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

Abstract

Microstructures govern the mechanical properties of materials and change dramatically during phase transformations. A 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 of transformation microstructures at the grain scale have been mostly based on ex situ observations of quenched products, which are difficult to correlate with bulk sample properties and transformation kinetics. Here, it is shown 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 the extraction of the crystallographic parameters and orientations of several hundreds of grains inside a transforming sample. Important bulk information on grain size distributions and orientation relations between the parent and the 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 versus incoherent growth), growth rates and orientation‐dependent growth of individual grains. The methodology is demonstrated on the α–γ phase transitions in hydrous Mg2SiO4·H2O 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 in their subduction behaviour.

Evolution of grain sizes and orientations during phase transitions in hydrous Mg2SiO4

“…Microstructure evolution with time in (a, c, and e) Experiment #3 [ Rosa et al , ] and (b, d, f, and g) Experiment #4 (this study). (Figures a and b) Volume fraction of phases ( α , β , and γ ( * ) ).…”

“…Table summarizes the P / T conditions and observations of all experiments. Experiments #1–#3 are from Rosa et al , . Experiments #4–#6 are new.…”

“…The principle of this analysis has been described previously [i.e., Lauridsen et al , ; Schmidt et al , ; Poulsen , ; Vaughan et al , ]. In this study, we follow the procedure described in Rosa et al [], including noise removal from individual ω oscillation images and the extraction, filtering, and assignment of individual diffraction spots to diffraction vectors using Peaksearch [ Sørensen et al , ] and ImageD11 [ Wright, ].…”

“…After crushing, the starting material comprised larger grains of 1–3 µm and a fine‐grained matrix in the submicron domain, while a small amount of tungsten was present in the sample due to the sample preparation procedure. For more details on the sample preparation see Rosa et al []. The water content was estimated from the ambient conditions unit cell volume (293.38 A 3 ) of forsterite which corresponds to 4500 ppm of H 2 O according to the calibration of Smyth et al [].…”

Unit cell determination of coexisting post-perovskite and H-phase in (Mg,Fe)SiO3 using multigrain XRD: compositional variation across a laser heating spot at 119 GPa

“…In recent years, the multigrain method (Sørensen et al 2012) has been successfully applied to study coarse-grained polycrystalline samples obtained under ultra-high P-T conditions in a DAC (Nisr et al 2012;Zhang et al 2013;Zhang et al 2014;Rosa et al 2015;Zhang et al 2016). These studies have demonstrated that integration of the multigrain method with synchrotron XRD is suitable for treating each sub-micron-sized crystallite as a single crystal in a coarse-grained polycrystalline material in a DAC.…”