Radiometric Dating: Principles and Applications

Radiometric dating determines an object's age by measuring the ratio of radioisotopes to their decay products.
It is also known as radioisotope dating or radioactive dating.
It is a method of absolute dating, providing the exact age of an object.
Relative dating in stratigraphy provides the relative age of stratigraphic layers compared to each other, but not the exact age.
Radiometric dating applies to geological materials (rocks, minerals), once-living materials (fossils), and wooden artifacts to establish timelines.

Radioactive decay is the natural process of unstable isotopes transforming, eventually, into stable isotopes of different elements.
Each radioactive isotope decays at a fixed rate, known as its half-life, which is calculable and measurable.
Half-life is the time it takes for half of an unstable radioactive isotope to decay.
By comparing the amount of the remaining radioactive isotope to the amount of its decay product in an object, scientists determine its age.
Knowing the half-life of a radioactive isotope is essential for radiometric dating.

Isotopic dating is another term for radiometric dating
Several radioisotopes are used, resulting in various isotopic dating methods.
Common methods include uranium-lead, potassium-argon, rubidium-strontium, and radiocarbon dating.
Uranium-lead dating is for uranium-bearing rocks/minerals, using the decay of uranium isotopes to lead isotopes.
Potassium-argon dating is for potassium-bearing rocks/minerals, tracking the decay of potassium to argon.
Rubidium-strontium dating is for rubidium-bearing rocks/minerals, measuring the decay of rubidium to strontium.
Radiocarbon dating is for dating once-living objects with carbon, based on the decay of carbon-14 to nitrogen-14.

A nucleus emits an alpha particle (helium-4 nucleus), consisting of two protons and two neutrons.

Most daughter isotopes emit gamma rays, high-energy photons that can damage molecules and DNA.

Results in a decrease of the parent isotope and an increase of the daughter isotope.
It's how an unstable nucleus loses energy by releasing radiation.

Uranium-238 decays to lead-206 and uranium-235 decays to lead-207; these processes are used to date zircon crystals.
Zircon crystals incorporate uranium (but reject lead) when forming; lead found within them results from radioactive decay.
Radioactivity and lead formation damage the crystal structure.
Scientists use lasers to measure lead-206 and lead-207, calculating the crystal's age using the half-lives of uranium-238 and uranium-235.
Half-life of Uranium-238 is about 4.5 billion years.
Half-life of uranium-235 is about 704 million years.
A concordia diagram is a graph of the decay ratios for both uranium isotopes to determine the material's age.
The oldest rocks have been dated using the uranium-lead method.
The concordia diagram compares lead loss values from multiple samples.

Potassium-40 decays into argon-40 and calcium-40.
Only argon-40 is compared to potassium-40 when calculating an object's age, since calcium-40 is too common.
Potassium-40's decay into argon-40 has a half-life of about 1.25 billion years.
Potassium-40 decays into calcium-40 through beta decay and into argon-40 through electron capture.
Electron capture is when a proton captures an electron and transitions to a neutron, also emitting a neutrino.
Rocks such as potassium feldspars, amphiboles, and clay contain potassium.
Argon gas is trapped once the rock solidifies.
The rock is melted to measure the amount of argon, and the radon gas is then released.
A mass spectrometer measures the mass-to-charge ratio of substances to measure the radon gas.
Absorption spectroscopy measures for detecting elements within a substance.
Molecules converted to a gas-ion phase are diverted through a spectrometer.
Ratios of mass to charge and abundance of each element are identified using a diagram showing compounds.

Rubidium-87 decays into strontium-87 and the isotopes ratios are compared in rocks.
Rubidium replaces potassium in minerals like plagioclase, hornblende, feldspar, biotite, and muscovite.
The half-life of rubidium-87 decaying into strontium-87 is 48.8 billion years.

Radiocarbon dating measures an object's age by determining its radioactive carbon content.
Carbon exists in stable isotopes of carbon-12 and carbon-13.
Carbon-14 is continuously created in the atmosphere from cosmic rays at a fixed rate.
Carbon-14 is found in rocks (limestone), carbon dioxide, the ocean, and all living beings.
A life form stops ingesting carbon when it dies.
Carbon-14 decays through beta decay into stable nitrogen-14, with a half-life of 5,730 years.
Radiocarbon dating is used on organic material that is relatively young.
It can date fossils, biological tissues, wood, cloth, and human remains.

Studies have been carried out on radiometric dating and half-life by scientists.

Zircon crystals from Western Australia (discovered in the 1990s) were radiometrically dated.
Zircons incorporate uranium (making them suitable for uranium-lead dating) and resist weathering.
The oldest zircon crystals on Earth are between 4.2 and 4.3 billion years old, calculated from the uranium-lead dating method.

The frozen remains of the Iceman were discovered in 1991 in the Otzal Alps (Italian-Austrian border).
Radiocarbon dating using mass spectrometry dated Otzi to have lived between 3350 to 3100 BCE.