Geology: Deep blue boron-bearing diamonds reveal rock recycling route

Boron-bearing blue diamonds may be among the deepest diamonds ever found and their geochemical history could shine light on the extent of the Earth’s crustal recycling, reports a paper in this week’s Nature.

Materials from the Earth’s surface are recycled into the mantle by plate tectonics. However, the extent of crustal recycling has been hard to determine. Evidence for possible deep recycling of surface materials comes through rare blue boron-bearing (or ‘type IIb’) diamonds, like the famous Hope Diamond. Boron is predominantly a crustal element, making its presence in diamond-forming mantle fluids mysterious and suggesting a crustal connection. However, the diamonds’ rare and precious nature, as well as their general lack of mineral inclusions that could yield clues to their formation, makes their origin hard to determine.

Evan Smith and colleagues used Raman spectroscopy to characterise rare inclusions in 46 type IIb diamonds, having screened hundreds of thousands of potential samples. Unlike diamonds originating in stable continental crust and the upper mantle, which typically have silicate or sulphide inclusions, blue diamonds contain mineral phases that one would expect to see after the subduction of an oceanic plate into the lower mantle. This would make the origin of type IIb diamonds among the deepest known. In addition, hydrogen and methane fluids surrounding some of the inclusions in the diamonds indicate hydration of the original minerals. The boron source in oceanic plates that is most likely to remain stable during subduction is serpentinite, which can also host water, so these findings also highlight a possible major pathway for the recycling of water deep into the Earth.

doi: 10.1038/s41586-018-0334-5

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