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

¹ Research Center for Planetary Science, College of Earth and Planetary Sciences, Chengdu University of Technology, Chengdu 610059, PR China
² Chinese Academy of Sciences Center for Excellence in Comparative Planetology, Hefei 230026, PR China
³ Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, PR China
⁴ State Key Laboratory of Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing 101499, PR China
⁵ Center for High Pressure Science and Technology Advanced Research, Beijing 100193, PR China
⁶ State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, PR China
⁷ Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, PR China
⁸ Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, PR China

^★ Corresponding author: zhaoyuyan@cdut.edu.cn

Received: 12 March 2025
Accepted: 19 May 2025

Abstract

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 Fa₅₀ olivine induced by shock, offering insights into impact-driven alterations on Fe-rich planetary surfaces.

Methods. Shock recovery experiments were conducted on synthetic Fa₅₀ 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 Fa₅₀ 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 Fe⁰, magnetite, and vesicles formed on Fe-rich silicates may contribute to spectral reddening and localized bluing on Phobos and Deimos.