Radiometric dating is how geologists determine the age of a rock. In a
closed system, the amount of radiogenic isotopes present in a sample is a direct function of time and the decay rate of the mineral. Therefore, to find the age of a sample, geologists find the ratio of
daughter isotopes to remaining parent isotopes present in the mineral through different methods, such as
mass spectrometry. From the known
parent isotopes and the
decay constant, we can then determine the age. Different ions can be analyzed for this and are called different dating. For thermochronology, the ages associated with these isotopic ratios are directly linked with the sample's thermal history. At high temperatures, the rocks will behave as if they are in an
open system, which relates to the increased rate of
diffusion of the
daughter isotopes out of the mineral. At low temperatures, however, the rocks will behave as a
closed system, meaning that all the products of decay are still found within the original host rock, and therefore more accurate to date. This temperature is dependent on several assumptions, including:
grain size and shape, a constant cooling rate, and chemical composition. To better study the fission tracks created, the natural damage tracks are further enlarged by
chemical etching so they can be viewed under ordinary
optical microscopes. The age of the mineral is then determined by first knowing the spontaneous rate of fission decay, and then measuring the number of tracks accumulated over the mineral's lifetime as well as estimating the amount of Uranium still present. Therefore, exact dating of samples is very hard. Absolute age can only be determined if the sample has cooled rapidly and remain undisturbed at or close to the surface. The environmental conditions, such as pressure and temperature, and their effects on the fission track on the atomic level still remains unclear. However, the stability of the fission tracks can generally be narrowed down to temperature and time. Approximate ages of minerals still reflect aspects of the thermal history of the sample, such as
uplift and
denudation. The age can be found by knowing the
half-life of potassium.
(U-Th)/He Dating (U-Th)/He dating is used to measure the age of a sample by measuring the amount of radiogenic
helium (4He) present as a result of the
alpha decay from
uranium and
thorium. This helium product is kept in the mineral until the closure temperature is reached, and therefore can be determinant of the thermal evolution of the mineral. As in fission track dating, the exact age of the sample is difficult to determine. If the temperature goes above the closure temperature the product of decay, helium, diffuses to the atmosphere and the dating then resets. == Applications ==