IAP2-22-383

Controlling the Earth’s climate by silicate mineral weathering

The weathering of Ca- and Mg-silicate minerals has helped to balance the Earth’s climate through geological time (Berner et al. 1983). Thus, processes enhancing Ca- and Mg silicate weathering at an industrial scale can help to combat anthropogenic climate change (e.g., Schuiling and Krijgsman 2006). Olivine is a Mg-Fe silicate that is of particular interest to geoengineering owing to its reactivity, and its abundance in the Earth’s crust. In terms of practical applications, the inclusion of olivine into ‘geocrete’ can reduce CO2 emissions from concrete production (~3.5 billion tons annually; e.g., Fennell et al., 2021), and there is much interest in spreading finely ground olivine in soils and the oceans (Köhler et al. 2010). Accordingly, many experimental studies have sought to explore how the dissolution rates of olivine and other Ca- and Mg-silicates are affected by variables including solution chemistry and temperature, and the chemical composition and grain size of the reactants (e.g., Fuhr et al. 2022). Such studies have provided the fundamental understanding of the mechanisms of mineral-water reaction, and a wealth of empirical data on dissolution kinetics. However, the experiments do not faithfully reflect natural processes – for example, rates are typically accelerated in order to generate measurable dissolution on laboratory timescales, and the experiments are sterile, i.e., microbial processes are not included. These limitations can be overcome by examining natural rock exposures, although the length of time that they have been subaerially exposed is difficult to determine, and in many cases their constituent minerals could have interacted with earlier generation of fluids (e.g., hydrothermal).

This project will take a new approach to understanding the mechanisms and rates of natural weathering of Ca- and Mg-bearing silicates by analysis of a set of mafic igneous rocks called the shergottites. These rocks have been delivered to the surface of Earth from Mars as meteorites, and have several properties that make them a unique and powerful source of information on the weathering of silicate minerals: (i) the shergottites have lain at the Earth’s surface for up to hundreds of thousands of years, and in environments ranging from Antarctica to the Sahara; (ii) the length of time that any one rock has been subaerially exposed can be quantified precisely; (iii) their petrology and mineralogy is very well understood and includes Ca- and Mg-bearing silicates that are key to the global carbon cycle (olivine, pyroxene, plagioclase); (iv) there is no evidence that they have interacted with liquid water prior to being exposed at the Earth’s surface (i.e., these rocks have not interacted with Martian aqueous fluids). As diagrammatically summarised in Figure 1, insights from the shergottites will be complemented by the study of terrestrial mafic lava flows of known eruption age, and by laboratory weathering experiments on shergottite and lava samples using a range of conditions.

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Image Captions

Methods of understanding rates and mechanisms of Ca- and Mg-silicate weathering that will be used in this study. A) Characterising mafic igneous lava flows with known eruption ages. B) Analysing mafic igneous meteorites. C) Experiments using mineral samples.

Methodology

Shergottite samples will be obtained from museums and from the ANSMET collections (https://caslabs.case.edu/ansmet/), and mafic lavas of known age will be collected from igneous centres such as Sicily and Iceland. The mechanisms and rates of weathering of Ca- and Mg-silicates in these samples will be determined by sample characterisation using a suite of high-resolution mineralogical, microstructural and chemical analysis techniques. An emphasis is on using the ‘correlative analytical pipeline’ that has been developed in the School of Geographical and Earth Sciences (GES) whereby samples are studied at length scales from millimetres to atoms, and often in three dimensions, using a powerful microanalytical toolbox as described below (e.g., Daly et al. 2021).

1. Visualising petrology in three dimensions
X-ray computed tomography (XCT) enables the non-destructive characterisation of the internal structure of rock samples in three dimensions and with sub-micrometre resolution. Constituent minerals including weathering products can be defined from their X-ray attenuation properties, and porosity produced by weathering can be quantified. Several lab-based systems are available for use in this project, and time can also be requested on one of the tomography beamlines at the Diamond Light Source.

2. Mapping mineralogy and microstructure
One of the important questions that this project asks is the extent to which mineral microstructures control mechanisms and rates of weathering. The mineralogy of entire thin sections can be mapped at high resolution using the scanning electron microscope (SEM) based technique of electron backscatter diffraction (EBSD). In addition to characterising the size and shape of individual mineral grains, their microstructures including twins and subgrains can be identified, and their crystallographic properties characterised. GES has recently installed an Oxford Instruments Symmetry2 system that is capable of rapid and automated acquisition of EBSD maps of entire thin sections.

