Geothermal waters of the Taupo Volcanic Zone, New Zealand Ashley Steffen NDSU Geol 428 Geochemistry 2010

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 Ma (Giggenbach, 1994 and Boothroyd, 2009) Tectonics and Geology Fig. 1: Plate tectonics of the New Zealand regionhttp://geosphere.gsapubs.org/

Fig. 2: Subduction model 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 Thermodynamic Systems

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 1pHNa + K+K+ Ca 2+ Mg 2+ Cl - SO 4 2- HCO 3 - Geothermal waters Geothermal steam Cold spring water Rhyolite pumice Welded ignimbrite Rhyolite pumice Stream Precipitation 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. 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 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] p l = ∑[i] p Total concentration from geothermal water ∑[i] g l = { } ∑[i] g [Cl - ]-2[Cl - ] p [Cl - ] g Total concentration from weathering by steam ∑ [i] s l = 2 { [SO 4­ - ] l - [SO 4­ - ] l s - [SO 4­ - ] l p - [H + ] l } Total concentration from weathering by H2CO3 ∑[i] w l = 2 {[HCO 3 -­ ] - [HCO 3 -­ ] l g } [Cl - ] l [Cl - ] p Total ion concentration of given lake: ∑[i] l =∑{ [i] p l + [i] g l + [i] s l + [i] h l + [i] w l } 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 HCO 3 -­ 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 Timperley and Vigor-Brown Lakes not influenced by geothermal waters Low [Cl - ] Total cation concentrations “almost equal” [HCO 3 -­ ] + [SO 4­ - ]

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 pH8 Cl57 Na220 SiO175 K43 HCO1.2 SO3177 Ca<1 Li.6 F.3 Geothermal waters (Timperley, 1986) pH8.3 Na1330 K198 Ca23 Mg.18 Cl2290 SO35 HCO66 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 Reacted along with gases #1 #2 Concentrations taken from “GEOTHERMAL WATERS: A SOURCE OF ENERGY AND METALS.” Department of Earth Sciences, University of Waikato

California State Polytechnic University Pomona: The QAP Triangle Composition of rhyolite 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) NaAlSi 3 O 8 K-feldspar KAlSi 3 O 8 Quartz SiO 2 Biotite KMg 3 AlSi 3 O 10 (OH)

PHREEQC Interactive 1) Define minerals need for reaction PHASE Biotite formula found in Example 16 Need log k and delta h

Finding log k and ∆H For this reaction, log k of K-mica (KAl 3 Si 3 O 10 (OH) 2 ) was used log k= For delta h, ∆h for all minerals/elements in reaction Mineral/element∆h (kcal mol -1 ) KMg 3 AlSi 3 O 10 (OH) (Robie and Hemingway, 1984) H H2OH2O K+K Mg Al(OH) H 4 SiO KMg 3 AlSi 3 O 10 (OH) + H 2+ + H 2 O = K + + 3Mg 2+ + Al(OH) H 4 SiO 4 ∆H R = ∑ ∆H products - ∆H reactants ∆H R = ( ) – ( ) = kcal mol -1 PHREEQC Interactive

SOLUTION 1 Geothermal water (Timperley) temp 100 pH 8.3 pe 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 2) Define solution pe= x kcal mol -1 (2.303)(1.98x10 -3 )(398) Faraday constant RT Eh=-.059 x 8.3 pe=-6.22 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 pH8 Cl57 Na220 SiO175 K43 HCO1.2 SO3177 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 CO 2 H2H2 H2OH2O H2SH2S NH 3 +

Results Second water composition, with added gases Precipitating phases Anorthite Aragonite Calcite Dolomite Fluorite K-mica Talc 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 Vol. 38, pp Graham, I.J., et al. Petrology and petrogenesis of volcanic rocks from the Taupo Volcanic Zone: a review. Journal of Volcanology and Geothermal Research Vol. 68, pp , Robie, Richard and Hemingway, Bruce. Heat capacities and entropies of phlogopite (KMg 3 [AlSi 3 O 10 ](OH) 2 ) and paragonite (NaAl 2 [AlSi 3 O 10 ](OH) 2 ) between 5 and 900 K and estimates of the enthalpies and Gibbs free energies offormation. American Mineralogist, Vol. 69, pp Timperley and Vigor-Brown. Water chemistry of lakes in the Taupo Volcanic Zone, New Zealand. New Zealand Journal of Marine and Freshwater Research, Vol. 20, pp