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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)

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TABLE OF CONTENTS

Abstract .................................................................................................................................................... 3

1. Introduction .......................................................................................................................................... 4

1.2. Fluid-basalt interaction .............................................................................................................. 6

1.2.1 Supercritical Conditions .......................................................................................................... 6

1.2.2. Previous basalt-fluid interaction experiments ........................................................................ 8

2. Proposed research and methods ........................................................................................................... 9

2.1. Research framework and questions .......................................................................................... 9

2.2. Experimental methodology ...................................................................................................... 11

3. Initial results ......................................................................................................................................... 13

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

6. References ............................................................................................................................................ 18

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ABSTRACT

The reactive environments of mid-ocean ridge and seafloor spreading centres are of enormous global

importance but surprisingly, there are few modern, experimental studies aimed at understanding the

associated hydrothermal reactivity and kinetics. In order to study fluid-rock interactions, at near-

supercritical and supercritical conditions that are typical of these systems, computer modelling is less

accessible due to a lack of thermodynamic data. The experimental approach offers the only alternative

to directly access the fluid-mineral interactions that are occurring in these environments.

In addition, submarine geothermal reservoirs contain a large amount of thermal energy that has, as yet,

not been used for commercial energy production. Their potential is much larger than those of onshore

geothermal resources and could provide a significant part of the global future energy demand in an

environmentally sustainable way. This is because the hydrothermal fluids are present under supercritical

conditions and can therefore transfer much higher amounts of heat than subcritical fluids.

This study concentrates on fluid-rock interactions in fresh mid-ocean ridge basaltic (MORB) rocks

under subcritical and supercritical conditions (350 – 400˚C, 500 bar). The experiments will be

conducted to investigate the reaction-path chemistry of subcritical and supercritical fluids as they react

with basalt and the mineralogical changes that result. In previous experimental studies, some insight

was gained into the fluid-rock exchanges that gave rise to the fluid chemical signature; however, many

fundamental questions involving the equilibrium and kinetic aspects of water-rock interaction, remain.

In addition, the chemical evolution of the fluid compositions and their fluxes are still poorly understood.

The experiments proposed are designed to further our knowledge of subcritical and supercritical fluid-

rock interactions in terms of the nature and timing of chemical exchange. We will use fresh basalt from

the Reykjanes Peninsula, Iceland, and three types of hydrothermal fluid: distilled water; geothermal

brine from Taupo Volcanic Zone (TVZ) and seawater. Using the flow-through apparatus, we will react

the basalt with fluids at temperatures and pressures up to 400°C and 500 bars. Effluent solutions will be

analysed using standard methods for aqueous samples and run products will be analysed by a

combination of XRD, SEM, EMPA and petrography.

Using these results, we will examine the relationship between fluid and secondary mineral compositions

to investigate reaction-path and elemental fluxes with respect to pressure-temperature conditions and

time.

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1. INTRODUCTION

The reactive environments of mid-ocean ridge and seafloor spreading centres are of enormous global

importance but surprisingly, there are few modern, experimental studies aimed at understanding the

associated hydrothermal reactivity and kinetics. The entire volume of the Earth’s oceans are considered

to cycle through these reactive, chemical “conveyor belts” approximately every nine to ten million

years. In addition, these seafloor environments are responsible for the discharge of enormous amounts

of heat and chemical components to the Earth’s surface and as such, they have contributed significantly

the ocean chemistry through geological time.

There are several approaches to investigate the processes that occur when high temperature fluids

reacted with rock materials. These include both computational and experimental approaches. Current

computer programmes (e.g. Geochemist’s Workbench®, (Bethke, 2008)) are used to model different

environments utilising thermodynamic data which are relatively well-known, at least below 350oC. In

order to study fluid-rock interactions at near-supercritical and supercritical conditions, computer

modelling is less accessible due to a lack of appropriate thermodynamic data. In sub-seafloor

environments, physico-chemical conditions are usually at these conditions and computer modelling is

unable to achieve meaningful results. Geochemical rock-fluid interactions in these special environments

are also poorly understood. Consequently, the experimental approach offers the only alternative to

directly access the fluid-rock-mineral interactions that are occurring in these environments (Mountain

and Sonney, 2011). In this thesis, I propose to conduct high temperature and pressure fluid-rock

interaction experiments at supercritical pressure and temperatures using a flow-through apparatus to

recreate the processes occurring natural sub-seafloor fluid-basalt interactions.

Submarine geothermal reservoirs (Fig. 1) contain a large amount of thermal energy that has, as yet, not

been used for commercial energy production. Their potential is much larger than those of onshore

geothermal resources and they could conceivably provide a significant proportion of the global future

energy demand in an environmentally sustainable way. There are two types of resources: (i) deep

resources along oceanic spreading centres where uprising magma heats fluid circulating through

fissured rocks, emerges at vents with temperatures up to 460˚C and mixes with seawater (1000 - 4000 m

below sea level) and; (ii) coastal shallow resources where geothermal fluids emerge at fractures (1 - 50

m bsl). The deep resources total ~65,000 km in length and at many sites, pressure and temperature are

high enough to create supercritical conditions (Fig. 2).