3. Characterisation at the atomic scale
Transmission electron microscopy (TEM) and atom probe tomography (APT) are the culmination of the analytical pipeline (Daly et al. 2021). For TEM, micrometer-size wafers are extracted from the surface of a sample using a Focused Ion Beam (FIB) microscope. By transmitting electrons through the wafers their microstructure can be visualised down to the atomic scale, and chemical composition quantified. APT is an emerging technique whereby a sub-micrometre size sample is ablated by laser, and using the data collected individual atoms and molecules can be located in the sample and in three dimensions. APT has been applied very little in the field of terrestrial weathering although has enormous potential. Prof. Lee have recently successfully used the technique to characterise the water content of olivine (Daly et al. 2021), and nanoscale properties of serpentinised peridotites.

Project Timeline

Year 1

Review of literature relevant to the project including climate geoengineering, mineral microstructures, and rock and mineral weathering.
Training on the analytical techniques: XCT, SEM, EBSD, FIB and TEM.
Identification and acquisition of suitable shergottite samples, and fieldwork to collect weathered basalt from lava flows.
Non-invasive sample characterisation by XCT.

Year 2

Construction of an analysis plan for the lava and shergottite samples based on XCT results
Correlative sample analysis by SEM, EBSD and TEM
Start of weathering experiments
Dissemination of initial results via conference presentations, and outreach

Year 3

Completion of the characterisation of naturally weathered samples, including APT work
Analysis of samples used in the first weathering experiments, modification of the experimental design where needed, then running a second set of experiments
Dissemination of results via conference presentations, and outreach

Year 3.5

Completion of writing up of the thesis, and dissemination of results via peer-reviewed publication, conference presentations, and public engagement

Training
& Skills

This project will provide training in a wide range of skills including fieldwork and microanalysis. The scientific topics that will be covered include important areas of Earth system science including anthropogenic and deep time environmental change, climate geoengineering, and sample characterisation.

Working closely with the supervisory team the candidate will benefit from their complementary experience. Prof. Lee will oversee the project and brings his knowledge of research on mineral-water interactions. He also brings his extensive experience of XCT and TEM. Dr Griffin is a leader in microstructural analysis of minerals using EBSD and is pioneering the computational analysis of large datasets. Dr Schröder researches mineral-carbon interactions and uses meteorites to probe planetary environments.

The candidate will benefit greatly from the training and support provided by IAPETUS, and will also be part of vibrant research communities in Glasgow and Stirling. The candidate will be able to exploit the supervisory team’s extensive networks of international collaborators, and will have the opportunity to present results at international conferences. As this subject area is one that is of great interest to the general public, the candidate will be encouraged to join in the extensive outreach activities undertaken by the supervisors and their colleagues.

References & further reading

Berner R.A., Lasaga A.C. and Garrels R.M. (1983) The carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years. American Journal of Science 283, 641–683.
Daly L., Lee M.R., Hallis L.J., Ishii H.A., Bradley J.P., Bland P.A., Saxey D.W., Fougerouse D., Rickard W.D.A., Forman L.V., Timms N.E., Jourdan F., Reddy S.M., Salge T., Quadir Z., Christou E., Cox M.A., Aguiar J.A., Hattar K., Monterrosa A., Keller L.P., Christoffersen R., Dukes C.A., Loeffler M.J. and Thompson M.S. (2021) Solar wind contributions to Earth’s oceans. Nature Astronomy 5, 1275–1285.
Fennell P.S., Davis S.J. and Mohammed A. (2021). Decarbonizing cement production. Joule 5(6), 1305–1311.
Fuhr M., Geilert S., Schmidt M., Liebetrau V., Vogt C., Ledwig B. and Wallmann K. (2022) Kinetics of olivine weathering in seawater: An experimental study. Frontiers in Climate 4, 831587.
Köhler P., Hartmann J. and Wolf-Gladrow D. A. (2010) Geoengineering potential of artificially enhanced silicate weathering of olivine. Proc. Natl. Acad. Sci. U.S.A. 107, 20228–20233.
Schuiling R. D. and Krijgsman P. (2006) Enhanced weathering: An effective and cheap tool to sequester CO2. Climate Change 74, 349−354.

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