The effects of minor intrusions on host rock properties: Testing alternative models for heat transfer from magma to host

Magmatic intrusion into porous sediments can result in localised host rock diagenetic effects (e.g., compaction of pore space, grain cataclasis, pore infilling, redistribution of cements), which can have significant impact on subsurface fluid flow. Analogue heat-flow studies for minor intrusions typically sample the host rock as line transects (1D), with the aim of measuring thermal effects as a function of distance (or time if a conduction model is assumed) from an intrusion.

However, results show complex maturation/thermal profiles suggesting that (a) heat transport is by convection, (b) heat transport is not 1D, and (c) there are no simple applicable models for heat conduction in minor intrusive systems. Our approach here is to use a combination of high resolution field data collection and experimental rock-property analysis to: (a) characterise the internal architecture of sheet intrusions (dykes as a primary target, but also sills), to consider the growth and magma throughput of the intrusion (using internal contacts, vesicle distributions, and fabric variations); and (b) characterise the pathways along which hot fluids might migrate around the intrusions through the surrounding host rock.

The study will focus on well-exposed intrusions in the San Rafael Sub-Volcanic Field (SRSVF), Utah, which hosts about 200 dykes with in excess of 2000 segments, and >10 sill complexes. SRSVF dykes and sills were intruded into Triassic and Jurassic strata of the Glen Canyon Group, and San Rafael Group between 3.7 to 4.6 Ma. The SRSVF covers an area of about 1200 km2, occupying an elevation range of ~500 m, and was probably emplaced within the upper 2 km of the crust. The excellent exposures of intrusions in the SRSVF, in a range of host rock materials (mudstones, siltstones, and sandstones) provide an opportunity to gain critical insights into the propagation and growth of intrusions at a range of scales. Importantly, the San Rafael Group outwith of the SRVF has experienced near-identical tectonic histories.

The area provides therefore the opportunity to study the effects of igneous intrusion within a layered sequence, at a range of scales, with the benefit of comparison to non-intruded host properties. Results of the study will have significant implications for hydrogeological systems in intruded regions, particularly concerning the subsurface transport of fluid resources (e.g., water and hydrocarbons).

The study will involve detailed structural characterisation of dykes that cut layered sedimentary host rocks. This includes: digital mapping (photogrammetry) of dyke segments and fabrics internal to the dykes, to determine emplacement phases; and detailed mapping of the surrounding host rock to determine the distributions of pre- and post-emplacement structures, including constraining the style of rock deformation related to dyke emplacement. Samples will be collected as transects across intrusions, as well as targeting the key structural features identified in mapping. Physical properties, including elastic moduli and permeability will be measured in experimental tests, and linked to physical and mineralogical variations through scanning electron microscopy. Flow fabrics and discrete textural variations in the dykes will be imaged using micro-CT scans of samples.

The project continues a strong working collaboration with the University of Aberdeen, and in particular, Dr Dave Healy, whose expertise spans structural and metamorphic geology, rock physics, and geomechanics. Dr Healy runs a geomechanics laboratory, and will host the student for experimental rock properties characterisation. Dr Miles is an igneous petrologist and geochemist at the University of Leicester and will advise on petrological means of determining magma emplacement temperatures and compositions.

Entry requirements
Applicants are required to hold/or expect to obtain a UK Bachelor Degree 2:1 or better in a relevant subject. The University of Leicester English language requirements apply where applicable.

How to apply
Please refer to the CENTA Studentship application information on our website for details of how to apply.

As part of the application process you will need to:
• Complete a CENTA Funding form – to be uploaded to your PhD application
• Complete and submit your PhD application online. Indicate project CENTA2-GGE21-WALK in the funding section.
• Complete an online project selection form Apply for CENTA2-GGE21-WALK