Geothermal waters of the Taupo Volcanic Zone, New Zealand
Ashley Steffen
NDSU Geol 428 Geochemistry 2010
Tectonics and Geology
Pivot point between two plate-converging systems
South Island, located on Pacific Plate
North Island on Australian Place
9 Volcanic Centres, 20 geothermal fields
Geothermal environments:
steep soil temperature gradients
extreme pH
highly mineralized soils and waters
High production of rhyolite beginning c. 1.23 Ma
(Giggenbach, 1994 and Boothroyd, 2009)
Fig. 1: Plate tectonics of the New Zealand
regionhttp://geosphere.gsapubs.org/
Subducting System of North Island
Pacific plated “pulled” down
Increase in depth causes increase
in temperature and pressure.
With the addition of water from
the subducting oceanic plate,
magma is generated.
Fig. 2: Subduction model
http://people.uncw.edu/grindlayn/revabstr_vol.pdf
Thermodynamic Systems
Convection system
Juvenile vs. meteoric waters
http://www.anaspides.net/earth_life_sciences/geothermal_fields_taupo_volcanic_zone_nz.html
Water chemistry of lakes in the Taupo Volcanic Zone, New Zealand
Timperley, M. H., and Vigor-Brown, R. J., 1986
Looked at 32 lakes and attempted to classify specific sources of their waters
Cold water springs and rivers carrying weathered rock
Precipitation
Geothermal water
Geothermal steam
Collected into polyethylene bottles from a depth of .5 at lake center or away from inflows
Table 1
pH
Na+
K+
Ca2+
Mg2+
Cl-
SO42-
HCO3-
Geothermal waters
8.3
1330
198
23.0
0.18
2290
35.0
66.0
Geothermal steam
3.3
0
0
0
0
0
148
0
Rhyolite pumice
7.2
9.6
1.4
3.0
1.4
2.9
2.4
40.8
Welded ignimbrite
6.8
8.0
2.7
2.3
1.3
5.2
2.4
27.0
Rhyolite pumice Stream
6.8
5.1
1.3
2.9
1.0
2.6
1.7
23.1
Precipitation
5.2
0.58
0.21
0.14
0.084
1.2
0.55
0
Cold spring water
Contributing sources and their major ion concentrations (ppm) (Timperley)
Timperley and Vigor-Brown
The 32 Lakes
1.Ngakoro
2. Rainbow North
3. Rotokawa
4. Rotomahana
5. Echo
6. Rotowhero
7. Tarawera
8. Rotoehu
9. Rainbow South
10. Rotorua
11. Rotoma
12. Rotoiti
13. Taupo
14. Rotoaira
15. Okataina
16. Opal
17. Rotongaio
18. Okaro
19. Otamangakau
20. Ngapouri
21. Okareka
22. Emerald
23. Tutaeinanga
24. Rotokawau
25. Rotokakahi
26. Ngahewa
27. Rerewhakaaitu
28. Tikitapu
29. Rotopounamu
30. Tama upper
31. Tama lower
32. Blue
Fig. 3: Taupo Volcanic Zone Lakes (Timperley and Vigor-Brown)
Timperley and Vigor-Brown
Origins of lakes
•Total ion concentration of given lake:
∑[i]l=∑{ [i]pl + [i]gl + [i]sl + [i]hl + [i]wl}
•Total concentration due to precipitation
∑
[i]pl=
[Cl-]l
∑[i]p
[Cl-]p
•Total concentration from weathering by steam
{
}
∑ [i]sl= 2 [SO4-]l - [SO4-]ls - [SO4-]lp - [H+]l
•Total concentration from geothermal water •Total concentration from weathering by H2CO3
[Cl-]-2[Cl-]p
∑[i]gl =
[Cl-]g
{
} ∑[i]
∑[i]wl= 2 {[HCO3-] - [HCO3-]lg}
g
Timperley and Vigor-Brown
Group A: small amounts of metal chlorides
-> precipitation is a major contributor
Group B: Stream and spring waters
-> final products of weathering
appreciable proportions of metal sulphates in their
dissolved salts
do not exceed expected from normal weathering
-> may result from titration of HCO3- in lake by
sulphuric acid rather than in catchment
Group C: >> metal chlorides, greater than precipitation would
contribute
-> geothermal waters
substantial concentrations of metal sulphates from
weathering by sulfuric acid
Groups E, D, F: >> metal chlorides, greater than precipitation
would contribute
-> geothermal waters
Lakes not influenced by geothermal waters
Low [Cl-]
Total cation concentrations “almost equal” [HCO3-] + [SO4-]
Timperley and Vigor-Brown
Results/Findings
In conclusion, Timperley and Vigor-Brown found it hard to find exact sources for most lakes.
Reaction of geothermal waters with host rock
(EQUILIBRIUM_PHASE)
Two waters with different compositions
Reacted with rhyolite to see what other minerals might form
Reacted at different temperatures to if there were different saturations
of the minerals present
Typical alkali carbonate
#2
#1
waters
Geothermal waters
pH
8
(Timperley, 1986)
Cl
57
pH
8.3
Na
1330
Na
220
K
198
SiO
175
Ca
23
K
43
Mg
.18
HCO
1.2
Cl
2290
SO
3177
SO
35
Ca
<1
HCO
66
Li
.6
F
.3
Concentrations taken from
“GEOTHERMAL WATERS: A
SOURCE OF ENERGY
AND METALS.” Department of
Earth Sciences, University of
Waikato
Reacted along with gases
Composition of rhyolite
The QAP Triangle
All rhyolites are not the same, and exhibit
different ratios of quartz, feldspar, and alkali
feldspar, along with variable amounts
hornblendes, pyroxenes, and biotite.
