Innovative numerical petrological methods for definition of metamorphic timescale events of southern European Variscan relicts via thermodynamic and diffusion modelling of zoned garnets

Abstract

Innovative numerical petrology methods have been developed using several computer programming languages, to investigate chemical-physical properties of metamorphic rocks at the microscale. These methods can help users to analyse the final aspect of the metamorphic rocks, which derives from the counterbalancing factors controlled by deformation vs. recovery processes, through a better quantification of the rock fabric parameters (e.g., grain and mineral size distribution) as well as of the rock volumes and the specific compositions that take part in the reactions during each metamorphic evolutionary stage. In this perspective, a grain boundary detection tool (i.e., Grain Size Detection - GSD) was created to draw grain boundary and create polygon features in a Geographic Information System (GIS) platform using thin section optical scans as input images. Such a tool allows users to obtain several pieces of information from the investigated samples such as grain surfaces and sizes displayed as derivative maps. These maps have been then integrated with the mineralogical distribution map of the entire thin section classified from the micro X-ray maps. This step has been made to enhance the grain size distribution analysis by associating a mineral label to each polygon feature, by developing a further tool called Min-GSD (i.e., Mineral-Grain Size Distribution). The image analysis of rocks at the microscale was further improved by introducing a new multilinear regression technique within a previous image analysis software (i.e., X-ray Map Analyser - XRMA), with the aim to calibrate X-ray maps per each classified mineral of the selected thin section microdomain. This enhancement (called Quantitative X-ray Map Analyser - Q-XRMA) allowed to compute: (a) the elemental concentration within a single phase expressed in a.p.f.u; (b) maps of the end member fractions defining the potential zoning patterns of solid solution mineral phases. Moreover, the classification through this new method of one or several microdomains per thin section, able to describe the potential sequence of recognized metamorphic equilibria, has been here used to a better definition of the effective bulk rock chemistries at the base of a more robust thermodynamic modelling, providing more reliable thermobaric constraints. These thermobaric constraints were here converted for the first time into Pressure-Temperature (PT) maps by the development of an add-on (i.e., Diffusion Coefficient Map Creator - DCMC) of the previous tool (Q-XRMA), for creating maps of compositionally-dependent diffusion coefficients, by integrating diffusion data from the literature. As a result, an articulated Local Information System (LIS) for the investigated mineral, involving data on composition, grain size, modal amounts and kinetic rates, is created and potentially useful for detailed investigations as, for instance, the determination of the timescales of metamorphic events. All of these methods mentioned above can be considered part of the Petromatics discipline, here for the first time defined as the science which integrates new computers technologies with different techno-scientific sectors related to the detection and handling of spatial minerochemical data characterising rocks at the microscale. Furthermore, the quantification of the rock parameters at the microscale laid the groundwork for the development of an innovative numerical petrological workflow here called Metamorphic Petrology Information System (MetPetIS). The latter is a new LIS able to store, manage and elaborate multidisciplinary and multiscale data collection from metamorphic basement rocks within a unique cyber-infrastructure.