In this multidisciplinary project the student will investigate the underlying mechanisms that control porosity generation by reaction of hydrothermal fluids with oceanic gabbro below the seafloor. Zones of high hydrothermal fluid fluxes are common in black smoker systems and are hypothesized to be one of the environments where life could develop and thrive both in early Earth similar to present-day deep-sea hydrothermal systems. The student will be able to explore porosity generation through a unique set of approaches including analysis of the root zones of hydrothermal vents, via newly obtained samples from IODP expeditions past and present, novel experiments and sample analysis using state-of-the-art analytical equipment along with thermodynamic fluid flow modelling.
Reaction induced porosity has been observed in gabbroic rocks and dolerite sills within the detachment fault zone of the Atlantis Massif oceanic core complex (Fig. 1) (http://publications.iodp.org/scientific_prospectus/399/ ). Preliminary data shows that dissolution of plagioclase and pyroxene by black smoker fluids may lead to significant permeability and focussing of fluid flow. . Chlorite geothermometry (Fig. 1D) shows that porosity from microns to cm in size in the detachment fault zone was generated by mineral dissolution at temperatures of ~ 400 °C, and filled over waning temperatures to below the known temperature limit of microbial growth (~121 °C). Higher cell counts close to zones of porosity (Fruh-Green et al., 2018) suggest that the porosity remained open during the cooling of the Atlantis Massif, and may have been infiltrated by fluids related to the famous Lost City Hydrothermal Field, and at that time occupied by microbes. A similar dissolution process has been documented in highly altered epidosites within sheeted dyke rocks of the Troodos ophiolite, Cyprus (Cann et al., 2015), which are also thought to be the upflow zones for black smoker systems that produced significant copper deposits on the seafloor.
This project is investigating the following questions:
- What is the extent of porosity generation when hydrothermal black smoker fluids interact with oceanic crust?
- What are the processes that lead to the porosity generation?
- What are the conditions under which porosity is generated and maintained over long time scales?
- Is reaction porosity a viable environment for life to thrive and even emerge?
This project takes advantage of the opportunity that the lead supervisor is co-chief scientist of IODP Expedition 399 http://iodp.tamu.edu/scienceops/expeditions/atlantis_massif_blocks_of_life.html (April to June 2023) which will drill several new holes through the detachment fault zone of the Atlantis Massif oceanic core complex. This zone has been interpreted as the upflow zone of hot hydrothermal fluids which would have vented on the seafloor as a black smoker vent field (McCaig et al., 2010). During the expedition, the team plans to extract and culture microbes, and analyse fluids and gases such as H2, from the porosity in the drilled rocks. This, together with shipboard characterisation of core and thin sections, will provide background to this project and enable sample selection. Furthermore, there is a wealth of past expedition samples that can supplement the sample collection for this study.
The student will:
- Use state of the art microanalytical and data analytics/machine learning techniques to quantify both porosity and the progressive filling of porosity by secondary minerals, on new and existing samples from the Atlantis Massif, and from epidosites in the Troodos ophiolite. Here new, unique equipment at University of Leeds that allows 3D characterization from nano- to mm scale will be of great advantage.
- Undertake novel mineral dissolution experiments in Leeds to establish far from equilibrium dissolution rates of plagioclase at temperatures up to 300 °C, and with a range of fluid pH and composition. To do this experimental samples will be analysed using the same techniques as for natural samples
- Construct thermodynamic reaction path models in collaboration with Dr Frieder Klein of Woods Hole Oceanographic Institution (WHOI) to investigate the effects of fluid composition, temperature and pH on the progress of different observed reactions in gabbro-dominated and serpentinite-dominated environments
The student will receive training in the thermodynamics of fluid-rock interaction, reactive transport processes, use of the SEM and other tools. The student will be fully integrated into post-cruise research of IODP Exp. 399, and will present work at a post-cruise meeting of the science party in 2024 or 2025, together with conference sessions. There will be an opportunity to undertake field work and sample collection in Cyprus, visit the IODP core store in Bremen, and make an extended visit to WHOI to work with Dr Klein.