HR: 14:40h
AN: P13E-05    [Abstracts]
TI: Thermal Infrared Spectroscopy of Experimentally Shocked Plagioclase and Basalt and Applications to Mars
AU: * Johnson, J R
EM: jrjohnson@usgs.gov
AF: USGS, 2255 N. Gemini, Flagstaff, AZ 86001, United States
AB: Laboratory thermal infrared emission spectra (250-1400 cm^-1) of experimentally shocked (17-60 GPa) plagioclase feldspars (bytownite, andesine, and albite), basalt, and basaltic andesite demonstrate the disordering of mineral lattices and increasing glass content with increasing shock pressure. These effects cause loss of spectral detail and shifts in absorption feature positions. Disordering in the feldspar structure begins at pressures >15-20 GPa, and diaplectic glass (maskelynite) formation is complete between ~30-45 GPa. As pressures increase the mutual existence of crystalline phases and diaplectic glasses cause the characteristic, fourfold (tetrahedral), strong coordination bonds of Si and Al in feldspars to alter to weaker, less polymerized bonds that approach sixfold (octahedral) coordination. This influences the characteristic vibrational frequencies in the thermal infrared. For example, the bands near 400-550 cm^-1 are caused by bending vibrations in the Si-O-Al planar ring structures in tectosilicates and diaplectic glasses. Si-O-Si octahedral bending vibrations cause absorptions between about 700-450 cm^-1 and SiO₆ octahedral stretching vibrations occur between 750-850 cm^-1. Absorptions in the 900-1200 cm^-1 region are due to Si-O antisymmetric stretch motions of silica tetrahedra. Many of these spectral features persist to higher pressures in albite compared to bytownite, possibly due to the relatively lower Al content in albite. With increasing Ca content, the main absorption band of highly shocked albite shifts from ~1050 cm^-1 to ~1000 cm^-1 for andesine and ~950 cm^-1 for bytownite. However, the other main absorption in highly shocked feldspars near 450-460 cm^-1 varies little with Ca content. Linear mixing models demonstrate that mineral and glass spectra cannot replicate shocked bytownite spectra beyond shock pressures of 20-25 GPa, coincident with the onset of diaplectic glass formation. Similar models of shocked basalt also exhibit increased errors beyond 20-25 GPa unless spectra of shocked feldspar are used in spectral libraries. Ongoing spectral mixing analyses of Thermal Emission Spectrometer (TES) data using spectra of shocked feldspars suggest that semi- contiguous regions of shocked materials worthy of further study are found in Solis Planum, Acidalia, Syrtis Major, and northern Utopia.
DE: 3934 Optical, infrared, and Raman spectroscopy
DE: 3944 Shock wave experiments
DE: 5420 Impact phenomena, cratering (6022, 8136)
DE: 5464 Remote sensing
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting