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 and Wuyi Wang
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Evan M. Smith: Gemological Institute of America
Steven B. Shirey: Carnegie Institution for Science
Stephen H. Richardson: University of Cape Town
Fabrizio Nestola: University of Padova
Emma S. Bullock: Carnegie Institution for Science
Jianhua Wang: Carnegie Institution for Science
Wuyi Wang: Gemological Institute of America

Nature, 2018, vol. 560, issue 7716, 84-87

Abstract: Abstract Geological pathways for the recycling of Earth’s surface materials into the mantle are both driven and obscured by plate tectonics1–3. Gauging the extent of this recycling is difficult because subducted crustal components are often released at relatively shallow depths, below arc volcanoes4–7. The conspicuous existence of blue boron-bearing diamonds (type IIb)8,9 reveals that boron, an element abundant in the continental and oceanic crust, is present in certain diamond-forming fluids at mantle depths. However, both the provenance of the boron and the geological setting of diamond crystallization were unknown. Here we show that boron-bearing diamonds carry previously unrecognized mineral assemblages whose high-pressure precursors were stable in metamorphosed oceanic lithospheric slabs at depths reaching the lower mantle. We propose that some of the boron in seawater-serpentinized oceanic lithosphere is subducted into the deep mantle, where it is released with hydrous fluids that enable diamond growth10. Type IIb diamonds are thus among the deepest diamonds ever found and indicate a viable pathway for the deep-mantle recycling of crustal elements.

Date: 2018
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DOI: 10.1038/s41586-018-0334-5

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