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Dehydrating on a Greek island: corundum formation in relation to fluid flow and deformation on Naxos


Project Description

Introduction:
Dehydration reactions are fundamental to how the Earth works, and how H2O is carried around bound into minerals. During dehydration, bound H2O is given off in reactions which are often complex and involve a number of minerals. The rates of such reactions are governed not just by the local movements of atoms but also by the fluid pressure, which in turn is governed by how easily the fluid can escape. The feedback between these processes is not well understood – reactions are complex, and they have often gone to completion so we cannot see the relationships of reaction products with initial minerals.

The mineral diaspore, AlO(OH), dehydrates to corundum, Al2O3. This is an unusual but very simple dehydration reaction, so should be relatively simple to understand. Such understanding, the focus of this project, will assist in unravelling more complicated dehydration reactions. The island of Naxos in the Aegean provides a superb natural laboratory in which to study metabauxite dehydration (Feenstra & Wunder 2002), bauxite being an Al-rich rock formed by weathering. The island preserves various states of dehydration of diaspore to corundum, allowing for direct insight into reaction mechanism. The area is deformed and remarkably shows more deformation in the more dehydrated parts despite the production of corundum, a very strong mineral. Naxos therefore provides a great opportunity to look at the links between dehydration, deformation and fluid flow, as highlighted by Urai and Feenstra (2001).

Project Summary: It is essential to become familiar with the field setting of these rocks, so structural and metamorphic mapping will form part of the study. Because the metabauxites are embedded in marbles, you will need to look at structures there too, and the relationships between the rock types. Microstructural characterisation will provide a detailed context to unravel the link between fluid pressures, deformation and metamorphism. This must include all minerals, not just the diaspore and corundum, because there may have been interactions between different minerals during deformation and metamorphism. For example, if carbonates metamorphose they can give off CO2 that then, if it mixes with H2O, changes the conditions for diaspore dehydration. Methods will include optical petrography and more detailed techniques such as Electron Backscatter Diffraction (EBSD, a Scanning Electron Microscope technique) that enables a detailed look at crystal orientation. The original bauxites contained pisoids (subspherical concentric structures) which will allow for quantitative strain analysis, in other words we can discover the actual amount of deformation that such rocks have undergone (a rare privilege). Whilst the chemistry of diaspore and corundum are simple, carbonate chemistry is more varied and you will investigate it using Energy Dispersive Microanalysis (on SEM or microprobe). You will interpret your data using conceptual and quantitative models informed by previous work including that by the supervisors on dehydration reactions (Leclere et al. 2018, Llana-Funez et al. 2012) and their more general metamorphic, structural (Wheeler & Butler 1993, Wheeler et al. 2010) and microstructural studies (Prior et al. 2009).

Funding Notes

Full funding (fees, stipend, research support budget) is provided by the University of Liverpool. Formal training is offered through partnership between the Universities of Liverpool and Manchester in both subject specific and transferable skills to the entire PhD cohort and at each University through local Faculty training programmes.

References

Feenstra, A. & Wunder, B. 2002. Dehydration of diasporite to corundite in nature and experiment. Geology 30(2), 119-122.
Leclere, H., Faulkner, D., Llana-Funez, S., Bedford, J. & Wheeler, J. 2018. Reaction fronts, permeability and fluid pressure development during dehydration reactions. Earth And Planetary Science Letters 496, 227-237.
Llana-Funez, S., Wheeler, J. & Faulkner, D. R. 2012. Metamorphic reaction rate controlled by fluid pressure not confining pressure: implications of dehydration experiments with gypsum. Contributions To Mineralogy And Petrology 164, 69-79.
Prior, D. J., Mariani, E. & Wheeler, J. 2009. EBSD in the Earth Sciences: applications, common practice and challenges. In: Electron Backscatter Diffraction in Materials Science (edited by Schwartz, A. J., Kumar, M., Adams, B. L. & Field, D. P.). Springer, 345-357.
Urai, J. L. & Feenstra, A. 2001. Weakening associated with the diaspore-corundum dehydration reaction in metabauxites: an example from Naxos (Greece). Journal Of Structural Geology 23(6-7), 941-950.
Wheeler, J. & Butler, R. W. H. 1993. Evidence for extension in the western Alpine orogen: the contact between the oceanic Piemonte and overlying continental Sesia units. Earth and Planetary Science Letters 117, 457-474.
Wheeler, J., Park, R. G., Rollinson, H. & Beach, A. 2010. The Lewisian Complex: insights into deep crustal evolution. In: Continental Tectonics and Mountain Building: The Legacy of Peach and Horne (edited by Law, R., Butler, R. W. H., Holdsworth, R. E., Krabbendam, M. & Strachan, R.). Special Publication 335. The Geological Society, London, 51-79.

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