Fig. 2: Phase diagram for the relative stability of the MgCO₃ + CaSiO₃ assemblage versus CaCO₃ + MgSiO₃.

The boundary sketched as a black dashed line with gray shadow as uncertainty inferred is based on experimental observations of carbonate–silicate exchange reactions CaC-to-MgC and MgC-to-CaC. Squares represent observations from this work starting with (Ca,Mg)CO₃ and (Mg,Fe)SiO₃, looking for newly synthesized CaSiO₃ to indicate the CaC-to-MgC reaction takes place. Circle symbols represent observations from this work of experiments starting with (Mg,Fe)CO₃ + CaSiO₃, looking for identification of newly synthesized MgSiO₃ to indicate the MgC-to-CaC reaction takes place. Open symbols indicate nonreaction and filled for confirmed reaction, and blue and red colors correspond to the inferred stable phase assemblage based on reaction products. Triangles indicate the P–T conditions for CaC-to-MgC taking place reported by Seto et al.²⁵, and blue-shaded region indicates approximate conditions of four experiments conducted by Biellmann et al.²⁴ using indirect methods for pressure and temperature calibration, which all produced the run products MgCO₃ + CaSiO₃. The error bars indicate uncertainties of pressure and temperature measurements (see “Methods” for details). The boundaries proposed by previous theoretical predictions are illustrated by yellow-shaded region^28,29,30.