THERMAL HISTORY: A new software to interpret diffusive zoning profiles in garnet

Mineral grains can record the cooling history of metamorphic terrains by preserving characteristic chemical zoning profiles caused by diffusion. A range of analytical and numerical models have been used to describe the relationship between the cooling rate and the shape of chemical zoning profiles. Most of these models are characterized by a deficit of usability to external users. This problem is overcome by the code THERMAL HISTORY presented here. The code is platform independent and runs without compilation or a hard disk install. Model results are stored in a basic database and displayed graphically. The code is controlled by an intuitive graphical user interface and uses a very fast diffusion algorithm. THERMAL HISTORY can be used to model zoning profiles as a function of a series of cooling histories, and is written so that it is particularly applicable for the Fe-Mg exchange between garnet and biotite. The code takes into account mass balance so that the volumetric ratio of garnet and biotite can be considered explicitly and it provides a facility to calculate sections through the grains. As some of these facilities have never been published before, the impact of cooling histories, mass balance and section position is explained in some detail. As an application example, THERMAL HISTORY is used to demonstrate that highly non-linear cooling histories, small biotite-garnet ratios and the section effect may result in zoning profiles that are misleading if interpreted in terms of the cooling history.

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