Helium isotopes in Icelandic geothermal systems: I. 3He, gas chemistry, and 13C relations

Helium isotopes in Icelandic geothermal systems: I. 3He, gas chemistry, and 13Crelations R.J.

H.CRAIG,' S. ARN~RSSON,~ andJ. A. WELHAN',+ ‘Isotope Laboratory,Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093, USA %cienee Institute, University of Iceland, Dunhagi 3, Reykjavik, Iceland POREDA,"*

(Received May 7, 1992; accep&d in revised form Jury 22, 1992)

Abstract-Gas

samples from seventeen high-temperature and twenty-two low-temperature geothermal systems have been analyzed for chemistry and ‘He/“He ratios. Within the Neo-Volcanic Zone the 3He/ 4He ratios show a consistent regional pattern: I4- 19 times the atmospheric ratio ( RA) in the southwest, 8-l 1 RA in the north, and 17-26 RA in central Iceland. Outside of the rift zones a mantle helium component also dominates with the highest 3He/4He ratios found in waters circulating through ~-MYold crust in Northwest Iceland (up to 29 RA). The minimum Icelandic 3He/4He ratio (excluding a methane seep east of the rift) is 8.5 RA at Kverkfjoll, in central Iceland at the southern end of the narrow Northern Rift Zone; throu~out the NRZ the ratios vary only from 8.5 to 10.7 RA. The Kverkfjoll ratio is precisely the mean MORB ratio: (8 t 1) RA. Thus, the mantle helium emerging at Iceland is a simple mixture of two components: MORB He (8 R,) and deep-mantle plume He with R/RA > 29. High-temperature systems have C02/3He ratios of lo9 to 10” that encompass the range found in MORB (i-3 X 10’). However, the COz/‘He values have been subjected to postmagmatic effects that alter and obscure the original magmatic CO>/ ‘He ratios. 6( ‘%) in the fluid-phase CO2 is well defined at -3.8’% in the high-CO2 fluids (up to I mol/kg fluid), very similar to MORB values. CH4/3He ratios vary widely, from 3 X lo4 to lo*. Most high-temperature systems from southwestern and northern Iceland have CH4/3He ratios less than 106, while those from central Iceland have consistently higher ratios of the order of 107. Local conditions and possible proximity to an organic source of methane can have a strong effect on this ratio. setting. We have studied the chemistry and the helium and carbon isotopic com~sition of gases from Icelandic wells, fumaroles, and hot springs in order to obtain a complete picture of the gas composition and isotope geochemistry relative to the regional tectonic and volcanic factors. In a following paper ( POREDAand ARN~RSSON, 1992 ) a model relating the 3He content of the reservoir fluid to the fluid enthalpy is developed, based on the data from this work. Helium isotope geochemistry of the volcanic rocks erupted on Iceland and the mid-ocean ridges to the north and south has played an irn~~ant role in the undemanding of mixing between the mantle plume and the source for Mid-Ocean Ridge basalts. POLAK et al. ( 1976) identified Iceland as a region with high 3He/4He ratios up to 23 times the atmospheric ratio ( RA), in excess of the ratios found in MORB. The “higher than MORB” 3He/4He ratios in mantle plume environments such as Hawaii (CRAIG and LUPTON, 1976) and Iceland showed that these volcanoes are erupting from a less-degassed, more primordial reservoir, in agreement with models based on Sr, Pb, and Nd isotopic systems ( SCHILLING, 1973). The effect of the 3He mantIe plume signature is observed not only on Iceland but also - 1,000 km to the south along the Reykjanes Ridge, and - 500 km to the north along the Kolbeinsey Ridge ( POREDA et ai., 1980, 1986). On lceland itself, helium in subglacial basalt glasses and olivine phenocrysts shows a clear plume signature in 3He/4He ratio plus the effects of mixing with mid-ocean ridge and Icelandic crustal sources ( CONDOMINES et al., 1983; KURZ et al., 1985; POREDAet al., 1986). The effects of the plume on Icelandic Iavas appear to be asymmetric. The maximum 3He/4He plume signature is found in lavas from the Southeastern NVZ near ~ndmann~augar, with lower ratios along the Southwest

THE INTENSE VOLCANIC

and geothermal activity of Iceland provides a unique opportunity to study the interactions between magmatic and hydrothermal systems. Volcanic lavas, principally basaltic, erupted over the past 20 My have built an island of - 105 km2. The volcanic rocks are the product of complex interaction between a mantle plume, the midocean ridge, and an Icelandic crustal source, with geochemical characteristics varying greatly in space and time (e.g., SCHILLING, 1986). Volcanically and tectonically active rift zones (Neo-Volcanic Zones or NVZ), constituting one-third of Iceland, form the Mod-Atlantic plate boundary on the island and connect the Reykjanes Ridge in the south to the Kolbeinsey Ridge to the north. Within the active rift zones some twenty high-temperature geothermal systems (>2OO”C) are associated with Quatemary to Recent volcanic centers that range from composite shield volcanoes with well-developed calderas (Askja, Krafla) in central and northern Iceland, to dominantly fissure swarms in the Reykjanes Peninsula of southwest Iceland f SAEMUNDSSON,1986). Outside of these active rifts, numerous low-tem~mture ( < 16O’C) waters circulate through Tertiary f 3-16 My) basalts. Apart from the submarine hydrothermal activity on the East Pacific Rise and Mid-Atlantic Ridge, the Icelandic geothermal systems represent the only extensive region of hightemperature water-rock interaction outside of a continental

* Present address: Department of Geological Sciences, University of Rochester, Rochester, NY 14627, USA. + Present address: Idaho Geological Survey and Department of Geology, Idaho State University, Pocatello, ID 83209, USA. 422 1

15.61

9.32 12.25

12.73 16.93 17.27 11.2 27.3 25.8

16.26 16.56 22.87 22.37 18.91 23.14 25.77 20.50

SNAEFELLSNES I83-102 Lysohull We11 183-104 RaudamelsolkeldaSpr

NORTHWEST ICELAND 183-107 SaellngsdalurWell 183-106 Reykholar Spring 183-108 BordeyriWell I85SvannrhollNW-4 Spr 185 Krossnes Nu#l Spr I85 Hveravik N'J#ZSpr

CENTRAL ICELAND 183-116 HveravellirFum 183-117 KerlingarfjollFum 181-9 ThiorsadalurSnr 181-33 LandmannalaugarSpt. 183.120 LandmannalaugarFu,,, 183-123 &ndmannalaugar Fun, 85-3092 KoldukvLslarbornarFum 85-3014 VonarskardFur,.

n.m.

