In-situ X-ray tomographic imaging under extreme conditions: a proof of concept study

Performing experiments under high pressure/temperature (P/T) conditions allows us to study how materials behave in Earth's deep interior, a key step in understanding processes which formed and constantly reform the Earth. Developing new techniques for studying earth materials under these conditions provides new insight into the workings of our planet. In X-ray microtomography (CT) a sample is rotated in an X-ray beam and transmission of radiation through the sample recorded by a detector. As with medical CAT scanning, data is then used to construct 3d models of the internal structure of solid objects, although CT can also give qualitative data on the distribution of components in complex systems. Most importantly, the technique is non-destructive and samples can be studied by CT many times to observe how they evolve with time. As such, CT is ideally suited to in-situ investigations, where changes in a sample under non-ambient conditions are observed. We will test the viability of developing a novel device to study the internal structure of materials at high P/T conditions, and also whilst they are deforming. This proof-of-concept study results from 2 recent advances: (1) development of the rotational Paris-Edinburgh cell (roPEC), a device designed to allow in-situ investigations of samples held at high P/T whilst they are deforming, and (2) development of a state-of-the-art CT instrument in the School of GeoSciences, University of Edinburgh, which was specially designed to facilitate in-situ CT studies of geologically important materials. To date, only one instrument has been developed to perform detailed CT under extreme conditions. This device was developed at the APS synchrotron in the USA. Synchrotrons are intense radiation sources which produce high energy X-rays capable of penetrating much more deeply into materials than lab-based sources, and are well suited for in-situ investigations. However, obtaining beamtime at synchrotrons is very competitive; in-situ high P/T CT investigations typically take several days for one experiment which prohibits detailed investigations, and the full potential of in-situ high P/T CT has yet to be realised. We hope to develop a new device (rotating tomography Paris-Edinburgh Cell, rotoPEC) which has the key advantage that it can used for in-situ CT using both synchrotron radiation and lower intensity lab sources. This device will be based on the roPEC, in which samples are pressurised between 2 carbide anvils, heated using an internal furnace, and deformed by rotating one of the anvils, but modified to allow full rotation of the entire sample in an X-ray beam, as required in CT. The roPEC was designed for in-situ studies, and allows X-ray beams to reach the sample with minimal, unwanted absorption. It is also small enough to be transported and installed at synchrotron sources or on other lab equipment, including the CT instrument at Edinburgh. However, before constructing a rotoPEC we need to conduct a feasibility study. Specifically we will: (1) test the potential and limitations of a rotoPEC (how much detail can we observe in samples under extreme conditions...are there limitations in the types of material we can study?). As well as testing the potential of a rotoPEC this information is also required in its future design; (2) test a new type of anvil which is X-ray transparent and would increase the volume of sample which could be 'seen' during CT -the performance of transparent anvils during deformation is critical, but remains untested; (3) further develop sample assemblies used in the roPEC to minimise unwanted absorption and increase sample resolution. Whilst conducting this work we will also study the structure of melt in 2 geologically important systems: Fe-rich melt in peridotite (did deformation help Earth to form an Fe-rich core during early stages of planet formation?) and basaltic melt in olivine (how is magma transported beneath mid-oceanic ridges?).