Mining diamonds for the history of Earth

More than just symbols of wealth and beauty, diamonds are a testament to the dynamic history of the Earth, according to new research published last week in the journal Nature.

Many people have thought for a long time that the carbon that forms diamonds had always been buried deep in the Earth’s mantle, says John Valley, a geology professor who contributed to the study. But, according to the new findings, the carbon originated from biological processes and was incorporated into the ocean’s crust.

Valley, along with the study’s lead investigator at the University of Toronto, analyzed the oxygen isotope ratios in tiny inclusions of the mineral coesite within diamonds mined from Venezuela. They compared the oxygen isotope ratios to those of volcanic rock found in the ocean’s crust and pristine mantle rocks found buried beneath the crust.

During diamond formation, volcanic rock of the ocean crust sinks deep into the Earth’s mantle. Here, high temperatures and pressure turn the rock into ecolgite, which sometimes forms diamonds.

“Everyone knows diamonds come from the mantle,” explains Valley. “But the question is where does the material — the carbon — that makes them come from. If we know where the carbon comes from, we can predict whether diamonds will form.”

By comparing the oxygen isotope ratios, the researchers have answered this question: The carbon in the Venezuelan diamonds came from the Earth’s surface, not its interior. The isotopes from the diamonds revealed a geochemical signature very similar to that of the volcanic rock, not the mantle.

Valley says this finding not only resolves a long-standing debate, but it also provides dynamic evidence of the tectonic engine that drives the geology of the Earth.