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Journal of Applied Crystallography, 48, 1346-1354, (2015) [doi: 10.1107/S1600576715012765]

In situ monitoring of phase transformation microstructures at Earth's mantle pressure and temperature using multi-grain XRD

A. D. Rosa, N. Hilairet , S. Ghosh, G. Garbarino, J. Jacobs, J.-P. Perrillat, G. Vaughan and S. Merkel

Microstructures govern the mechanical properties of materials and dramatically change during phase transformations. The detailed understanding of microstructures at different stages of a transformation is important for the design of new materials and for constraining geophysical processes. However, experimental studies on transformation microstructures at the grain scale have been mostly based on ex situ observations of quenched products, which are difficult to correlate to the entire sample properties and transformation kinetics. Here, we show how multi-grain crystallography on polycrystalline samples, combined with a resistively heated diamond anvil cell, can be applied to investigate the microstructural properties of a material undergoing a phase transition in situ at high pressure and high temperature. This approach allows extracting crystallographic parameters and orientations of several hundreds of grains inside a transforming sample. Important bulk information on grain size distributions and orientation relations between parent and newly formed phase at the different stages of the transformation can be monitored. These data can be used to elucidate transformation mechanisms (e.g., coherent vs. incoherent growth), growth rates, and orientation dependent growth of individual grains. The methodology is demonstrated on the α-γ phase transitions in hydrous Mg2SiO4 up to 22 GPa and 940 K. This transformation most likely occurs in the most abundant mineral of the Earth’s upper mantle (Mg0.8Fe0.2SiO4) in deep cold subducted slabs and plays an important role for their subduction behaviour.

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© Sébastien Merkel, Université de Lille, France

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