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Loss (leakage) of lead within the sample will result in a discrepancy in the two decay schemes, resulting in a different age determined by each decay scheme.

This effect is referred to as discordance, and provides a check on the reliability of the age.

The data lie on a discordia, which indicates a metamorphic date of ~65Myr; indeed the degree of collinearity of data from different sites and from the crater itself is remarkable.

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In this paper, Krogh et al study zircons from the KT layer at a site in Colorado, and conclude that the degree of shock is proportional to the degree of resetting and that the original date of crystallisation is 550My and the date of impact is 65My.

Further work on crystals from the Chicxulub crater itself and from Haiti confirm these results (9) and link the zircons at distant sites with the crater itself, through the date of original crystallisation and the date of the metamorphic event.

It has been shown that the physical signs of shocking are proportional to increasing shock and can be revealed by etching (7).

Furthermore these signs correlate with the degree of resetting of the U-Pb clock (8) .

The method relies on the coupled chronometer provided by the decay of 238U to 206Pb, with a half-life of 4.46 billion years and 235U to 207Pb, with a half-life of 704 million years.