6.00

251 34.8

8.38 72.6

78.57 47.26 1.79

90.37

91.75 84.90 77.97 27.95 38.68 49.36 96.28 84.86 90 00 98.56 82.70 4.20 82.70

54.80

94.68

h 6 6 0

13.5;

32.88 33.53 8.52 2.59 5.33 5.48 0.75 4.32

8.06

6.18 4.65

4.4;

5.0: 8.12

75.20 74.30

0.00 60.30

1.69 11.84 0.00

42.70 71.67

first two numbers in sample I.D. are the year the sample YPS collected.

- H28 not measured: value in CO2 column is %(co2 + HIS)

n.!: not measured

h

ii -

250 57 11.2 1.6

NON-RIFT ZONE SAMPLES 85-3088 Thveit Spr 9.73 85-3089 BaejarstadaskogurWeLL 13.57 85-3086 Seljavallalaug 17.98 80 Lagarfloe Gas Seep 2.70 9.76 7.97 13.76 66.5 19.2 5.83 4.4-5.5 0.34

9.46 10.7 9.42 15.3 9.49 39.0 9.40 21.4 10.28 175 10.66 19.1

320 840 1000 200 40 140

6.47 3.80 8.02 3.78 14.2 21.4 3.97

10.45 a.54 8.84 9.46 9.91 9.74 9.68

16.40 18.2 16.68 6.42 23.27 315 22.93 20.0 23.4 2.22 23.74 2.18 25.95 36.0 20.87 2.57

0.0:

0.10 0.01

0.06

0.00

0.00

6

6

6

12.2;

2.9: 1.20 0.80 4.00 11.47

&

6 6

15.93

n.m. n.m. n.m. n.m.

% H2S

0.01 0.00

98.02

96.83

41.36

n.m. n.m. 14.39 80.25 20.14 43.70 7.26 34.07 31.34 96.31 94.83 97.89 84.10 76.20 97.10 81.10

a.m.

73.8 n.m. 13.2 49.2 175 43.1 27.0 23.6 72.5 7.89 5.87 12.0 7.80 13.2

76.7

% CO2

Systems

96.9

13.60 n.m. L7.40 101 17.28 2002 14.2 10.10 28.8 7.40 154 26.2

12.25

19.23 15.76 14.54 15.23 14.62 14.42 15.0 18.09 15.86 19.53 14.53 15.85

18.0 19.5 18.8 18.7 13.6 18.73

100 100 220 34 57 4:;::

100 120 50 55 5 38 125 54

14 34 1300 4.3 15.8 60

n.m. 1000

55

2::

2::

2;: 10.5

il; 19.2

29

5": 45

23.7

Rc/R* Re(PPa

Geothermal

NORTHERN RIFT ZONE 183-110 Askja Fum. 10.37 183-111 KverkfjcU Fur,. 8.47 183.112 Kverkfjoll Fum. 8.81 183-114 Namafjall Well-11 9.24 183-115 Namafjall Fum. 9.77 85.3020 Namafjell-Hverarondh un 0.00 183-113 Krafla Fur,.(Viri) 9.74 85-3015 Krafla Well-14 9.44 85-3016 Krafla Well-15 9.41 85-3017 Ktafla 8.8 Fur,. 9.48 85-3019 Krafla-LeirhnukurFum. 9.36 I85 AerlaekjarselSpr 10.05 85-3018 TheistareykirFum. 10.60

14.30 14.38 13.68 17.69 15.80 19.07 14.46

14.62

17.4 18.9 18.2 18.3 11.41 18.56 17.82 18.72 15.40 14.23

(He/Ne)A

l&J.&

of Icelandic

R/R*

1. Gas Chemistry

SOUTHERN ICEUND Fludir We11 79-1 HlemmiskeldWell 79-2 79-3 Sydra Langholt Well Selfoss Well 79-4 I81-6 Geysir Spring 183-118 Geysir Spring 181-7 Reykholt Well 181-E Fludir "el1 181-10 SeltjarnesWell 181.17 Reykir Well-KG-17 Reykir Well-~~-39 181-28 181-19 Reykjanes Well-7 181-20 Reykjanes Fum. 181-21 SvartsengiWell-8 181-22 HveragaerdiWell-2 183-119 NesjavellirFum. 183-124 KlaustarhalarWell Fum. 84-3105 Krisuvik I83-101 Arhver Spring

SAMPLE#

Table

0

n.m. n.m. n.m.

0.94

n.m.

13.00 36.92 16.09 15.10 0.80 5.31 4.23 0.54 2.22

8.60

1.64

n.m. 4.37 17.00 Tl.m. 4.30

0.009 14.12 n.m.

n.m.

n.m.

0

0 0

0

5.42

n.m.

0.009 0.626 0.001 2.90 10.12

0 0 0

0

0.019 0 0.389

n.m. n.m. n.m.

n.Ir.

