Modelling the effect of arbitrary P-T-t histories on argon diffusion in minerals using the MacArgon program for the Apple Macintosh

Abstract

Argon diffusion in mineral grains has been numerically modelled using P-T-t histories that may be relevant to multiply metamorphosed orogenic terranes and for rocks that have resided at high ambient temperatures in the Earth's crust for long durations. The MacArgon program 1 1 MacArgon can be obtained by e-mail from MacArgon artemis.earth.monash.edu.au with enquiries to gordonartemis.earth.monash.edu.au generates argon concentration profiles in minerals assuming argon loss occurs via volume diffusion. It can be run on an Apple Macintosh computer, with arbitrary P-T-t histories used as input. Finite-difference equations are used in the calculation of 40Ar∗ concentration profiles across individual diffusion domains. The associated MacSpectrometer generates model spectra after a P-T-t history has been specified. The form of model 40Ar 39Ar apparent age spectra suggests that considerable caution needs to be exercised in the use of the closure temperature concept and in the interpretation of the significance of plateaux observed in many 40Ar 39Ar apparent age spectra, particularly in cases involving metamorphic rocks, where complex P-T-t histories might apply. Although modelled spectra cannot be directly compared to experimentally determined 40Ar 39Ar age spectra, especially when hydrous phases are involved or in cases where loss of argon has not occurred via volume diffusion, they do provide insight into theoretically expected age spectra for samples that have experienced complex P-T-t histories.