Although the vast majority of diamonds form in Earth’s lithospheric mantle at depths between 140 and 200 kilometers, about 1% of mined diamonds originate from much greater depths. As the only direct samples from Earth’s sublithospheric regions, these “superdeep diamonds” offer unique geochemical information about our planet’s inaccessible interior.
Despite the seeming ubiquity of temperature and pressure conditions favorable to diamond formation throughout the deep mantle, analyses of inclusions in superdeep diamonds indicate that most form within two narrow zones, between 250 and 450 kilometers and between 600 and 800 kilometers in depth. To date, no hypothesis has satisfactorily explained the cause of the intermediate diamond-forming gap, which lies within the mantle transition zone.
Now Zhu et al. propose a novel explanation for superdeep diamonds’ puzzling depth distribution. Building on previous research suggesting these unusual gems result from reactions between iron in the mantle and carbonates in subducting slabs of oceanic crust, the team conducted a series of laser-heated, diamond anvil cell experiments to test whether these interactions are feasible under the pressure-temperature conditions present in these settings.
By tracking diamond formation in real time, the team was able to determine the rate and conditions under which diamonds were produced from the reactions between metallic iron and magnesite, a magnesium carbonate mineral. The results indicate that diamonds can form at the mantle-slab interface and that higher temperatures promote carbonate-metal reactions, whereas higher pressures inhibit them.
Intriguingly, the authors observed a threefold drop in reaction rate at pressures and temperatures corresponding to depths below about 475 kilometers. The only exception they found was at conditions equivalent to 600 to 800 kilometers depth, where subducting slabs encounter the top of the lower mantle. The researchers suggest the resulting stagnation causes the accumulation of the reactants and the slabs to warm up, creating conditions once again favorable for diamond formation.
In addition to illuminating the importance of reaction rates on the depth distribution of superdeep diamonds and offering an explanation for their rarity within the mantle transition zone, this study demonstrates the feasibility of using real-time tracking to boost our understanding of the reaction kinetics of complex mantle-slab interactions. (Geophysical Research Letters, https://doi.org/10.1029/2018GL080740, 2018)
—Terri Cook, Freelance Writer