What is GRTMOD and what does it do?

GRTMOD is a program for modeling garnet growth and resorption stages in metamorphic rocks. It can be used for thermobarometry based on reactive bulk compositions. This version of GRTMOD requires Theriak-Domino to run Gibbs energy minimisation.


GRTMOD was developed and is maintained by Pierre Lanari (University of Bern).

Lanari, P., Giuntoli, F., Loury, C., Burn, M., Engi, M., (2017). An inverse modeling approach to obtain P-T conditions of metamorphic stages involving garnet growth and resorption. European Journal of Mineralogy,  29, 181-199. pdf

Last information

May 2021 – GRTMOD 2 is now available via Github

Nov. 2018 – If you do not have X-ray maps of garnet, you can use my converter file that generates pseudo X-ray intensities from garnet spot analyses. NOTE: The weighting procedure can significantly affect the results if the weights are not correctly defined (e.g. below Fig. 11 from Lanari & Duesterhoeft 2019).

Example of forward thermodynamic analysis of the metapelite 12TM16 from the Flinton Group of the Canadian Grenville Province (modified from McCarron et al., 2014). (b) P–T diagram showing the range of garnet isopleths (Xalm, Xprp, Xgrs, Xsps) for garnet core compositions. The star shows the best match empirically derived by McCarron et al. (2014); the circles mark the two solutions obtained by GRTMOD. (c) Unweighted and (d) weighted objective functions of GRTMOD, corresponding to C0 and L0 of Lanari et al. (2017). Mineral abbreviations are from Whitney & Evans (2010). Interpretations: S1 has the lowest residual, and its P–T values are close to the unweighted conditions (see equation (13) of Lanari et al. (2017)) and the solution proposed by McCarron et al. (2014), whereas S2 has a slightly higher residual (still within an acceptable range), and lies in the staurolite stability field. Although the intersection of isopleths would seem tighter at S1 when judged visually, the two solutions are equivalent if the uncertainties are taken into account. In this case, the P–T differences are rather small (􏰃0􏰀025 GPa and 􏰃15􏰂 C) but S2 is of interest because it suggests that garnet may have grown in equilibrium with staurolite. This example indicates that the P–T conditions determined by isopleth thermobarometry may not be optimal if all the compositional variables (and the modes) are not used, or if a weighting scheme taking into account the analyses’ uncertainty is not applied.