The boron in natural blue diamonds originates from which source?

• A. Meteorites
• B. Subduction zones
• C. Mantle plumes
• D. Oceanic plates

Answer: (D)

Boron impurity in the crystal lattice creates the blue color, so the source of boron is crucial. In natural blue diamonds, boron enters the mantle environment via fluids released from subducted oceanic plates. As oceanic plates descend, seawater and boron-rich minerals are carried downward; dehydration reactions release fluids enriched in boron into the surrounding mantle, where diamonds crystallize and incorporate boron, giving them their blue hue. So the oceanic plates are the source because they carry the seawater-derived boron that, during subduction, becomes available in the mantle region where these diamonds form.

Meteorites are not the primary boron source for mantle diamonds; mantle plumes bring deep material but do not specifically account for seawater-derived boron enrichment in blue diamonds; subduction zones describe the process that releases the boron, but the origin of that boron is the oceanic plates themselves.