% Hq

@

*

-

-

9.35 19.21 48.04 2.50

0.379 1.490 0.836 1.970 7.95 25.00 0.296 4.32 0.257 0.135 9.04 83.40 2.37

45.61 2.08 98.21 37.15 15.00 0.49 3.05 31.34

95.94

95.36

77.89 98.00 97.25 90.56

2.928 1.723

96.48 97.84 97.30 n.m. 8.12 18.54 78.73 54.79 90.68 64.12 66.09 0.76 2.25 1.05 8.70 1.86 2.77 1.12 50.90

$ N2

n.m,

0.265 0.505 1.407 0.050

0.007 n.m. 0.013 0.040 0.165 0.770 0.005 0.097 0.015 0.003 0.223 2.685 0,047

0.059 1.450 0.753 0.202 0.003 0.090 1.090

0.839

1.892

1.468

1.665 1.951 1.429

n.m.

0.072 0.039

0.050 1.097

0.059

0.0046 0.0036 0.050 95.10

0.0026 0.085 0.062 0.015 1.737 0.102 0.001 0.050 0.001 0.0049 0.095 5.659 0.138

0.014 0.123 0.334 L.bOP 0.119 0.053 1.137 0.222

0.010 0.125 0.238 0.002 0.013 0.020

0.047 0.020

0.055 0.140

0.286 0.015

Cl.067 0.146 0.001 0.023 0.001 0.030

0.136

0.016 0.460 0.292 0.270

n.m.

0.116 0.345 ".Ul. 1.198 1.685 1.596 1.555 0.011 0.688 0.013 n.m. 0.051

0.971 0.100 0.356

% CH4

2.04 2.22 2.36

8 Ar

35.2 38.0 34.1 49.2

51.7 n.m. 64.0 49.0 47.7 32.6 63.0 44.6 17.0 40.9 40.5 31.1 90.1

54.4 35.0 67.3 49.3 74.2 196.0 42.7 28.6

65.0 44.7

63.4

n.m. 52.5 49.7

40.6 44.1

47.1 44.0 41.2 n.m. 70.1 53.9 n.m. 45.6 53.7 40.2 42.9 69.4 32.8 80.4 n.m. 36.4 46.8 22.2 46.3

(L

CO

8324 622 3222 13930

9769 12344 7878 5709 1996 10124 3296 6004 4466 1905 2949 17 2914

1031 4815
".?ll. 0.04 10.01 3.8 3.5 0.01

295 1643

3:; 125 725 211 6097 8025 4252 4354 2620 6062 4702 261

3525 620

n.m. n.m. n.m. n.m.

6.9 0.69

154

1417 330 202 56.4 66.6 185 2310 334 900 1959 156

400

62 407

44.3 159

284

34.5 1.24 0.06 0.46 0.51 163 401 151 26.6 158

10.6) mi&h8

CO2/‘He

CO2

CH& Reservoir*

-72.3

-27.7 -3.5

-6.0 -5.7

-26 8 -32.4 -36.4 -29.9 -37.1

-39.6

-26.2

-23.6

-17.8

-32.4 -37.3

-21.7 -24.7

-22 4 -23.4

W.G and S are water. gas and steam‘samples.respectively. GGC. ICE, $10 are gas chromatographicanalyses made at Global Geochemistry Corp.. University of Iceland and Scripps Institution of Oceanograph,v,respectively

70 70

6

254

278 315 295 320 306 239

262

316 269 270 272

260 285 239 271

265 93

80

61 LOO 110

168 5

62

66

W,SlO

W,SLO W,SIO W,SIO G,GGC

S.GGC S,GGC S,GCC S,GGC S,GGG S,ICE S,ICE S,ICE S,ICE S,ICE S,ICE G,SIO S,ICE

S.SIO S,GCC C.SIQ C,SIO S,ICE S.ICE S.SIO S,ICE

G,GGC G,SIO W,SIO G,SID G,SIO G.SIO

G,GGC G,GGC

W,SIO w,sxo W,SIO 26620 G,GGC G,SIO 260 G,ICE 133 100 W.SIO W.SIO 114 W,SlO W,SIO :: S,GGC 248 285 S,GGC S,GGC 240 S,ICE 182 S,ICE 281 V.SIO 160 S,ICE 267 G,SIO 120

27

(O/oo) Temp.(C) 'ODE@

13c

-3.4 -4.1

-4.7

-3.7

-4.6

-5.5

-11.2

-4.0

-3.3 -2.5

-12.8 -6.7

-22.6

-6.2

-6.4

(O/00)

13c

for reservoirs less than 100". +he sampLing remperature is reported

0.014 1.706 3.237 0.580

0.035 0.244 0.051 0.227 1.846 0.870 0.026 0.000 0.002 0.003 1.340 4.058 0.188

9.13 0.112 0 0.247 0.083 0 0.228 3.760

22.10 0 0.560 7.95 3.15 2.15

0.123 0.194

: 6.511

1.412 0.408 0 0.042 0.238 0.143 0.069 0 0.383 0.240 n.m. 0

0 0 n.m

0.505

% 02

N2/Ar

M

e

Helium isotopes in Icelanldic geothermal systems NVZ decreasing to a minimum on the Reykjanes Peninsula ( KURZ et al., 1985). To the north the plume signature ends abruptly, with a ratio of -8 RA at Askja (CONDOMINES et al., 1983) the characteristic 3He/4He ratio in normal MORB. In addition to providing a unique fingerprint for the influence of mantle plumes, 3He/4He ratios and the ratio of the other gas species (especially CO2 and CH4) to mantle-derived 3He may provide insights into the origin of these gas components. Comparison of the CH4/3He and C02/3He ratios in Icelandic thermal waters with the purely igneous ratios in mid-ocean ridge basalt glass can be used to constrain the variability of these ratios in the mantle source and the effects of modification in the geothermal systems. SAMPLE

