Insights into magmatic processes from the crystal orientation mapping of igneous microstructures

The crystallisation of magma to form solid rock involves many complex physical and chemical processes that remain poorly understood. Mapping crystallographic orientations using electron backscatter diffraction (EBSD) has revolutionised the study of deformed rocks, and is now emerging as a useful tool for studying igneous rocks. In this thesis, I combine EBSD with field, petrographic and geochemical observations to tackle long-standing problems in igneous petrology. The first half of the thesis utilises EBSD to explore crystal interactions in magma, focusing on interactions between K-feldspar megacrysts and crystals of other minerals in granite. There is a long-running debate about whether megacrysts grow in melt-rich magma, or when crystallisation is almost complete. I combine EBSD and phase equilibria modelling to demonstrate that megacrysts grew in melt-rich magma, when crystals were able to rotate relative to each other and attach in low-energy orientations in a process known as synneusis. Synneusis is generally assumed to occur only between crystals of the same mineral, and this study is the first modern, quantitative demonstration that synneusis can occur between crystals of different minerals. My second chapter therefore comprises a detailed investigation into the characteristics of polymineralic synneusis in granites. The second half of my thesis develops intracrystalline orientation mapping as a tool for exploring igneous crystal growth processes. Crystals that grow close to equilibrium typically form euhedral shapes without internal misorientation. However, crystals that grow at high supersaturation commonly display complex morphologies with high internal misorientation. I use EBSD to characterise these crystals in order to explore the poorly understood topic of disequilibrium magma crystallisation. I begin by describing the morphologies and misorientation characteristics of feldspar, clinopyroxene and olivine crystals from a volcanic rock that displays unequivocal disequilibrium growth textures. I find that the crystals display unusual growth directions and internal misorientations that point towards continuous crystallisation mechanisms and may result from nonclassical crystallisation pathways. Then in my final chapter, I bring together my findings to undertake a detailed case study on an enigmatic orbicular granite. I demonstrate that the orbs’ unique textures result from a combination of disequilibrium crystallisation and subsequent recrystallisation.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2024

Author dc:creator

• Gordon, Charlotte

Advisor dc:contributor.advisor

• Wallis, David

• Electron backscatter diffraction
• Igneous petrology

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/05dcefc2-02f6-45e5-b377-ba5344dbd679/download
• https://creativecommons.org/licenses/by/4.0/

Language dc:language

eng

Author Identifier

0009-0002-7400-6686

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/377548