Environmental isotope investigations of New Zealand geothermal systems—A review

Geothermics, Vol. 12, No. 2 3, pp. 223 - 232, 1983.

0375 ~ 6505]83 $3.00 * 0.00 Pergamon Press Ltd. : 1983 CNR.

Printed in Great Britain.

E N V I R O N M E N T A L ISOTOPE I N V E S T I G A T I O N S OF NEW Z E A L A N D G E O T H E R M A L S Y S T E M S - - A REVIEW J. R. H U L S T O N Institute o f Nuclear Sciences, DSIR. Lower Hurt. New Zealand

AbslraeI--Environmenlalisotope investigations of geothermal systems at the Ne~ Zealand Institute of Nuclear Sciences have concentrated in recent years on combining several isotopes with chemical analyses. Geothermal hydrology has been studied by the use of hydrogen and oxygen isotopes with chloride and other water analyses. Rocks and minerals in well cores have added information to the geology and chemistry through the use of oxygen isotope techniques. Oxygen, hydrogen, carbon and sulphur isotope geothermometry have been combined in an attempt to derive temperature profiles with depth. :::Rn measurements in soil indicate leakage through faults, while :::Rn measurement of ~ell discharges show evidence of underground processes. Future work will include noble ga~ isotope measurements. Measurements of 'H and "C are reported in an associated paper on tracing of underground water moxements.

INTRODUCTION New Z e a l a n d ' s geothermal activity derives from large scale tectonic processes in the region. W a r m springs in the South Island occur along the m a j o r t r a n s c u r r e n t Alpine Fault (Fig. I) while the North Island g e o t h e r m a l features are d o m i n a t e d by the Pacific plate s u b d u c t i o n zone along the east coast ( H i k u r a n g i - Kermadec Trench), f o r m i n g the central volcanic zone (Fig. 2) with a variety of volcanoes, t h e r m a l areas a n d g e o t h e r m a l p r o d u c t i o n zones stretching from Mr. R u a p e h u to White Island. There is some further activity in the N o r t h e r n geothermal region, particularly at Ngawha, a n d in the H a u r a k i geothermal region, e n c o m p a s s i n g older geothermal areas which have been studied as ore deposits ( R o b i n s o n , 1974; R o b i n s o n and Christie, 1980). Since the last review of our work ( H u l s t o n , 1977), a concerted effort has been made to investigate most of the m a j o r g e o t h e r m a l areas o f the North island. Stable isotope a n d chemical samples have been collected s i m u l t a n e o u s l y to m a k e i n t e r p r e t a t i o n more complete. The areas studied include all those for which p r o d u c t i o n depth wells are available, i.e. Ngawha (Fig. I), Wairakei, B r o a d l a n d s a n d Kawerau (Fig. 2). We have also looked at other areas where geophysical resistivity lows have been f o u n d . These are at W a i o t a p u , Tikitere, O r a k e i k o r a k o and Mokai. GEOTHERMAL HYDROLOGY A n i n t r o d u c t i o n to the use of d e u t e r i u m a n d '+O ratios in New Z e a l a n d waters has been published by Stewart a n d T a y l o r (1981) a n d the techniques used in our l a b o r a t o r y described by H u l s t o n et al. (1981b). M e a s u r e m e n t s o f g e o t h e r m a l waters made by Stewart (1978), G i g g e n b a c h (1971), Lyon and S h e p p a r d (1981), M c D o n a l d (1966), Sheppard and Lyon (1981), Henley and Stewart (1983), H u l s t o n et al. (1981a) a n d Stewart, M. K. (in p r e p a r a t i o n ) are s u m m a r i z e d in Fig. 3. The meteoric waters fall on a line 8D = 8 8 ' " 0 + 13],~,with the d e u t e r i u m a n d '"O values of surface waters of the central volcanic zone sho~ving the typical hydrological features of low d e u t e r i u m a n d 'sO in the high altitude areas in the centre of the North Island, where 8D = - 56~,,, with a steady increase in d e u t e r i u m c o n t e n t to - 32t,,, near the coast. The only a n o m a l o u s n a t u r a l waters in this respect are Lake T a u p o and the R o t o r u a Lakes, where sufficient e v a p o r a t i o n occurs from the surface to enrich the water isotopes to the extent that deuterium has a value of - 32~,,, in Lake T a u p o c o m p a r e d to approx. - 43g,,, in the s u r r o u n d i n g 223

