Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v12b..02c&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #V12B-02
Mathematics
Logic
3625 Petrography, Microstructures, And Textures, 5225 Early Environment Of Earth, 5420 Impact Phenomena, Cratering (6022, 8136), 5455 Origin And Evolution, 9619 Precambrian
Scientific paper
Impact processes dominated the Early Earth and are believed to have culminated in a Late Heavy Bombardment at ca. 3.9-3.8 Ga. No direct geologic evidence of this stage of planetary evolution has been found, however Hadean zircons up to 4.4 Ga have been identified. A new method for identifying impact evidence in siliciclastic sediments has recently been reported (Cavosie et al., 2008; Quintero et al., 2008 this volume), based on the discovery of detrital grains of shocked quartz and zircons eroded from the 2.02 Ga Vredefort Dome in channel sediments of the Vaal River. Here we describe grain scale characteristics of these shocked minerals, and discuss aspects of their preservation. In grain mount, detrital grains of shocked quartz range from sub-angular to round, and characteristically preserve a single set of parallel decorated planar deformation features (PDFs) with spacing of less than 5 microns, analogous to shocked quartz reported from Vredefort impactites. While the PDFs are continuous across most grains, numerous examples were found where a single orientation of PDFs is heterogeneously distributed in an optically continuous grain, such as only being preserved within 15 microns from a grain margin. Where sub-grains are present, PDFs can occur in multiple orientations, or in single isolated sub-grains as small as 100 microns that are surrounded by PDF-free sub-grains. Well-preserved PDFs are found in grains as small as 100 microns. In addition to quartz, detrital shocked zircons were also found. The shocked zircons are subhedral to anhedral, and show evidence of sedimentary abrasion. Planar fractures (PFs) were observed parallel to (010), (100), and in two {hkl} orientations. Up to three PF orientations are present in single zircons, with variable spacing that usually ranges from 5-20 microns. Despite the complex history of the shocked grains from the Vredefort Dome, including post-impact granulite facies metamorphism, exhumation, erosion, and fluvial transport, detrital quartz and zircon provide a robust record of the Vredefort impact in modern sediments. The recognition that shocked quartz and zircon can survive erosion and fluvial transport in siliciclastic sediments opens up new avenues in searching for impact evidence from the Hadean, as shocked grains from the early Earth may be preserved in Hadean detritus. Given the abundance of evidence supporting an origin for the Hadean detrital zircons in quartz-saturated granitoids, large volumes of shocked quartz may have been produced, eroded, and deposited in younger sediments. This is particularly promising, given that all known populations of Hadean detrital zircons occur in quartz-rich siliciclastic rocks. Cavosie et al., 2008. Geology, in review Quintero et al., 2008. AGU, this volume Cavosie et al., 2007. Earth's Oldest Rocks.
Cavosie Aaron J.
Moser Danielle E.
Quintero R. R.
Radovan H. A.
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