Experimental investigation of the garnet formation in the CMNAS system at high pressure under deviatoric stress

Abstract:

The formation of eclogite in basaltic rocks during subduction of crustal material is a crucial process in the geodynamic cycle. This transition and also the formation of garnetite at greater depth (>10 GPa) have been investigated closely in experiments under hydrostatic conditions. However studies of naturally occurring eclogites indicate concurrent plastic deformation and deviatoric stresses during the phase transition may influence the microstructures as well as the rheology of the forming eclogite. In the present project we aim to investigate the influence of plastic deformation on the gabbro to eclogite transition in a simplified basaltic material as well as the transition of eclogite to garnetite at higher pressures.

Starting materials with a simplified CMNAS composition were synthesized from glass in piston cylinder experiments at 0.5/3 GPa and 950-1000°/1200°C respectively, yielding fine grained (~10-20μm) mixtures of orthopyroxene, clinopyroxene and plagioclase ('gabbro') and omphacite, garnet and quartz ('eclogite'). Transformation experiments were then performed in a pressure range from 3 to 12 GPa at 1200°C in a Dia-type mulitanvil press with six independently movable rams. Pure shear deformation of up to 30% was imposed in the deformation runs with strain rates ranging from 5x10^-5 to 5x10^-6 sec^-1. For comparison static transformation experiments were performed with the same duration (100 to 1000 min). The reaction of the crystalline starting materials was generally sluggish and relatively large overstepping of phase boundaries was needed to induce notable reaction progress as determined by SEM-EDS, -EBSD and EPMA. Preliminary results suggest that concurrent deformation enhanced reaction progress in comparison to static experiments for both investigated transitions, however more experiments are needed to quantify this effect. Deformation was accommodated by intracrystalline plasticity of omphacite as well as diffusion assisted grain (phase) boundary sliding. In the samples deformed at high pressures also garnets developed subgrain structures and dispersions of the crystal lattice orientation consistent with the external deformation geometry indicating intracrystalline plasticity during growth.