Uranium lead dating method
First, measuring the isotope ratio of a single element can be done much more precisely than measuring isotope ratios of two differing elements.
Second, using two isotopes of the same element makes the sample immune to chemical fractionation during a post-crystallization disturbance (Dalrymple 208). This model ultimately led to the development of isochrons, in which two isotopes are plotted against each other to calculate an age for the mineral or rock.
The commonly accepted 4.5 billion year age of the earth is derived from radiometric dating of lunar rocks and meteorites in addition to dating methods based on the Gerling-Holmes-Houtermans model. Those who developed the method utilized Pb, lead isotopes that are the product of radioactive decay, normalized to 204Pb.
The possibility that dinosaur eggs might survive extreme climatic conditions is also a possible avenue to be explored.
If the new U-Pb dating technique is borne out by more fossil samples, then the KT extinction paradigm and the end of the dinosaurs will have to be revised, Heaman and his colleagues believe.
The in situ U-Pb technique involves laser ablation to remove minute particles of the fossil which then undergo isotopic analysis.
An example isochron from Dalrymple (2004) is shown in Figure 4.8 below. Unlike other isochrons, the slope of the Pb-Pb isochron decreases with increasing age.This is because Pb, causing the isochron to decrease in slope with increasing age.
These properties mean that the radioactive decay of uranium to lead has previously been used to measure the age of rocks, including those of some of the oldest on Earth, but its use in direct dating of fossils is new. During fossilisation – typically within 1000 years after death – bone becomes enriched in elements including uranium, which decays spontaneously to lead over time.The use of lead isotope ratios makes this isochron self-checking.A large scattering of measurements would indicate the sample is multi-stage rather than single-stage, making the isochron unreliable.A Canadian research team has used a new uranium-lead (U-Pb) dating technique to show that a fossilised dinosaur bone found in New Mexico is only 64.8 million years old, meaning the creature was alive about 700,000 years after the mass extinction event that is believed to have wiped out all non-avian dinosaurs.A team led by Larry Heaman of the University of Alberta's Department of Earth and Atmospheric Sciences used the method to determine the age of the fossilised femur of a sauropod, a herbivorous dinosaur.Two other characteristics of lead isotope measurements make it superior to other methods.