In Diamonds’ Flaws, Finding Clues to Earth’s Carbon Cycle

Diamonds that once lay more than 435 miles beneath the earth’s surface have provided researchers with an unexpected window into the planet’s history.

The diamonds, during their formation, captured evidence that slabs of the ocean floors descend deep beneath the earth’s surface, recycling carbon between the oceans and the earth’s mantle, the shell of rock, about 1,800 miles thick, that lies directly beneath the earth’s surface.

Understanding the fate of the slabs will help scientists better understand the earth’s carbon cycle and all the processes that depend on it, from the carbon dioxide in the atmosphere to the carbon compounds in living organisms and the formation of hydrocarbons in oil and gas.

Objects that resemble ocean slabs can be seen in seismic recordings, but they lie far too deep for any drill to sample. Impurities in the diamonds contain chemical signatures of the extinct ocean floor, evidence that the slabs have been cycled deep into the earth’s mantle, says a research team led by Michael J. Walter of the University of Bristol in England.

These microscopic impurities, derived from rock and from organic material in creatures that once lived on an ancient ocean floor, have undergone an amazing journey. The ocean floor rock, basalt, along with the sediment that built up on top of it, was drawn down at the edge of an ocean as part of the conveyor-belt mechanism that moves the continents.

When the slab of ocean floor had plunged 435 miles beneath the surface, minerals from the basalt were encapsulated inside the diamonds that formed at these depths.

The diamonds continued to descend with the slab of ocean floor until they experienced two elevator rides back to the surface. A rising mass of solid rock known as a mantle plume carried them slowly back toward the upper mantle, and the heat of the plume then propelled to the surface an explosive jet of molten kimberlite, a volcanic rock that preserves diamonds.

Eons later, the diamonds were mined by the Rio Tinto Group from Juina in Brazil. The company allowed members of the research team to sift through stones not deemed to be of gem quality. After examining thousands of diamonds, the researchers found just six that seemed to be of superdeep origin.

Despite their deep origin, the Juina diamonds are comparatively young as diamonds go. They were formed only 100 million years ago. Most gem-quality diamonds are 1 billion to 3.5 billion years old, and originate at shallower depths, in the keels beneath the cratons, the ancient blocks of rock that form the hearts of the earth’s continental masses.

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