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Melt segregation and strain localization in oceanic mafic mushes: an experimental approach
Lola-Lou Baudry  1@  , Laurent Arbaret  1@  , Rémi Champallier  1@  , Jacques Précigout  1@  , Marine Boulanger  2@  , Aneta Slodczyk  1@  
1 : Institut des Sciences de la Terre d'Orléans - UMR7327  (ISTO)
Université d'Orléans, CNRS, BRGM
2 : Laboratoire Magmas et Volcans  (LMV)
Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, Clermont-Ferrand, France

Intensive research over the last decades led to a reappraisal of magmatic systems beneath oceanic ridges. The classical vision of a melt-dominated magma chamber has been shaken up in favour of crystal-dominated bodies and only minor localized melt-rich lenses. This led to a change in apprehending melt mobilisation processes at play to feed the extrusive magma flux at mid-oceanic ridges. One explored mechanism is deformation, and how it can lead to the formation of melt-rich and melt-poor areas.

This study aims to further constrain the behaviour of a partially molten mafic system (mush). We investigate the parameters and characteristics of deformation-induced melt segregation, and we explore the topology of the liquid and the stress recording through associated microstructures.

The starting product is an oceanic gabbro sampled from a “diabase” dyke cored at Atlantis Bank (Southwest Indian Ridge). Partial melting of the gabbro was performed in a Pressure Vessel (IHPV) at 200 MPa to characterize the melt fraction and phases present prior to deformation. We explored temperatures ranging from 1075°C to 1125°C resulting in partial melt ratios comprised between 8 and 20%. Deformation (torsion and compression) experiments on partially molten gabbros were conducted in a gas-medium apparatus (Paterson press), following the previous results. EBSD used alongside SEM helped investigate deformation-related changes in microstructures, including melt distribution, shape (SPO) and crystallographic (CPO) preferred orientation, as well as intragrain misorientations, and crystal defects.

Results show that the initially homogeneously distributed deformation becomes rapidly overprinted by brittle strain localisation initiating along with melt-rich pockets. With increasing strain, these tensional gashes evolve as transtensional shear bands. High finite strain experiments are in progress to decipher the potential control of strain localisation on melt segregation. This constitutes a significant lead towards the understanding of the influence of deformation on MOR mafic systems and melt extraction processes.


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