At seafloor hydrothermal vents, fluid-rock reactions create zones of alteration due to the exchange of

chemical components between cold, alkaline seawater and hot basaltic rocks. Cold seawater descends at

recharge zones away from the ridge, becomes heated and then rises buoyantly, reacting with the rocks

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on its permeability path. These subcritical and/or supercritical fluids can achieve temperatures over

400˚C while ocean floor seawater normally has a temperature range between 2-4˚C (Kingston, 1995).

The formations of vents, or smokers, occur due to the contact between two fluids with extremely

different physical characteristics. Hot fluids to ascend to the ocean floor, transporting heat and high

concentrations of chemical elements, some of which precipitate immediately after mixing with cold

seawater to form large deposits of base and precious metals, that are contemporary analogous for

volcanic-associated massive sulphide (VMS) deposits.

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)).

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1.2.2. PREVIOUS BASALT-FLUID INTERACTION EXPERIMENTS

The first experiments investigating seawater-rock interaction were carried out in the mid-1970’s in

which basalt and gabbro were reacted at low water/rock ratios (<5:1). These allowed a better

understanding of the role of rock composition in buffering pH and redox conditions. The major problem

was the lack of reliable thermodynamic data able to explain the relevant alteration minerals that formed

as well as the conditions of equilibrium between fluid and rock (Hajash, 1975; Mottl and Holland,

1978).

An improvement in experimental techniques by Dickson, who first used a Teflon cell for on-line

sampling, allowed higher water/rock ratios (Bischoff and Dickson, 1975). A similar approach was used

to study the chemistry and the mineral reactions as a function of time by Seyfried and Bischoff (1979).

These initial experiments were innovative for that period and permitted the analysis of the behaviour of

elements during changes in temperature and pressure, as well as allowing investigation of kinetic

parameters. It was, however, difficult to achieve the temperature and pressure regime to reproduce

natural conditions of a sub-seafloor hydrothermal system and the experiments were static rather than

flow-through. A principle problem was the pressure cell material utilised for the experiments. Teflon

reacted with the cell wall contaminating the chemical results and could not be employed above 300°C.

Using gold-lined pressure cells, the experiments could be conducted at higher temperature and pressure,

closer to the conditions in sub-seafloor hydrothermal systems (Seyfried and Dibble, 1980; Shanks et al.,

1981; Seyfried and Bischoff, 1981; Seyfried and Mottl, 1982). The results were significant in terms of

understanding the chemical changes that occurred in the seawater and it was possible to study the

behaviour of some important base metals. The release and uptake of elements to and from the fluid

phase such as Mg, Fe, Mn and Cu cause changes in H+ concentration and consequently a change in

solution pH (Seyfried and Janecky, 1985). In particular, it was possible to understand how fluid-rock

interactions play a fundamental role in mobilising heavy metals and trace elements at supercritical

conditions.

During the mid-80s, the study of fluid-rock interaction became more advanced, using better equipment

which offered better control of temperature and pressure (Seyfried, 1987). This produced new and more

reliable thermodynamic data allowing better numerical modelling and prediction of pH and redox

conditions in sub-seafloor hydrothermal systems (Saccocia and Seyfried, 1994; Bischoff, 1991; Seyfried

and Ding, 1993).

Although all the above experimental studies provided useful insights into the interpretation of chemical

cycles in sub-seafloor hydrothermal systems, it remains important to improve on these studies with

better simulation of the natural environment, particularly at supercritical conditions. In my proposed

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research programme, I plan to improve on these earlier reconnaissance studies through the use of a

sophisticated hydrothermal apparatus able to generate fluid-rock interactions at more extreme conditions

and most importantly, with continuous flow. The hydrothermal apparatus permits the control and

adjustment of parameters in order to simulate accurately the chemical conditions active during basalt-

fluid interactions. It is also capable of variable fluid flow (as low as 1.4 ml day-1

) which allows for long

term experiments with higher fluid/rock ratio and more extensive sampling.

2. PROPOSED RESEARCH AND METHODS

2.1. RESEARCH FRAMEWORK AND QUESTIONS

The study concentrates on fluid-rock interactions in fresh mid-ocean ridge basaltic (MORB) rocks under

subcritical and supercritical conditions (350 – 400˚C, 500 bar). The experiments will be conducted to

investigate the reaction-path chemistry of subcritical and supercritical fluids as they react with basalt

and the mineralogical changes that result. An important aim will be to thermodynamically inter-relate

the evolved, reacted fluid chemistry with the observed, hydrothermally produced mineralogy. In

previous experimental studies, some insight was gained into the fluid-rock exchanges that gave rise to

the fluid chemical signature, however, several fundamental questions remain, including:

1) the effect of fluid-rock interactions on the chemistry in supercritical zones;

2) the elemental fluxes emanating from the reaction of basalts with supercritical (hydrothermal)

fluids;

3) the time scale of chemical alteration (i.e. kinetics);

4) the fractionation of stable isotopes at subcritical to supercritical conditions.