Minerals used
Albite (Sodium plagioclase)
NaAlSi3O8
K-feldspar
KAlSi3O8
Quartz
SiO2
Biotite
KMg3AlSi3O10(OH)
California State Polytechnic University Pomona:
http://geology.csupomona.edu/alert/igneous/igclass.htm
PHREEQC Interactive
1) Define minerals need for reaction
PHASE
Biotite formula found in Example 16
Need log k and delta h
PHREEQC Interactive
Finding log k and ∆H
KMg3AlSi3O10(OH) + H2+ + H2O = K+ + 3Mg2+ + Al(OH)4- + 3H4SiO4
For this reaction, log k of K-mica
(KAl3Si3O10(OH)2) was used
log k=12.703
For delta h, ∆h for all
minerals/elements in reaction
Mineral/element
∆h (kcal mol-1)
KMg3AlSi3O10(OH)
-1488.2 (Robie and
H2+
0.0
H2O
-68.315
K+
-60.32
Mg2+
-111.58
Al(OH)4-
-356.2
3H4SiO4
∆HR= ∑ ∆H products - ∆H reactants
Hemingway, 1984)
-348.3
∆HR = (-1796.16) – (-1762.46) = -34.7 kcal mol -1
PHREEQC Interactive
PHREEQC Interactive
2) Define solution
SOLUTION 1 Geothermal water (Timperley)
temp 100
pH
8.3
pe
-6.22
redox pe
units ppm
density 1
Cl
2290
Na
1330
Alkalinity 66
Mg
0.18
S(6) 35
Ca
23
K
198
water 1 # kg
Eh=-.059 x 8.3
Faraday constant
pe=
23.06 kcal mol-1
x -.472
-3
(2.303)(1.98x10 )(398)
R
pe=-6.22
T
PHREEQC Interactive
3) EQUILIBRIUM PHASE
Select/type in
desired minerals
Biotite phase
SI – kept at 0
keeps mineral
in saturation, but
never dissolution
-> May precipitate
Decide amount
desired for reaction
PHREEQC Interactive
When changing temperatures
always change pe
(same when changing pH)
Done at 100 C and 195 C
PHREEQC Interactive
Water #2
GAS_PHASE
PHREEQC Interactive
Done at 100 C and 150 C
Results
Waters from Timperley and Vigor-Brown
10
0
-10
-20
SI at 100 C
-30
SI at 195 C
-40
-50
-60
Precipitating phases
Anorthite
Aragonite
Calcite
Gibbsite
K-mica
Kaolinite
pH went from 8.3 to 8.1 for 100 C
8.3 to 8.01 for 195 C
Water #2
Typical alkali carbonate
waters
pH
8
Gases
Cl
57
CO2
Na
220
H2
SiO
175
K
43
HCO
1.2
SO
3177
Ca
<1
Li
.6
F
.3
Concentrations taken from “GEOTHERMAL
WATERS: A SOURCE OF ENERGY AND METALS.”
Department of Earth Sciences, University of
Waikato
+
H2O
H2S
NH3
Results
Second water composition, with added gases
20
10
0
-10
-20
SI at 100 C
-30
Si at 150 C
-40
-50
-60
-70
Precipitating phases
Anorthite
Aragonite
Calcite
Gases
Dolomite
Fluorite
K-mica
Talc
CH4(g)
N2(g)
pH: went from 8 to 10.4 at 100 C
8 to 11.3 at 150 C
In conclusion
Unfortunately, PHREEQC I is limited in it’s temperature gradient. Temperatures of geothermal
systems can reach up to 300 C and higher.
Many other variations in the rock types that occur. Not only rhyolite, but andesite, dacite, and
basalt, all with varying degrees on plagioclase, alkali-feldspar, quartz, and other minor (but
important) minerals.
Reference
•Boothroyd, Ian. Ecological characteristics and management of geothermal systems of the Taupo
Volcanic Zone, New Zealand. Geothermics. 2009 Vol. 38, pp. 200-209.
•Graham, I.J., et al. Petrology and petrogenesis of volcanic rocks from the Taupo Volcanic Zone: a
review. Journal of Volcanology and Geothermal Research. 1995. Vol. 68, pp. 59-87,
•Robie, Richard and Hemingway, Bruce. Heat capacities and entropies of phlogopite
(KMg3[AlSi3O10](OH)2) and paragonite (NaAl2[AlSi3O10](OH)2) between 5 and 900 K and
estimates of the enthalpies and Gibbs free energies offormation. American
Mineralogist, 1984. Vol. 69, pp. 858-868.
•Timperley and Vigor-Brown. Water chemistry of lakes in the Taupo Volcanic Zone, New Zealand.
New Zealand Journal of Marine and Freshwater Research, 1986. Vol. 20, pp. 173-183.