Closure temperature

This article is about radiometric dating. For blocking temperature in a ferromagnet, see Superparamagnetism.

In radiometric dating, closure temperature or blocking temperature refers to the temperature of a system, such as a mineral, at the time given by its radiometric date. In physical terms, the closure temperature at which a system has cooled so that there is no longer any exchange of parent or daughter isotopes with the external environment. This temperature varies broadly between different minerals and also differs depending on the parent and daughter atoms being considered.^[1] It is specific to a particular material and isotopic system.^[2]

These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace. As the mineral cools, the crystal structure begins to form and diffusion of isotopes is less easy. 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 isotopes. ^[2]Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the blocking temperature. 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 and thermochronology to date events and determine rates of processes in the geologic past.

Table of values

The following 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.

Potassium-argon method

Mineral	Closure temperature (°C)
Hornblende	530±40
Muscovite	~350
Biotite	280±40

Uranium-lead method

Mineral	Closure temperature (°C)[3]
Titanite	600-650
Rutile	400-450
Apatite	450-500
Zircon	>1000
Monazite	>1000

References

1. ^ Earth: a Portrait of a Planet Glossary W.W. Norton & Company
2. ^ ^{a b} Rollinson, 1993. Using Geochemical Data: Evaluation, Presentation, Interpretation Longman Scientific & Technical. ISBN 978-0-582-06701-1
3. ^ Flowers, R.M.; Bowring, S.A.; Tulloch, A.J.; Klepeis, K.A. (2005). "Tempo of burial and exhumation within the deep roots of a magmatic arc, Fiordland, New Zealand". Geology 33: 17. doi:10.1130/G21010.1.