Bursting the Bubble: Copper Trapped Inside Volcanic Crystals

Using rocks erupted from San Cristóbal volcano, researchers analyzed the contents of shrinkage bubbles inside melt inclusions, which form within olivine crystals as they cool inside rising volcanic magma. While looking for carbon dioxide, they discovered water and copper inside the bubbles; a finding that may have implications for developing new ways to locate copper ore deposits.

Turrialba volcano in Costa Rica

After lying quiet for three centuries, the San Cristóbal volcano, which lies at the northwestern edge of Nicaragua, erupted in March 1976. Melted rock, called magma, bubbled up toward the surface, causing the mineral olivine to solidify into crystals, which trap droplets of magma known as melt inclusions. As these crystals cooled, carbon dioxide and other volatile compounds coalesced into tiny “shrinkage bubbles” inside the melt inclusions. Scientists have examined the content of these bubbles from other volcanoes and from ones they created in the lab. But shrinkage bubbles in melt inclusions from the San Cristóbal crystals held a surprise inside. These bubbles contained traces of copper minerals. 

DCO members Philippe Robidoux (Universidad Mayor, Chile), Maria Luce Frezzotti (University of Milano Bicocca, Italy), Erik Hauri (deceased, formerly of Carnegie Institution for Science, USA), and Alessandro Aiuppa (Università degli Studi di Palermo, Italy) analyzed shrinkage bubbles in melt inclusions from rocks erupted by San Cristóbal, finding carbon compounds, liquid water, and copper sulfides inside. They report these findings in a new study in the Journal of Petrology. The research may have applications for geologists trying to find new deposits of copper ore and for volcanologists looking for more accurate ways to estimate the depth at which magma formed. 

Robidoux and his colleagues didn’t begin this project mining for metals in magma. Originally, he started working on the San Cristóbal volcano as a graduate student in Aiuppa’s lab at the Università degli Studi di Palermo, analyzing gases and the composition of volcanic rocks. He discovered numerous melt inclusions, several of which contained shrinkage bubbles. As part of his doctoral work, he and his coauthors used multiple analytical techniques to measure the carbon dioxide, water, and hydrated and sulfur mineral species inside the bubbles, and also to detect minerals that contain copper, magnesium, and iron. 

Inside of shrinking bubble
 Inside the shrinkage bubbles within olivine crystals, researchers discovered liquid water, carbon dioxide, and copper, iron, and magnesium minerals. Image courtesy of the Journal of Petrology; modified by P. Robidoux. 

The contents of a shrinkage bubble can reveal the fluids and gases existing in the magma, allowing researchers to reconstruct the conditions the magma experienced during the trip to the surface. In general, the more volatile compounds dissolved in magma, the deeper it originated below the volcano. As magma rises and pressure decreases, however, the volatile compounds inside the melt inclusions separate from the melt into bubbles, through a mechanism that still is poorly understood. The gases and fluids inside the shrinkage bubble can’t escape, so they record a fingerprint from the original magma.

Initially Robidoux detected carbon dioxide, which is a well-established component of shrinkage bubbles. “But then I discovered liquid water and copper and that was the first time they were found in this kind of natural environment, instead of produced in lab experiments,” he said. “That was not at all expected.” The shrinkage bubbles also contained iron and magnesium minerals that formed at the interface of the shrinkage bubble and the olivine.

The copper and water inside the shrinkage bubbles signifies that “there is a natural mechanism that separates the liquid water from magma and that copper sulfides is separated at the same time,” said Robidoux. If researchers can figure out the conditions, and thus the depth, that cause copper and water vapor to separate from the magma, then geologists potentially could use that information to locate new ore deposits. “I think this research has many applications for developing exploration tools for copper,” Robidoux said. 

Thus far, scientists have primarily looked at shrinkage bubbles from arc volcanoes, which occur at the edges of a continental plate, where an oceanic plate sinks beneath it. Next, Robidoux hopes to collaborate with other volcanologists to analyze shrinkage bubbles from other types of volcanoes and from other tectonic environments. Robidoux’s home institution is located in Chile, which is the largest copper producer in the world. He is interested in studying Chilean volcanoes to see if he finds the same surprising mix of carbon, copper, and water that he discovered in the Nicaraguan samples.

Robidoux also is talking with nanotech researchers who specialize in metallogenesis. He is interested to see if this natural process can be applied to separate copper from other metals and compounds for use in industry.

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Top photo: Philippe Robidoux analyzes volcanic gases at the Turrialba volcano in Costa Rica. Credit: V. Conde

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