PhD Thesis defense of Monica Spagnoli

Abstract

Planetary differentiation and the societal impact of volcanic activity are all linked to the transfer of magmas into the crust, which remains relatively unconstrained. In recent years, a paradigm shift has emerged: instead of being essentially liquid, magma bodies could be made up mainly of mush: a mixture rich in crystals (>>50%) forming a weak solid framework imprisoning pockets of liquid that are more or less connected (Bachmann & Bergantz, 2004, 2008). This model has recently been generalised to the entire continental crust (Trans-Crustal Magmatic System; Cashman et al., 2017).
Geological evidence suggests that ubiquitous differentiation by extraction of silicate liquids from mush appears to be controlled by the physics of chains of forces in the crystal framework. The aim of this thesis is to constrain the mechanical behaviour (rheology, structuring) of crystal-rich magmas using an experimental approach to deformation at HP-HT. The chains of forces and their influence on the rheology of mush and on the initiation of deformation localisation have been characterised. Particular emphasis is placed on the effect of this deformation localisation on the segregation capacities of liquids and late fluids.
The innovative strategy of this thesis is to use an experimental approach coupled with a study of natural systems. The experiments are carried out in a Paterson press at ISTO (3 Kbars). The starting products are plagioclase mush resulting from the crystallisation of hydrated haplotonitic synthetic liquids (Picard et al., 2013). Post-mortem analyses of the crystal frameworks (crystal framework topology, CPO, SPO, CSD, crystal breakage) are carried out in 2D using SEM, in 3D using EBSD. The natural samples consisted of phonolitic lavas from a neck and a dyke from the Eastern Velay volcanic province (French Massif Central). These samples were also prepared and analysed in 2D and 3D, using the same techniques as the synthetic samples.
The results of the torsion experiments carried out with the Paterson press show localised deformation structures that evolve as a function of the intensity of the chains of forces; this intensity itself being highly dependent on the anisotropy of the crystals. The segregation of residual liquids appears for large finite deformations and particularly in mush composed of crystals with low anisotropy. Similar localised deformation structures, in which late liquids are segregated, were also found in natural phonolite samples. These results show that the extraction window for residual liquids in mush subjected to shear deformation extends to very high crystal contents (>90%).