Experimental investigation of the stability of Fe‐rich carbonates in the lower mantle

The fate of carbonates in the Earth's mantle plays a key role in the geodynamical carbon cycle. Although iron is a major component of the Earth's lower mantle, the stability of Fe‐bearing carbonates has rarely been studied. Here we present experimental results on the stability of Fe‐rich carbonates at pressures ranging from 40 to 105 GPa and temperatures of 1450–3600 K, corresponding to depths within the Earth's lower mantle of about 1000–2400 km. Samples of iron oxides and iron‐magnesium oxides were loaded into CO 2 gas and laser heated in a diamond‐anvil cell. The nature of crystalline run products was determined in situ by X‐ray diffraction, and the recovered samples were studied by analytical transmission electron microscopy and scanning transmission X‐ray microscopy. We show that Fe (II) is systematically involved in redox reactions with CO 2 yielding to Fe (III) ‐bearing phases and diamonds. We also report a new Fe (III) ‐bearing high‐pressure phase resulting from the transformation of FeCO 3 at pressures exceeding 40 GPa. The presence of both diamonds and an oxidized C‐bearing phase suggests that oxidized and reduced forms of carbon might coexist in the deep mantle. Finally, the observed reactions potentially provide a new mechanism for diamond formation at great depth.