Problems with radiometric dating of rocks online dating websites info
Detectors at the end of the tube record the number of charged particles of a particular atomic mass and provide a ratio of the isotopes present in a sample.This is the most widely used system for radiometric dating of sedimentary strata, because it can be used to date the potassium-rich authigenic mineral glauconite and volcanic rocks (lavas and tuffs) that contain potassium in minerals such as some feldspars and micas.Potassium is a very common element in the Earth’s crust and its concentration in rocks is easily measured.However, the proportion of potassium present as 40 K is very small at only 0.012%, and most of this decays to 40 Ca, with only 11% forming 40 Ar.The decay series of most interest to geologists are those with half-lives of tens, hundreds or thousands of millions of years.
This dating method is principally used for determining the age of formation of igneous rocks, including volcanic units that occur within sedimentary strata.
Measurement of the concentrations of different isotopes is carried out with a mass spectrometer.
In these instruments a small amount (micrograms) of the sample is heated in a vacuum to ionise the isotopes and these charged particles are then accelerated along a tube in a vacuum by a potential difference.
Argon is an inert rare gas and the isotopes of very small quantities of argon can be measured by a mass spectrometer by driving the gas out of the minerals.
K–Ar dating has therefore been widely used in dating rocks but there is a significant problem with the method, which is that the daughter isotope can escape from the rock by diffusion because it is a gas.
It is also possible to use it on authigenic minerals, such as glauconite, in some sedimentary rocks.