Stability of carbonated magmas at the base of the Earth's upper mantle

We measured the density of carbonated basaltic melt containing 5.0 wt.% CO2 at 2573 K and in the pressure range from 16.0 to 20.0 GPa by using the sink/float method with single crystal diamond as a density marker. We observed sinking of diamond at 19.0 GPa and flotation of diamond at 20.0 GPa and 2573 K. Using the third order Birch‐Murnaghan equation of state, the calculated isothermal bulk modulus (KT) of the carbonated basaltic melt (5.0 wt.% CO2) and its pressure derivative (K′) are 16.0 ± 1.0 GPa and 5.2 ± 0.2, respectively. Our result implies that magmas can contain up to 3.0–4.0 wt.% CO2 to be denser than the surrounding mantle at the top of the 410 km discontinuity.