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PROgrade metamorphism MOdeling: a new petrochronological and compuTING framework

Periodic Reporting for period 2 - PROMOTING (PROgrade metamorphism MOdeling: a new petrochronological and compuTING framework)

Reporting period: 2021-12-01 to 2023-05-31

The main goal of PROMOTING is to quantify fluxes of aqueous fluids and silicate melts released during prograde metamorphism in the Earth interior and to model fluid pathways. Understanding the behavior of these liquids is critical as they play an important role for earthquake generation, arc magmatism, the growth of continental crust and for global geochemical cycles. However, the task is challenging as direct observation in metamorphic environment is impossible and (indirect) geophysical observations do not have the required spatial resolution. In addition, the presence of fluid in the grain boundary network of a metamorphic rock is important as it acts as catalyst reducing the kinetic barriers for mineral dissolution and growth. The presence of fluid remains often invisible when the rock has been dried and exhumed to the surface. The only pieces of evidence are preserved in isotope compositions of minerals and textures of dissolution-precipitation.

The objectives of PROMOTING are to develop new analytical tools, modeling frameworks and computer programs for tracking and simulating fluid-rock interaction processes and fluid flows in the lithosphere. The project is focused on the quantification of (1) aqueous fluid fluxes for example during oceanic subduction using natural examples from the Alps and the Cyclades and (2) silicate melt fluxes for example during partial melting of continental crust using the El Oro massif in Ecuador as a natural case study. Estimates from the natural record are critical to quantify key model parameters such as permeability at conditions for which they cannot be estimated experimentally. The investigation of the collected samples requires advanced surface analysis and 3D characterization of grain shapes and textures. To achieve this objective, several data analysis and reduction techniques are being developed in PROMOTING. In addition, new computers models for simulating prograde metamorphism are being developed: (1) a multi-rock petro-geochemical model containing a rock-failure model for simulating fluid extraction and (2) A multi-phase reactive flow model for simulating melt production and migration including chemical advection.
The work performed since the beginning of the project PROMOTING along the three axes: “new analytical developments”, “application to the study of natural rocks”, “development of the new modeling programs” is reported bellow.

a. Analytical developments:
(a1) An advanced analytical technique for quantitative compositional mapping by LA-ICPMS has been developed and is implemented in the open-source software XMapTools which is already available for public use (https://github.com/xmaptools). New tools for classification of complex datasets were also developed and implemented.
(a2) A new machine learning strategy and a program garNET were developed for the classification of tomography data and the automated identification of grain shapes in 3D. This technique was used to localize in key samples where spatially garnet was dissolved by interacting with an aqueous fluid at high-pressure conditions. This new technique has a significant potential to easily distinguish in any sample If the metamorphic fluid responsible for garnet dissolution was flowing along channels or pervasively through the grain boundary network.

b. Applications: Several case studies were selected to study (b1) aqueous fluid flows (b2) and the consequences of fluid-rock interaction processes on the rock record, and (b3) melt flows.
(b1) Several localities in the Western Alsp (Lago di Cignana; Zermatt; Gran Paradiso) and in the Cyclades (Syros, Sifnos) were selected for further investigation along the topic of “fluid flows in subduction zones”. Key samples and geological data were collected during several field campaigns in the various localities. The samples were investigated in the lab and are prepared for further isotope analyses. A proof-of concept study was published (Bovay et al. 2021) demonstrating that in-situ oxygen isotope analyses can be used to quantify critical parameters of fluid flow during high-pressure metamorphism.
(b2) The investigation of the possible consequences of element redistribution by aqueous fluid during prograde metamorphism was investigated using a database of bulk rock compositions (Forshaw & Pattison, 2022).
(b3) key samples and geological data were collected in the El Oro metamorphic complex in Southern Ecuador during a field campaign in 2022. The samples are ready for chemical analyses and geochronology.

c. Modeling programs:
(c1) A thermodynamic model for aqueous speciation calculation is in development to be implemented in a Gibbs energy minimizer. This program will be the first to perform accurate speciation calculations in metamorphic environments.
(c2) A multi-rock petro-geochemical model containing a rock-failure model for simulating fluid extraction has been developed. We are currently working on a new approach of energy calculation for systems with metastable relics to better consider the possible kinetic barriers for mineral re-equilibration in petrological models.
(c3) A multi-phase flow model has been developed and is now tested with several advection algorithms to ensure an optimal quantification of element transport by a fluid phase such as melt.
The newly developed methodologies have already proved to be critical for producing preliminary results beyond the state of the art. The following results are expected until the end of the project:
- The determination of fluid fluxes and permeability values for various rock types in different subduction systems (e.g. cold, warm, recent, old subduction environments) using both modeling and estimates from natural rocks.
- The simulation of elements transport by aqueous fluid during prograde metamorphism to quantify open-system behavior and possible consequences for element cycling.
- The quantification of melt production and melt fluxes during partial melting of the continental crust using new modeling techniques (multi-phase reactive flow) and estimates from natural rocks.
- A new method and case study for constraining ultra-fast mineral growth caused by fluid flow in dry rocks during regional metamorphism.