224

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,~;~ri_._n9Tc~perafar es m Max Temp >70"C • flax. Temp 40-70*(: o flax. Temp 20-40*[ + Tempnot known • Cities Oi liOO 200kmi Fig. 1. Thermal and geothermal regions of New Zealand. rainfall. Analysis of deuterium and '80 in steam and water phases of well discharges has allowed the estimation of the isotopic composition of the deep water feeding the systems at Wairakei, Broadlands, Waiotapu and Kawerau. These are shown as black circles in Fig. 3. The oxygen isotopes are enriched in 5'80 due to reaction with rocks but the deuterium values generally associate with the local ground water. This figure indicates a hydrogen isotope shift for the Broadlands area but further investigations are under way to see if this water may have an origin further to the north (e.g. Waiotapu) in which case no hydrogen isotope shift would need to be postulated. Studies of the correlation of these oxygen shifts with the chloride content of the waters have been made and, in particular, it has been found that variations in chloride at Kawerau correlate with '80. It is suggested that cold water infiltration may occur in this particular area. Stable isotope studies have also been made on thermal waters from Rabaul caldera, Papua New Guinea (Green et al., 1978), and Fiji (Cox and Hulston, 1980). '~0 IN ROCKS AND M I N E R A L S In a joint laboratory with the New Zealand Geological Survey '80 measurements of altered and unaltered fractions of reservoir rock from Wairakei, Broadlands, Kawerau and Ngawha are proceeding. The example of Fig. 4 shows c a l c i t e - w a t e r deduced temperatures for four related wells at Kawerau. The depth profiles show an interesting feature, suggested

Environmental Isotope Investigations of New Zealand Geothermal Systems

225

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Fig. 3. Plot o f 6 ~ O vs 6 D for deep geothermal waters and their source waters from a number o f New Zealand geothermal areas. Lake Taupo and the Rotorua lakes show isotopic enrichment because o f evaporation, and the geothermal waters are enriched in " O compared to local ground waters because o f w a t e r - r o c k interaction. (WR, Wairakei; BR, Broadlands; THo Tauhara; WT0 Waiotapu; KA, Kawerau; and NG, Ngawha.) (After Stewart and Taylor, 1981.)

independently beforehand by well performance data, in that relatively cool surface water seems to have infiltrated two of the well areas at 7 0 0 - 800 m depth (Blattner, 1979). In general, the data lead to refined models o f pre-drilling hydrology and allow estimates of cumulative water/rock ratios. In the case of Ngawha, the deep well water as inferred from calcite analyses

226

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shows a very much larger oxygen isotope shift than is apparent at Wairakei, Broadlands and Kawerau (also compare Fig. 3). This may indicate a major magmatic contribution to the Ngawha discharge or we may be observing a very young, incipient, meteoric water system, so that the input water suffers a large isotope shift. At the opposite end of a scale of evolution, Wairakei seems to represent a very late stage of a meteoric water system (Blattner, 1981, 1982).

