Raman spectroscopy of olivine in dunite experimentally shocked to pressures between 5 and 59 GPa

Abstract

Previous Raman investigations on experimentally shocked single-crystal olivine indicated that the olivine Raman bands seemingly shift to a higher wave number with increasing shock pressure. If this effect could be confirmed, Raman analysis of natural shock-metamorphosed minerals could potentially provide an important shock barometric tool. We carried out a Raman spectroscopic study on olivine in a series of natural dunite samples experimentally shocked to pressures between 5 and 59 GPa. In addition, we analyzed olivine grains in a sample of the Cold Bokkeveld C1 meteorite. We studied samples of several dunites with olivine of 90.64-92.00 mole% Fo to determine Raman effects in the region from 200 to 900 cm(-1). Several olivine grains per sample/shock pressure stage were analyzed. Raman analysis, however, showed little or no shift with increasing shock pressure. The shifts to higher or lower frequencies observed were not specific for a given pressure stage, with some grains within a sample showing more shift than others. This finding is unrelated to the crystallographic orientation of analyzed grains and cannot be related systematically to the different degrees of optically determined shock metamorphism of the analyzed grains. We identified an increase in full width at half maximum (FWHM) for the 824 cm^(-1) band with increased shock pressure in the shocked heim samples above 45 GPa and, to a lesser extent, for the 856 cm^(-1) band. Evaluation of band broadening of olivine in the Cold Bokkeveld meteorite showed FWHM values that were much greater (920 cm^(-1)) than those of olivine in the shocked dunite samples (712 cm^(-1)). We concluded that these differences in FWHM are due to differences in chemical composition between the meteoritic and the experimentally shocked olivine. Therefore, using Raman spectroscopy to detect small shifts in wave numbers to higher frequencies with increased shock pressure does not yield consistent effects for polycrystalline dunite. An extra band at 650 cm^(-1) was identified in the Raman spectra of the unshocked Mooihoek dunite and the heim dunite samples shocked to 5, 29.3, and 59 GPa, as well as another at 696 cm^(-1) in all the spectra of the 59 GPa heim sample. The cause of these extra bands is not known. Comparison of these results with Raman spectra of olivine from the Cold Bokkeveld C1 meteorite did not allow us to determine shock pressures for the meteoritic olivine.