Shock effects of Fa₅₀ iron-rich olivine: Spectral and microstructural implications for Mars and Phobos

Context . Global remote sensing and in situ investigations indicate that the Martian crust is enriched in iron, with olivine averaging an iron number (Fa#) of ~50. However, due to scarce terrestrial analogs, the shock behavior and preservation potential of Fe-rich olivine remain poorly constrained, limiting our ability to assess impact-induced mineralogical and spectral changes on Mars and potentially Phobos. Aims . This study examines the microstructural and spectral modifications in Fa 50 olivine induced by shock, offering insights into impact-driven alterations on Fe-rich planetary surfaces. Methods . Shock recovery experiments were conducted on synthetic Fa 50 olivine using one- and two-stage light gas guns at pressures of 18, 31, 41, and 47 GPa, corresponding to impact velocities between 0.89 and 1.95 km/s. Post-shock samples were analyzed using scanning electron microscopy (SEM), focused ion beam sectioning (FIB), and transmission electron microscopy (TEM) as well as Raman, visible-near-infrared (VNIR), and mid-infrared (MIR) spectroscopy to assess microstructural, compositional, and spectral modifications. Results . Shocked Fa 50 olivine exhibited systematic changes with increasing pressure. At ≥31 GPa, Fe migrated to form a three-layer zoning pattern with nanoscale α -/ γ -Fe particles and minor magnetite. Vesicles associated with dislocations and grain boundaries indicated localized gas release. Raman spectra showed progressive peak shifts, increasing separation, and the full width at half maximum (FWHM) broadening. VNIR spectra exhibited spectral bluing at 18 GPa, followed by reduced reflectance, redshifted and weakened absorption features at 31–47 GPa. MIR spectra showed pressure-dependent shifts in the Christiansen feature (CF) and Reststrahlen bands (RB1 and RB4), particularly at ≥31 GPa. Conclusions . Natural impacts likely induce more extensive Fe migration and secondary particle growth than observed in laboratory simulations. On Mars, impacts into Fe-rich olivine may contribute surface redox changes independent of water. Nanoscale Fe 0 , magnetite, and vesicles formed on Fe-rich silicates may contribute to spectral reddening and localized bluing on Phobos and Deimos.