Closure temperature
Encyclopedia
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. It is specific to a particular material and isotopic system.
These temperatures are experimentally determined in the lab by artificially resetting sample mineral
s using a high-temperature furnace. As the mineral cools, the crystal structure begins to form and diffusion of isotope
s 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. 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.
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
, closure temperature or blocking temperature refers to the temperature of a system, such as a mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
, 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. It is specific to a particular material and isotopic system.
These temperatures are experimentally determined in the lab by artificially resetting sample mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s using a high-temperature furnace. As the mineral cools, the crystal structure begins to form and diffusion of isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s 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. 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
Geochronology
Geochronology is the science of determining the age of rocks, fossils, and sediments, within a certain degree of uncertainty inherent to the method used. A variety of dating methods are used by geologists to achieve this, and schemes of classification and terminology have been proposed...
and thermochronology
Thermochronology
Thermochronology is the study of the thermal evolution of a region of a planet. Thermochronologists use radiometric dating along with the closure temperatures that represent the temperature of the mineral being studied at the time given by the date recorded, to understand the thermal history of a...
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 Hornblende Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole.... |
530±40 |
| Muscovite Muscovite Muscovite is a phyllosilicate mineral of aluminium and potassium with formula KAl22, or 236. It has a highly-perfect basal cleavage yielding remarkably-thin laminæ which are often highly elastic... |
~350 |
| Biotite Biotite Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers... |
280±40 |
Uranium-lead method
| Mineral | Closure temperature (°C) |
|---|---|
| Titanite Titanite Titanite, or sphene , is a calcium titanium nesosilicate mineral, CaTiSiO5. Trace impurities of iron and aluminium are typically present... |
600-650 |
| Rutile Rutile Rutile is a mineral composed primarily of titanium dioxide, TiO2.Rutile is the most common natural form of TiO2. Two rarer polymorphs of TiO2 are known:... |
400-450 |
| Apatite Apatite Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal... |
450-500 |
| Zircon Zircon Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y... |
>1000 |
| Monazite Monazite Monazite is a reddish-brown phosphate mineral containing rare earth metals. It occurs usually in small isolated crystals. There are actually at least four different kinds of monazite, depending on relative elemental composition of the mineral:... |
>1000 |