STABLE ISOTOPE G E O T H E R M O M E T R Y For a number of years we have been investigating the possibility of using isotope equilibria of differing reaction rates to establish temperature profiles with depth and this has appeared reasonably successful (Hulston, 1977; Hulston and McCabe, 1962). The h y d r o g e n - w a t e r isotope temperatures generally agree with temperatures at accessible drilling depths, as do sulphate- water '~O temperatures, provided that near surface oxidation of hydrogen sulphide does not occur (Robinson, 1978a, 1978b). Recently, Hulston (1978) attempted to establish a suitable temperature scale for the m e t h a n e - w a t e r isotope equilibrium (Fig. 5). The curves 1000

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Environmental Isotope Investigations of New Zealand Geothermal Systems

227

m a r k e d (a) a n d (b) represent the two possible theoretical curves o f f e r e d by Richer et al. (1977). The points with h o r i z o n t a l e r r o r lines show e x p e r i m e n t a l values derived by s u b t r a c t i n g the m e t h a n e - h y d r o g e n curves o f C r a i g (1975) f r o m h y d r o g e n - water i s o t o p e e x p e r i m e n t a l data. The solid line shows the curve which H u l s t o n (1978) chose to a d o p t after m a k i n g s o m e estimates o f the z e r o - p o i n t energy shifts from curve (b). T h e m e t h a n e - water r e a c t i o n should be faster than the m e t h a n e - c a r b o n d i o x i d e exchange b e c a u s e only the o u t e r a t o m s need to be involved in the exchange process a n d , hence, this g e o t h e r m o m e t e r should indicate t e m p e r a t u r e s nearer the surface than m e t h a n e - c a r b o n d i o x i d e e q u i l i b r i a (Truesdell a n d H u l s t o n , 1980). U n f o r t u n a t e l y , there is very little change in the e q u i l i b r i u m values in the region f r o m 250 to 350°C, m a k i n g this g e o t h e r m o m e t e r sensitive to errors in this region. Figures 6 a n d 7 show the a p p l i c a t i o n o f these t e m p e r a t u r e s to results from Tikitere, N g a w h a a n d B r o a d l a n d s ( L y o n a n d H u l s t o n , 1980, 1983).

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Fig. 6. Hydrogen isotope ratios (+D values with respect to SMOW) of methane and hydrogen from some Ne~+Zealand geothermal areas. The temperatures shown are those for equilibrium between methane and water (vertical lines) and between hydrogen and water (horizontal lines), using water ,SD values of -40',I,o for Broadlands, -31%o for Ngawha and - 35%o for Tikitere. The Ngawha well samples are outlined by dashes and the springs by the solid line (after Lyon and Hulston, 1980). Fig. 7. Carbon isotopic ratios (6"C values with respect to PDB) of methane and carbon dioxide from some New Zealand geothermal areas. The temperatures shown are those for isotopic equilibration between the molecules (after Lyon and Hulston, 1980).

Figure 6 shows the d e u t e r i u m i s o t o p e values o f the h y d r o g e n plotted against that o f m e t h a n e f r o m the s a m e sample. T h e h o r i z o n t a l lines show t e m p e r a t u r e e q u i l i b r i u m values for the h y d r o g e n - w a t e r e q u i l i b r i u m in the range 1 5 0 - 4 0 0 a c . Tikitere is shown as the lowest t e m p e r a t u r e , N g a w h a a r o u n d 250°C for the n a t u r a l features and 3 5 0 a c for the well results (shown in the d o t t e d area). B r o a d l a n d s also shows t e m p e r a t u r e s a r o u n d the 3 2 5 - 3 5 0 ° C region. The vertical lines (slightly d o g l e g g e d because o f the differing water 6D values in each area) indicate the implied m e t h a n e - water e q u i l i b r i u m t e m p e r a t u r e s . S t a r t i n g from the b o t t o m l e f t - h a n d c o r n e r , the first vertical line refers to t e m p e r a t u r e s o f 150°C to 250°C at Tikitere, with N g a w h a in the 2 0 0 - 2 5 0 ° C region and B r o a d l a n d s in the 2 5 0 - 3 0 0 ° C region. A l t h o u g h this figure is s o m e w h a t c o m p l i c a t e d by s h o w i n g the two t e m p e r a t u r e scales it does indicate that there is some c o r r e s p o n d e n c e between h y d r o g e n - w a t e r and m e t h a n e - w a t e r isotopic t e m p e r a t u r e s in these areas.

