Earth is the only planet known to have plate tectonics, where pieces of crust gradually move, collide, and sink back into the mantle through subduction. This crustal recycling system regulates the balance of carbon and other elements between the surface and subsurface and is a major controller of the global climate. Scientists aren’t exactly sure when subduction began, and estimates vary widely, from around 3.2 to 1 billion years ago.
Now, an international team of researchers including DCO Reservoirs and Fluxes Community member Alexander Sobolev (Université Grenoble Alpes, France and Vernadsky Institute, Russia) has produced evidence that some kind of global recycling program began during Earth’s first billion years. The researchers analyzed the contents of tiny blobs of ancient melted rock, called magma, preserved within crystals that erupted 3.3 billion years ago in South Africa. The surprising amount of water in the magma, along with other lines of evidence, suggest that sinking ocean crust was already bringing surface water into the mantle before the eruption, providing the earliest evidence of subduction, or a more primitive system for crustal recycling. The team describes their findings in a new paper in Nature.
The discovery moves back the existence of a deep water cycle by at least 600 million years. Sobolev and his colleagues first reported surprisingly high concentrations of water in ancient mantle samples in a 2016 paper that described 2.7-billion-year-old rocks from the Abitibi belt in Canada [2]. These rocks were komatiites, which come from extremely hot plumes of mantle material that erupted primarily during the Archaean eon (4.0 to 2.5 billion years ago). Billions of years at the surface changed the komatiites, but they still contain tiny crystals of the mineral olivine that formed around bits of magma, called melt inclusions. These crystals act like time capsules that retain the magma’s original composition.
The water content in the 2.7-billion-year old inclusions was twenty times higher than typical mantle material. The researchers interpreted the results to be evidence of a reservoir of water in the mantle leftover from when Earth first formed, or possibly from subduction. “We thought at that time, this is not so probable because that means that subduction was very, very early in Earth’s history,” said Sobolev.
In the new study, the team performed a more in-depth analysis of melt inclusions in olivine crystals from even older komatiites, 3.3 billion-year-old rocks from the Barberton greenstone belt in South Africa. To get accurate measurements of the water content in the inclusions, they had to turn the crystalline melt inclusions into glass, which is the best proxy for the original melt. A team at the Vernadsky Institute of Geochemistry and Analytical
Chemistry in Russia heated the crystals to more than 1500 degrees Celsius and then rapidly cooled them in ice water to prevent them from recrystalizing.
Once in their glassy form, the samples were transported to ISTerre in Grenoble, France, to CRPG in Nancy, France and to the GEOMAR - Helmholtz Centre for Ocean Research Kiel in Germany. “We were successful because of large international team that was involved in this study,” said Sobolev. At the various institutions, scientists used cutting-edge techniques to measure water content, trace elements, and the hydrogen isotope deuterium, which has an extra neutron in the nucleus.
The extensive analyses showed that the melt inclusions contained several signatures of subducted ocean crust, including elevated water and chlorine content from seawater and extremely low levels of deuterium. When crustal rocks altered by seawater sink into the mantle, the high heat and pressure squeeze out some, but not all, of the water and chlorine and also drive off much of the deuterium, leaving lighter hydrogen isotopes behind. By the time the ocean crust reaches the mantle transition zone (410 to 660 kilometers beneath the surface), it contains more water and chlorine but much less deuterium than the surrounding mantle material.
The evidence suggests that komatiite magma picked up water and chlorine from a reservoir in the mantle transition zone and carried it back to the surface 3.3 billion years ago. For this to happen, the crust must have left the surface hundreds of millions of years earlier. The results could be evidence of an earlier start to subduction, or may indicate a more rudimentary recycling process, such as the “dripping” of cooler, dense crust back into the mantle, like a lava lamp.
“Nobody knows when this global cycle of water started,” said Sobolev. “But we see that this cycle started very early – in the first billion years of Earth’s history.”