DESCRIPTIONS

AND ANALYTICAL

METHODS

In Table I we give the results of measurements on seventeen hightemperature geothermal systems (reservoir T > 1SO’C) within the Neo-volcanic zones at the locations shown in Fig. I. In three cases, both geothermal wells and natural fumaroles were sampled for comparison of the gas chemistry (Krafla, Namaljall, and Reykjanes). One sample from a high-temperature system (Geysir) consisted of “free gas” bubbling through a hot pool (92°C). Only the remote localities of Torfajokull (southern ENVZ) and Grimsvton (within the Vatnajokull glacier) are not included in this comprehensive survey. In addition, thirteen low-temperature (T -c 160°C) wells, eight hot springs, and a COz-rich spring outside of the Neo-Volcanic Zones were sampled by collection of gas when a free-gas phase existed, or by collection of the total fluid (Table 1). It is beyond the scope of this paper to describe the more than thirty sampling sites in detail; ARN~RSSON(1977) and ARN~RSSONet al. (1978, 1983a) provide descriptions of the sample locations. Many of our samples were collected from the features described by ARN~RSSONand GUNNLAUGSSON(1985).

4223

Gases from fumaroles and “free gases” from hot springs were collected in 5Occ flasks made of Corning-1720 glass and fitted with a high-vacuum stopcock, using standard inverted funnel techniques. Samples of total fluid from low-temperature wells and hot springs were also collected in these 5Occ 1720-glass flasks. For high-temperature well discharges, the “gas + dry steam” fraction was separated from the total-fluid discharge using a standard well-head separator (e.g., ELLISand MAHON, 1977; WELHAN, 1981), and collected in an evacuated 1720-glass flask. The gases in the flasks were extracted on a I720-glass high-vacuum line. The COz + H2S fraction was condensed at liquid-nitrogen temperature, purified of water with dry ice, and the total amount of gas was measured in a Hg manometer. The C02/HzS ratio was determined by thermal-conductivity gas chromatography (measured at Global Geochemistry Corp.). The noncondensable (NC)gases were split into three aliquants (to.I %). One fraction was used to measure the He and Ne concentrations and the 3He/4He ratio. Another was used to measure gas concentrations (N2, H2, Cz, Ar, CH4) by gas chromatography, using either a thermal conductivity or ultrasonic detector at Global Geochemistry Corp. or Scripps (“GGC” and “SIO,” respectively, in Table I ). Errors are - 1% for CO* and Hz, -5% for N.C. gas concentrations greater than 0.05% of total gas, and -20% for concentrations less than 0.05%. Additional information on gas analysis is given by WELHAN( 1981). Samples were also collected in evacuated bulbs containing IO mL of saturated KOH solution and analyzed at the University of Iceland for gas chemistry and gas/steam ratios (labeled ICE in Table 1). CO2 was measured by titration of the KOH-solution with HCL. H2S was measured by titration with mercuric acetate using dithizone to indicate the endpoint. The noncondensable (NC) gases were analyzed by thermal-conductivity gas chromatography. Gas concentrations in the steam fraction (mmol of gas/ kg of steam) are based on the volume of gas divided by the amount of condensate collected in the flask. (Errors are the same as mentioned earlier). Additional samples of the high-temperature systems are reported by ARN~RSSONand GUNNLAUCSSON ( 1985). Duplicate gas analyses (GGC and ICE) were made for ISL Samples 110, 1 I I, 112, 115, and 117. 3He/4He ratios were measured by previously described techniques ( RISON and CRAIG, 1983). Neon is routinely removed from helium

FIG. I. Geothermal sampling localities in Iceland, showing the 3He/4He ratios (as R/R,)for each site. The NeoVolcanic Zone is outlined by dashes, and the three areas of highest ratios ( -20 RA or more) are stippled. The minimum ratios are all slightly greater than MORB values ( R/RA = 8 f I ) except for the low-temperature, high-CH., seep (Iagartlot ) in eastern Iceland.

4224

R. J. Pore& et al

prior to the isotopic analysis, and the 3He/4He ratios are compared to our secondary standard of Yellowstone Park He (MM = 16.45 R,,) and are precise to - 1%. Helium concentrations (nmol of He/

kg steam) are based on the He/(C02 + H2S) ratio measured at SIO, and the CO1 and H2Sconcentrations (mmol of gas/ kg steam) measured at the University of Iceland. Helium concentrations in water samples are the measured amount of He divided by the weights of water. The measured 3He/4He ratios are corrected for the addition of atmospheric helium, either in nature or during sampling, by assuming that all the neon is atmosphe~c in origin (CRAIG et al., 1978a,b): i.e.. Rc = (RX - R,,&)/(X

- X,)

