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(STFC DTP) Combining neutron diffraction experiments and microstructural analyses to investigate the fracturing of pyroxenes


Project Description

The link between aqueous fluids and potential habitability means that assessing the distribution of liquid water at or near the surface of Mars throughout its history remains a strong focus of missions to Mars. Eons of impact bombardment coupled with limited tectonic resurfacing has left the martian crust in a highly fractured condition, and this fracture network must exert a profound effect on how fluids move through the near-surface environment, and on where they are stored in the sub-surface. It follows that understanding the distribution of fluids in the near-surface of Mars requires knowledge of the density and connectivity of fractures in the subsurface. Central to that is knowledge of the fracture properties of those minerals that form a volumetrically significant part of the crust.

In terms of volume proportion, pyroxenes are one of the most important mineralogical constituents of the igneous part of the upper martian crust. Most of what we currently know about the mechanical properties of pyroxenes applies at the high temperature and pressure conditions that occur deep within planetary interiors; much less is known about those properties at the lower pressure conditions in which fracturing is prominent. From a mechanical perspective, the behaviour of pyroxenes at these lower pressure conditions is intrinsically interesting because not only do they fracture, they also undergo a range of crystallographically-controlled processes (e.g., mechanical twinning) that initiate and influence the subsequent development of the fracturing. Some of these processes are especially helpful because they allow the deformation conditions (e.g., orientation and magnitude of the stresses that caused the deformation) to be quantified from the deformation microstructures.

The aim of this PhD project is to investigate the processes that influence the onset and development of fractures in pyroxenes, and thereby to improve the information that we can recover about deformation conditions from martian meteorites, and inferences that we can make about martian tectonic stresses. The project will focus on: a. examining the interactions between fracturing and crystallographically-controlled processes such as twinning by performing mechanical tests within two of the neutron beamlines (POLARIS and ENGIN-X) at the UK neutron source near Oxford (ISIS Facility), and b. comparing fracture microstructures in pyroxene-bearing samples that have been experimentally deformed at known conditions and naturally deformed during terrestrial tectonic processes, with those present in Martian meteorites (which are the product not only of martian tectonic stresses but also of the effects of shock deformation during the impact event that generated the meteorite).

The mechanical experiments will be performed in a neutron beamline because this allows the onset and progress of any crystallographically-controlled process (including fracturing along cleavage planes) to be monitored from diffraction patterns collected during the experiment. These deformation processes include ones that are reversible upon unloading and so are not visible in the deformation microstructures, and yet they undoubtedly have a significant influence on fracturing (e.g., elastic twinning). The neutron diffraction approach also allows the contributions of individual minerals to the overall properties of the deforming sample to be measured, and so the significance of the pyroxene deformation response on the other minerals present (and vice versa) can be quantitatively assessed. The mechanical experiments will be designed to monitor stresses during thermal cycling as well as during loading. The microstructural analyses will involve a range of electron microscopy, spectroscopy (FTIR, Raman) and optical microscopy techniques. These will be deployed to highlight compositional influences on the fracture behaviour as well as to evaluate deformation conditions. In both the mechanical experiments and microstructural analyses, pyroxenes of different chemical composition and crystal structure will be investigated.

Pyroxenes are a significant component of the crusts of other planetary bodies, e.g., on Earth, the Moon and Venus, and so the findings of the project will have implications for those bodies as well. However, the primary focus will be on generating results that will be of direct relevance for ongoing planetary exploration programmes to Mars, including the InSight mission which aims to investigate the interior structure of Mars and the ExoMars2020 mission which plans to investigate habitability and the martian hydrous cycle. We seek an enthusiastic person for this project with a strong background in the physical sciences or material sciences or geology, and with an interest in applying their work in a planetary science context.


References

Suggested reading

Heap MJ, Byrne PK, Mikhail S, 2017, Low surface gravitational acceleration of Mars results in a thick and weak lithosphere: implications for topography, volcanism and hydrology. Icarus 281: 103-114
Taylor SR, McLennan SM, 2009, Planetary Crusts: Their Composition, Origin and Evolution, Cambridge University Press
McSween HY, 2015, Petrology on Mars. American Mineralogist 100: 2380-2395
Covey-Crump SJ, Schofield PF, Oliver EC, 2017, Using neutron diffraction to examine the onset of mechanical twinning in calcite rocks. Journal of Structural Geology 100: 77-97
Leroux H, Jacob D, Marinova M, Hewins RH, Zanda B, Pont S, Lorand J-P, Humayun M, 2016, Exsolution and shock microstructures of igneous pyroxene clasts in the Northwest Africa 7533 Martian meteorite. Meteoritics and Planetary Science 51: 932-945

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