April 25, 2013
Oceanic Volcanoes Reveal Signs Of Ancient Earthen Crust
April Flowers for redOrbit.com - Your Universe Online
For a long time now, scientists have been convinced that lava that has erupted from certain oceanic volcanoes contains material from the crust of early Earth, but decisive evidence has been elusive. A new study, published in the journal Nature, reveals that oceanic volcanic rocks contain samples of recycled crust dating back to the Archean era 2.5 billion years ago.
Oceanic crust sinks below the Earth's mantle where two tectonic plates come together. These areas are called subduction zones. Much of what happens to the crust during this process is unknown and model-based studies have been uncertain in estimating how long subducted material could exist in the mantle. Until now, "evidence of very old crust material returning to the surface via upwellings of magma has not been found."
The research team, which included Carnegie Institution for Science's Erik Hauri, studied volcanic rocks containing iron sulfide inclusions within crystals from the island of Mangaia in Polynesia's Cook Islands. Analysis of the chemical makeup of these samples yielded interesting results.
The team focused on isotopes of sulfur found in the crystals, looking at three of the four naturally occurring isotopes - isotopic masses 32, 33, and 34. Graduate student Rita Cabral performed the measurements, finding that sulfur-33 isotopes showed evidence of a chemical interaction with UV radiation that has not occurred in Earth's atmosphere since 2.45 billion years ago after the Great Oxidation Event. The Event was a point when the oxygen levels in Earth's atmosphere skyrocketed because of oxygen-producing photosynthetic microbes. Before this event, there was no ozone in the atmosphere. Once ozone was introduced, however, it started to absorb UV and prevent the chemical process found in the sulfur isotope.
These results indicate that the sulfur bearing rocks come from a deep mantle reservoir containing crustal material subducted before the Great Oxidation Event.
"These measurements place the first firm age estimates of recycled material in oceanic hotspots," Hauri said. "They confirm the cycling of sulfur from the atmosphere and oceans into mantle and ultimately back to the surface," Hauri said.