Blue boron-bearing diamonds from Earth’s lower mantle

Abstract

Geological pathways for the recycling of Earth's surface materials into the mantle are both driven and obscured by plate tectonics. Gauging the extent of this recycling is difficult because subducted crustal components are often released at relatively shallow depths, below arc volcanoes. The conspicuous existence of blue boron-bearing diamonds (type IIb) 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 growth. Type IIb diamonds are thus among the deepest diamonds ever found and indicate a viable pathway for the deep-mantle recycling of crustal elements.

The water and fluid-mobile element cycles during serpentinite subduction. A review

“…As shown by the water and halogen-rich composition of OIB glasses (Kendrick et al, 2017), by the isotopic compositions of serpentinites presented here (Fig. 14), and by the B content of ultradeep blue-diamonds (Smith et al, 2018), the presence of de-serpentinized materials can be identified in the lower mantle and might well be increasingly documented in the near future.…”

“…Retention of boron anomalies in nominally anhydrous phases produced by de-serpentinization represents a viable mechanism for recycling B and other fluid-mobile elements into the Earth's lower mantle. This has been recently proposed in the case of super-deep B-bearing blue diamonds, hosting up to 10 ppm B potentially recycled by serpentinite precursors, and crystallized at minimum depths of 350-400 km down to 700 km (Smith et al, 2018).…”

Subducting carbon

“…SLD are identified by virtue of the mineral inclusions that they hold, which signal a depth greater than that of the deepest lithospheric keels (>200 km). SLD are younger than lithospheric diamonds, show highly complex growth histories and display strong chemical links to subducted crust 72,73 . Carbon isotope data for SLD are quite variable, extending to very low δ 13 C values consistent with carbon derivation from subducted sediment or altered basalt 30,72,74 (Fig.…”

Natural-Color Fancy White and Fancy Black Diamonds: Where Color and Clarity Converge

“…Several researchers have studied "milky" diamonds and concluded, based on mineral inclusions, that these type IaB diamonds have an origin from the transition zone or the lower mantle (400-670 km depth; Rudloff-Grund et al, 2016;Kagi et al, 2016;Gu and Wang, 2017;Gu et al, 2019). Most diamonds shown to have a superdeep origin are type II diamonds (Smith et al, 2016(Smith et al, , 2018. Meanwhile, the vast majority of nitrogen-containing diamond (D-to-Z color type Ia and yellow type Ib) form in the continental lithosphere (depths of ~140-200 km; Shirey and Shigley, 2013).…”