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Life and death of chemical reaction fronts ? a field, experimental and numerical study

Dr Thomas Mueller (SEE), Dr Sandra Piazolo (SEE)

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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 chemical 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 – a feature that is ubiquitous in the natural world and of high importance to questions of high societal impact including ore deposit generation, nuclear waste disposal safety and CO2 sequestration. In this project you will get to the bottom of the feedbacks between different processes occurring during reaction front progression and/or stagnation. As such you will 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 reaction fronts and compare results to reaction fronts in nature.

We have chosen dolomitisation, the secondary replacement of calcite (CaCO3) by dolomite (CaMg[CO3]2), as our case study system. This reaction is readily accessible to low/moderate temperature experimentation and can therefore be used to infer process in many other mineralogical settings. The main aim is to develop a theoretical framework to predict the progression and stagnation of chemical fronts, by quantitatively describing the coupling and feedback between element transport including element fluxes and fluid/elemental pathways, mineral reaction, recrystallization, evolution of fluid pathways, and their associated rates. Specifically, the project aims to answer the following questions:

1) How does fluid composition and rock texture influence reaction affinity, nucleation and growth kinetics at the chemical front? 

2) What is the temporal and spatial link between pore type, connectivity and reaction progression? 

3) 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?

Our group has not only been conducting experiments in carbonate systems for several years building substantial expertise in sample preparation, reaction rates, and analytical methods, but also performed numerical simulation focussing on the processes involved. The project has three objectives: 

O 1: Experimentally quantifying the evolution of reaction fronts during dolomite replacement in single and polygrain samples to measurable physio-chemical parameters such as flow-rate, permeability, grain size, fluid chemistry, and temperature controlling nucleation and growth rates. 

O 2: Detailed parameterization of natural and experimental carbonate replacement domains.

O 3: Development of a numerical model that predicts the evolution of a dolomitisation front based on feedbacks between permeability development, reaction and chemical transport, to allow upscaling and harmonisation of results from 1 and 2.

To characterize the samples (both experimental and natural) and fluid pathways you will use 4D state-of-the-art analytical and visualisation techniques. Evolution of permeabilities will be monitored during the flow-through experiments using a custom-made permeameter. The formation of secondary porosity will be quantified (SEM, µ-CT) and linked to quantitative physicochemical analysis of both fluid and solid phases (ICP-MS/TIMS – fluid chemistry including isotopes, NanoSIMS – isotope tracers, EBSD – grain size and EMPA – solid chemistry). Comparison of results with natural dolomitisation fronts will provide natural evidence of the character of reaction fronts and facilitate upscaling of experimental data to reactive transport models. Data from outcrops of reaction fronts and from samples taken from this front will also ground truth model output and provide support for code refinement such that existing numerical codes can be upgraded to realistically simulate reaction front progression from mm to cm scale.

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Related undergraduate subjects:

  • Chemistry
  • Earth science
  • Geochemistry
  • Geological science
  • Geology
  • Geoscience
  • Hydrology
  • Materials science
  • Natural resource management
  • Natural sciences