where R = 3He/4He, Rc is the corrected ratio, X is the He/Ne ratio in air, OYthe atmospheric solubility ratio in water at - 10°C and the subscript “A” denotes either the atmospheric ratios, or the atmospheric solubility ratio. In general we use the NZ/Ar ratio as a guide to the proper value of X to use when it is not obvious. This correction is most important in low-helium hot springs in which the dissolved atmospheric-helium component is significant. RESIJLTS Figure 1 shows the helium isotope distribution in geothermal fluids in Iceland. In general the results confirm the findings of POLAK et al. ( 1976) and show that there has not been a significant (> 10%) long-term change in the 3He/4He ratio of the geothermal systems over a ten-year period. (A detailed comparison cannot be made because Polak and coworkers did not include a method for evaluating the effects of air contamination on their 3He/4He ratios.) The data are also in general agreement with the survey results of HILTON et al. ( 1990) on samples collected in 1982-1983, although again a detailed comparison cannot he made because their He samples and standards were not purified of Ne, which can cause errors of several percent in the measured ratios. Their 3He/4He ratios tend to be somewhat lower than our results as expected for charge exchange of neon with helium (RISEN and CRAIG, 1983). The accumulated data from these three studies indicate that Iceland can be divided into several distinct provinces on the basis of the 3He/4He ratios in volcanic and geothermal gases. The SWNVZ and WNVZ from Reykjanes to Kerlingafjcill have ratios from 14.4 to 19.5 & with the highest values in several low-temperature wells near Geysir. The northern NVZ samples, from Kverkfjoll to the northern coast, have much lower ‘He/“He ratios (8.8-10.7 RA) in five high-temperature volcanic centers. There is a sharp increase in 3He/ 4He ratios south of Kverkfiiill in central Iceland. Fumaroles from Kiildukvislarbotnar have the highest 3He/4He ratios (26.0 RA) within the Neo-Volcanic Zones and may mark the center of the Icelandic mantle plume because these gases contain the greatest amount of p~mordial 3He relative to radiogenic 4He. The other high-tem~rature systems at Vonarskard (20.9 RA) and Landmannalaugar (23.7 R,) in the eastern NVZ are nearly as high as the maximum value. Outside of the Neo-Volcanic Zones, the gases are still dominated by a mantle helium component. The most striking feature of the non-NVZ samples is the presence of extremely high 3He/4He ratios in the northwestern part of Iceland, as observed both by POLAK et al. ( 1976) and HILTON et al. ( 1990). The highest 3He/4He ratios in Iceland (28.8 RA at Krossnes Spring and 26.2 RA at nearby Hveravik Spring)

occur in moderate-temperature ( <70°C) springs in this region (Table I). (Hveravik samples were reported by Polak and coworkers and Hilton and coworkers to have R IRA = 23.6 and 26.2, respectively.) Volcanism ceased in this area - 9 My ago so that the presence of the high 3He/4He fluids cannot be due to the active upwelling of a mantle plume. The other gases are dominantly of atmospheric origin and the ratio of C02/3He is less than 0.1% of a typical mantle or hip-tem~rature geothermal value. Presumably the helium in the fluids is extracted from the IO-My-old basalt as the water circulates through the crust. Although the helium concentration in subaerial basalts is low, mantle helium has been observed in other Tertiary volcanic provinces in Southern California, Guatemala, and East Africa (SIO unpublished data). The 3He/4He ratios in northwest Iceland must also be regarded as minimum estimates for the original ratio in the basalt& because circulating water will also extract radiogenic 4He, produced by in-situ decay of U and Th since eruption, although the magnitude of the radiogenic production cannot be easily quantified. The large-scale transport of mantle gases is usually associated with volcanic activity. The possibility that deep fractures in the basalt have facilitated the transport of gases from the mantle into the crust cannot be disproved, but the process would have to fractionate CO2 and He, since very little CO2 is observed in these gases. If the high 3He content is due to the decay of tritium from nuclear weapons testing, the helium at Hveravik (26.2 RA) must have degassed from a water that contained 56,000 TU in 1961 if all of the ‘He is due to tritium decay. (This calculation assumes all the nitrogen is from atmospheric solubility at 5°C and that during degassing nitrogen and helium behave similarly.) For the well at Bordeyri, the tritium content in I96 1 must have been 500,000 TU. Rain as high as 56,000 TU has not been observed, and it is most improbable that the aquifer at Hveravik is almost all water from the years I96 I - 1963. Measured tritium contents for Bordeyri are - IO rt 3 TU (SIO data). Within the NVZ, 3He/4He ratios of the geothermal fluids are remarkably similar to the 3He/4He ratios in nearby basaltic rocks (CON~MINES et al., 1983; KURZ et al., 198.5; POREDA et al., 1986). This close correspondence between fluid and basalt indicates that the transfer of helium from a volcanic to a geothermal system occurs with only a minor change in the helium isotopic composition. Submarine basalt glass from the Reykjanes Ridge ( 13.4 RA) in the south and the Kolbeinsey Ridge ( 12.0 RA) to the north ( POREDA et al., 1986) are similar to the adjacent subaerial Reykjanes Peninsula and northern NVZ. This indicates that the transition from the subm~ne to subaerial portions ofthe Mid-Atlantic ridge is a smooth one and not marked by major discontinuities in the mantle sources of the basalt. SANOet al. ( 1985) measured 3He/4He ratios at five localities in Iceland and proposed that the measured neon contents of the gases could be used to derive proportions of MORBtype and Plume-type helium “components” by using a Plumet.ypeHe/Ne ratio = 1000 (no precision estimate given). They also assumed that no crustal or “radiogenic” helium is present. It would be nice if useful information could be gained by such assertions, but it will be obvious that the assumed

Helium isotopesin Icelandic geothe~ai

Plume-type He/Ne value of 1000 is simply an assumption of the “whole-cloth” variety, with no validity as a measured value of any kind applicable here. This is a case in which the desire to produce important results has outstripped the available objective information. CO#He

