Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 247

deformation mainly consists of thin, open cracks and are interpreted as
micro-cleavages as the result of shock-induced shear stresses produced
during the compression stage. Plastic deformation is indicated by abun-
dant screw dislocations in a characteristic glide configuration. The large
density of dislocations at crack tips suggests that plastic deformation is
initiated by the micro-cracking process.

(2) PDFs were found in the 40 and 60 GPa samples. They consist of thin

planar defects filled with amorphous material of zircon composition.
The formation of these PDFs is likely to occur at the shock front.

(3) In the 40 GPa sample, a fraction of the zircon was converted to a high-

pressure phase having a scheelite crystal structure. This phase deforma-
tion was complete in the 60 GPa sample. The phase transformation was
found tobedisplacive.Withregardtothe shock process, it was estab-
lished that PDF formation is an earlier process than transformation to
ascheelite structure.

(4) The optically resolved planar features in zircon are tentatively explained

as micro-cleavage.

One of the main problems with using zircon as an indicator of extraterrestrial
impacts is that it is much less abundant than other shockable minerals (Gucsik
et al. , 2002). However, the reverse is true when attempting to examine impact
features on extraterrestrial bodies such as meteorites and the Moon where quartz
is in low abundance (Bohor et al. , 1993).
U--Pb isotopic dating can be applied successfully to zircon because it incorpo-
ratesasmall amount of uranium upon crystallisation, has one of the highest
thermal blocking temperatures, is resistant to weathering and does not anneal
under the same conditions as other minerals. U--Pb analysis of zircons and dat-
ing of the 'fireball' layer of the K--T boundary layer in the Western Interior of
the USA was undertaken by Krogh et al. (1993). They found a limited range of
primary ages from the target area rocks because of a lack of zircon in the pri-
mary rock. They determined an impact age of 65.5 ± 3 Ma, which concurs with
previously established dates for the K--T boundary.
U--Pb dating of zircons has also provided strong evidence that the Chicxulub
crater was the impact site for the K--T boundary event. Kamo and Krogh (1995)
compared dated zircons from the impact site with zircon ejecta found at the K--T
boundary in Colorado and Haiti. All the zircons have source ages of 545 ± 5 Ma,
which is the age of some of the basement rock into which the crater was formed.
The U--Pb dating technique is regarded as very reliable because it has two inde-
pendent parent--daughter decay schemes permitting evaluation of both closed
and open system behaviour (Deloule et al ., 2001). U--Pb ages, shock features,