MODELING THE EFFECT OF ARBITRARY P-T-T HISTORIES ON ARGON DIFFUSION IN MINERALS USING THE MACARGON PROGRAM FOR THE APPLE-MACINTOSH

Argon diffusion in mineral grains has been numerically modelled using P-T-t histories that may be relevant to multiply metamorphosed orogeni c terranes and for rocks that have resided at high ambient temperature s in the Earth's crust for long durations, The MacArgon program(1) gen erates argon concentration profiles in minerals assuming argon loss oc curs via volume diffusion, It can be run on an Apple Macintosh compute r, with arbitrary P-T-t histories used as input. Finite-difference equ ations are used in the calculation of Ar-40 concentration profiles ac ross individual diffusion domains. The associated MacSpectrometer gene rates model spectra after a P-T-t history has been specified. The form of model Ar-40/Ar-39 apparent age spectra suggests that considerable caution needs to be exercised in the use of the closure temperature Ar -40/Ar-39 apparent age spectra, concept and in the interpretation of t he significance of plateaux observed in many particularly in cases inv olving metamorphic rocks, where complex P-T-t histories might apply. A lthough modelled spectra cannot be directly compared to experimentally determined Ar-40/Ar-39 age spectra, especially when hydrous phases ar e involved or in cases where loss of argon has not occurred via volume diffusion, they do provide insight into theoretically expected age sp ectra for samples that have experienced complex P-T-t histories.