Research team pinpoints the source of unique chemical composition of volcanic rocks

Boston College Professor of Earth and Environmental Sciences Ethan Baxter, at work on Greek island of Sifnos. Analysis of garnet samples collected on the island confirmed theories that water flowing from deep in the Earth millions of years ago is the source of oxidation found in the remnants of ancient arc volcanoes.
Credit: Boston College

A new analysis has revealed the source of oxidation found in rock samples from the coast of Greece, where geological activity spawned explosive arc volcanoes about 45 million years ago, a team of researchers reports in the journal Nature Geoscience.

Arc  are highly oxidized, which has led scientists to speculate that the fluids coming from subducted oceanic rocks might be the , said Boston College Professor of Earth and Environmental Sciences Ethan Baxter, a co-author of the report.

To test that hypothesis, Baxter’s team at Boston College along with colleagues from the Sorbonne, the University of South Carolina, and Durham University (UK) sought to identify a fingerprint of the fluid source in the remnants of ancient subducted oceanic crust found on the Greek island of Sifnos.

The team of researchers studied samples of large garnet crystals, which contain concentric rings of growth, much like the rings of a tree. Within these rings are chemically unique zones that change from the core of the crystal to the rim, said Baxter. These zones reflect the evolution of the rock system within which the garnet grows over millions of years.  In these rocks, the iron isotope composition varies from core to rim in a way that supports the release of an  fluid.

The team used electron microprobe analysis and iron isotopic analysis to document progressive changes in the  system recorded in these strongly zoned garnets, said Baxter, whose research is funded by the National Science Foundation.

“Garnet chemical and iron isotope zonation supports the idea that these rocks released oxidizing fluids during subduction,” said Baxter. “We have chemically ‘fingerprinted’ a source of these oxidizing fluids in subduction zones.”

These novel measurements reveal for the first time that garnet crystals preserve zonation of iron isotopic composition from early formed core to later forming rims, the team reports.

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