Institute of Geophysics and Tectonics (IGT) PhD Projects
Permeability and fluid-rock interaction in oceanic hydrothermal systems
Supervisors: Dr Andrew McCaig, Prof Bruce Yardleyand Prof Joe Cann
Black smoker systems at mid ocean ridges are a vital part of the Earth System, influencing the rate of heat loss from the mantle, the chemistry of the ocean, and probably the internal structure of the ocean crust. The two key parameters of seafloor hydrothermal systems are the supply of heat and the permeability structure of the crust. Long lived systems require there to be a thin conductive boundary layer between molten magma and vigorously convecting fluid in order to match the heat output of black smokers (Lowell and Germanovitch, 2004). Recent modelling (Driesner, 2010) suggests that permeability must be “just right” to allow venting at black smoker temperature of up to 400 °C. If it is too low, little circulation occurs, while if it is too high, heat is extracted too efficiently and cooler vents result.
Conventional wisdom is that permeability in the ocean crust is dominated by fractures produced either through deformation or contraction during cooling. New evidence from IODP drill core (Fig.1) and from epidosites in the Troodos ophiolite indicates that significant permeability can be created during metamorphic hydration reactions.
Fig.1: SEM photos of relict porosity in oceanic diabase (dolerite) intrusions from IODP Site 1309, in the footwall of an oceanic detachment fault in the mid-Atlantic (see Ildefonse et al 2005). Isotopic data suggests the fault zone acted as a discharge pathway for black smoker fluids (McCaig et al.,2007, 2010). A: Backscatter SEM image of zoned amphiboles replacing clinopyroxene and formed at temperatures ranging from 800 to <500 °C. Black areas are plagioclase. At bottom right is relict porosity partly filled by fibrous amphibole. B: Combined secondary electron/backscatter image of the broken surface of a sample showing fibrous amphiboles partially filling porosity.
Field evidence in epidosites (Edwards et al. 2009) indicates that each dyke in the sheeted dyke complex was replaced by epidote-quartz assemblages before emplacement of the next dyke, and that fluid exploited dyke centres in preference to more fractured margins. Modelling (Steefel and Maher 2009) shows that reaction-induced permeability leads to fingering instabilities in fluid flow that can positively reinforce dissolution reactions. It is quite possible that reaction-enhanced permeability is the dominant permeability generation mechanism in black smoker discharge zones. This mechanism also allows permeability to be generated above the brittle-ductile transition, and in the intact wall rocks between fractures, promoting the isotopic alteration of the crust and the release of metals into hydrothermal fluids.
Aims of this project are:
- To investigate the extent of reaction-enhanced permeability in a range of oceanic and ophiolite settings
- To quantify porosity and estimate permeability before occlusion by secondary minerals
- To make direct measurements of relict permeability and porosity in selected specimens
- To constrain the conditions under which hydration occurred and the fluid-rock reactions between that promoted generation of permeability
- To constrain the rates of these processes by comparison with experimental data
This project will make use of new SEM and electron microprobe equipment installed in Leeds in 2010. Permeability measurements will be made in the Wolfson multi-phase flow laboratory. Reactions and fluid flow will be modelled using Geochemist’s Workbench. Field work will be undertaken in Cyprus, together with visits to core repositories in Bremen and Texas. There may be the opportunity to spend time at IPG in Paris incorporating results into hydrothermal models being developed there.
References:
Driesner, T. The interplay of permeability and fluid properties as a first order control of heat transport, venting temperatures and venting salinities at mid-ocean ridge hydrothermal systems. Geofluids 10, 132-141, doi:10.1111/j.1468-8123.2009.00273.x (2010).
Edwards SJ, Edwards H.-E. K., Cann JR, Malpas J, Xenophontos C. Classic Geology in Europe 7: Cyprus. (Terra, 2010).
Ildefonse, B. et al. Oceanic core complexes and crustal accretion at slow-spreading ridges. Geology 35, 623-626, doi:10.1130/g23531a.1 (2007).
Lowell, R. P. & Germanovich, L. N. Hydrothermal processes at mid-ocean ridges: Results from scale analysis and single-pass models. Mid-Ocean Ridges: Hydrothermal Interactions between the Lithosphere and Oceans 148, 219-244 (2004).
McCaig, A. M., Cliff, R. A., Escartin, J., Fallick, A. E. & MacLeod, C. J. Oceanic detachment faults focus very large volumes of black smoker fluids. Geology 35, 935-938, doi:10.1130/g23657a.1 (2007).
McCaig, AM, Delacour A, Fallick AE, Castelain T and Fruh-Green GL (2010), Detachment Fault Control on Hydrothermal Circulation Systems: Interpreting the Subsurface Beneath the TAG Hydrothermal Field Using the Isotopic and Geological Evolution of Oceanic Core Complexes in the Atlantic. In: Diversity of Hydrothermal Systems on Slow Spreading Ocean Ridges, PA Rona, CW Devey, J Dyment and BJ Murton (eds), Geophysical Monograph 108, p. 207-240.
Steefel, C. I. & Maher, K. Fluid-rock interaction: a reactive transport approach. In: Thermodynamics and Kinetics of Water-Rock Interaction Vol. 70 Reviews in Mineralogy & Geochemistry eds E. H. Oelkers & J. Schott) 485-532 (Mineralogical Soc Amer, 2009).