An inverse modeling approach to obtain P-T conditions of metamorphic stages involving garnet growth and resorption

作者：Lanari Pierre^*; Giuntoli Francesco; Loury Chloe; Burn Marco; Engi Martin

来源：European Journal of Mineralogy, 2017, 29(2): 181-199.

DOI：10.1127/ejm/2017/0029-2597

• thermodynamic modeling
• garnet
• resorption
• X-ray mapping
• XMapTOOLS

This contribution presents an approach and a computer program (GRTMOD) for numerical simulation of garnet evolution based on compositions of successive growth zones in natural samples. For each garnet growth stage, a new local effective bulk composition is optimized, allowing for resorption and/or fractionation of previously crystallized garnet. The successive minimizations are performed using the Nelder-Mead algorithm; a heuristic search method. An automated strategy including two optimization stages and one refinement stage is described and tested. This program is used to calculate pressure-temperature (P-T) conditions of crystal growth as archived in garnet from the Sesia Zone (Western Alps). The compositional variability of successive growth zones is characterized using standardized X-ray maps and the program XMapTools. The model suggests that Permian garnet cores crystallized under granulite-facies conditions at T> 800 degrees C and P = 6 kbar. During Alpine times, a first garnet rim grew at eclogite-facies conditions (650 degrees C, 16 kbar) at the expense of the garnet core. A second rim was added at lower P (similar to 11 kbar) and 630 degrees C. In total, garnet resorption is modeled to amount to similar to 9 vol% during the Alpine evolution; this value is supported by our observations in X-ray compositional maps.

• 出版日期2017-4