RATIOS AND S13C OF CO2

The C02/3He ratio in high-tem~rature

Icelandic systems (Table 1: reservoir temperatures = 160 to 320°C) vanes from 0.63 X lo9 to 12.8 X lO’(Geometric Mean = 4.2 X lo9 at 26O’C). This range may be compared to the MORB CO*/ 3He ratio of -2 X lo9 (CRAIG et al., 1980; WELHAN and CRAIG, 1983; MARTY and JAMBON, 1987) and appears to be independent of the 3He/4He ratio or volcanic province (Fig. 2). The five high-temperature well samples fall within relatively narrow limits (4.2 X 109-6.1 X IO’), while the fumaroles display a much wider range, due to the effects of degassing. These data indicate that the Icelandic plume ratio is probably higher than the MORB value by a factor of 2 to 5, or so, but the data do not allow a more precise estimate. Figure 3 shows the distribution of 613C vs. the COz concentrations in total-fluid samples collected from eleven hightemperature geothermal fumaroles and wells (>26O”C) and four lower temperature wells. These samples are from eight high-temperature fields throughout the Neo-Volcanic Zones (southwest, central, and northern). There is no apparent correlation of 6 13Cwith volcanic province or ‘He14He ratios (not shown), which range from 9 to 24 ii,. Eight high-temperature samples (265-316’C) with the highest COz concentrations (> I50 mm/kg), have Sr3C values relative to PDB in the narrow range (-3.8 +: 0.7.1%0, and C0,13He ratios in the range from 2.6 to 12.3 X lo9 (mean = 7.3, geometric mean = 5.7 X 109). The values of -3.8%0 and ~6 X lo9 are thus our best estimate for the magmatic values of 6 13C( COz) and CO2 / 3He for the Iceland plume. The isotopic composition is 0.5%0 heavier than the mean MORB value (-4.3 f 0.1 %o)in three

5

10

15

20

25

30

35

FIG. 2. C02/‘He vs. 3He/4He. Most of the C02/‘He ratios, including the high-temperature reservoirs ( T > 16O”C), are greater than 0.6 X IO9 and show no apparent correlation with 3He/4He ratios. All samples from northwest Iceland have low C02/3He ratios. The open symbols with central points mark geothermal well samples. Lagarflot seep plots just above the upper left corner ( 13.9 X IO’) with a unique signature.

systems

4225

1

MORB

-5

--

r”lL) c z_+ -10 R FI

--

E hV -Lo

-15

--

/ I

-20

--

I

/

0

I

0

-25

Saltjarnes 114°C. 15.8

A Central Cl Northern

R,

t 10-2

10-l

too

CO, (mmoles/kg

10’

to2

t t

Southwest

lo3

steam)

FIG. 3. 6°C in CO2 as a function of the fluid CO2 content in samples of total fluid collection (fumaroles and wells). The eight high-temperature sites bracketed (265-3 16°C) have a mean 613C = -3X%0 f 0.7%0vs. the PDB isotopic standard. Lower temperature fluids appear to have lost r3C-enriched carbon by precipitation of carbonates. The MORB hydrothermal vent CO2 range includes SIO data (unpubl.) for two Galapagos fields and three 2 1ON fields, with CO2 concentrations ranging from 5.7 to I 1.6mmol/kg, and a mean &13C= -4.3%~ (range = -4.2 to -4.4%~) for all five fields.

high-temperature ( 2 1‘N ) and two low-temperature (Galapagos) hydrothermal vent fields (Fig. 3) with perhaps 3-6 times higher COz/3He ratios. Obviously, many factors can change the original magmatic 6 ‘IC and COz/‘He values during crustal residence (e.g., loss of CO* from the magma or geothermal fluids, oxidation of organic matter in the crust, precipitation of CaC03 at depth). However, the narrow range observed for these diverse high-temperature fields argues against major crustal m~i~cations to the 6 “C and the COz/ 3He values and indicates that the carbon source in Iceland is slightly but distinctly different from MORB values in the Pacific. It is noteworthy that the absolute He concentrations of the Icelandic fluids sampled here range up to 1.74 ccSTP/ kg (Krafla fumarole), more than 50 times the mean MORB values ( WELHANand CRAIG, 1983 ). However, the mean He contents of the high-temperature fluids are -0.045 cc/kg ( POREDAand ARN~RSSON, 1992), much closer to the MORB mean of 0.03 cc/kg (WELHAN and CRAIG, 1983). Lower temperature fluids in these systems have lighter carbon (lower ‘3C/‘2C ratios) and lower CO? concentrations (Fig. 3 ) . This effect is almost certainly due to calcite precipitation in the geothermal flow systems (ARN~RSSON and GUNNLAUGSSON, 1985) for the four or five fluids with only minor isotopic shifts (to about -7%0). Three samples (Table 1) have distinctly lower 613C values (- 11 to -23%0). All three are from low-temperature systems and have C02/3He ratios well below the magmatic ratio. The gases obtained from these waters are dominantly atmospheric with CO2 making up less than 50% of the total gas. At such low COz concen-

trations (< 1 mmol/kg),

small additions of CO2 can greatly

R. J. Poreda et al.

4226

affect the 6 13C values. Low 6 13C values for CO2 in thermal waters is often due to bacterial oxidation of isotopically light ( - -25%0) organic matter to COz . All of these samples have measurable O2 in either the gas or liquid phase. CHd /3He RATIOS The ratio of CH, / ‘He in geothermal and volcanic systems is an indication of the possible sources of methane (e.g., WELHAN and CRAIG, 1983; POREDA et al., 1988). Because ‘He is certainly mantle derived, the CH,/3He ratio may reflect the proportions of crustal and mantlederived methane, based on the CH4/3He ratio in mid-ocean ridge hydrothermal systems ( - lo6 on the East Pacific Rise) and oceanic mantle plume environments (- lo5 at Loihi Seamount; WELHAN and CRAIG, 1983; CRAIG et al., 1984). Figure 4 shows the CH4/3He ratios in the Icelandic data plotted against the 3He/4He ratios of the samples. The measured CH4/3He ratios have a very large range: from 3 X 10“ to lo8 with most of the high-temperature systems having ratios below 10’. There seems to be no correlation with 3He / 4He ratios or volcanic province. High-temperature fumaroles from adjacent localities in the northern NVZ span the range in CH4/3He, from 3.4 X lo4 at Krafla to 8.8 X lo7 at Namafiall. The very high value at Namafjall fumarole may represent a very localized phenomenon, as an adjacent well has a CH4/3He ratio which is 25 times lower. Three of the hightemperature systems from central Iceland (Landmannalaugar, Koldukvislarbotnar, and Vonarskard) have high CH,/ 3He ratios (7-27 X 106) and 3He/4He ratios greater than 20 R,, ,in marked contrast to the very low CH4/3He ratios ( 10 5, associated with high 3He/4He ratios (30 RA)at Loihi (CRAIG et al., 1984). Low CH4/3He ratios in northwest Iceland may reflect the original basalt concentrations of CHI and 3He, as the temperature of the water is too low (~70°C) to generate significant CH, by thermogenic breakdown of organic matter (e.g., SCHOELL, 1983). However, the CH4 values must be regarded as minimum estimates because CH4 consumption by bacteria may be occurring in these low-temperature springs.

