Fig. 5: Solid Fe-Ni metal blebs as geothermometers.

a T-X_Fe,Ni binary phase diagrams (top) and artworks (bottom) illustrating the stages of fcc-to-bcc (martensitic) and bcc-to-fcc (austenitic) transformations in a solid Fe-Ni metal bleb, resulting in the nutshell-like microstructure displayed in Fig. 1. The Fe and Ni concentrations have been retrieved from profiles AA’ and CC’ (Fig. 4) and normalized to 100 wt% (Table S1). M_F and A_F have been sourced from existing experimental data³⁴ (all converted to wt%), and their position and shape have been refined with respect to the presence of alloying elements⁹³, particle, and grain sizes^{68,113,114,115,116,117}, and local compression^71,118 (see Supplementary Section 4). The equilibrium phase boundaries^119,120 (light gray) are shown for reference only. Note that the abscissa is discontinuous at the core-intermediate band interface (between 19.9 wt% and 23.5 wt% Ni) to more clearly illustrate the compositional step (seen as double-kinks in M_F and A_F). The low-temperature double-kink in the rightmost T-X_Fe,Ni phase diagram reflects the change in grain size between the intermediate band and the rim. For the sake of simplicity and clarity, the squeezing of the bleb shape during shock has not been illustrated in the artwork. b Chemical Fe/Ni ratios for the stages shown in (a). c Crystallographic orientations (in arbitrary colors) for the stages shown in (a, b).