CU-Boulder GEOL 5700 - Closure Temperature in Cooling Geochronological and Petrological System (16 pages)

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Closure Temperature in Cooling Geochronological and Petrological System



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Closure Temperature in Cooling Geochronological and Petrological System

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Lecture Notes


Pages:
16
School:
University of Colorado at Boulder
Course:
Geol 5700 - Geological Topics Seminar

Unformatted text preview:

Contr Mineral and Petrol 40 259 274 1973 9 by Springer Verlag 1973 Closure Temperature in Cooling Geochronological and Petrological Systems M a r t i n I D o d s o n Department of Earth Sciences The University Leeds Received March 5 1973 Abstract Closure temperature Tc of a geochronological system may be defined as its temperature at the time corresponding to its apparent age For thermally activated diffusion D Doe E RT it is given by T c R E In A Do a2 i in which R is the gas constant E the activation energy z the time constant with which the diffusion coefficient D diminishes a is a characteristic diffusion size and A a numerical constant depending on geometry and decay constant of parent The time constant z is related to cooling rate by R E d T 1 dt R T Ed T dt ii Eq i is exact only if T 1 increases linearly with time but in practice a good approximation is obtained by relating T to the slope of the cooling curve at T c If the decay of parent is very slow compared with the cooling time constant A is 55 27 or 8 7 for volume diffusion from a sphere cylinder or plane sheet respectively Where the decay of parent is relatively fast A takes lower values Closure temperatures of 280 300 C are calculated for R b S r dates on Alpine biotites from measured diffusion parameters assuming a grain size of the order 0 5 mm The temperature recorded by a frozen chemical system in which a solid phase in contact with a large reservoir has cooled slowly from high temperatures is formally identical with geochronological closure temperature 1 Definition of Closure T e m p e r a t u r e W h e n t h e a g e of a r o c k or m i n e r a l is c a l c u l a t e d f r o m its a c c u m u l a t e d p r o d u c t s of r a d i o a c t i v e decay w h e t h e r those p r o d u c t s be c r y s t a l s t r u c t u r a l changes caused b y fission fragments or radiogenic isotopes such as sTSr or a t h e result i d e a l l y r e p r e s e n t s a p o i n t in t i m e a t which a c o m p l e t e l y



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