Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufmmr11a0925x&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #MR11A-0925
Physics
5410 Composition, 6022 Impact Phenomena, 3662 Meteorites, 3924 High-Pressure Behavior, 3944 Shock Wave Experiments
Scientific paper
High-pressure minerals are generally found within or adjacent to shock-induced melt veins and melt pockets in highly shocked chondrites. The minerals that crystallize in the melt veins and pockets and the distribution of these minerals provide a record of crystallization and quench histories that can be used to constrain shock pressure and pulse duration. Most previous investigations have focused on relatively thick veins (>100 μ m in width) because they tend to contain high-pressure minerals that are observable using petrography or scanning electron microscopy. However, the mineralogy of thin shock veins can provide additional constraints on the pressure history of shocked meteorites. Because shock veins cool predominantly by conduction to the surrounding matrix, rather than by adiabatic decompression, the timing of shock-vein crystallization depends strongly on vein thickness and position within the veins. Therefore, the thinnest melt veins, which solidify within tens of nanoseconds after melting, provide a brief crystallization history at the time of formation whereas thicker veins provide a longer history that may reflect crystallization during decompression. If thin veins form during compression or early in the shock pulse, they will likely record the equilibrium shock pressure or the peak pressure. The goal of this study is to characterize the mineralogy of thin melt veins and to compare the results to those of thicker veins in the same samples. We have investigated three L chondrites that contain a wide range of melt vein sizes. These include Tenham (several μ m to 600 μ m in width), Roy (10 μ m to 150 μ m in width) and Umbarger (35 μ m to 300 μ m in width). Thick veins in these samples have been previously investigated using FESEM and TEM, resulting in crystallization pressures of approximately 25, 20 and 18 GPa for Tenham, Roy and Umbarger, respectively. Thin veins from these samples were investigated using TEM. Three thin veins in Tenham show three different assemblages including glass, high-pressure minerals and low-pressure minerals. The thin vein from Roy contains uniform majorite throughout, whereas a thin vein from Umbarger contains fine ringwoodite grains and Ca-clinopyroxene. The fact both low- and high-pressure assemblages occur in the thin veins from Tenham indicates that melt-vein formation occurs during pressure release as well as during compression. Further details of the shock-pressure history of these samples will be presented.
Decarli P.
Sharp Tristan
Xie Zhaohua
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