Understanding how microstructures develop in Earth's materials is critical to constrain mantle processes from geophysical observations, such as seismic anisotropy. In the past, however, high pressure experiments on microtructures could not truly be performed at both mantle pressures and temperatures, in part due to the sample microstructure which was not fully appropriate for in-situ sample characterization techniques based on powder X-ray diffraction.

In recent years, multigrain X-ray diffraction emerged as a new technique for experiments in the diamond anvil cell, allowing tracking of the orientation and size of hundreds of individual microstructural elements, in-situ, at synchrotron beamlines. The method is compatible with other diamond anvil cell procedures, such as double-sided laser-heating and the use of pressure medium to limit the sample's deviatoric stresses. Multigrain X-ray diffraction combined with double-sided laser heating was at the core of the joint ANR-DFG TIMEleSS project, in which we investigated phase transformations and deformation of deep Earth materials.

In this presentation, we will present the main concepts and principles of multigrain X-ray diffraction in the laser-heated diamond anvil cell, how it can be used at the pressures and temperatures of a mantle geotherm, and results relevant for a pyrolitic composition in the Earth's lower mantle, between 24 and 110 GPa. We will also discuss how the technique offers a range of new opportunities for high pressure research, well beyond the field of geosciences.