New research led by scientists from the Gemological Institute of America and the Carnegie Institute of Washington did some detective work on blue diamonds like this one, and they believe they’ve tracked that boron to where it started. Blue diamonds, like this one, start off as carbon in the oceanic crust. They are subducted all the way to the lower mantle, at least 700 kilometers deep, before they are brought back to the surface in the special type of volcanic eruption that occasionally brings diamonds up.

Scientists have a number of tools to figure out where the carbon in diamonds originated. They can measure the carbon isotopes, as certain reservoirs of carbon on Earth have distinct abundances of Carbon 12 and Carbon 13 (both stable isotopes with different numbers of neutrons). Many blue diamonds are missing some of their Carbon 13 – a characteristic most common of organic matter found at Earth’s surface. That isotope signature argues that this diamond likely started as organic matter in the Earth’s crust. How, then did they track the carbon through a subduction zone?

When diamonds form at pressures deep inside the planet, they sometimes overgrow the minerals that surround them, trapping those minerals as inclusions inside the diamond. For this study, the scientists exposed some of those inclusions in a series of (lower quality than this) blue diamonds to see what they were. In some samples, they found evidence of the mineral bridgmanite, which only exists at depths greater than 660 kilometers in the planet. In other samples, they found evidence of the high-pressure mineral stishovite, which cannot exist in the normal mantle because it will react with most minerals in the mantle. Stishovite can only exist in the mantle in rocks that haven’t reacted with the full mantle, such as subducted oceanic crust.

By finding these and other inclusions, the scientists verified that these diamonds started their life in ocean crust and went down a subduction zone. There is a little bit of boron that gets stuck in oceanic crust as it reacts with the ocean. That boron rides down into the mantle along with the minerals, carbon, and water, and eventually a little bit of that boron gets stuck into the diamond structure as it grows, creating this blue color.

-JBB