X-ray absorption spectroscopy (XAS) is a widely used technique to probe the local environment around specific atomic species. Applied to samples under extreme pressure and temperature conditions, XAS is sensitive to phase transitions, including melting, and allows gathering insights on compositional variations and electronic changes occurring during such transitions. These characteristics can be exploited for studies of prime interest in geophysics and fundamental high-pressure physics. Here, we investigated the melting curve and the eutectic composition of four geophysically relevant iron binary systems: Fe-C, Fe-O, Fe-S and Fe-Si. Our results show that all these systems present the same spectroscopic signatures upon melting, common to those observed for other pure late 3d transition metals. The presented melting criterion seems to be general for late 3d metals bearing systems. Additionally, we demonstrate the suitability of XAS to extract melt compositional information in situ, such as the evolution of the concentration of light elements with increasing temperature. Diagnostics presented in this work can be applied to studies over an even larger pressure range exploiting the upgraded synchrotron machines, and directly transferred to time-resolved extreme condition studies using dynamic compression (ns) or fast laser heating (ms). The coupling of X-ray absorption spectroscopy (XAS) with laser heated-diamond anvil cell (LH-DAC) is a challenging technical development only recently implemented in synchrotron facilities 1-3. The opportunities that stem from this combination are of great interest in physics, chemistry and planetary science. The element selectivity and the local structure sensitivity are two of the distinctive characteristics that make XAS a very informative and multipurpose technique. Combined together, they provide the possibility of microscopic investigation of the local atomic environment of a specific element in a compound or an alloy, irrespectively of the solid, liquid or amorphous state. The X-ray absorption near edge spectroscopy (XANES) region of the XAS spectrum, also probes the unoccupied electron density of states right above the Fermi level 4-7. Recent high pressure/high temperature absorption experiments 6,8-14 and theoretical calculations 8,9,11,15-19 on pure metals directly correlate changes in the XANES spectra and/or in the pre-edge to phase transitions, but to which extent these findings could be generalized to multi-component systems remains to be assessed. XANES sensitivity to the electronic structure further grants the possibility to probe the chemical environment, through relatively simple approaches like the linear combination analysis (LCA), as demonstrated in many studies at ambient conditions (Benfatto et al. 20 , among others), but never attempted under extreme high-pressure, high-temperate conditions. open