Living organisms have a complex relationship with metals. In small amounts, some metals are vital nutrients for cells, while others are highly toxic. Human mining and industry have made metal pollution a problem, however, for most of Earth’s history volcanoes have gradually brought metals from deep Earth to the surface.
A new study published in Nature Geoscience  shows that we can learn a lot about a volcano from the metals it spews into the air. DCO Reservoirs and Fluxes Community members Marie Edmonds (University of Cambridge, UK), and Tamsin Mather (University of Oxford, UK), along with Emma Liu (University of Cambridge, UK) looked at the metals emitted from two volcano types. They found distinct differences in the composition of metals from hotspot volcanoes like Hawaii, where a superheated plume from deep in the mantle causes an isolated eruption, and arc volcanoes, which occur along the edges of subducting tectonic plates. Their findings suggest that the type of volcanoes erupting on Earth when life first began may have influenced the metals that early cells encountered as they evolved.
“Most people wouldn’t immediately think of volcanoes as sources of metals to the atmosphere,” said Edmonds, a co-chair of the Scientific Steering Committee for the Reservoirs and Fluxes Community. “It’s not just carbon and water that volcanoes degas. They also outgas lots of volatile metals like copper and zinc.”
After compiling existing data on metal emissions from six basaltic volcanoes, the researchers noticed a pattern. Arc volcanoes typically released gases with large amounts of tungsten, arsenic, thallium, antimony, and lead compared to hotspot volcanoes, which outgassed more cadmium, selenium, and tellurium. The researchers focused on basaltic volcanoes because they typically erupt along mid-ocean ridges, and likely occurred early in Earth’s history, during the Hadean and Archaean eons. Also, unlike with other volcano types, basaltic magma has not “evolved” through cooling and crystallization steps that might alter the metal content, muddying the results.
The researchers propose that these different metal signatures can be explained by how much water and oxygen the magma holds. Magma can only hold onto a certain amount of water, and as it ascends and the pressure decreases, it can hold less and less. The more water the magma holds, the deeper its water will start to bubble and come out of solution. Arc magmas contain a lot of water because hydrous minerals in the crust sink into the mantle during subduction. The abundance of water causes the magma to release its gases deep in the mantle. These metal-rich gases rise and can either end up as ore deposits, or be released through arc volcanoes at levels rivaling industrial smelters.
In comparison, hotspot volcanoes contain less water but greater amounts of sulfides. In these volcanoes, the magma releases its gases closer to the surface, emitting smaller amounts of metals, including those that typically bind with sulfide.
In future work, the group plans to explore how the presence of carbon dioxide might affect these patterns. They expect that carbon dioxide would cause magma to give up its metals via the gases even deeper within the volcano. “We know arc magmas are really rich in carbon dioxide,” said Edmonds. “Potentially, these metal-rich fluids are actually being generated much deeper than we think they are.”
Arc volcanoes didn’t erupt on Earth until the start of subduction, around two billion years ago, but hotspot volcanoes have erupted for much longer. The shift in volcanic activity to include arc volcanoes likely changed the metal composition of the atmosphere and in the oceans. “Some metals are really vital for biological systems, such as copper and zinc, which are used in enzymes for microbial life in the ocean and even in people,” said Edmonds. “We think this may have had implications for how life evolved in the ocean since the onset of plate tectonics.”
Now, Edmonds hopes to apply these findings to make sense of previous volcanic eruptions recorded in ice cores. Scientists have a few hundred thousand years of history contained in ice cores from Greenland, and Edmonds hopes that by looking at the metal signatures from each eruption, she can figure out which ones were small, local hotspot eruptions in Iceland, and which were large, climate-altering arc volcanoes erupting near the equator.
The group also is collecting more metal data from volcanoes and is headed to Hawaii to take gas measurements from the recent eruption of Kilauea. “The story is not over yet,” said Edmonds. “I think the next steps will be to look at the more evolved magmas and make the link between these fluids and how ore deposits are formed.”
Image: Mount Etna, Italy, an arc volcano, erupts at night. Credit: Alessandro Aiuppa