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Shock-Induced Intragranular Breakdown of Quartz and Feldspar:
Implications for Kevin Pope’s KT-Boundary Impact Clouding Model

From: Andrew Glikson
To: Benny Peiser

Dear Benny,

I have now read Kevin Pope's Geology article "Impact dust not the cause of the Cretaceous-Tertiary mass extinction" (Geology, 30:99-102), and like to thank him for sending me a copy of his article.

Kevin questions the long-held assumption of a silicate-dominated dusting scenario associated with the KT boundary impact. In the following I suggest that an extrapolation from the coarser-grained (~0.1-1.0 mm) dust fraction incorporated in the KT-boundary unit to finer fractions, using volcanic clastic dust size-frequency distribution, is unlikely in view of (1) intra-crystalline breakdown of shock-metamorphosed mineral grains in impacted target rocks along planar deformation features (PDF), and (2) solid-state amorphisation (diaplectic transformation from crystalline structure to glass) of quartz and feldspar (cf. Stoffler and Langenhorst, 1994; French, 1998 and references therein), resulting in a high proportion of low-density micron-scale sub-crystal grains and silicate glass in the ejecta.

The penetrative development of crystallographically controlled PDF planes and glass ensues in loss of mechanical coherence of individual grains and rock fragments. This results in an increased production of micron to submicron-scale particles. Cores of impact-shocked granite or sandstone are commonly pulverised on touch and disintegrate upon sample preparation, for example shocked granitoids from the Woodleigh impact structure (Mory et al., 2000a,b; Glikson, 2000).

The proportion of shocked grains may be expected to increase with distance from the impact site, due to high angle ejection of shocked ejecta from inner-impact aureoles as contrasted with lower angle ejection of less shocked ejecta from outer crater aureoles. Whereas the relatively high proportion of shocked quartz grains in the KT boundary unit at Pacific sites (~65%, Bostwick and Kyte, 1996) may conceivably be interpreted in such terms, further studies are required to determine the ratio of shocked to unshocked quartz grains with distance from Chicxulub.

Pope (2002) states: "Assuming a grain size distribution typical for distal volcanic ash deposits, the submicrometer size component of the clastic debris in the fireball layer is probably <1%". However, intragranular disintegration of shocked ejecta should result in higher proportion of submicron ultra-fine dust, and thus in different grain size distribution frequencies of impact-released particles and volcanic dust. It follows that extraterrestrial impacts may be more effective than volcanic activity in producing long-term stratospheric clouding, and thereby photosynthesis blocking and extinction.

Evidence for impact-produced ultra-fine dust would be difficult to identify due to (1) fallout of micron-scale dust would postdate the KT boundary "fireball layer", and may be incorporated in post-impact sediments, and (2) some or much of the ultra-fine silicate dust may have been dissolved in the acid atmosphere/acid rains consequent on flash-triggered oxidation reactions.

Andrew Glikson
Research School of Earth Science
Australian National University
Canberra, ACT 0200
01/27/02