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Clumped isotope characterization of fluid-rock interactions in fault zones: The Gulf of Corinth

Primary Supervisors: Dr Victor Bense and Mr Paul Dennis

Project Description:

Faulting in Earth’s crust and fluids, primarily water, are intimately related. Both physical and chemical fluid-rock interactions can affect the mode and style of deformation. For example high pore fluid pressures might promote brittle behaviour and seismic slip, whilst pressure solution can lead to aseismic creep. Once formed, faults may become a locus for widescale fluid migration with the focusing of mineralising fluids as has been suggested for many Mississippi Valley Type (MVT) Pb-Zn deposits e.g. The Irish Midlands and impacts on shallow groundwater resources e.g. the Lower Rhine Graben (Bense et al., 2008). However the transport properties of faults are generally anisotropic and complex with large variations in hydraulic conductivity both within and across fault zones. Characterizing the source, fluxes and conditions of water-rock interaction in fault zones is difficult. Our understanding of fluid-rock interactions in fault zones is clearly limited. One factor is the absence of suitably exposed, well characterised fault zones. Another is the technical challenge of recovering mineral growth temperatures and fluid isotope compositions from suitable syn and post kinematic fault calcites. In this project we address both these issues using the south side of the Gulf of Corinth as a study area and the new technique of clumped isotope geothermometry. Clumped isotope thermometry is based on determining the ordering of 13Cand 18O within the carbonate lattice. The greater stability of the 13C-18O bond results in a temperature dependent ordering, or clumping of 13C and 18O in the carbonate anion. This is a unique geothermometer, suitable for temperatures to 200 degrees C, in which it is not necessary to know the parent water isotope composition. Thus we can decouple measurement of temperature from measurement of the fluid oxygen isotope composition. In combination with conventional isotope ratio measurement (e.g. d18O) it is now possible to estimate independently the parent fluid oxygen isotope composition. This can be used, together with fluid inclusion measurements of both d 18O and d2H (e.g. Dennis et al., 2002), to characterise the parent fluid isotope composition and constrain it’s likely source. To date, as far as we are aware, there are no extant measurements, or estimates of fault mineral growth temperatures based on clumped isotope measurements. This project represents the first such application. The student will work in a varied and highly stimulating environment with other stable isotope geochemists, geophysicists and environmental scientists.

References:

Benedicto, A., Plagnes, V., Vergely, P., Flotte, N. and R.A. Schultz, 2008, Fault and fluid interaction in a rifted margin: integrated study of calcite-sealed fault-related structures (southern Corinth margin), in Wibberley et al. (eds) The Internal Structure of Fault Zones: Implications for Mechanical and Fluid-Flow Properties, Geol. Soc. Sp. Publ. 299, 257-275

Bense, V.F., M.A. Person, K. Chaudhary, Y. You, N. Cremer, and S.Simon (2008), Thermal anomalies as indicator of preferential flow along faults in an unconsolidated sedimentary aquifer system, Geophysical Research Letters, 35, L24406, doi:10.1029/2008GL036017.

Dennis, P.F., P.J. Rowe, T.C. Atkinson (2001) The recovery and measurement of water from fluid inclusions in speleothem. Geochimica et Cosmochimica Acta, 65, 4081-4098 DOI: 10.1016/S0016-7037(00)00576-7