Other
Scientific paper
May 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agusm.p21a..01l&link_type=abstract
American Geophysical Union, Spring Meeting 2005, abstract #P21A-01
Other
5420 Impact Phenomena (Includes Cratering)
Scientific paper
We have collected samples from the Chicxulub impact breccias in Yaxcopoil-1, from the global K-T iridium-rich layer found at distances greater that ~1400 km from Chicxulub, and also from the proximal spherule layer close to Chicxulub. We will use these samples to: try to determine the angle and direction of impact at Chicxulub, provide the compelling evidence that Chicxulub is K-T, and ascertain whether there were multiple impacts at the K-T boundary. The clearest indicator of angle of impact for circular craters on other planetary bodies is in the ejecta pattern. Experiments, numerical modelling and field observations all indicate that the plumes of oblique impacts expand initially in the downrange direction. We are currently documenting how the size of the coarse-grained ejecta particles and the geochemistry of spinels varies with geographical location. If the distribution of the size and/or geochemistry of the ejecta is asymmetric it is likely to be diagnostic of the direction of vapour plume expansion, and hence an indicator of impact direction. The majority of planetary scientists agree that Chicxulub is the K-T impact crater - but dissenters argue that the evidence is not yet compelling. To link Chicxulub unequivocally to the K-T boundary we must be able to prove that components of this global K-T ejecta layer originate from the target rocks at the Chicxulub impact site. All the evidence presented so far (the dating of melt rocks, the change in ejecta size with distance from Chicxulub, the dating of zircons,) is compatible with a genetic link but does not prove it. Dating of melt-rich rocks at Chicxulub at ~64.98 Myr show that the Chicxulub impact occurred at about K-T time. Zircons dates at Chicxulub (main age of ~545 Myr, minor component of ~420 Myr) are similar to those found at a few North American K-T sites, suggesting that these zircons could have originated from Yucatan basement rocks. However, some of the ages found within the global K-T ejecta layer have not yet been found at Chicxulub. This fact, as well as the small number and limited geographic distribution of the zircons, means that the link between Chicxulub and the global K-T layer is not yet established. The size of ejecta particles within the global K-T layer have been measured and there appears to be a gradual increase in size towards Chicxulub. However, the size has been only been measured at a small number of locations, with very few measurements in the southern hemisphere. We have collected a large number of samples, and will use these to obtain a statistically meaningful answer to the hypotheses that Chicxulub is not K-T, and that there is more than one impact at the K-T boundary. We will analyse the size distribution of the shocked quartz, the age of the shocked zircons, and the trace element chemistry of the quartz. If Chicxulub is the K-T impact site then: 1) The size of the ejecta fragments should gradually increase towards Chicxulub in every direction; 2) The shocked zircons in the global K-T layer will have similar age components as zircons at Chicxulub; 3) The shocked quartz in the global K-T layer will have similar trace element chemistry as quartz at Chicxulub. If the zircon ages or trace element chemistry does not match, and the K-T ejecta layer has a wider range than the Chicxulub basement, then this may indicate that there were multiple impacts at the K-T boundary (e.g. at Silverpit or Boltysch). The spinel chemistry might also be diagnostic of one or multiple impacts.
Lana Cristiano
Morgan Jaqueline
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