-45 lo5

10’

lo6 CH,/‘He

FIG. 5. Carbon isotope composition of CH4 carbon (vs. PDB) vs. the CH4/‘He ratio. Based simply on the CH4/3He ratios the potential sources for Icelandic methane are very complex. The variability in CH4 / 3He ratios between wells and fumaroles from the same field highlights this complexity (e.g., Namafjall). It is unreasonable to suppose that the mantle beneath Iceland can maintain such disparate CH4/3He ratios on such a small scale; if the variation in the ratio of C02/CH4 (20 at Namafjall to 10’ at Krafla) represented equilibrium conditions, the oxygen fugacity of the mantle would have to vary by orders of magnitude beneath Iceland. Figure 5 shows the carbon isotopic composition (6 values vs. the PDB standard) of CH4 vs. the CH,/‘He ratio. There is a large range in 6 13C of CH4 (- 18 to -40%0) with progressively lighter values as the CH4/3He increases by about three orders of magnitude. The heaviest (most “C-enriched) methane samples and the lowest CH4/3He ratios, approximately -20%0 and 10 5, respectively, are associated with the high-temperature fields and are similar to values in the oceanic hydrothermal vents; they appear to represent dominant mantle source values as an endmember. In Fig. 6 the isotopic delta values for CH4 are plotted vs. the associated CO2 values together with the isotopic equilib-

”

0

-35

--

Lo

0 282. -40~-

__--

200” _____-----s&r

-45,

: -7

:

:

:

I -6

:

:

:

:

I

vs. 3He/4He ratios. Samples are labeled by location within the Neo-Volcanic Zone. Note that there is no apparent correlation with 3He/4He and adjacent samples can span the range in CH.J’He.

:

-5 613C (CO,)

FIG. 4. CHJ’He

:

zw_o_ 0 0 A 0

_ _ - - ..

Southwest Snasfsllnss.~ Central ‘: Northern : ” : r -4

-3

(“/..I

FIG. 6.6 “C in CH4 vs. CO2 with reservoir temperatures indicated for each isotopic pair. The isotopic equilibrium lines (CRAIG, 1953; BOTTINGA,1969) are shown for temperatures from 200 to 600°C for comparison with the measured reservoir temperatures printed by the sample points.

Helium

isotopes in Icelandic

rium contours for temperatures from 200 to 600°C. The measured reservoir temperatures are shown beside the points for the isotopic pairs, and one sees that the correspondence between measured and equilibrium temperatures is quite poor, indicating that isotopic equilibrium at the reservoir temperatures is generally not observed, and that other factors probably control the isotopic differences. In general the gas concentrations in these high-temperature systems do not correspond to equilibrium proportions for the reaction COz + 4H2 = CH, because

there

is too

little

CHI

+ 2H20,

at any

given

tem-

CONCLUSIONS Helium

isotope

ratios in Icelandic

4221

systems

The isotopic 13C endmember is well defined at -3.8% in the maximum CO* concentration fluids. In contrast, the CH4/ 3He ratio is extremely variable, ranging from 3 X lo4 to 10R. The high CHJ3He ratios appear to reflect either thermogenic production of CH4 at depth from organic matter or limited reduction of CO*. CO2 /3He ratios do not unequivocally reflect the mantle source as gas loss and CO> - calcite equilibria can dramatically alter the original magmatic ratio. The best estimate for the Icelandic CO2 / 3He ratio is - 6 X IO 9. thank Alan Jeffrey and Ian Kaplan of the Global Geochemistry Corporation for measuring the gas and carbon isotope compositions of some of the samples. E. Hernandez and H. Kueker expertly maintained the mass spectrometers. This research was supported by grants from the Mantle Geochemistry Program, Earth Sciences Division of NSF (EAR 82- 13237. EAR 85- 19230) to the Isotope Laboratory, SIO. Travel and field expenses were supported by NATO Grant 038 I /83 to J. A. Welhan at SIO. The manuscript benefitted from reviews by J. Varekamp and M. D. Kurz and discussions with D. Hilton.

Acknowledgmenfs-We

reservoir

perature (ARN~RSSON and GUNLAUGSSON, 1985). Thus, our interpretation of the CH4/ 3He data in the high-temperature fields is that the lowest CH4/3He and highest 13C/‘*C ratios in CH4 are close to the Icelandic mantle source values ( - 10 5 and -18%0), while the higher CH4/3He values and lighter isotopic ratios (Fig. 5 ) are due to formation of CH4 in the fluid by limited reduction of CO2 and/or thermogenic CH4 production from organic matter at high temperature. Either explanation conforms with the 6 13Cdata for methane, which show decreasing 613C values with increasing CH4/3He, consistent with the addition of a -40% thermogenic CHI component or limited reduction of CO> (SIO, unpublished data). An exception to the lack of isotopic equilibrium for carbon may occur in the Northern Rift zone, where the three heavy “C CO2 samples in Fig. 6 indicate equilibrium temperatures very close to the reservoir temperatures (270-3 16”C), in the region of a MORB-like 3He component. Time was not available to study these gases of this section in detail, but it appears that a detailed study of chemical vs. isotopic approach to equilibrium would be worthwhile in this relatively “clean” section of the rift. The highest apparent equilibrium temperature ( -580°C) is from Lysokull, a low-temperature well on the Snaefellsnes Peninsula. This water is highly charged with CO*, and the gas sampled evolves as the well discharges to one atmosphere. As shown in Fig. 6, the reservoir temperature is only - 168°C and this well may be producing gases with a quenched hightemperature history. By contrast, the isotopically lightest CH, encountered in Iceland, the Lagarflot seep east of the rift zone has 6( 13C) = -72.3% and is associated with the minimum 3He/4He ratio found, 5 R,_,: this is clearly methane of organic origin in an essentially pure occurrence.

