A blob of magma entombed in a bubble smaller than the width of a human hair and found in South Africa may turn back the clock on Earth’s first slow dance of the rocky slabs that make up its outer shell.
The chemicals inside that little blob suggest so-called plate tectonics revved up during the first billion years of Earth’s existence.
Since the 1950s, scientists have known Earth’s crust is made of giant slabs called that float above Earth’s molten mantle. These colossal plates meet in , where the lighter slab slides under the heavier one into the depths of the mantle. The sinking crust, infused with minerals collected from Earth’s surface, melts into magma under the extreme pressures and temperatures of Earth’s interior. 
When exactly this planetary recycling began has been hotly debated. Estimates range from 1 billion to 4 billion years ago. Now, an international team of scientists has discovered that the subduction of Earth’s crust likely began more than 3.5 billion years ago. Their results were published July 15 in the journal
The microscopic bead of cooled magma at the root of their discovery laid dormant for more than 3.3 billion years, protected by its
The olivine crystal, no bigger than a grain of sand, was found in a komatiite rock, named after the Komati River in South Africa where such rocks were discovered. They formed when extraordinarily hot plumes of
The crystal-entombed magma blob was found in a komatiite rock, named after the Komati River (shown here) in South Africa.
Credit: Alexander Sobolev
To study the tiny magma inclusion, Sobolev and his team remelted the ovaline crystals by heating them to more than 2,700 degrees Fahrenheit (1,500 degrees Celsius) and rapidly cooling them in ice water to form a glassy sample. They then used state-of-the-art instruments to measure the chemical makeup of the glassy magma and determine its origin.
The researchers discovered the magma contained a number of signatures of subducted oceanic crust, including high concentrations of water and chlorine, and low levels of
„If that is the case, it means a lot,“ Sobolev said. „It means that seawater-altered crust from the surface went down into the mantle nearly 3.3 billion years ago. Because all these processes are slow, you can expect that from the point from when this source went down to the point where it reached the surface again, it took at least 100 to 200 million years. That means this process started within the first billion years of Earth’s history.“
Originally published on .
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