URI scientist uncovers surprising relationship between lavas erupting on the sea floor and the deep-carbon cycle
Todd McLeish, 401-874-7892
NARRAGANSETT, R.I. – May 3, 2013 -- Scientists from the University of Rhode Island and the Smithsonian Institution have found unsuspected linkages between the oxidation state of iron in volcanic rocks and variations in the chemistry of the deep Earth. Not only do the trends run counter to predictions from recent decades of study, they belie a role for carbon circulating in the deep Earth.
The team’s research was published this week in Science Express.
Katherine Kelley, associate professor at the URI Graduate School of Oceanography, and Elizabeth Cottrell at the Smithsonian’s National Museum of Natural History measured the oxidation state of iron, which is the amount of iron that has a 3+ versus a 2+ electronic charge, in bits of magma that froze to a glass when they hit the freezing waters and crushing pressures of the sea floor. In their analysis, the researchers found very subtle variations in the iron-oxidation state that had been overlooked by previous investigations.
“It’s important that we do studies like this because it provides us with a way to quantify how much carbon is in the Earth’s mantle,” said Kelley. “Carbon in the mantle is expelled by volcanoes at mid-ocean ridges when they erupt, and that’s ultimately important for understanding our climate, the evolution of the Earth, and the carbon budget.”
The variations the scientists found correlate with what they described as the “fingerprints” of the deep Earth rocks that melted to produce the lavas—but not in the way previous researchers had predicted. The erupted lavas that have lower concentrations of 3+ iron also have higher concentrations of elements such as barium, thorium, rubidium and lanthanum that concentrate in the lavas, rather than staying in their deep Earth home. More importantly, the oxidation state of iron also correlates with elements that became enriched in lavas long ago, and now, after billions of years, show elevated ratios of radiogenic isotopes. Because radiogenic isotopic ratios cannot be modified during rock melting and eruption, Cottrell called this “a dead ringer for the source of the melt itself.”
Carbon is one of the “geochemical goodies” that tends to become enriched in the lava when rocks melt. “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.”
The rocks that the team analyzed that were reduced also showed a greater influence of having melted in the presence of carbon than those that were oxidized. “And this makes sense because for every atom of carbon present at depth it has to steal oxygen away from iron as it ascends toward the surface,” said Cottrell.
This is because carbon is not associated with oxygen at depth; it exists on its own, like in the mineral diamond. But by the time carbon erupts in lava, it is surrounded by oxygen.
In this way, concluded Kelley, “carbon provides a means of reducing the iron and also creating an enriched chemical signature in the magma, both of which are consistent with melting carbon-rich mantle rocks.”
URI Associate Professor Katherine Kelley (right) and Smithsonian scientist Elizabeth Cottrell display samples of rocks from the Earth’s mantle that they study.
Photo courtesy of Katherine Kelley.