SCIENCE

Diamonds Hold Secret About Plate Tectonics

When it comes to diamonds in jewelry, perfection is everything. But imperfections are a clue to the past

July 25, 2011

A hexagonal grain of iron sulfide in a diamond may be a flaw for jewelers, but it's useful data for scientists Jeffrey Harris, University of Glasgow

When it comes to diamonds in jewelry, perfection is everything. But tiny little inclusions–imperfections in the crystal structure–are a clue to the past. In a study published last week in Science, scientists have now analyzed more than 4,000 inclusions found in diamonds to determine just when plate tectonics began.

As you probably know, the Earth is covered with tectonic plates that grow and move and dive under and crash into each other, creating and destroying continents and oceans over billions of years. Scientists call this the Wilson Cycle, but just when it began has been a mystery.

Diamonds are created in the Earth’s mantle, the hot and viscous layer between the core and the crust. Volcanic eruptions then bring them to the surface. There are two types of inclusion in diamonds: Peridotitic inclusions come from the melting of the mantle, which has happened continuously through Earth’s history. Eclogitic inclusions derive from shallow, partial melting that most often occurs during the formation of oceanic crust.

The scientists used two types of isotopic dating to determine when each of the diamonds in the study formed. They found that diamonds with peridotitic inclusions formed before 3.2 billion years ago, and after 3 billion years ago, eclogitic inclusions were far more common. The researchers concluded that the cycle of plate tectonics must have started around 3 billion years ago.

“The simplest explanation” for the emergence of eclogitic inclusions as the dominant type, says the study’s lead author, Steven Shirey of the Carnegie Institution of Washington, 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.”