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Rotten Rocks at the Heart of Black Smoker Systems

Dr Andrew McCaig (SEE), Dr Thomas Mueller, Prof. Susan Lang

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It is normally assumed that permeability in ocean floor hydrothermal systems is largely fracture controlled. However work in epidosites (metasomatic epidote-quartz rocks replacing dolerite dykes) in the Troodos ophiolite has shown that dissolution of primary minerals in hydrothermal fluid creates large local porosity and permeability. This acts to focus fluid flow and also dissolution is an extremely effective way of extracting metals from rocks in the formation or ore deposits.

This project focusses on new evidence for reaction porosity and permeability in the Atlantis Massif, mid-Atlantic Ridge. This includes dolerite dykes where clinopyroxene was dissolved and then the porosity filled by amphibole, tubular vugs 5 cm in diameter in gabbro, and gabbro lenses in serpentinised peridotite where plagioclase was dissolved and then the space filled by chlorite over a temperature range from >400 to

The project has three strands:

1) Quantification of the evolution of porosity using SEM and microCT scanning. This aims to establish how much porosity was present at the time of reaction, and how connected it was, and also how the porosity and permeability evolved as the porosity was progressively filled by secondary minerals. Image analysis will be used to reconstruct the 3-D geometry of growth zones in secondary minerals such as epidote, quartz, amphibole and chlorite as a function of time. 

2) Experimental work on the dissolution rates of minerals in hot fluids, and on the evolution of porosity during flow through experiment, aimed at quantifying the rates of this process. Most previous experiments on dissolution rates have been carried out at low temperatures, but rates can be very fast in “far from equilibrium” conditions. Experiments at higher temperatures are needed to extrapolate dissolution rates to black smoker conditions. Bothe static experiments with a large excess of fluid, and flow through experiments will be performed, in both cases seeming to reproduce “far from equilibrium” conditions

3) Analysis of fluids trapped in the porosity, including potential organic molecules such as acetate and formate which are building blocks for life. Much attention recently has focussed on abiotic synthesis of methane and higher hydrocarbons, which is an essential precursor to life. Finding such molecules in relict porosity from black smoker systems would be very exciting.

The project will involve fieldwork to resample epidosites in Cyprus, a search for reaction porosity in both in recently collected drill core from the Oman drilling project, and in IODP core from the Atlantis Massif, and a visit to the laboratories of project partner Prof. Susan Lang in South Carolina to search for hydrocarbons in relict porosity.


Related undergraduate subjects:

  • Earth science
  • Geological science
  • Geology
  • Geoscience