Boron Isotopes in Blue Diamond Record Seawater-derived Fluids in the Lower Mantle

by Margo E.Regiera, Karen V.Smit, Thomas B.Chalk, Thomas Stachel, Richard A. Stern, Evan M. Smith, Gavin L. Foster, Yannick Bussweiler. Chris DeBuhr, Antony D. Burnham, Jeff W. Harris, D. Graham Pearson

Earth and Planetary Science Letters  Volume 602, 15 January 2023, 117923

A 15.1-carat diamond from the Cullinan mine in South Africa gets its blue color from the element boron. Credit: Justin Lane/EPA-EFE/Shutterstock

Rare blue diamonds arose from the watery remains of deep-diving slabs of Earth’s crust

The famous Hope diamond and similar azure gemstones owe their color to the element boron, which is common in crustal rocks. Plate tectonics causes slabs of such rocks to plunge into the deep Earth, where high temperatures and pressures can forge diamonds. But the details of what precisely is carried to great depths have not been clear.

Margo Regier at the University of Alberta in Edmonton, Canada, and her colleagues analyzed the chemistry of ten blue diamonds, most of which came from the Cullinan mine in South Africa. The amounts of boron and carbon isotopes in the diamonds were markedly different from those expected in the deep Earth — but similar to levels found in fluids derived from seawater.

This suggests that the diamonds formed from crustal rocks that interacted with seawater and were dragged deep into the planet by plate tectonics. Such geochemical connections link the shallow and deep Earth, shaping the planet’s chemical diversity.

The link HERE will take you to as much of the scholarly article by Regan et al that can be obtained in front of the Earth and Planetary Science Letters (EPSI) paywall. The Abstract is shown below in an effort to spur your interest in clicking.

Earth and Planetary Science Letters

Editors note: many thanks to MSDC member Jurate Landwehr for providing this super interesting article about the source of the boron in blue diamonds! ~KRock


Boron is a quintessential crustal element but its conspicuous presence in diamond – a mantle mineral – raises questions about potential subduction pathways for boron and other volatiles. It has been a long-standing goal to characterize the isotopic composition of boron in blue, boron-bearing (Type IIb) diamonds to reveal its origin. Mineral inclusions indicate that Type IIb diamonds crystallize at transition zone to lower mantle depths, meaning that if the boron is subducted it would trace a pathway of volatile elements into the deep mantle. Here, using off-line laser ablation sampling, we present the first boron isotope compositions, along with trace element contents and carbon isotope compositions, of a suite of blue diamonds mainly sourced from the Cullinan diamond mine in South Africa. The ten analyzed blue diamonds have a wide range in δ11B, between −9.2 ± 2‰ and −0.5 ± 2‰, compared to the more restricted range for mid-ocean ridge basalts (−7.1 ± 0.9‰). Carbon isotope values for the blue diamonds range between −20.6 and −1.8‰, with a mode at −17‰, significantly more negative than the main mantle mode at −5‰. Combined, the boron and carbon isotope compositions require fluid input from subducted oceanic lithosphere down into deep mantle source regions of blue diamonds. This finding highlights a deep subduction pathway for volatiles to stoke the source regions of deeply-derived magmas.