228

J. R. H u l s t o n

The m e t h a n e - c a r b o n dioxide isotopic temperatures shown in Fig. 7 again correspond to increasing temperatures from Tikitere through Ngawha and Broadlands to Ketetahi (a fumarole area on the slopes of Nit. Tongariro). We feel that these temperatures probably indicate temperatures at considerable depth, because of the slow isotopic equilibrium rate of the m e t h a n e - carbon dioxide exchange [probably slower than that originally indicated bv Hulston (1977) and Hulston and McCabe (1962)], but field evidence suggests that this reaction is probably not as slow as predicted by Giggenbach (1982). Des Marais et al. (1981) have suggested that the hydrocarbon gases are kinetically derived rather than a result of chemical equilibrium, but have agreed that the samples of relatively positive methane 6'~C compositions probably indicate higher temperatures at depth. 1 prefer to leave this as an open question at this stage, as I am not entirely convinced that their measurements of hydrocarbons have proved that isotopic equilibrium is not attained particularly where the methane content is greater than that of nitrogen. We do not, however, in our laboratory use samples of very low methane content for isotopic equilibrium estimations, partly because we find it extremely difficult to obtain a 100% oxidation of methane to carbon dioxide in our conventional chemical apparatus when the methane/nitrogen ratio falls below 1%. Sulphate (and bisulphate) ions occurring in geothermal discharges can be analysed for both their sulphur and oxygen isotopic compositions. Isotopic equilibration with the hydrogen sulphide and water of the fluids produces temperature-dependent fractionations which make two geothermometers, s u l p h a t e - h y d r o g e n sulphide and s u l p h a t e - w a t e r , practicable. The calibration curves on which these geothermometers are based are discussed in Robinson (1978a, 1978b). These curves can be more rigorously applied to natural systems if the kinetics of the isotope exchange reactions are also known (Hulston, 1977). The rate of oxygen isotope exchange between sulphate and water is both pH- and temperature-dependent, but at pH 6, 97% equilibrium is achieved in about 0.5 ,,'ears at 300°C and 4 ,,rears at 200°C. Sulphur isotope exchange between sulphate and hydrogen sulphide is dependent on total sulphur concentration as welt as temperature and pH. Times for the above conditions and total sulphur of 10-' mol/kg are 5 years at 300°C and 500,000 years at 200°C. Robinson (1978a) estimates that at Wairakei hydrogen s u l p h i d e - s u l p h a t e reaches 70% isotopic equilibrium in 1000 years--a value considerably longer than that estimated by Hulston (1977). In order to obtain meaningful results for the s u l p h a t e - w a t e r geothermometer, it is necessary to exclude samples where a significant fraction of the sulphate has been derived from near surface oxidation of dissolved hydrogen st, lphide. The particular criteria depend on the type of discharge being sampled but low temperature pools are particularly suspect. 22-'R n M E A S U R E M E N T S Dr. N. E. Whitehead of our Institute has been using ' : : R n measurements in two ~vavs. Firstly (Whitehead, 1981), radon has been measured in the top layer of soils in Wairakei and Karapiti using an c~ technique in which a cellulose nitrate film is placed in the bottom of a plastic cup and this cup is buried in an inverted position at 30 cm depth for several weeks. The film is then treated with alkali and tracks show cc particles from the daughter product decay of :::Rn. These are counted in order to give a measure of the radon content in the soil. The result of a survey at Wairakei is shown in Fig. 8. The contours showing the radon counts obtained are superimposed on a map showing the wells (black dots} and the faults identified by Grindley (1965). It vdll be seen that the radon concentrations are correlated with the presence of fault traces, though the concentrations are not uniform along the trace. There appears to be also a slightly elevated general radon level in regions of thermal activity. These results indicate such measurements may be useful for geothermal prospecting. An extensive survey has recently been made of the Nga',vha area and the results are currently being assembled.

