Rice researchers track zinc, iron in magma to see how oxygen travels inside planet

Rust reveals Earth’s Secrets

BY MIKE WILLIAMS
Rice News staff

The Earth gets rusty. That’s helpful, because it’s key to a mystery that has long puzzled Earth scientists.

Researchers at Rice University and the Ecole Normale Superieure in Lyon, France, led by Rice geochemist Cin-Ty Lee are tracking zinc and iron in magmas that emerge from volcanoes to see where and how the iron was oxidized. What they’ve learned challenges a tenet of Earth science.

Scientists have long believed oxygen introduced to the planet’s interior at subduction zones, where oceanic plates are pushed by tectonic forces down into the mantle, combines with iron to oxidize it.

Cin-Ty Lee

In the generally accepted model, Lee said, magma that flows from mid-ocean ridges oxidizes when it comes into contact with the atmosphere and the ocean. “When oceanic plates sink back into the mantle at the subduction zones, this rusty iron is introduced into the deep Earth, gradually oxidizing the Earth,” he said. “Subduction zones should therefore be the most oxidized regions of the Earth’s interior.”

That’s not what happens, said Lee, an associate professor of Earth science and lead author of a paper that appears in today’s online edition of the journal Nature. Lee and his colleagues said magma may appear to be carrying oxidized iron from the mantle, but that transformation is actually happening much closer to the surface than deep in the mantle. “During its upward journey, the magma cools, decompresses, crystallizes, degasses and mixes with surrounding rocks,” he said. “These processes can modify the oxidation state of magma, masking any memory of the original oxidation state of the mantle.”

The proportion of iron to zinc, a constant in the magma, does not change during its ascent, Lee said, even though the iron oxidizes as the magma bubbles while approaching the surface. Using ancient basalts from volcanic arcs in the Cascades, the Aleutians and the Marianas for a baseline zinc-to-iron ratio, researchers determined the ratio doesn’t change even though oxidation of the iron in new magma does; this suggests that oxidation is not a feature inherited from the mantle. ”I compare it to a biological process,” Lee said. “It is not what you eat that is important, but what your body does with the food.”

Lee suggested a hidden, but important, implication relates to such metal ore deposits as the giant copper mines found in the Western United States and South America. “A widely held view is that copper deposits are genetically related to magmas that derive from oxidized mantle, which allows large amounts of copper to be scavenged from the mantle and the subducting oceanic plate,” he said. “If these conclusions are correct, there is nothing fundamentally unique about the mantle in subduction zones in terms of the oxidation state and ore metals. The formation of metal ores in subduction zones is more likely controlled by how magmas crystallize at shallow depths than what happens deep in the mantle.”

The research also has implications for how oxygen is involved in Earth’s history, Lee said.

“Nearly all the oxygen in the Earth is tightly bound in rocks,” he said. “The atmosphere gets its oxygen from life, primarily through photosynthesis. Although the Earth doesn’t give free oxygen to the atmosphere, it releases oxygen indirectly via carbon dioxide through volcanoes. This carbon is then reduced during photosynthesis to form organic carbon, liberating free oxygen to the atmosphere.

“The real problem is that in order to generate oxygen in the atmosphere, it had to have been reduced in some other part of the planet,” Lee said. “Earth is mostly a closed system. So to have oxygen rise in the atmosphere to where it is today, it had to have been reduced somewhere in the Earth.

“That calls into question whether subduction is actually oxidizing the mantle. I would go so far to say that subduction has actually reduced oxygen in the mantle throughout Earth’s history.”

Co-authors of the paper are Peter Luffi and Véronique Le Roux, both postdoctoral research associates in Lee’s lab; Rajdeep Dasgupta, a Rice assistant professor of Earth science; Francis Albaréde, a visiting professor at Rice and a professor at the Ecole Normale Superieure in Lyon; and William Leeman, a Rice professor emeritus of Earth science and a program director in the Directorate for Geosciences, Division of Earth Sciences, of the National Science Foundation.