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Question

Most radioisotopic systems are associated with a closure temperature that is the temperature below which daughter product does not escape from the mineral. Suppose an eclogite crystallized at temperature of 750°C and this rock was dated by the Ar-Ar technique using muscovite, which has a closure temperature for Ar diffusion of about 400°C, It is most likely that the age obtained will be YOUNGER or OLDER than the actual crystallization age? 

(IGNORE the possibility of the possibility of the phenomenon known as excess Argon)

Answer 1

Hi Sam!

The obtained age from the muscovite dating will be YOUNGER than the actual crystallization age!

We know that the eclogite began forming at a temperature of 750degrees. However, muscovite (which we are dating) doesn't begin to form until the system reaches 400degrees. No muscovite can form if the temperature is above 400degrees, so by dating the muscovite, we are not accounting for the time it took the rock to go from 750 to 400 degrees, which can take tens of thousands of years in some instances.