This garnet pyroxenite xenolith from Sierra Nevada, Calif., is an example of the deepest products of crystallization within the magmatic belts of subduction zones. Rice University’s Cin-Ty Lee and colleagues showed that most of the copper in arc magmas eventually end up in these deep-seated rocks. Image: Cin-Ty Lee/Rice University |
Earth is clingy when it comes to copper. A new Rice University
study in Science finds 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 3 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 oxidized 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 oxidized 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 crystallized 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 oxidizing, Lee said, these
sulfide minerals would be destabilized 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.”
Lee said deciding where to look for undiscovered copper
deposits requires an understanding of the conditions needed to overcome the
forces that conspire to keep it deep inside the planet.
“As a continental arc matures, the copper-rich sulfides are
trapped deep and accumulate,” he said. “But if the continental arc grows
thicker over time, the accumulated copper-bearing sulfides are driven to deeper
depths where the higher temperatures can re-melt these copper-rich dregs,
releasing them to rejoin arc magmas.”
These conditions were met in the Andes
Mountains and in western North America. He said other potential sources of
undiscovered copper include Siberia, northern China,
Mongolia, and parts of Australia.
Lee noted that a high school intern played a role in the
research paper. Daphne Jin, now a freshman at the University
of Chicago, made her contribution to
the research as a high school intern from Clements
High School in the Houston suburb of Sugarland.
“The paper really wouldn’t have been as broad without
Daphne’s contribution,” Lee said. “I originally struggled with an assignment
for her because I didn’t and still don’t have large projects where a student
can just fit in. I try to make sure every student has a chance to do something
new, but often I just run out of ideas.”
Lee eventually asked Jin to compile information from
published studies about the average concentration of all the first-row of
transition elements in the periodic table in various samples of continental
crust and mantle collected the world over.
“She came back and showed me the results, and we could see
that the average continental crust itself, which has been built over 3 billion
years of Earth’s history in Africa, Siberia, North America, South
America, etc., was all depleted in copper,” Lee said. “Up to that
point we’d been looking at the building blocks of continents, but this showed
us that the continents themselves followed the same pattern. It was all
internally consistent.”
