Other
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmmr54a..07k&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #MR54A-07
Other
3630 Experimental Mineralogy And Petrology, 3672 Planetary Mineralogy And Petrology (5410), 3924 High-Pressure Behavior, 3954 X-Ray, Neutron, And Electron Spectroscopy And Diffraction
Scientific paper
Presence of amorphous plagioclase and high-pressure phases of plagioclase in shocked meteorites can be important constraints on the history of shock events. Here we report new experimental results on amorphization and crystalization kinetics of plagioclase (albite and labradorite) examined by in-situ X-ray observations at high pressure and temperature using Kawai-type high-pressure apparatus and synchrotron radtion. We have found that amorphization pressures become lower at higher temperatures. For example, we have observed that albite and labradorite become amorphous at 23 GPa and 1173 K, and 13 GPa and 973 K, respectively. These amorphization pressures are much lower than those reported in previous shock experiments and DAC studies at room T. We have also found that crystallization kinetics from amorphous plagioclases, which was examined by time-resolved X-ray diffraction measurements, is rather different among minerals. Jadeite first appears from amorphous plagioclases of both chemical compositions. Nucleation of other minerals such as stishovite, grossular, and Ca ferrite-type NaAlSiO4 is significantly delayed. Preliminary analysis of kinetic data based on Avrami rate equation indicates that the n-value for the formation of Jadeite is around 0.5, whereas the n-values for other phases are relatively large 3-4. Based on kinetic data obtained in albite, we have estimated P-T-t kinetic boundaries for the presence of amorphous albite, jadeite (no stishovite), and jadeite+stishovite. In the time scale of 1 sec, only jadeite is present between 1473-1973 K at around 20 GPa. Stishovite appears at higher T, and jadeite is not formed at lower T. It has been reported that jadeite is present without silica phase in shocked meteorites, which is possibly due to the difference in crystallization kinetics as demonstrated in the present study. These findings and further quantitative analysis of kinetic data make it possible to constrain P-T-t conditions of shocked meteorites based on the plagioclase breakdown.
Funakoshi Kenichi
Kato Taichi
Kikegawa Takumi
Kimura Masahiro
Kubo Takayuki
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