2021 | Coupling of experiments and numerical models to investigate early stages of de-hydration process and magma decompression

Aims: This project has two major themes that will be investigated by the same approach – ex-periments coupled with numerical models. The implementation of numerical modelling in the experimental work is still rare. However, based on our previous experience, the nu-merical models provide important insights not only for interpretations of the experimental results but also for improvements of the experimental set-ups (Cionoiu et al. 2019- joint publication Mainz-Heidelberg). The key point for this kind of research is, that we have to start with a simple chemical system with well-defined properties, in our case - serpentine dehydration. After investigating this simple system, we can proceed with more complex and more topically relevant rock systems for TeMaS. The aim is to establish a solid base for a future tandem (modelling-experiments) PhD project/projects for the SFB proposal. The first theme’s aim focuses more on the coupling of the models and the experimental results itself and on tuning it. The second theme is to investigate a particular effect of pressure drop, e.g. magma decompression.

Theme 1: Early stages of dehydration processes in subduction zones Dehydration of the subducting slab and migration of fluids generates magma below back-arcs. Particularly, serpentinites hydration and subsequent dehydration critically influence the generation and chemistry of arc-related magmas. In fact, antigorite is considered as the most important source of water and a major carrier of fluid-mobile elements in sub-duction zones, which influences the geochemical signature of arc magmatism (e.g. high abundances of B, Ba, Sr, B, As, Mo and Pb). Since such metamorphic reactions frequently occur during lithospheric deformation, understanding of such processes should involve a coupled approach – rock deformation, fluid flow and metamorphic reactions. This can be investigated with computer simulations. Such simulations can either provide important insights into the processes that can be verified by experiments or the simulations might lead to a new kind of experiments. Here, we want to focus in the initial stages of the dehydration of serpentinite and compare the experimental and numerical results. The modelling part will build on the ongoing re-search of Evangelos Moulas with Stefan Schmalholz (Schmalholz et al., 2020), that focuses on the development of hydro-chemo-mechanical models for dehydration processes in de-forming heterogeneous rocks. The idea is to start with a simple chemical system and see if the numerical models results, e.g. channelized vein network development, are compat-ible with experimental results. Even though the project will focus on simple systems, it will provide the basis for reactive transport models that include supercritical solutions and melts. The deformation experiments will be done with Griggs apparatus based in Heidel-berg. The output of this project will serve as a basis for more advance chemical systems that can then lead to a better understanding of following topics, closely related to TeMaS: partial melting in subduction zones, localization of deformation, seismicity associated with magma flow.

Theme 2: Insights into magma chamber processes and decompression The nature of magma ascent is not yet well understood. There are temporal and spatial variations among volcanic systems. Some can be characterized by rapid accelerations, whereas other it may consist of steady fluid-like flow. This has an impact, for example, on crystallization kinetics and microstructures, porosity, volatile exsolution, bubble nuclea-tion and fragmentation. Some of the processes are difficult to do experimentally or might be risky. One way to take the step forward is to use numerical models to simulate the effect and improve the experimental design. However, for tackling such processes the numerical models must consider the effects of compressibility and reaction in rocks. Focusing on simple, yet highly compressible systems, such as the serpentine dehydration, will provide the basis for future research of this phenomena. This will provide insights to mechanical changes in rocks during the devolatilization processes related to fast pressure drops, which is closely related to the safety issues (volcanic eruptions). This theme is closely related to the new Volkswagen funded project of the Heidelberg group that is focused on modification of the existing piston cylinder to allow for fast com-pression and decompression. This modification will take some time to implement. How-ever, we can do preliminary experimental tests in a cold-seal (externally heated) pressure vessel in combination with numerical predictions to bring insights into magma chamber processes and decompression.

Cooperation: This project involves experimental and analytical labs at Heidelberg and modelling capa-bilities of Mainz. The Frankfurt team has a long term experience with processes serpen-tinites and mélange rocks and will participate in discussions. This project will be a part of a bigger initiative on this experimental-modelling topic, that involves representatives from all three participating universities. These representatives will meet in September in two days’ workshop organized by TeMaS to discuss more concrete plans related to this research direction.

Work plan: The experiments will be performed between August-October 2021. Analytical work (EPMA and EBSD) will be done simultaneously, i.e. maximum till the end of November. The mod-elling will be performed on parallel with the experimental and analytical investigations.

Expected outcome: This is a pilot project. With the suggested experiments and models, we want to create a basis for the future coupled projects of that kind for the SFB proposal. This project will be in a mutual connection to the experimental-modeling workshop within TeMaS organized in September. First experimental results from this project can be presented there. A joint publication is highly possible.