January 19, 2013
Proof Of Ancient Subduction Discovered In Volcanic Rocks
redOrbit Staff & Wire Reports - Your Universe Online
Researchers for the first time have discovered evidence supporting the theory that the processes that act as catalysts for volcanic activity today are similar to those that occurred nearly four billion years ago.
Writing in the journal Geology, Frances Jenner of the Carnegie Institution for Science and colleagues report that 3.8 billion-year-old volcanic rocks recovered from an island in southwestern Greenland support previous geochemical studies that have suggested that subduction-style tectonic activity had been occurring as early as the Eoarchean era.
Those studies had used “similarities between modern subduction zone magmas and those erupted about 3.8 billion years ago,” to argue that subduction-related tectonics had been occurring early on in our planet´s history, the Institution said in a statement Friday.
Previously, scientists had been unable to “locate any suites of volcanic rocks with compositions comparable to modern mid-ocean ridge or oceanic island magmas that were older than 3 billion years and were also free from contamination by continental crust,” they added. However, rocks discovered by Jenner´s team are the “missing piece of the puzzle” because they are comparable to modern-day samples obtained from oceanic islands.
The subduction process takes place at plate boundaries, and begins when the upwelling and melting of the mantle at mid-ocean ridges and the eruption of new seafloor magmas spur on the continued production of oceanic crust, the researchers explain. As that crust moves away from mid-ocean ridges, it cools and becomes denser than the mantle beneath it, ultimately sinking back into the mantle and triggering additional eruptions.
“Volcanic eruptions that are triggered by subduction of oceanic crust are chemically distinct from those erupting at mid-ocean ridges and oceanic island chains, such as Hawaii,” representatives from the Institution said. “The differences between the chemistry of magmas produced at each of these tectonic settings provide 'geochemical fingerprints' that can be used to try to identify the types of tectonic activity taking place early in the Earth's history.”
“The Innersuartuut samples may represent the world's oldest recognized suite of oceanic island basalts, free from contamination by continental crust,” added Jenner said. “This evidence strengthens previous arguments that subduction of oceanic crust into the mantle has been taking place since at least 3.8 billion years ago.”