Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 246

of this event. However, biological and chemical factors that can also produce Ir
anomalies must always be eliminated before Ir alone can be taken as a reasonable
indicator of an extraterrestrial source.

Zircon as an indicator for extraterrestrial impacts

Zircon is ideally suited as an indicator of shock-metamorphic impact
events because of its transparency, lack of significant twinning or well-developed
perfect cleavages and uniaxial optical character. Zircon is also more refractory
than quartz and is therefore better able to resist subsequent thermal annealing
of shock features in addition to being datable using the U--Pb isotopic method.
Bohor et al. (1993)state that although only irregular radial cracking and open
planar fractures had been reported in zircon from shock-impacted environments,
these features were very similar to those developed tectonically in zircon from
mylonitic terranes. With weathering, zircon decomposes into baddeleyite and
silica from highly shocked impact glasses, and it exhibits a shock-induced dis-
placive transformation to the scheelite structure before breaking down into
mixed oxides. Bohor et al. (1993)found that impact-shocked zircon displays PDF
when exposed to etching, and that shock-induced textures can be identified from
distal ejecta, impact glasses and target rocks at known impact sites. The textures
observed in zircon range from PDF to combined PDF/granular to fully developed
granular (polycrystalline) texture and incipient melting. The described sequence
is assumed to represent increasing shock pressure during impact (Bohor et al. ,
1993).
Deformation effects have been identified in zircon crystals from impact brec-
cias or shock-metamorphosed basement rocks at a number of confirmed impact
structures and also at several K--T boundary sites. Bohor et al. (1993)found that
etching of the zircon grains revealed a series of shock-induced features that sug-
gest atextural response to progressive increases in impact shock pressure. The
most common deformation features consist of planar features and strawberry or
granular texture. The series of shock features extends from no shock features at
all to those showing a continuous gradation from PDF alone, through PDF com-
bined with granular texture, to a well developed granular texture and incipient
melting phenomena.
Leroux et al. (1999) attempted to shock-deform zircon experimentally at shock
pressures of 20, 40 and 60 GPa to establish the mineral behaviour under such
conditions. They found the following behaviour.

(1) Deformation in the 20 GPa sample, and partially in the 40 and

60 GPa samples, includes both brittle and plastic deformation. Brittle