Micro-Raman spectroscopy of plagioclase and maskelynite in Martian meteorites: Evidence of progressive shock metamorphism

We present the first systematic Micro-Raman spectroscopic investigation of plagioclase of different degree of shock metamorphism in Martian meteorites. The equilibrium shock pressure of all plagioclase phases of seventeen unpaired Martian meteorites was determined by measuring the shock-induced reduction of the refractive index. Systematic variations in the recorded Raman spectra of the plagioclase phases correlate with increasing shock pressure. In general, the shock induced deformation of the plagioclase crystal lattice leads to an increase of luminescence background and a broadening of all Raman bands. The most persistent bands are those at 505 and 590cm^(-1). For shock pressures above ～40GPa, the post shock temperature in the meteorites was high enough to initialise reordering of the O-tetrahedra in the structure of the diaplectic glass, leading to a decrease of intensity for the band near 590cm^(-1). At higher shock pressure (>45GPa) and post-shock temperature plagioclase starts to recrystallise. This results in a reduced luminescence background and a decrease of the full width of half maximum of the characteristic Raman bands