In Situ Monitoring of Microstructures during Subsequent Phase Transitions in the Olivine System up to 30 GPa and 1100 K Using 3D-XRD Single-Grain Analysis. Effects of Grain Size Evolution on the Stagnation of Slab.

Abstract:

Microstructures significantly influence the rheological properties of rocks and are important to understand geodynamical processes such as the descent of slabs. For example, grain size reductions during phase transitions in the dominant upper mantle constituent, olivine, to its high-pressure polymorphs wadsleyite and ringwoodite have been related to the stagnation of slabs in the transition zone. Detailed studies of the microstructure development, underlying transformation mechanism and transformation kinetics are needed in order to assess the effects of these phase transformations on the rheological properties of the slab material and to build reliable models of mantle flow and slab subduction behaviors. So far, the experimental studies dedicated to this theme were mostly based on ex situ techniques (e.g. electron microscopy of quenched products).

In this contribution, we present detailed results form in situ single-grain analysis on the evolution of microstructures during the succession of phase transitions in Mg₂SiO₄*H₂O. We used a new approach based on in situ three dimensional-X-ray diffraction (3D-XRD) experiments performed up to 30 GPa and 1100 K using a resistively heated diamond anvil cell at the beam lines ID11 and ID27 of the ESRF. The individual orientations, crystallographic parameters and growth rates of numerous grains inside a polycrystalline sample have been monitored in situ at the high pressure and temperature conditions and while the material was transforming. These parameters have been used to infer grain size distributions, textural relations between parent and newly formed phase and their evolution with ongoing transformation, as well as changing PT conditions and transformation kinetics. This original dataset allows drawing a refined picture of phase transitions in the most abundant minerals of the Earth’s upper mantle, shed new light on the origin of seismic anomalies at transition zone depth and provide new grounds for complex simulations of geodynamical processes in the Earth’s mantle.