Scientists from the University of Portsmouth (UK) and the ESRF have revealed that activity at the Earth’s surface could have been influencing the deep mantle much earlier than previously thought. By measuring the oxidation state of sulphur trapped inside mineral inclusions, researchers have traced the recycling of oxidised surface material into the Earth’s interior to at least 3.4 billion years ago. The results are published in Nature Geoscience.
Today, plate tectonics drives a long-term exchange of material between the Earth’s surface and its interior. At subduction zones, plates carry water, carbon, sulphur and other volatile elements down into the mantle, while volcanoes return material to the surface. The cycling helps regulate the atmosphere, oceans and climate, and is thought to have played an important role in making the Earth habitable. But no-one knows exactly when this cycling started. “We are not dating the origin of life, but establishing when the processes allowing life to thrive got underway,” says Moreira.
To find out, Hugo Moreira, Royal Society University Research Fellow at the University of Portsmouth and colleagues turned to sanukitoids. Found in ancient continental crust, these mantle-derived rocks contain tiny crystals of the mineral zircon, which in turn can encase inclusions of another mineral, apatite. The apatite preserves chemical traces of surface material carried downwards at the time the sanukitoids formed, billions of years ago. “I see sanukitoids as potential recording tapes of Earth’s early surface processes – perhaps the only rock-type that can do this confidently,” says Moreira.
Sanukitoids first appear in the geological record around 3.5 billion years ago. Moreira and colleagues took samples ranging from nearly this old to much younger, providing snapshots of the magma’s chemistry over time – in particular its “oxygen fugacity”, or how oxidising or reducing its chemical environment was, as recorded by the different oxidation states of sulphur. Using sanukitoids throughout allowed the researchers to compare the same record across billions of years. But detecting trace amounts of sulphur in microscopic inclusions is no easy task.
Read more on the ESRF website