Diffarg: A program for simulating argon diffusion profiles in minerals

Abstract

An understanding of argon transport is essential for the interpretation of K Ar , 40Ar 39Ar stepped heating and laser probe apparent ages of metamorphic rocks. The DIFFARG program, written in the Matlab 4.1 interpreted language, uses a finite-difference algorithm to simulate argon diffusion profiles and bulk ages of individual mineral grains for any thermal history and grain boundary argon history. Volume diffusion through the mineral lattice is assumed to be the dominant process, so the Ar diffusion coefficient is spatially homogeneous (though temperature dependent). The 40Ar concentration at the edge of the grain can be varied with time to model the diffusion of “excess” Ar from the surroundings. Plane sheet, cylindrical, or spherical grain shapes may be selected. The temperature history may be a linear decrease with time, to simulate slow cooling; a temperature pulse, to simulate a reheating event; or any temperature history composed of linear T- t segments. The grain boundary 40Ar history is defined indirectly via the apparent age at the grain edge, itself dependent on time. Nonlinear temperature and grain boundary argon histories can be defined by the user if required. Numerical models are compared to analytical predictions which concern simple scenarios and real data. The program is useful particularly for interpreting 40Ar profiles obtained by laser microprobe 40Ar 39Ar dating.