In the geosphere, fluid-mediated mineral reactions are of pivotal importance in governing the redistribution of elements and isotopes. Incomplete elemental redistribution is preserved in the rock record in the form of geochemical reaction fronts, the boundaries between reacted and unreacted material. Such fronts control geochemical exchange between the hydrosphere and the geosphere, the formation of mineral deposits, and migration of aqueous fluids and melt in the lithosphere. Associated mineralogical changes can dramatically change the physicochemical properties of Earth materials affecting their flow properties (rheology), strength, porosity and permeability. Recent experimental work has shown that reaction rates of metasomatic fronts are likely controlled by the transport of elements contradicting previous assumptions of dissolution/precipitation-controlled reaction rates. Understanding the dynamics of progression and stagnation of reactive fronts is therefore one of the most important challenges in geoscience.
In this exciting and innovative project, you will explore reaction front dynamics with a focus on the evolution of fluid transport to and from reaction sites. This transport depends on a balance between transport-enabling reaction steps such as dissolution and pore formation and transport-inhibiting reaction steps such as recrystallization and mineral precipitation. In this project, you will link experimental and theoretical studies of dolomitisation to provide a new qualitative and quantitative framework for predicting reaction front behaviour in general.
We have chosen dolomitisation, the secondary replacement of calcite (CaCO3) by dolomite (CaMg[CO3]2), as our case study. This reaction is readily accessible to low/moderate temperature experimentation, and can therefore be used to infer process in many other mineralogical settings. In addition, it is one of the most volumetrically important carbonate metasomatic processes, with dolomite hosting numerous ore deposits. The project will develop a process-oriented, parameterized toolbox for numerical modelling that describes the evolution of metasomatic reaction fronts on a grain scale. The results will be a radical change in our ability to reliably predict the complex interplay between the highly dynamic processes (nucleation, recrystallization, transport) controlling element fluxes and will have impact on several areas of science and industry including studies of crustal rheology, hydrocarbon reservoirs and, mineral deposits.
Specifically, the PhD project aims to answer the following three questions:
How does fluid composition and rock texture influence reaction affinity, nucleation and growth kinetics at the chemical front? An answer to this question will allow us to predict reaction front progression based on an assumed or known fluid and/or reactant.
What is the temporal and spatial link between pore type, connectivity and reaction progression? Answering this question will allow us to predict reaction front progression as a function of permeability and develop a numerical protocol to predict chemical front behaviour in space and time. This will be particularly important for economic prospectivity that is reliant upon knowledge of the spatial distribution of porosity such as hydrocarbon and ore exploration.
What are the parameters controlling chemical front progression and/or stagnation and do these parameters vary under different conditions of temperature, prior permeability or rock texture? An answer to this question will inform and enable reliable large scale chemical transport models to be universally applied to reactive transport processes in many socio-economically and geodynamically important settings.
The student will work under the supervision of Dr Thomas Mueller and Prof Sandra Piazolo within the Institute of Geophysics and Tectonics. This project provides a high level of specialist scientific training in: (i) experimental petrology (ii) cutting-edge geochemical analytical approaches (iii) numerical modelling (iv) integrated textural-metamorphic reconstruction. Co-supervision will involve regular meetings between all partners, use of Leeds analytical facilities including the state of the art TIMS, FE-SEM, EPMA, Cohen laboratories and extended visits for the student to the NERC Isotope Geosciences Laboratory. The successful PhD student will have access to a broad spectrum of training workshops run by the Faculty, including numerical modelling, managing your degree, and preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/