What Makes Diamonds Blue?

Photo by Evan Smith/GIA

The research group—which included Carnegie’s Steven Shirey, Emma Bullock, and Jianhua Wang—determined that blue diamonds form at least as deep as the transition zone between the upper and lower mantle—or between 410 and 660 kilometers below the surface. Several of the samples even showed clear evidence that they came from deeper than 660 kilometers, meaning they originated in the lower mantle. By contrast, most other gem diamonds come up from between 150 and 200 kilometers.

So how did the boron get down there if it is an element known for residing predominately in the shallow crust?

According to the hypothesis put forth by the research group, it came from seafloor that was conveyed down into the Earth’s mantle when one tectonic plate slid beneath another—a process known as subduction.

A blue, boron-bearing diamond containing mineral inclusions that were examined as part of this study. This gem is 3.81 carats and 1.26 centimeters long.

 Photo by Robison McMurtry/GIA.

The new study proposes that boron from the Earth’s surface was incorporated into water-rich minerals like serpentine, which crystallized during geochemical reactions between seawater and the rocks of the oceanic plate. This reaction between rock and water is a process called serpentinization and can extend deep into the seafloor, even into the oceanic plate’s mantle portion.

The group’s discovery reveals that the water-bearing minerals travel far deeper into the mantle than previously thought, which indicates the possibility of a super-deep hydrological cycle.

Cover of the journal Nature featuring the paper revealing the superdeep origin of blue diamonds, on which Evan Smith of the Gemological Institute of America is lead author and Carnegie’s Steven Shirey, Emma Bullock, and Jianhua Wang are co-authors.
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Credit: Carnegie Institution for Science / Nature

“Most previous studies of super-deep diamonds had been carried out on diamonds of low quality,” Shirey said. “But between our 2016 finding that the world’s biggest and most-valuable colorless diamonds formed from metallic liquid deep inside Earth’s mantle and this new discovery that blue diamonds also have super-deep origins, we now know that the finest gem-quality diamonds come from the farthest down in our planet.”

Other co-authors on the paper are Stephen Richardson of University of Cape Town, Fabrizio Nestola of University of Padua, and Wuyi Wang of the Gemological Institute of America.

Contacts and sources:

Citation: Blue boron-bearing diamonds from Earth’s lower mantle
Evan M. Smith, Steven B. Shirey, Stephen H. Richardson, Fabrizio Nestola, Emma S. Bullock, Jianhua Wang, Wuyi Wang. . Nature, 2018; 560 (7716): 84 DOI: 10.1038/s41586-018-0334-5