TYPE IIB BORON-BEARING DIAMONDS ARE SUPERDEEP DIAMONDS (Invited Presentation)

Type IIb diamonds, those defined as having trace amounts of substitutional boron, are prized for their blue colors. The famous Hope diamond is a perfect example. Very little is known about how type IIb diamonds form, but they are especially intriguing because boron is typically regarded as a crustal element and its occurrence in mantle-derived diamonds is unexpected. However, type IIb diamonds are so rare and so valued for their color that accessing samples for research is problematic. Even when samples are available, these diamonds often do not contain inclusions.

To investigate the petrogenesis of type IIb diamonds, the day-to-day diamond grading operations of the Gemological Institute of America were systematically screened to identify type IIb diamonds with inclusions. Over the course of about two years, more than twenty prospective diamonds were encountered. These were analyzed using Raman spectroscopy in order to identify included mineral phases in-situ. This approach allowed the examination of multiple examples of what are otherwise exceedingly rare and inaccessible diamonds. The identified suite of inclusions deviates from the familiar mineralogy of lithospheric diamonds, instead replicating the expected mineralogy of eclogitic to peridotitic host rocks at lower mantle depths, beyond 660 km. The implication that type IIb diamonds originate from the sublithospheric mantle qualifies them as “superdeep” diamonds, whereas most diamonds (>98%) form in the continental lithospheric mantle at about 150 to 200 km deep. This is a surprising result given that until recently it was believed that all superdeep diamonds were small and unsuitable as gemstones.