Ferromagnesian carbonates are the stable high pressure carbonates in eclogite and peridotite and their physical properties are thus important for understanding the dynamics of reintroduction of carbon into the Earth's mantle. Using Brillouin scattering spectroscopy, we have determined the sound velocities and elasticity of carbonates along the (Mg1−xFex)CO3 join (with x = 0, 0.65, 0.95 and 0.99) at room conditions to evaluate the effect of carbonates on the seismic velocity structure of the subducting lithosphere. Within experimental uncertainties, Fe substitution has a negligible effect on the adiabatic bulk modulus Ksof Mg‐Fe carbonates whereas the shear modulusμ decreases by 34% from MgCO3 to FeCO3. We find that the seismic velocity contrast between carbonate‐bearing eclogite and peridotite with 5 wt% CO2and their carbon‐free counterparts is less than 1%, indicating that moderately carbonated regions in the subducting mafic crust and mantle may be difficult to detect seismically. The threshold of seismic detection with present methods (∼2%) would require significantly high carbonate contents of 24.5 vol% (Mg0.79Fe0.21)CO3 (15 wt% CO2) in eclogite and more than 30 vol% (Mg0.93Fe0.07)CO3 (>20 wt% CO2) in peridotite. Key Points Elasticity‐composition systematics in Mg‐Fe carbonates Seismic velocities and density of carbonated eclogite and peridotite lithologies Implications for the detection of carbonated regions at depth