Carbon Holds the Key to Mantle Redox Heterogeneity

In a paper released this week in Science, DCO researchers Elizabeth Cottrell and Katie Kelley describe the central role of carbon in controlling mantle chemistry.

Earth’s mantle is a complex and heterogeneous mixture. Understanding the chemical composition of the mantle, which comprises over 80% of Earth’s volume, and how its activities influence our atmosphere, are crucial. In a paper released this week in Science (2 May 2013), Elizabeth Cottrell and Katie Kelley of the Reservoirs and Fluxes community describe the central role of carbon in controlling mantle chemistry [1].

We have yet to directly sample Earth’s mantle, the thick, viscous layer of rock lying beneath the thin crustal shell; however, volcanic activity is constantly liberating mantle-derived material that researchers can use. Of particular interest are mantle-derived mid-ocean ridge basalts, or MORBs, which hold clues both to the origin and chemistry of the rock.

The study of such basalts has shown that the upper mantle is composed of enriched and depleted reservoirs. These distinct regions of the mantle have different elemental composition, and therefore play host to different chemical reactions. One important facet of the different mantle reservoirs is their oxygen fugacity, an interpretation of their redox potential, often measured as a ratio of iron oxidation states. Previous work suggested that if any difference in oxygen fugacity existed between enriched and depleted mantle, it was that enriched mantle was more oxidized [2].

Cottrell and Kelley, however, have challenged this concept. By measuring the oxidation state of iron in 35 MORBs from 13 different geographical locations, they have now shown that enriched mantle is in fact more reduced than depleted mantle. This observation led them to question why this counter-intuitive situation should exist, and the answer they came up with is based on the chemical behavior of carbon.

"Despite its importance to life on this planet, carbon is a really tricky element to get a handle on in melts from the deep Earth," said Cottrell. "That is because carbon also volatilizes and is lost to the ocean waters such that it can't easily be quantified in the lavas themselves. As humans we are very focused on what we see up here on the surface. Most people probably don't recognize that the vast majority of carbon — the backbone of all life—is located in the deep Earth, below the surface—maybe even 90 percent of it."

Listen to Elizabeth Cottrell's interview as part of the Science Podcast (3 May 2013) here.

Current estimates of carbon content are ~16 parts per million (ppm) in depleted mantle and >300 ppm in enriched mantle [3], however precise measurement of carbon in MORB is hindered by partial degassing of the element as carbon dioxide during volcanic eruptions. Deep in the mantle, carbon exists in its reduced, elemental form (graphite or diamond), but as the mantle moves upwards beneath mid-ocean ridges, it oxidizes to carbon dioxide. This chemical reaction requires that reduced carbon steal oxygen from another compound, and in the case of enriched melts the victim of such theft is iron. Thus, in MORBs, the measured oxidation state of iron is skewed by the starting carbon content.

These observations, backed up by recent experimental work by Stagno et al [4], have important implications for deep carbon science. If we are to ever comprehend how much carbon Earth possesses, we need an appreciation of how it interacts with other elements in the mantle. Moreover, understanding deep Earth contributions to the atmospheric carbon cycle is crucial, especially as we tease apart human contribution to current climate change.

Caption: Molten magma erupted onto the seafloor freezes to glass that contains clues to its origin in Earth’s deep interior and ancient past (field of view ~1cm). Volcanic glasses like this one may reveal a link between Earth’s oxidation state and the deep carbon cycle.

 
Image Credit: Glenn Macpherson and Tim Gooding, as featured on the cover of the June 14th issue of Science. 

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