1 Doctoral Research Proposal Thesis title EXPERIMENTAL STUDIES OF BASALT-FLUID INTERACTIONS AT SUBCRITICAL AND SUPERCRITICAL HYDROTHERMAL CONDITIONS Mauro Passarella Student ID#: 300324610 Ph.D. Candidate in Geology School of Geography, Environment and Earth Sciences Victoria University of Wellington Ph.D. SUPERVISORS: Prof. Terry M. Seward (Victoria University of Wellington) Dr. Bruce W. Mountain (GNS Science Wairakei, Taupo)
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Mauro Passarella - Doctoral Research Proposal_October 2015
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Doctoral Research Proposal
Thesis title
EXPERIMENTAL STUDIES OF BASALT-FLUID
INTERACTIONS AT SUBCRITICAL AND SUPERCRITICAL
HYDROTHERMAL CONDITIONS
Mauro Passarella Student ID#: 300324610
Ph.D. Candidate in Geology
School of Geography, Environment and Earth Sciences
Victoria University of Wellington
Ph.D. SUPERVISORS:
Prof. Terry M. Seward (Victoria University of Wellington)
Dr. Bruce W. Mountain (GNS Science Wairakei, Taupo)
3.1. Basalt-distilled water interaction at hydrothermal conditions .................................................. 13
3.2 First interpretation of data ......................................................................................................... 14
4. Research timeline ................................................................................................................................. 17
5. Funding and resources.......................................................................................................................... 18
Figure 1. Interactive map for the InterRidge Vents Database Version 2.0 (S. Beaulieu, K. Joyce, and S.A. Soule (WHOI), 2010)
Figure 2. Schematic drawing illustrating the portions of submarine hydrothermal system. Seawater enters the crust in widespread recharge zones and reacts at increasing temperature during penetration into the crust. High-temperature (>400C)
reactions occur in the reaction zone above the magmatic or hot rock heat source, and buoyant fluids rapidly rise
upward in focused or diffuse discharge zones (Susan E., Humphris, Robert A.Z., Lauren S.M., Richard E.T., (2013).
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1.2. FLUID-BASALT INTERACTION
1.2.1. SUPERCRITICAL CONDITIONS
At subcritical conditions, liquid water is nearly incompressible and has a low thermal expansion and
molar heat capacity. It also has an elevated dielectric constant. When a compound such as water reaches
temperatures and pressures above its critical point, only one phase exists and it is referred to as a
supercritical fluid (Fig. 3). Under supercritical conditions, these properties change significantly and the
fluid becomes more compressible, has a much higher heat capacity, lower viscosity, and a diminished
dielectric constant (Anisimov et al., 2004).
In terms of heat content, a supercritical H2O has a higher enthalpy than steam produced from boiling
below the critical point. Its low viscosity allows it to transport large amounts of mass and energy at
faster rates (Dunn and Hardee, 1981). However, its ability to dissolve solid compounds such as minerals
strongly depends on the density of the fluid and hence, on the fluid dielectric properties. A higher
density supercritical fluid can dissolve significant concentrations of chemical elements thus playing a
major role during water–rock interaction (Norton, 1984; Norton and Dutrow, 2001; Friðleifsson et al.,
2013).
Figure 3. The liquid-vapour critical point in a pressure-temperature phase diagram is at the high-temperature extreme of the
liquid-gas phase boundary. The dotted green shows the anomalous behaviour of water.
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Figure 4 shows the pressure-enthalpy diagram for water (Fournier, 1999). A supercritical fluid at 500
bars (2 km lithostatic pressure) and a high enthalpy (e.g., at Point A in Figure 4) can ascend along a
number of P-T paths. If the fluid ascends without heat loss (i.e., no change in enthalpy) it can pass
below the critical point (Point B on the solvus) and separate into two phases (liquid and vapour, Points
E and D, respectively). If the fluid loses heat by conductive cooling, it can reach a higher level in the
crust without boiling (Point A to L). This is typical of circulating fluids in geothermal systems where
water convects without boiling due to heat loss to the surrounding rocks. Another situation would be a
high temperature supercritical fluid (e.g., Point H) where during ascent the two phase boundary is
reached and phase separation occurs (Point D). This is the situation in steam-dominated geothermal
systems. An extreme example is the case of a superheated fluid (Point F). As this fluid rises, enthalpy
decrease by conduction is insufficient to allow phase separation and the fluid reaches the surface as
superheated steam. This is, for example, the situation at the Icelandic Deep Drilling Project borehole
(IDDP-1). The IDDP project was initiated to investigate whether sufficient superheated fluid could be
accessed by a deep borehole (4 km) to produce electrical power. The ultimate objective is to utilise
superheated steam to gain 4-5 times the energy produced by a conventional production well
(Fridleifsson and Elders, 2005).
