Earth scientists routinely monitor the levels of carbon they find in the planet’s biomass, atmosphere, and oceans. But to judge by the composition of basaltic magma and the flux of carbon dioxide coughed out by volcanoes, the amount of carbon sequestered within Earth’s voluminous mantle—estimated to be a few hundred parts per million (ppm) by weight—is roughly a thousand times larger than the amount on the surface. The presence of so huge a reservoir of carbon separated by mere kilometers from the comparatively tiny one on the surface has spurred researchers to figure out where and in what form the carbon is stored within Earth’s interior.
As the dominant constituent of the upper 400 km of the mantle, olivine is a natural suspect. It is a solid mixture of 90% magnesium silicate and 10% iron silicate. Like any ionic mineral, it accepts small levels of impurities into the crystal structure....
