A new study suggests that 120 to 150 kilometers (75 to 93 miles) under the Earth’s surface there is 1000 trillion tons of diamonds. The diamonds are in the “roots” of cratons, which are large sections of rock. Cratons lie beneath most continental tectonic plates.
They created a three-dimensional model of the velocities of seismic waves that traveled through the planet’s major cratons.
Vibrations from earthquakes and tsunamis tend to speed up when passing through cratonic roots. The speedup was greater than would be expected from the fact that cratons tend to be colder and less dense than surrounding structures.
The best explanation for the speeds actually observed underground was that 1 to 2% of the roots of the cratons was made up of diamonds, while the rest was made up of peridotite (the main type of rock in Earth’s upper mantle) and a little bit of eclogite rocks (from the ocean’s crust).
Some seismic models derived from tomographic studies indicate elevated shear‐wave velocities (over 4.7 km/s) around 120–150 km depth in cratonic lithospheric mantle. These velocities are higher than those of cratonic peridotites, even assuming a cold cratonic geotherm (i.e., 35 mW/m2 surface heat flux) and accounting for compositional heterogeneity in cratonic peridotite xenoliths and the effects of anelasticity. We reviewed various geophysical and petrologic constraints on the nature of cratonic roots (seismic velocities, lithology/mineralogy, electrical conductivity, and gravity) and explored a range of permissible rock and mineral assemblages that can explain the high seismic velocities. These constraints suggest that diamond and eclogite are the most likely high‐Vs candidates to explain the observed velocities, but matching the high shear‐wave velocities requires either a large proportion of eclogite (over 50 vol.%) or the presence of up to 3 vol.% diamond, with the exact values depending on peridotite and eclogite compositions and the geotherm. Both of these estimates are higher than predicted by observations made on natural samples from kimberlites. However, a combination of less than 20 vol.% eclogite and ~2 vol.% diamond may account for high shear‐wave velocities, in proportions consistent with multiple geophysical observables, data from natural samples, and within mass balance constraints for global carbon. Our results further show that cratonic thermal structure need not be significantly cooler than determined from xenolith thermobarometry.