Other
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmmr43b1232q&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #MR43B-1232
Other
3939 Physical Thermodynamics, 3944 Shock Wave Experiments, 6022 Impact Phenomena (5420, 8136), 6240 Meteorites And Tektites (1028, 3662)
Scientific paper
The effects of impact-induced shock metamorphism can be preserved in the microstructure of individual mineral grains in target rocks, and thus provide a record of past impact events. This undergraduate research project is a petrographic study of shock metamorphism preserved in minerals from the Vredefort Dome impact structure, South Africa, with the goal of documenting shock features using standard petrographic techniques. The Vredefort Dome is widely regarded as the oldest and largest preserved impact crater on Earth at 2.02 Ga (Kamo et al., 1996 EPSL), and is thus important in studies of impact processes on the Precambrian Earth. Five samples representing three rock types were collected from the Vredefort Dome: two different quartzites were sampled from the collar zone, including Dominion Group quartzite (Ro) on R53 north of Parys, and quartzite Rjo1 on the maps of Bisschoff (1999). In addition, three granitoids were sampled, including two pseudotachylite breccias, from the center of the dome. Two pseudotachylite samples from quarries within the amphibolite zone contain granitoid clasts; a third sample of granulite-facies granitoid (charnockite) was collected from the amphibolite-granulate transition near Vredefort. In general, all of the samples are quartz-rich, and exhibit quartz grains with variably developed planar deformation features (PDFs). Two and three compelling sets of PDFs within individual grains were only observed in quartz from the collar zone; most samples contain quartz with only one set of conspicuous PDFs, usually decorated, that are readily visible with optical light microscopy. Shock microstructures were also observed in accessory minerals, including zircon. Detrital zircons in quartzite (unit Ro) contain PDFs that are readily apparent with a 10x to 40x objective. The zircons are average size (e.g. 125 microns), and contain parallel PDFs with an apparent regular spacing of 5 microns. Zircons were observed in all granitoid and pseudotachylite samples, however no grains with PDFs have (thus far) been observed, although Kamo et al (1996) report zircons from Vredefort pseudotachylite that contain PDFs. In addition to zircon, a single grain of apatite was observed in the Ro quartzite with possible PDFs. The grain is average size, 135 x 50 microns, and contains one dominant set of parallel fractures that are less-regularly spaced than in zircon, and are oriented 72 degrees from parallelism with the c- axis. While the identification of PDFs in apatite from Vredefort Dome is tentative, apatite has previously been described with shock microstructures from other impact sites (French, 1998).
Cavosie Aaron J.
Quintero R. R.
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