Using estimates of the total amount of iridium in the K-Pg layer, and assuming that the asteroid contained the normal percentage of iridium found in chondrites, the Alvarez team went on to calculate the size of the asteroid.
Spectra for whole rocks all indicate some amount of disturbance, and ages based on plagioclase bulk samples seem to be consistently more reliable [Hofmann et al., Earth Planet. This was responsible for erosion and destruction of part of the uppermost sediments (including spinels if there were any) and heterogeneous and non-uniform redeposition of Ir at a number of underlying sedimentary levels.
Eight basalt samples provided Ar results, four on plagioclase bulk samples, four on whole rocks. Magnetic analyses (both thermal and by alternating fields) uncovered clear reversed primary components in the upper flows, and more disturbed normal components in the lower flows, with evidence for an additional reversed component. Volcanism resumed shortly after the K/T boundary, within reversed chron C29R, as witnessed by the three reversely magnetised overlying basalt flows dated ∼65 Ma.
Despite careful search, no Ni-rich spinels were found. The three flows underlying the Ir-bearing sediments are dated at ∼66.5 Ma, and two overlying flows at ∼65 Ma. The K/T bolide impact was recorded as a deposit of Ir, and possibly (though not necessarily) spinels. There is no indication contradicting the view that the bulk of Deccan trap volcanism occurred over only three chrons (C30N, C29R, C29N) [Courtillot, Evolutionary Catastrophes: the Science of Mass Extinctions, Cambridge University Press, 1999; Courtillot et al., Earth Planet.
At the Scottish Universities Environmental Research Centre in East Kilbride, a team led by Dr Darren Mark conducted independent argon-argon analyses which confirmed the results from the US.
Argon-argon dating uses a mass spectrometer to measure the ratio of radioactive potassium in a sample of rock to its decay product, argon.
The amount of argon in a sample allows researchers to use rocks as incredibly slow clocks.