In which region of the mantle are super-deep diamonds associated?

• A. Upper mantle
• B. Transition zone
• C. Lower mantle
• D. Crust

Answer: (B)

Diamonds that form under the highest pressures found in the mantle tend to come from the zone between the upper and lower mantle, the transition zone. This region, roughly 410 to 660 kilometers deep, hosts minerals and phase changes that create extreme pressures necessary for deep crystallization. When scientists study super-deep diamonds, they look for inclusions of high-pressure minerals such as ringwoodite, which only stabilize at those depths. Finding such inclusions inside a diamond serves as a fingerprint that the diamond formed in the transition zone, not in the shallower upper mantle or the much deeper lower mantle. The crust sits above all these layers, so it isn’t the source region for these deep-formation diamonds.