gases don’t have time to reach equilibrium before the bubbles and magma emerge from mid-ocean ridges. By developing a model that takes into account this disequilibrium, the researchers make new estimates of the carbon concentration in the mantle, and how much carbon dioxide leaves the mantle each year. They report their findings in a new paper [1] in Earth and Planetary Science Letters.

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Tucker et al.’s new paper uses a disequilibrium model to give a more accurate estimate of the deep carbon flux depicted as #1 in this diagram. Credit: Deep Carbon Observatory

As tectonic plates move apart at mid-ocean ridges, degassing occurs in a process very similar to opening a can of soda. The new opening releases the pressure, causing gases that were once dissolved in magmas to form bubbles and escape. Within Earth, those gases are not only carbon dioxide, which is what makes soda bubbly, but also helium and other noble gases. Helium is small so it can move quickly into the rising bubbles, but larger gases, like xenon and carbon dioxide move more slowly, and don’t diffuse fully into bubbles before they escape. “The bubbles leave the magma before all of the gases have had time to separate to their full equilibrium extent,” said Tucker. “So they miss the bus, basically.”

Early on in this project, Tucker had compiled data from the scientific literature on noble gas concentrations in the mantle. He was puzzled when existing models of degassing, which assumed that the gases were in equilibrium between the magma and the bubbles, could not fully explain the data. He and his colleagues constructed a simple model of degassing that took into account disequilibrium between the magma and the bubbles, which could better explain the data.

The new model is quantitative, so if researchers know how much gas was lost, they can estimate the original gas content of the magma, and how much is left behind. The researchers estimate that the mantle holds 110 parts per million of carbon dioxide, and that it loses 60 megatonnes of carbon dioxide from the mantle each year, which falls on the low end of previous estimates. The model will likely enable researchers to assign more accurate numbers to Earth’s global carbon fluxes and cycles in future studies.

Tucker cautions that by introducing this process of disequilibrium into the degassing model, he also introduces additional sources of uncertainty. A better understanding of fundamental physical parameters associated with volcanic degassing, such as constants that allow the estimation of solubility or rates of diffusion, would further refine the model’s results.

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A slice of gas bubbles trapped in a basalt glass. Credit: Jonathan Tucker

Now, Tucker is studying degassing from other volcanic environments, and estimating how much carbon is coming out of the Hawaiian volcanoes specifically. He also is looking at water and its movement as a volatile compound in magma, within a similar global context.

Currently Tucker is a DCO Postdoctoral fellow in the Department of Terrestrial Magnetism, at the Carnegie Institution for Science, but his first exposure to the DCO was at a 2015 meeting in Berkeley, California, co-sponsored by the Cooperative Institute for Dynamic Earth Research (CIDER). “I realized that my interest in global volatile cycles extended beyond the noble gases,” said Tucker. “Being exposed to the deep carbon community gave me a broader perspective on my own work and its applicability. I really dove into that much more substantially since I’ve been here at Carnegie.” 
 

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