Figure 4. Pressure-enthalpy diagram for pure H2O with selected isotherms. The shaded area showing the conditions under which
steam and liquid water co-exist is bounded on the left by the boiling point curve and to the right by the dew point
curve. The arrows show various different cooling paths of ascending fluids (Barton and Toulmin 1961, Fournier 1999, (2007), Friðleifsson G.Ó., Elders W.A. and Albertsson A., (2013)).
Figure 6. Two 3D views of the high P-T hydrothermal apparatus: (a) double piston pump; (b) accumulator containing the metal
piston below which distilled water is pumped and above which contains the experimental fluid; (c) pressure vessel
containing rock material which is surrounded by the oven; (d) back pressure regulator control unit; (e) back pressure regulator; (f) collector syringe; (g) oven to heat the pressure vessel. The red arrows show the direction of the flow in
the system.
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3. INITIAL RESULTS
3.1. BASALT-DISTILLED WATER INTERACTION AT HYDROTHERMAL CONDITIONS
The first basalt-fluid interaction experiment was carried out at supercritical conditions (i.e. 400˚C and
500 bar). The basalt was crushed, sieved to obtain the 355-500 µm size fraction (Fig. 7a-b) and then
cleaned in water in an ultrasonic bath. The clean basalt fragments (~26 g) were then reacted with
distilled water in the flow-through autoclave (Fig. 7c-d). The distilled water was de-oxygenated with a
N2:H2 gas mixture and prior to being pumped into the main titanium accumulator. The high P-T
hydrothermal apparatus was run for a total time of 37 days with the first five days at room temperature
and 500 bar and the remaining 32 days at 400˚C and 500 bar. The flow rate was initiated and maintained
at 1 ml∙ hr-1
for 33 days and then changed to 0.5 ml hr-1
for the last 4 days to test for equilibrium.
Figure 7. (a) Icelandic basalt rock. (b) 355-500 micron size of basalt used for the experiment. (c-d) Pressure vessel
where occur the interaction between rock and fluid at 400˚C and 500 bar.
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3.2. FIRST INTERPRETATION OF DATA
Effluents (i.e. the reacted hydrothermal fluid) were analysed every day for cations (including Li, Na, K,
Mg, Ca, Sr, Mn, Fe, As, Al, B, and Si by ICP-OES) as well as for the anions (Cl- and SO4
- by ion
chromatography) (Fig. 8) and pH. Optical microscopy (Fig. 9a-b), SEM (Fig. 10a-d) and X-ray
diffraction (XRD) were used to characterise the mineralogical and chemical composition, both for
unreacted and reacted basalt.
Petrographic analysis of a polished thin section of unreacted basalt showed the presence of abundant
plagioclase (labradorite/bytownite) and clinopyroxene (augite), a lesser amount of opaque minerals
(magnetite-ilmenite) and a minor quantity of olivine.
Figure 8 shows the chemical analysis for the major cations and anions (in ppm) and pH of effluent
samples collected during experiment. Vertical red lines on the graphs represent the change in
temperature conditions from 25˚C, 500 bar (first 6 days) to 400˚C, 500 bar. This is to study the response
of elements due to temperature shift which allows intuitive conclusions about minerals reactions that
occur. It is evident that after increasing temperature, the solubility of major elements in the fluid phase
confirms that kinetics play an important role in terms of ion exchange between the rock material and
solution, at least at the early stages of the experiment. Notable are the concentration of silica (2000 mg
kg-1
); high concentrations of Na and Al; minor amounts of Ca and K; and the total absence of Fe, Mg,
Mn. The pH values increase to 9.2 by the end of experiment, indicating measurable hydrolysis of
silicate phases.
SEM photographs of the unreacted basalt are shown in Figure 10a-b. Preliminary SEM analysis of the
reacted basalt (Fig. 10c-d) showed visible corrosion of the primary minerals. This is evident in the
SEM photos where primary minerals covering the surface are partially dissolved (Fig. 10c) and
converted to what appears to be chlorite (Fig. 10d). This would be consistent with the conversion of
olivine and clinopyroxene to chlorite, according the schematic reactions:
↔
which is consistent with the presence of Ca and Na and the absence of Fe and Mg as well as the alkaline
pH values in the effluent samples. These reactions are here presented in a schematic and unbalanced
form, pending microprobe analysis of the solid phase.
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Figure 8. Final water chemistry analysis for major cations, anions (in ppm) and pH of first experiment in supercritical conditions. Red vertical lines indicate the changing point in temperature of basalt-fluid interactions from 25˚C to 400˚C.