A study of the observed shift in the peak position of olivine Raman spectra as a result of shock induced by hypervelocity impacts

Kuebler et al. (2006) identified variations in olivine Raman spectra based upon the composition of individual olivine grains, leading to identification of olivine composition from Raman spectra alone. However, shock on a crystal lattice has since been shown to result in a structural change to the original material, which produces a shift in the Raman spectra of olivine grains compared with the original un-shocked olivine (Foster et al., 2013). This suggests that the use of the compositional calculations from the Raman spectra, reported in Kuebler et al. (2006), may provide an incorrect compositional value for material that has experienced shock. Here we have investigated the effect of impact speed (and hence peak shock pressure) on the shift in the Raman spectra for San Carlos olivine (Fo91) impacting Al. foil. Powdered San Carlos olivine (grain size 1 to 10 ?m) was fired at a range of impact speeds from 0.6 – 6.1 km s-1 (peak shock pressures 5 – 86 GPa) at Al. foil to simulate capture over a wide range of peak shock pressures. A permanent change in the Raman spectra was found to be observed only for impact speeds greater than ~5 km s-1. The process that causes the shift is most likely linked to an increase in the peak pressure produced by the impact, but only after a minimum shock pressure associated with the speed at which the effect is first observed (here 65 – 86 GPa). At speeds around 6 km s-1 (peak shock pressures ~86 GPa) the shift in Raman peak positions is in a similar direction (red shift) to that observed by Foster et al. (2013) but of twice the magnitude.