Diamonds Pinpoint Start of Colliding Continents

Researchers used diamond inclusions to determine that plate tectonics began about three billion years ago.

Diamond inclusionJewelers abhor diamond impurities, but these mineral inclusions serve as time capsules and tell the story of Earth's distant past. As discussed in a recent Science article [1], Steve Shirey and other researchers used diamond inclusions to determine that plate tectonics began about three billion years ago. The inclusions provided age and chemical information for a span of more than 3.5 billion years and included clues to the evolution of the atmosphere, the growth of the continental crust, and the beginning of plate tectonics.  

The largest diamonds come from ancient continental cratons with deep mantle roots or keels around which younger continental material gathered. The cratons' keels extend into the mantle more than 125 miles (200 km) where pressures are sufficiently high, but temperatures sufficiently low, for diamonds to form. The diamonds may be stored here for billions of years until they find their way to the surface during deep magma eruptions that form kimberlites.  Diamond inclusions are primarily peridotite - the upper mantle's most abundant rock - and eclogite, which is generally thought to be the remnant of oceanic crust.

Shirey, who has been involved in the DCO since its beginning, and his co-author Stephen Richardson, University of Cape Town, combined their work with 25 years of literature data to consider more than 4,000 diamond inclusions from five continents. They focused on when the inclusions were encapsulated using rhenium-osmium and samarium-neodymium dating techniques and associated compositional trends.

They found that only diamonds with peridotitic compositions formed before 3.2 billion years ago, whereas since 3 billion years ago, eclogitic diamonds have dominated. According to Shirey, "The simplest explanation is that this change came from the initial subduction of one tectonic plate under the deep mantle keel of another as continents began to collide on a scale similar to that of the supercontinent cycle today. The sequence of underthrusting and collsion led to the capture of eclogite in the subcontinental mantle keel along with the fluids that are needed to make diamonds."

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