Carbonates stability at lower mantle conditions: implications for the deep carbon cycle
Carbon is recycled into the deep Earth mainly as carbonates, therefore stability of carbonates during the subduction in the deep mantle plays a key role in the geodynamic carbon cycle. The goal of this thesis is to provide new experimental information concerning constraints on the stability of carbonates at pressure and temperature conditions relevant to the Earth's lower mantle. In this experimental study, samples were synthesized at high-pressure and high-temperature in diamond anvil cells and analysed in situ by X-ray diffraction. Once quenched to room pressure (P) and temperature (T), samples were prepared using the focused ion beam (FIB) method for ex situ analyses: transmission electron microscopy and transmission X-ray microscopy. This study shows a high stability of carbonates versus decarbonatation and provides evidence of two new high-pressure polymorphs of carbonates: (1) a new high-pressure phase described in the case of a FeCO3 starting material. This phase is observed for pressure and temperature above 40 GPa-1500K (corresponding to depth within the Earth of about 1000 km). (2) A second phase described in the case of (Fe,Mg)CO3 solid solutions for P-T conditions above 80 GPa-2000 K, in agreement with previous theoretical studies. Ex situ analyses show that carbon in these two new high-pressure phases is present as CO4 tetrahedral groups and iron in its oxidized form Fe(III). The presence of Fe(II) in starting materials induces redox reactions from which Fe(II) is oxidized and a part of the carbon is reduced. This leads to an assemblage of magnetite, diamonds, and carbonates or their Fe(III) - bearing high-pressure polymorphs. Our results show the possibility for carbon to be recycled in the lowermost mantle and provide evidence of a possible coexistence of reduced and oxidized carbon at lower mantle conditions. This latter result might be important for better modelling redox state and melting in the Earth's lowermost mantle.