Fault systems and fluid-rock interaction in the Samail peridotite: implications for carbon capture in mantle rocks.

The Samail peridotite in the Oman Mountains is namely 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 extensive 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).
Such 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).
While models of alteration processes in peridotite have been proposed, there are gaps in our knowledge on the timing of serpentinisation and carbonation in the Samail peridotite and on the mechanisms that enable replacement reactions to continue and go to completion. In addition, large scale structural studies of the peridotite are mainly those of Searle and co-workers (1980-2000), and only few microstructural investigations have been carried out to date (e.g. Plumper et al. 2012).
To test hypotheses at the cutting edge of research, including the factors highlighted above, the Oman Drilling Project (OmanDP) initiative (www.omandrilling.ac.uk), funded by the International Continental Scientific Drilling Program (ICDP) and numerous partner, has successfully completed drilling and geophysical logs in the deserts of Oman and has cored key sections of the Samail peridotite.
In the Wadi Batin area three boreholes, drilled through serpentinites and a major hydrothermal fault zone (Fig. 1), have provided ~1000 m of core. This represents an excellent, continuous and well preserved section into the Samail peridotite and its fault, vein and fracture networks. This project aims to study the microstructure and microchemistry of carefully selected samples from this core to unravel the mechanical and chemical processes that characterise fluid-rock interaction and the implications for replacement reactions and their timing and extent.

Project Summary:

This project will entail: 1) microstructural and microchemical characterisation of serpentinites, calcite and serpentine veins and their relationships with rodingitic dykes; 2) microstructural and microchemical characterisation of cataclasites and fault gouges in the serpentinites; 3) detailed fieldwork in the Wadi Batin and Wadi Fins areas of Oman, to reconstruct the field relationships between deformation and serpentinisation at the meso-scale. 4) Experiments to determine the mechanical properties of different varieties of serpentine (e.g. serpentine gel).
Specific objectives are:
- To study brittle fault lithologies (gouges, cataclasites, breccias, veins) and their microstructures at the nano- to meso-scales and determine spatial and temporal relationships between them and with the serpentinite host-rock;
- To determine spatially resolved and bulk rock mineralogies and micro-chemistry as well as ppm chemical variations in fault rocks and veins and 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 at the macro- and micro-scale 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.
The correlation and interpretation of all datasets will provide the framework upon which a new conceptual model of serpentinisation can be developed.

The student will benefit from the microstructural and chemical expertise of the supervisory team as well as from interactions with the multi-national team of scientists involved in the Oman Drilling Project.