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Title: The dynamics of magma intrusion : using magnetic anisotropy to understand magma emplacement dynamics
Author: Martin, Simon
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2020
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Sheet intrusions provide key pathways for magma transport and storage as it flows through the Earth's crust. Many studies focus on the relationship between the intruding magma and the host rock through which it propagates, however the flow and emplacement processes occurring within the magma are often poorly constrained. Understanding this is vital for understanding the behaviour of active volcanic systems and the development of bodies of economic interest. To address this, I study how magnetic fabrics are preserved in a natural sill and dyke in combination with laboratory experiments using analogue materials to investigate the evolution of intrusion propagation and solidification. The geochemistry and petrology of sills of the Little Minch Sill Complex, Isle of Skye, are well documented, however the physical processes of emplacement, i.e. magma flow and solidification, are relatively understudied. The studied sill was 6 m thick, of crinanite composition and a sheet with evidence of magma fingers. Anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM) techniques were combined with petrology, to understand how magnetic fabrics vary across the sill length and thickness. AMS fabrics identified two groupings: Group A at the sill margins showed an initial magma flow aligned with the long-axis of the magma fingers, whereas Group B in the sill interior showed flow reorientation which was inferred to have occurred due to magma finger coalescence. AARM fabrics identified a post-emplacement flow regime, associated with migration of interstitial melt through the solidifying crystal mush. The contrasting fabrics demonstrate that multiple flow regimes were operational within the sill, and that these can be identified using multiple rock magnetism techniques. Within a basaltic dyke from the Skye Dyke Swarm, multiple magnetic fabrics are recorded and originate from contrasting processes. Within the dyke core, magnetic fabrics originate from titanomagnetite, however in margin regions pyrrhotite becomes the dominant source of the fabrics. I identified that within the dyke core a record of lateral magma flow was preserved, however closer to the margins the fabrics were more indicative of magnetite breakdown and growth of pyrrhotite from a sulphide rich hydrothermal fluid which had overprinted any primary flow indicator. There was also variation in fabrics along strike of the dyke, with alteration of the dyke core also evident nearer to branching in dyke strike. This study shows the complexity and variation in magnetic fabrics, how this can vary over short (~13 m) distances and can be affected by post-emplacement alteration. To understand how magnetic fabrics, develop during the propagation of sheet intrusions, a series of laboratory experiments were conducted. In these experiments plaster of Paris (magma analogue) seeded with magnetite particles, a pseudoplastic fluid, was injected into a box filled with flour (host rock analogue), a cohesive granular material. These materials were used to identify the evolution of AMS fabrics across multiple slices of model intrusions. The experimental intrusions formed a range of magma bodies including plutons, dykes, laccoliths and fingers, which indicate a range of processes occurring during initial emplacement and subsequent intrusion growth. AMS analyses of three parallel slices cut perpendicular to the flow axis, identified compressional and shear fabrics closer to the intrusion margins and towards the intrusion leading tips. Magnetic fabrics indicative of flow in the propagation direction were formed closer to the intrusion core and are like those observed in nature, thus demonstrating the potential of laboratory modelling for studying emplacement processes. In combination, these studies of natural intrusions and laboratory experiments investigate the development and evolution of magnetic fabrics across the length and thickness of intrusions. This is important as understanding the evolution of flow during dyke and sill emplacement is vital for determining and improving current emplacement models.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral