Geothermal waters of the Taupo Volcanic Zone, New ZealandAshley Steffen

NDSU Geol 428 Geochemistry 2010

Pivot point between two plate-converging systemsSouth Island, located on Pacific PlateNorth Island on Australian Place

9 Volcanic Centres, 20 geothermal fields

Geothermal environments:steep soil temperature gradientsextreme pHhighly mineralized soils and waters

High production of rhyolite beginning c. 1.23 Ma

(Giggenbach, 1994 and Boothroyd, 2009)

Tectonics and Geology

Fig. 1: Plate tectonics of the New Zealand regionhttp://geosphere.gsapubs.org/

Fig. 2: Subduction modelhttp://people.uncw.edu/grindlayn/revabstr_vol.pdf

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.

Convection system Juvenile vs. meteoric waters

http://www.anaspides.net/earth_life_sciences/geothermal_fields_taupo_volcanic_zone_nz.html

Thermodynamic Systems

Water chemistry of lakes in the Taupo Volcanic Zone, New ZealandTimperley, 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

Cold spring water

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

Contributing sources and their major ion concentrations (ppm) (Timperley)

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. Okareka22. Emerald 23. Tutaeinanga 24. Rotokawau 25. Rotokakahi 26. Ngahewa 27. Rerewhakaaitu 28. Tikitapu 29. Rotopounamu 30. Tama upper 31. Tama lower 32. Blue

The 32 Lakes

Fig. 3: Taupo Volcanic Zone Lakes (Timperley and Vigor-Brown)

Timperley and Vigor-Brown

Origins of lakes

•Total concentration due to precipitation

∑ [i]pl= ∑[i]p

•Total concentration from geothermal water

∑[i]gl = { } ∑[i]g

[Cl-]-2[Cl-]p

[Cl-]g

•Total concentration from weathering by steam

∑ [i]sl= 2{[SO4

-]l - [SO4 -]l

s - [SO4 -]l

p - [H+]l}

•Total concentration from weathering by H2CO3

∑[i]wl= 2 {[HCO3

- ] - [HCO3- ]l

g}

[Cl-]l

[Cl-]p

•Total ion concentration of given lake:

∑[i]l=∑{ [i]pl + [i]g

l + [i]sl + [i]h

l + [i]wl}

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 weatheringappreciable 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 waterssubstantial concentrations of metal sulphates from weathering by sulfuric acid

Groups E, D, F: >> metal chlorides, greater than precipitation would contribute

-> geothermal waters

Timperley and Vigor-Brown

Lakes not influenced by geothermal waters Low [Cl-] Total cation concentrations “almost equal” [HCO3

- ] + [SO4 -]

In conclusion, Timperley and Vigor-Brown found it hard to find exact sources for most lakes.

Timperley and Vigor-Brown

Results/Findings

Reaction of geothermal waters with host rock(EQUILIBRIUM_PHASE)

Typical alkali carbonate waters

pH 8

Cl 57

Na 220

SiO 175

K 43

HCO 1.2

SO 3177

Ca <1

Li .6

F .3

Geothermal waters (Timperley, 1986)

pH 8.3

Na 1330

K 198

Ca 23

Mg .18

Cl 2290

SO 35

HCO 66

Two waters with different compositionsReacted with rhyolite to see what other minerals might form Reacted at different temperatures to if there were different saturations

of the minerals present

Reacted along with gases

#1#2

Concentrations taken from “GEOTHERMAL WATERS: A SOURCE OF ENERGYAND METALS.” Department of Earth Sciences, University of Waikato

California State Polytechnic University Pomona: http://geology.csupomona.edu/alert/igneous/igclass.htm

The QAP Triangle

Composition of rhyolite

All rhyolites are not the same, and exhibit different ratios of quartz, feldspar, and alkalifeldspar, along with variable amounts hornblendes, pyroxenes, and biotite.

Minerals used Albite (Sodium plagioclase)

NaAlSi3O8

K-feldsparKAlSi3O8

QuartzSiO2

BiotiteKMg3AlSi3O10(OH)

PHREEQC Interactive

1) Define minerals need for reactionPHASE

Biotite formula found in Example 16Need log k and delta h

Finding log k and ∆H

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 Hemingway, 1984)

H2+ 0.0

H2O -68.315

K+ -60.32

Mg2+ -111.58

Al(OH)4- -356.2

3H4SiO4 -348.3

KMg3AlSi3O10(OH) + H2+ + H2O = K+ + 3Mg2+ + Al(OH)4- + 3H4SiO4

∆HR= ∑ ∆H products - ∆H reactants

∆HR = (-1796.16) – (-1762.46) = -34.7 kcal mol -1

PHREEQC Interactive

PHREEQC Interactive

SOLUTION 1 Geothermal water (Timperley)temp 100pH 8.3pe -6.22redox peunits ppmdensity 1Cl 2290Na 1330Alkalinity 66Mg 0.18S(6) 35Ca 23K 198water 1 # kg

2) Define solution

pe= x -.47223.06 kcal mol-1

(2.303)(1.98x10-3)(398)

Faraday constant

R T

Eh=-.059 x 8.3

pe=-6.22

PHREEQC Interactive

PHREEQC Interactive

3) EQUILIBRIUM PHASE

SI – kept at 0 keeps mineral in saturation, but never dissolution-> May precipitate

Decide amount desired for reaction

Select/type in desired minerals Biotite phase

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

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

Typical alkali carbonate waters

pH 8

Cl 57

Na 220

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

Water #2

Gases

CO2

H2

H2O

H2S

NH3

+

Results

Second water composition, with added gases

Precipitating phases

Anorthite Aragonite Calcite

DolomiteFluoriteK-micaTalc

Gases

CH4(g) N2(g)

pH: went from 8 to 10.4 at 100 C 8 to 11.3 at 150 C

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.

In conclusion

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.