Rare blue diamonds are born deep in Earth’s mantle

Blue diamonds, among the rarest gems on Earth, are born deep inside the planet’s mantle. Yet their blue hue comes from boron, an element far more abundant in Earth’s crust than its mantle. Using tiny flaws encased within the diamonds, scientists now think they’ve figured out how boron could have ended up at depths where the diamonds form: Subducting ocean plates carried the boron deep into Earth’s interior. The flaws also suggest that blue diamonds are among the deepest to form on Earth.   

Geologist Evan Smith of the New York City–based Gemological Institute of America and colleagues analyzed mineral inclusions, tiny bits of nondiamond material trapped inside the diamond’s crystal structure, found within 46 blue diamonds. The chemical structure and makeup of these minerals, which were trapped in the diamonds as they formed, point to an origin more than 660 kilometers deep, below the boundary between upper and lower mantle, Smith’s team reports August 1 in Nature. Although all diamonds form within the mantle, most form above that boundary layer (SN 4/30/16, p. 8).

Boron-bearing blue diamonds are extremely rare — and extremely pure, Smith says. “They’re quite often completely flawless.” That made it difficult to gather a large enough sample of inclusion-bearing blue diamonds to be able to say anything about their origins.  

First, the researchers studied the inclusions to try to determine how deep the 46 diamonds were born. The team used Raman spectroscopy, a noninvasive technique that illuminates the diamond inclusions with a laser beam and then measures the wavelengths of light scattered by the inclusions. Those wavelengths serve as chemical fingerprints for the elements present in the sample.

Certain minerals, such as bridgmanite, are stable only at the high temperatures and pressures found deep inside Earth. Although trapped in the diamonds, the minerals change in structure as they rise through the Earth. But the inclusions also retain traces of the deep, high-pressure forms, which the researchers used to better understand the diamonds’ origins.

“It’s a bit of detective work to trace back what you see in the diamond,” Smith says. For example, seeing the two lower mantle minerals bridgmanite, which becomes common at about 660 kilometers deep, and ferropericlase together indicates the diamond was formed in the deep. That assemblage wouldn’t be stable at shallower depths, he says. “It’s like oil and water. Under certain conditions they mix, but under others, they don’t.”

Because boron tends to be a crust-preferring element, scientists wondered how it could have reached the diamonds to be incorporated into their crystal structures. The likeliest explanation, Smith and colleagues say, is that the boron hitched a ride with a subducting ocean plate.

Boron is dissolved in seawater, which circulates through ocean seafloor and reacts with certain minerals to form a water-rich mineral called serpentinite. During a tectonic collision, an oceanic plate bearing serpentinite may subduct, or sink beneath another plate. The sinking plate then carries the serpentinite — including the boron — deep into the mantle.

The hypothesis also points to a possible way that water may travel deep into the mantle; scientists have long wondered about just how deep Earth’s water cycle goes (SN Online: 3/8/18). Such deep recycling of water could be important for driving plate tectonics and volcanic eruptions. “It’s not proof, but a glimmer of hope that we’ve got actual water being transported from the ocean down to the lower mantle,” Smith says.

Using diamonds to examine boron moving through Earth is a clever insight, says Graham Pearson, a geochemist at the University of Alberta in Canada who wasn’t connected with the study. “Virtually nothing is known about the boron cycle in the deeper parts of the mantle,” Pearson adds. He and other researchers are now investigating blue diamonds’ boron isotopes — forms of the element with different masses — to help pin down whether the boron in the crystals originated in the crust or the mantle.

Because the blue diamonds in the study contain inclusions, they would be less pricey than flawless blue diamonds such as the famed Hope Diamond. But their role in this scientific study may boost their value, Smith suggests. “I think it makes them a little more special.”