Environmental Isotope Investigations of New Zealand Geothermal Systems

113w

229

112w

Fig. 8. Map of 2::Rn study area at Wairakei. Co-ordinates are from Grindley (1965). Numbered spots are geothermal wells. Contour lines (italic values) are counted tracks per week of exposure (after Whitehead, 1981).

A second use of radon measurements (Whitehead, 1980) has been to measure the '"Rn in the gas discharge of geothermal wells and springs. The most interesting results come from radon analyses of wells at Wairakei where the radon concentrations tend to correlate with carbon dioxide (see Fig. 9). Since there is also a negative correlation with "C content, the radon

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230

J. R. H u l s t o n

p r o b a b l y does not originate from local g r o u n d water. At W a i r a k e i , boiling u n d e r g r o u n d causes the r a d o n to be most c o n c e n t r a t e d in the high e n t h a l p y wells, since r a d o n is relatively insoluble in water. F o r s o m e reason which we do not yet u n d e r s t a n d , the r a d o n c o n c e n t r a t i o n at W a i r a k e i correlates best with the m e t h a n e - c a r b o n d i o x i d e isotopic t e m p e r a t u r e with a very strong negative c o r r e l a t i o n coefficient. W e are currently looking at o t h e r c o r r e l a t i o n s in o r d e r to fully explain these results. WARM SPRINGS Stable i s o t o p e and chemical studies o f w a r m springs o f N o r t h Island have been studied by Downes et al. (1980) and those o f the S o u t h Island by Barnes et al. (1978). C h e m i c a l and isotopic t e m p e r a t u r e s show a wide range from surface t e m p e r a t u r e s to g e o t h e r m a l area t e m p e r a t u r e s a n d , in fact, it is s o m e t i m e s difficult to decide if an area should be classified as w a r m spring, or a g e o t h e r m a l area. Figure 10 shows the 6~'C value plotted against total c a r b o n content o f these samples. It will be seen that the m a j o r i t y o f these give 6 " C values in the range - 8 to - 10~',o, which is slightly m o r e negative than the c o m m o n l y accepted range o f -5g,,, to - 8 g , o for m a n t l e c a r b o n (Barnes and M c C o y , 1979).

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NOBLE GASES H e l i u m i s o t o p e v a r i a t i o n s in the thermal areas o f New Z e a l a n d have been studied by T o r g e r s e n et al. (1982). The 3 H e / ' H e ratios are generally in the range 5 - 8 times that o f air, in general a g r e e m e n t with earlier work on s u b d u c t i o n zones. T h e large v a r i a t i o n s o b s e r v e d t h r o u g h o u t the T a u p o zone and within the W a i r a k e i g e o t h e r m a l field are not c o r r e l a t e d with gas or water c h e m i s t r y but d o correlate with a b s o l u t e helium c o n c e n t r a t i o n and with ~°Ar/36Ar ratios, suggesting that the o b s e r v e d helium isotope variations are due to a mixing betsveen a ' H e rich m a n t l e end m e m b e r a n d a r a d i o g e n i c c o m p o n e n t a d d e d either at depth or near the surface. H o p e f u l l y , we will now be able to m a k e further m e a s u r e m e n t s in i n d i v i d u a l areas to see if this technique can be used to define the best p r o d u c t i o n areas for g e o t h e r m a l p o w e r p r o d u c t i o n . Dr. M. K. Stewart in our l a b o r a t o r y , is also p l a n n i n g to study the contents and stable isotope ratios o f the heavier rare gases. M e a s u r e m e n t s o f tritium and '~C are r e p o r t e d in an associated p a p e r on tracing o f u n d e r g r o u n d water m o v e m e n t s ( M c C a b e et al., 1981).

Environmental Isotope Investigations of New Zealand Geothermal Systems

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