radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares ...

, closure temperature or blocking temperature refers to the temperature of a system, such as a

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ...

, at the time given by its radiometric date. In physical terms, the closure temperature is the temperature at which a system has cooled so that there is no longer any significant diffusion of the parent or daughter

isotopes Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbe ...

out of the system and into the external environment. The concept's initial mathematical formulation was presented in a seminal paper by Martin H. Dodson, "Closure temperature in cooling geochronological and petrological systems" in the journal '' Contributions to Mineralogy and Petrology'', 1973, with refinements to a usable experimental formulation by other scientists in later years. This temperature varies broadly among different minerals and also differs depending on the parent and daughter atoms being considered.''Earth: a Portrait of a Planet'
Glossary W.W. Norton & Company
It is specific to a particular material and isotopic system.Rollinson, 1993. ''Using Geochemical Data: Evaluation, Presentation, Interpretation'' Longman Scientific & Technical. The closure temperature of a system can be experimentally determined in the lab by artificially resetting sample

s using a high-temperature furnace. As the mineral cools, the crystal structure begins to form and diffusion of

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers ...

s slows. At a certain temperature, the crystal structure has formed sufficiently to prevent diffusion of isotopes. This temperature is what is known as blocking temperature and represents the temperature below which the mineral is a closed system to measurable diffusion of isotopes. The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to blocking temperature. These temperatures can also be determined in the field by comparing them to the dates of other minerals with well-known closure temperatures. Closure temperatures are used in

geochronology Geochronology is the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is p ...

and thermochronology to date events and determine rates of processes in the geologic past.

Table of values

The following table represents the closure temperatures of some materials. These values are the approximate values of the closure temperatures of certain minerals listed by the isotopic system being used. These values are approximations; better values of the closure temperature require more precise calculations and characterizations of the diffusion characteristics of the mineral grain being studied.

References

{{reflist Radiometric dating

Table of values

Potassium-argon method

Uranium-lead method

Electron spin resonance dating

References