Geologists locate source of chemical signature in ancient volcanic rocks

Geologists have linked ancient volcanic activity with a unique geochemical signature measured in rock samples recovered from the coast of Greece.

According to a new study published this week in the journal Nature Geoscience, an ancient ring of explosive arc volcanoes, which erupted some 45 million years ago, accounts for the highly oxidized rocks found along the coast of Greece.

Researchers hypothesized that fluids from subducted oceanic rocks can explain the oxidation found in rocks formed by the arc volcanoes. To test their hypothesis, scientists went looking for the fluid source's geochemical signature in ancient subducted oceanic crust located on the Greek island of Sifnos.

Oxidizing fluids influence the iron isotope composition found large garnet crystals, and because garnet crystals feature rings of growth like a tree, researchers can plot geochemical changes across hundreds of thousands of years of crystal growth.

"Garnet chemical and iron isotope zonation supports the idea that these rocks released oxidizing fluids during subduction," Ethan Baxter, professor of earth and environmental sciences, said in a news release. "We have chemically 'fingerprinted' a source of these oxidizing fluids in subduction zones."

The measurements suggest an influx of oxidizing fluid from subducted oceanic crust fueled the oxidation of rocks produced by an arc of volcanoes some 45 million years ago.

Scientists hope that in followup studies they can figure out what caused the fluids to be oxidizing.

"These oxidizing fluids carry certain agents capable of oxidizing rocks they enter," Baxter said. "The most familiar example would be the way that oxidizing fluids can cause iron-bearing materials to rust as they weather. Based on the garnet chemical zonation, we know the fluids liberated from our samples are oxidizing, but we don't know why they are oxidizing or what the oxidizing agents are."

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