The experiments proposed are designed to further our knowledge of subcritical and supercritical fluid-

rock interactions in terms of the nature and timing of chemical exchange. With time permitting, we also

wish to investigate the fractionation of lithium isotopes during basalt-fluid interaction (Le Roux, 2010).

The fractionation of lithium isotopes during seawater-MORB interaction is a measure of the degree of

water-rock interaction. This is because lithium is fractionated into secondary minerals phases such as

clay minerals which commonly occur at moderate temperatures during seawater-basalt interaction. This

implies that lithium isotopic exchange significantly affects the lithium isotopic composition of the

oceans (Brandt et al., 2012).

In summary, the aim is to study the real mineral phases and elemental changes in fluid composition that

occur when fluids in equilibrium with basaltic rock react at sub-critical and supercritical conditions in

sub-seafloor hydrothermal systems, filling gaps in knowledge of previous studies. Using laboratory

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experiments, quantitative results will be obtained and used to compare with stability diagrams (e.g. Fig.

5) to evaluate the degree to which natural analogues match theoretical representations. The study will

also advance experimental techniques in that they will introduce fluid flow into the method, a feature

not previously seen in earlier experiments.

Figure 5. An example of activity-activity diagram versus logarithm of activity of silica and logarithm of activity of Ca2+ and pH

ratio. The diagram shows the mineral stability phases, delimited from red contours, in equilibrium with fluid phase at

300˚C.

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2.2. EXPERIMENTAL METHODOLOGY

I will conduct various experiments (Table 1) using fresh basalt (Eldvarpahraun basalt) from the

Reykjanes Peninsula, Iceland. This basalt was erupted in the years 1226-1227 and the sample taken is

devoid of any secondary mineralisation or alteration. Using the flow-through apparatus (Fig. 6), we will

react this rock with fluids at temperatures and pressures up to 400˚C and 500 bars. Each experiment will

be conducted for a total time of eight weeks. The fluids to be used in the experiments include: distilled

water, re-injection brine from a New Zealand geothermal power station and seawater. In addition, two

batch experiments using a gold-lined cell are planned to evaluate any contamination by the titanium

apparatus (i.e. the autoclave walls).

In selected experiments, we will also add a mixture of CO2 and H2S to the fluid to simulate re-injection

of waste gases that are emitted by geothermal power stations. Currently, the large majority of

geothermal plants emit these gases to the atmosphere. The problem is that it is uncertain what the effects

of high concentration of CO2 and H2S have on mineral formation and mineral storage (sequestration) in

the re-injection aquifer.

Fluid samples will be analysed for cations (by inductively-coupled plasma optical emission

spectrometer, ICP-OES), anions (by ion chromatography), as well as for pH, and CO2 and H2S

concentrations. X-ray diffraction (XRD) and scanning electron microscopy using energy dispersive

spectroscopy (SEM-EDS) will be used to characterise the mineralogical and chemical composition of

the reacted basalt.

Table 1. Proposed basalt-fluid interaction experiments.

Experiments Inlet Conditions T (˚C) P (bar) Type

1 Distilled water Supercritical 400 500 Continuous flow

2 Geothermal brine Supercritical 400 500 Continuous flow

3 Seawater Supercritical 407 500 Continuous flow

4 Geothermal brine + Non Condensable Gases Supercritical 400 500 Continuous flow

5 Seawater + Non Condensable Gases Supercritical 407 500 Continuous flow

6 Geothermal brine Subcritical 350 500 Continuous flow

7 Seawater Subcritical 350 500 Continuous flow

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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.

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Figure 9(a-b). Microscopy (direct light source): (a) unreacted basalt grain; (b) reacted basalt grain.

Figure 10(a-d). Electron Scanning Microscopy (SEM): (a) unreacted basalt grain; (b) unreacted basalt grain surface (plagioclase

phenocrystals); (c) reacted basalt grain; (d) reacted basalt grain surface covered by secondary mineralisation possibly chlorite.

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4. RESEARCH TIMELINE

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5. FUNDING AND RESOURCES

Funding for the project comes from the GNS Science Geothermal Core Funded Research Programme.

The experimental work will be carried out at GNS Science, Wairakei Research Centre, Taupo, in the

Experimental Geochemistry Laboratory of Dr. Bruce Mountain, GNS Science.

Sample preparation and analyses will be done at GNS Science and at the Victoria University of

Wellington. The high P-T flow-through facility is located at GNS Wairakei. Analyses of effluents

sample composition will be completed at GNS Science, Wairakei. ICP-MS and electron microprobe

facilities are available at the Victoria University of Wellington, and SEM work will be conducted at the

University of Auckland and/or at Victoria University.

6. REFERENCES

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