Optical photomicrograph of a sulfide-inclusion-bearing rough diamond from the Orapa kimberlite, Botswana. Below the natural diamond growth surface, at center, are several hexagonal grains of iron sulfide (FeS) surrounded by an irregular black rim. This rim is caused by internal fracture of the diamond on its 150-km ascent to the Earth’s surface in the explosive volcanic eruption of kimberlitic magma. Sulfide grains such as these are removed for rhenium-osmium isotopic analysis to reveal the age of the diamond and the composition of the sulfide. The sulfide grains are about 0.25 mm in diameter. Photo courtesy Steven B. Shirey, Carnegie Institution of Washington.

DTM’s Steven Shirey and long-time colleague and coauthor Stephen Richardson of the University of Cape Town have determined that continents started the so-called Wilson cycle of breaking apart, drifting, and colliding about 3 billion years ago. Their research, published in the July 22 issue of Science, involved the analysis of data from thousands of mineral inclusions encased in otherwise gem-quality diamonds.

Using their own work with other collaborators in more than 20 papers published over a 25-year period, Shirey and Richardson synthesized the data for more than 4,000 diamond inclusions of silicate—the crust’s most abundant material—and more than 100 inclusions of sulfide from five ancient continents. The host diamonds come from cratons, the ancient continental interiors that have deep mantle roots or keels and around which younger continental material gathered. Cratons contain the oldest rocks on the planet, and their keels extend into the mantle more than 125 miles (200 km), where pressures are sufficiently high, but temperatures sufficiently low, for diamonds to form and be stored for billions of years. The diamonds arrived at the surface as accidental passengers during younger volcanic eruptions of deep magma that solidified into rocks called kimberlites.

The most crucial aspects of the research were to look at when the inclusions were encapsulated and their associated compositional trends. Compositions vary depending on the geochemical processes experienced by the precursor components before they were encapsulated.

Two systems used to date inclusions—the rhenium-osmium and samarium-neodymium techniques—were compared. Both rely on natural isotopes that decay at exceedingly slow but predictable rates— one disintegration about every ten years on the scale of an inclusion—making them excellent atomic clocks for determining absolute ages. The researchers found that before 3.2 billion years ago, only diamonds with peridotitic compositions formed—whereas subsequent to 3 billion years ago, eclogitic diamonds dominated. Since the capture of eclogite and the formation of eclogitic diamonds is associated with continental collision in a manner similar to the modern Wilson or supercontinent cycle, this result establishes the beginning of this major plate tectonic process in Earth history.

For more information, see Carnegie’s Press Release.  To hear Steve being interviewed in a segment on BBC's Material World, click here.