Both the Earth's crust and atmosphere are the result of magmatic activity, expressed as volcanoes and great bodies of granite. Describing their dynamics is essential to understanding planetary growth, differentiation and the social impacts from volcanic activity. However, there is currently a paradigm shift in progress: instead of mostly liquid, magma bodies are now inferred to be bodies dominated by crystal-rich mush, a mix of crystals, and silicate liquid, that can be as large as 500,000 km3 (Ward et al., 2014; Delph et al., 2017). Furthermore, this model of a mush zone with limited pockets of mobile magmas (Bachmann and Bergantz, 2004, 2008) has been generalized as a possible continuum over the entire crust from the deep ductile hot crust to the shallower chambers opened to volcanic systems. This is conceptualized as the so-called Trans-Crustal Magmatic System (TCMS, Cashman et al., 2017; Fig. 1).
An obstacle to progress is that the physics of magma mush is very poorly understood, invoking ill-constrained processes such as separation of melts and fluids from low porosity source regions, mush destabilization, and segregation of magmatic fluids from underplated magmas (Christopher et al., 2015). It is nearly impossible to obtain examples of a 'living mush’, or to make detailed real-time measurements in the field. The object of this proposal, MECAMUSH, is to describe and rationalize the behavior of magma mush by a combination of novel, linked experimental and numerical modeling techniques.
The experiments will be conducted at ISTO by using the Paterson and Griggs apparatus that are pressure vessels capable of measuring the rheology of magmas up to 12 Kbars in order to explore the entire TCMS. The starting products are synthetic hydrous haplotonalitic mushes of plagioclase crystals (Picard et al., 2013). The applicant will measure the rheology in situ and analyses after experiments the textures of the deformed mush : mineral shapes fabrics, strain localisation, crystal damage by using SEM, 3D tomographic imaging (Tomograph ISTO, 1 μm and synchroton TOMCAT, Paul Sherrer Institut, Villingen, switzerland), EBSD (ISTO) and Raman spectrometry (SEM TESCAN et 514, 632, and 785 nm lasers, ISTO).
The experimental results will be coupled to numerical simulations done at ISTerre and Seattle (force chains development) and at Montpellier (LMGC, shear strain localisation and crystal brakeage). This thesis is part of the ANR MECAMUCH project (ANR2019, PI L. Arbaret).