Jessai Flores, Staff Illustrator

A new study led by Jay Ague, professor of earth and planetary sciences at Yale, has provided crucial insights into the role of oxidation in volcanic formation. 

The team behind the study is an international group of researchers at universities from the United States, China, Germany and Greece. The researchers’ goal was to understand the oxidation states of magmatic arcs, also known as volcanic arcs. Magmatic arcs are chains of volcanoes that take on arcuate shapes, such as the arc of the Aleutian Islands and form as a result of magmatic activity beneath the surface of the Earth over subduction zones — areas where one oceanic plate is compressed beneath another into the mantle. According to Ague, magmatic arcs are responsible for some of the most powerful volcanic eruptions on Earth.

“The relatively oxidized nature of magmatic arcs, which form above Earth’s subduction zones, has been a challenge to interpret for many years,” Ague said. “When studying subducted sediments in the Cycladic islands of Greece, I noticed that many were highly oxidized and sought to test whether this oxidized character was connected to the genesis of oxidized arc magmas.”

Magmatic arcs are the result of subduction by oceanic plates. The plates, or “slabs,” are layered with sediment originating from processes like hypothermal seafloor activity or deposition from continents. When this sediment contains a high ratio of ferric acid to ferrous acid, it is highly oxidized, which is essential to magmatic arc formation.

When an oceanic plate undergoes subduction, it is heated and dehydrated. This process produces aqueous fluids that begin to rise. These aqueous fluids become oxidized as they travel through the highly-oxidized sediment layer, which the team found acts as a reduction-oxidation, or redox, filter. Once the aqueous fluids ascend to the mantle, they become the driving forces behind partial melting, which starts the formation of the oxidized magma that feeds the arcs.

“What our group has done here, what Jay has really pioneered here, is the idea of the sediments that are going down on the subducting slab, acting like a redox filter,” said Megan Holycross, an assistant professor of earth and atmospheric sciences at Cornell and a co-author on the paper. “[…] The fluid itself could be reduced, but if it passes through these sediments which are oxidized, those sediments act as a redox filter and actually oxidize that fluid. It’s suggesting that sediments are really key when we’re thinking about the transfer of oxygen throughout Earth’s deep interior.” 

To study this sediment as a redox filter, the team analyzed subducted and metamorphosed sediments called metasediments from the Cyclades in Greece. The analysis was carried out with an electron-probe microanalyzer, or EMPA, to determine the chemical compositions of the minerals such as hematite and magnetite within the rock samples. Based on this data, it was possible to calculate the redox state of the metasediments with thermodynamics and evaluate how the calculated redox state may affect the aqueous fluids that travel through these kinds of rocks.

Magmatic arcs can contain significant deposits of metals such as copper, molybdenum and gold and also produce extremely violent eruptions. These eruptions typically release sulfur gas into the stratosphere, resulting in the subsequent presence of sulfur aerosols that lower the temperature of the atmosphere via transient cooling, which can affect the climate. An instance of this was observed in 1816 –– “the year without a summer” –– as a result of the 1815 eruption of the Mount Tambora volcano, a magmatic arc volcano that caused a decrease in global temperatures. 

Understanding the geochemistry behind magmatic arcs –– and how they become oxidized –– could lead to a better understanding of their characteristics and effects.

“This work was truly an interdisciplinary effort involving collaborators from around the globe,” said Santiago Tassara, a Bateman Postdoctoral Scholar and a co-author on the paper. “It reveals the dynamic nature of the Earth and demonstrates how its evolution is shaped by the complex interaction between surficial and endogenic processes in subduction zones.”

Ague is also the curator-in-charge of mineralogy and meteoritics at the Yale Peabody Museum of Natural History.