What are Isotopes?
The 94 natural existing chemical elements are a result of the different numbers of protons in an atom's nucleus, which controls the chemical and physical properties on an element. The atomic nucleus also contains neutrons and the number of these can vary, this creates several isotopes of each element. All of the isotopes of an element behave similarly with the exception of the stability of the nucleus.
What is Radioactive Decay?
The number of neutrons in an atom controls how stable the isotope is with most configurations being unstable. This instability is radioactive decay. The nucleus will emit energy and smaller particles to change to a different number of protons or neutrons, thus creating different elements as the number of protons change. Unstable isotopes vary in their half-lives (the time for half of a sample of the starting parent isotope to change to the resulting daughter isotope from less than a second to trillions of years.
Decay of Uranium to Lead
Our lab measures isotopes of uranium-238 and -235, which decay to lead-206 and -207, respectively, and have half-lives of 4.47 billion and 710 million years, respectively. These long half-lives allow geologists to date most rocks from the age of the Earth (4.5 billion years ago) to almost now. The radioactive decay of U to Pb involves many steps (see adjacent image) with only the first step having a long half-life.
The Mineral Zircon
Zirconium silicate (zircon) is the mineral of choice for geologist to date rocks. The crystal structure can accept trace amounts of uranium (U) and excludes lead (Pb). Zircon is also a very robust mineral that crystalizes at high temperatures and is resistant to chemical weathering. The oldest minerals known on Earth are zircons that are 4.4 billion years old and are older than the oldest rock on Earth, which is a young 4.0 billion years old (dated using U-Pb radioactive decay in zircon).
Simplified Radiometric Dating
The half-life of an isotope is the time for half of the sample to undergo radioactive decay to its daughter isotope, so the simplest radiometric age dating would be a sample fo zircon with equal amounts of 238U and 206U. For zircons, the initial amount of lead is zero so any lead is a product of radioactive decay, this measuring equal amounts of 238U and 206U is the result of radioactive decay over one half-life of the isotope 238U. The half-life of 238U is 4.468 billion years (4,468,000,000 years).
Measuring Tiny Amounts of Isotopes Precisely
We use mass spectrometers to measure amounts of isotopes in our samples. These allow separation of ions based on their mass. Isotopes vary in mass, 238U weighs slightly more than 235U and much more than 206Pb. A high voltage pulls the ions from the sample to the analyzer, passing by a powerful electromagnet. The electromagnet pulls the ions away from a straight path with the heavier ions traveling in a straighter path due to their greater mass and thus momentum. The lighter ions' path are bent more. Changing the amount of magnetic force via the electric current to the electromagnet changes the isotope mass reaching the analyzer, thus allowing measurement of most heavy isotopes.
IGL has Two Types of Mass Spectrometers
Thermal Ionization Mass Spectrometry
(TIMS) - Dissolving a zircon crystal and then ionizing the sample on a heated filament. Very precise but lacks spatial control and takes hours for one analysis to run.TIMS explained
Laser ablation inductively coupled plasma mass spectrometry
(LA-ICPMS) - Blasting a zircon crystal with a laser and analyzing the resulting ablated ions. Each analysis takes less than 2 minutes and different parts of a crystal can be analyzed.LA-ICPMS explained
Why Determine the Age of Rocks?
Determining the U-Pb age of a zircon helps define the age of the rock. The ages of rocks are used for studies of plate tectonics, meteorites, biotic evolution, climate change, natural hazards evaluation, mining and other applications. The Geologic Time Scale is a critical tool for study of the Earth and is based on high precision dates derived from U-Pb isotope geochronology like that performed in IGL. This image is a thumbnail of the latest Geologic Time Scale published by the Geological Society of America. Follow the link to see the detailed age data applied to the different increments of time.