The mapping-experiment duration for individual zircons was as fast as 10 minutes (Agilent 7900 setup).
91500 zircon was employed as the primary reference material, and secondary-standard data (Temora, Plešovice, WRS-1348, FC-1, Fish Canyon, Tardree) typically reproduce within 1% of their published crystallization ages.
High-resolution U–Pb laser-ablation quadrupole inductively coupled plasma mass spectrometry (LA-Q-ICPMS) zircon age maps were undertaken on zircon sampled from syn-tectonic granitoid suites from the Peruvian Eastern Cordillera that exhibit textural evidence for clearly-defined cores and rims.
The images were acquired by rastering a 7 μm spot with a 3.5 J cm and a 45 Hz repetition rate (Agilent 7900 system).
On the contrary, other than Gentry, the principle sources that he references will be seen to provide compelling evidence for the date of a major bolide event at Chicxulub at 65 million years, coinciding with the date of a major extinction, which occurred geologically at the K-T (Cretaceous – Tertiary) boundary and which included the extinction of dinosaurs.
Harris’s paper is rather disjointed and I can see no better way to address its fallacies than by categorising my comments into those about the quality of the essay and the errors of fact and reasoning it contains.
Among them is the presence of non-radiogenic Pb of unknown composition, often referred to as common or initial Pb.
There is also primordial Pb that the earth acquired when it formed, its isotopic composition determined as that of troilite in the Canyon Diablo iron meteorite.
In this way, even different growth zones in individual crystals can be analyzed and thus "dated." An alternative procedure is to take all the zircon grains liberated from a rock sample, and if they are of uniform composition, chemically digest them into solution for standard mass spectrometer analysis.In the case of metamorphic rocks this has been interpreted as inheritance of those zircon grains from the original sources of the sediments, the zircons somehow surviving metamorphism without resetting of the U-Pb isotopic system.These situations are enigmatic, given the dramatic effect of similar temperatures during contact metamorphism.In order to put the rest of the argument in context, I include here a very brief review of the background to Harris’s claims.Throughout the world, there is a sudden extinction of a large number of taxa, including the dinosaurs, at the geological boundary known as the K-T (Cretaceous – Tertiary) boundary.