Issues Magazine

Earth’s Deep-Seated Hold on Copper

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Rice University

Earth is clingy when it comes to copper. A new Rice University study published in April 2012 in the journal Science found that nature conspires at scales both large and small – from the realms of tectonic plates down to molecular bonds – to keep most of Earth’s copper buried dozens of miles below ground.

“Everything throughout history shows us that Earth does not want to give up its copper to the continental crust,” said Rice geochemist Cin-Ty Lee, the lead author of the study. “Both the building blocks for continents and the continental crust itself, dating back as much as three billion years, are highly depleted in copper.”

Finding copper is more than an academic exercise. With global demand for electronics growing rapidly, some studies have estimated the world’s demand for copper could exceed supply in as little as six years. The new study could help because it suggests where undiscovered caches of copper might lie.

But the copper clues were just a happy accident.

“We didn’t go into this looking for copper,” Lee said. “We were originally interested in how continents form and more specifically in the oxidation state of volcanoes.”

Earth scientists have long debated whether an oxygen-rich atmosphere might be required for continent formation. The idea stems from the fact that Earth may not have had many continents for at least the first billion years of its existence and that Earth’s continents may have begun forming around the time that oxygen became a significant component of the atmosphere.

In their search for answers, Lee and colleagues set out to examine Earth’s arc magmas — the molten building blocks for continents. Arc magmas get their start deep in the planet in areas called subduction zones, where one of Earth’s tectonic plates slides beneath another. When plates subduct, two things happen. First, they bring oxidised crust and sediments from Earth’s surface into the mantle. Second, the subducting plate drives a return flow of hot mantle upwards from Earth’s deep interior. During this return flow, the hot mantle not only melts itself but may also cause melting of the recycled sediments. Arc magmas are thought to form under these conditions, so if oxygen were required for continental crust formation, it would mostly likely come from these recycled segments.

“If oxidised materials are necessary for generating such melts, we should see evidence of it all the way from where the arc magmas form to the point where the new continent-building material is released from arc volcanoes,” Lee said.

Lee and colleagues examined xenoliths, rocks that formed deep inside Earth and were carried up to the surface in volcanic eruptions. Specifically, they studied garnet pyroxenite xenoliths thought to represent the first crystallised products of arc magmas from the deep roots of an arc some 50 km below Earth’s surface. Rather than finding evidence of oxidation they found sulfides – minerals that contain reduced forms of sulfur bonded to metals like copper, nickel and iron. If conditions were highly oxidising, Lee said, these sulfide minerals would be destabilised and allow these elements, particularly copper, to bond with oxygen.

Because sulfides are also heavy and dense, they tend to sink and get left behind in the deep parts of arc systems, like a blob of dense material that stays at the bottom of a lava lamp while less dense material rises to the top.

“This explains why copper deposits, in general, are so rare,” Lee said. “The Earth wants to hold it deep and not give it up.”