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  Closure temperature


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In radiometric dating </wiki/Radiometric_dating>, *closure temperature*
or *blocking temperature* refers to the temperature of a system, such as
a mineral </wiki/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
</wiki/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] <#cite_note-0> It is
specific to a particular material and isotopic system.^[2]
<#cite_note-Rollinson-1>

These temperatures are experimentally determined in the lab by
artificially resetting </wiki/Petrology#Branches_of_petrology> sample
minerals </wiki/Mineral> using a high-temperature furnace. As the
mineral cools, the crystal structure begins to form and diffusion of
isotopes </wiki/Isotope> 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] <#cite_note-Rollinson-1> 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 </wiki/Geochronology> and
thermochronology </wiki/Thermochronology> to date events and determine
rates of processes in the geologic past.


    Contents

[hide <javascript:toggleToc()>]

    * 1 Table of values <#Table_of_values>
          o 1.1 Potassium-argon method <#Potassium-argon_method>
          o 1.2 Uranium-lead method <#Uranium-lead_method>
    * 2 References <#References>


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    </w/index.php?title=Closure_temperature&action=edit&section=1>]
    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.


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      Potassium-argon method

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


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      Uranium-lead method

Mineral 	Closure temperature (°C)^[3] <#cite_note-2>
Titanite </wiki/Titanite> 	600-650
Rutile </wiki/Rutile> 	400-450
Apatite </wiki/Apatite> 	450-500
Zircon </wiki/Zircon> 	>1000
Monazite </wiki/Monazite> 	>1000


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    References

   1. *^ <#cite_ref-0>* /Earth: a Portrait of a Planet/ Glossary W.W.
      Norton & Company
      <http://www.wwnorton.com/college/geo/earth2/glossary/b.htm>
   2. ^ ^/*a*/ <#cite_ref-Rollinson_1-0> ^/*b*/
      <#cite_ref-Rollinson_1-1> Rollinson, 1993. /Using Geochemical
      Data: Evaluation, Presentation, Interpretation/ Longman Scientific
      & Technical. ISBN 9780582067011
      </wiki/Special:BookSources/9780582067011>
   3. *^ <#cite_ref-2>* Flowers, R.M. (2005). "Tempo of burial and
      exhumation within the deep roots of a magmatic arc, Fiordland, New
      Zealand". /Geology/ *33*: 17. doi
      </wiki/Digital_object_identifier>:10.1130/G21010.1