Fault systems and fluid-rock interactions in the oceanic lithosphere: Implications for carbon capture


  • Supervisors: Dr Elisabetta Mariani, University of Liverpool
    Dr Pablo Gonzales, University of Liverpool
    Prof. John Wheeler, University of Liverpool

  • External Supervisors: Prof. Damon Teagle, University of Southampton

  • Contact:

    Dr Elisabetta Mariani, University of Liverpool, mariani@liverpool.ac.uk

  • CASE Partner:

Application deadline: 10 January 2020

Introduction:

Processes of peridotite alteration are important globally because they influence the rheology of subducting slabs, play a key role in the water and carbon cycles on Earth, provide an environment suitable to the development of chemosynthetic organisms and could represent a geological means of carbon capture and storage in mantle rocks (Kelemen and Hirth, 2012).

To assess if mantle rocks can be a suitable means for carbon capture, this project tackles the challenges related to determining the space and time scales of serpentinisation and carbonation reactions within the environment of the Samail ophiolite, an ancient mid-ocean ridge in Oman (Nicola et al., 2000). This ophiolite includes one of the largest peridotite bodies exposed on land and it was emplaced by obduction of oceanic lithosphere between 95 and 70 Ma (e.g Searle et al. 2004). Intense fluid-rock interactions before, during and after obduction resulted in hydration and carbonation of the peridotite by means of retrograde metamorphic reactions. These reactions involve the replacement of olivine, the main constituent mineral of peridotites, by serpentine (Mg,Fe)3Si2O5(OH)4 (hydration) and magnesite (Mg,Fe)CO3 (carbonation).

While models of alteration processes in peridotite have been proposed (Plumper et al. 2012), there are gaps in our knowledge on the timing and mechanisms of serpentinisation and carbonation. One key hypothesis is that the large-scale fault system associated with a mid-ocean ridge exerts important controls on fluid pathways, and therefore on alteration reactions in the oceanic lithosphere. Additionally, to determine whether present-day carbonation is still occurring, it is necessary to understand the history of seismic activity and slow creep on such fault systems.

Project Summary:

The Oman Drilling Project (OmanDP) initiative (www.omandrilling.ac.uk) has successfully completed drilling and geophysical logs in the deserts of Oman, where ~1000 m of core was drilled through serpentinites and a major hydrothermal fault zone (Kelemen et al., 2013). This represents an excellent section into the Samail ophiolite and its fault system. This project will combine fieldwork with microstructural and microchemical studies of this core.

Specific objectives are:

-   To study brittle fault lithologies, their microstructures and their spatial and temporal relationships with the serpentinite host-rock;

-   To determine mineralogies in fault rocks, veins and in the serpentinite host-rock (e.g. distinguish between different serpentine populations), for a full characterisation of alteration processes and their conditions and timing;

-   To characterise the type, extent, distribution and connectivity of porosity at the microscale. From this we will be able to infer permeability and determine the geometry and extent of fluid pathways;

-   To determine which structures and alterations may be associated with ocean detachment faulting versus obduction or more recent uplift-related deformations.

Structural and petrologic investigations will be combined with the analysis of selected samples using the EBSD/EDS SEM systems in Liverpool, to quantify fabrics, textures, chemical changes, and their spatial and temporal relationships. Fine grained samples will be studied using FIB-SEM equipped for 3D EBSD and EDS, and TEM. Fracture/vein geometries, interconnected porosity, surface area of pores and permeability will be investigated by micro X-ray tomography at Southampton on selected 1 to 2.5 cm diameter mini-cores of “fresh” and variably altered peridotite from the OmanDP cores.

References:

Aslin, J., Mariani, E., Dawson, K. & Barsoum, M. W. 2019. Ripplocations provide a new mechanism for the deformation of phyllosilicates in the lithosphere. Nature Communications, 10, 686.

Kelemen, P. B. and G. Hirth (2012). "Reaction-driven cracking during retrograde metamorphism: Olivine hydration and carbonation." Earth and Planetary Science Letters 345-348: 81-89.

Kelemen, P., et al. (2013). "Scientific drilling and related research in the samail ophiolite, sultanate of Oman." Scientific Drilling(15): 64-71.

Nicolas, A., et al. (2000). "Accretion of Oman and United Arab Emirates ophiolite - Discussion of a new structural map." Marine Geophysical Researches 21(3-4): 147-179.

Plümper, O., et al. (2012). "The interface-scale mechanism of reaction-induced fracturing during serpentinization." Geology 40(12): 1103-1106.

Searle, M. P., et al. (2004). Subduction zone polarity in the Oman mountains: Implications for ophiolite emplacement. Geological Society Special Publication. 218: 467-480.

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