Diamonds can be a geologist’s best friend too — especially if that geologist has a mass spectrometer and is looking for clues about what Earth looked like billions of years ago.
These precious rocks occasionally contain impurities trapped inside during formation billions of years ago. And with the right tools, scientists can mine these traces for date details and chemical composition to get a rare snapshot of early Earth. From such miniscule grains—sulfides and silicate in a new analysis—a pair of researchers is now proposing a date for the beginning of the modern plate tectonic cycle: 3 billion years ago.
Formed under ancient intense pressure deep in the mantle, these diamonds were occasionally spouted to the surface via volcanic eruptions. The cargo carried inside these marred diamonds started to look different starting around 3 billion years ago, containing traces of a rock, eclogite, that would have been more common with shallow melting of basalt. And that scenario is likely during the emergence of thick, moving continents like the ones we have today, assert the researchers in a paper published July 22 in Science.
With the tiny fragments of rock gleaned from these rare minerals, “we are seeing the beginning of a major period of slab subduction that is fundamentally different,” says Steven Shirey, of the Carnegie Institution of Washington’s Department of Terrestrial Magnetism and co-author of the new study.
Diamonds are for…Billions of years
In the dynamic history of Earth, precise dates can be hard to come by—especially when they might extend back for billions of years.
To gather rough dates, “we’ve always had the rock record” from the surface, Shirey says. But continents and seafloors are in a constant state of recycling via weathering and plate tectonics, leaving very few masses safe from the forces of time.
Some extremely old chunks of continent do exist, however. Known as cratons, these masses have deep mantle roots that can reach down some 200 kilometers below the surface. And they contain diamonds that were formed by subsurface high pressures billions of years ago but have been protected by the relatively low temperatures there.
For the first half of its existence, Earth’s surface was more of a fluid place, with bits of crust being formed here and there from rising hot mantle. But at some point, as the planet cooled, larger masses started to form and the cycle of supercontinents and plate tectonics as we know it today got underway.