geothermal

fluids

to those in recent basalts and appear to correlate

are

with volcanic province within the Neo-Volcanic Zones. The highest ratios within the NVZ are in central Iceland and probably indicate the center of Icelandic mantle plume. Outside ofthe NVZ, northwest Iceland has higher 3He/4He ratios (to 29 RA) in fluids circulating through 9-My-old basalt, perhaps marking the plume center at -9 My ago. Minimum ratios in geothermal fluids are 8.5 RA at the south end of the Northern Rift: this is the mean MORB ratio and indicates that helium in Iceland is a simple two-component mixture of MORB He (R/R* = 8)and deep-mantle plume He with R/ RA > 29. similar

geothermal

Editor&l

bundling:

G. Faure

REFERENCES ARN~RSSON S. ( 1977) Changes in the chemistry of water and steam discharged from wells in the NamfJall geothermal field, Iceland, during the period of 1970-76. Jokul/27,47-58. ARN~RSSON S. and GUNNLAUCSSON E. ( 1985) New gas geothermometers for geothermal exploration-Calibration and application. Geochim. Cosmochim. Acta 49, 1307-l 325. ARN~RSSON S., GRONVOLD K., and SICURDSSON L. ( 1978) Aquifer chemistry of four high-temperature geothermal systems in Iceland. Geochim.

Cosmochim.

Acta 42, 523-536.

ARN~RSSON S., SICURDSSON S., and SVAVARSSON H. ( 1982) The chemistry ofgeothermal waters in Iceland: I. Calculation of aqueous speciation from 0 to 370°C. Gco&irn. C’o.smochim. Acta 46, I5 I31532. ARN6RSSON S.. GUNNLAUGSSON E., and SVAVARSSON H. (1983a) The chemistry of geothermal water in Iceland, II. Mineral equilibria and independent variables controlling water composition. Guchim. Cosmochim. /Icla 47, 547-566. ARN6RSSON S., GUNNLA~JGSSON E., and SV~VARSSOY H. ( 1983b) The chemistry of geothermal water in Iceland: III. Chemical geothermometry in geothermal investigations. Gcvchim. C’o.smochim. Acta 47, 567-577. BOTTINCA Y. ( 1969) Calculated

fractionation factors for carbon and hydrogen isotope exchange in the system calcite-carbon dioxidegraphite-methane-hydrogen-water vapor. Gcv&irn. Cosmochim Acra 33, 49-64.

CONDOMINES M., GRONVOLD K.. HOOKER P. J.. MuEHLENaAcHs K., O’NIONS R. K.. OSKARSSON N.. and OXBURGH E. R. ( 1983) Helium. oxygen. strontium and neodymium isotopic relationships in Icelandic volcanics. Eurth PIunrr. Sci. Lru. 66, 125-l 37. CRAIG H. ( 1953) The geochemistry of the stable carbon isotopes. Gcochim. Cosmochim. .lcta 3, 53-9-7. CRAIG H. ( 1963) The isotopic geochemistry of water and carbon in geothermal areas. In Mrciear Gcolog~ on Grorhrrmal .,Irea.y: Proceedings o/’ the Firvt Spolc~io C’on/ivcvxc. Spc~lc~o, Italy, ( ed. E. TONGIORGI). pp. 17-53. V. Lischi & F&Ii. CRAIG H. and LUPTON J. E. (1976) Primordial neon. helium and hydrogen in oceanic basalts. Eurlh PluncTf. Sb. Lcrt. 31, 369-385. CRAIG H., CLARKE W. B.. and BECKM. A. ( 1975) Excess 3He in deep water on the East Pacific Rise. Eurlh P/uncf. .%i. Let!. 26, 125132. CRAIG H., LUP~ON J. E., and HORIBE Y. ( 1978a) A mantle helium

component in Circum-Pacific volcanic gases: Hakone. the Marianas. and Mt. Lassen. In Tcrreclricrl Rure Gases (ed. E. C. ALEXANDER and M. OZIMA). pp. 3-16. Japan Scientific Societies Press.

4228

R. J. Poreda et al.

H., LUFTON J. E., WELHANJ. A., and POREDAR. ( 1978b) Helium isotope ratios in Yellowstone and Lassen Park volcanic gases. Geophys. Res. Lett. 5, 897-900. CRAIG H., WELHANJ. A., KIM K., POREDAR., and LU~TONJ. E. ( 1980) Geochemical studies of the 2 I “N EPR hvdrothermal fluids. Eos 6i, 992. CRAIG H., KIM K. R., and RISONW. ( 1984) Easter Island Hotspot: I Bathymetry, helium isotopes and hydrothermal methane and helium. Eos 65, 1140. ELLISA. J. and MAHONW. A. J. ( 1971) Chemistry and Geothermal Systems. Academic Press. HERMANCEJ. F. ( 198 1) Crustal genesis in Iceland: Geophysical constraints on crustal thickening with age. Geophys. Rex Lett. 8,203206. HILTOND. R., GR~NVOLDK., ONIONSR. K., and OXBURGHE. R. ( 1990) Regional distribution of ‘He anomalies in the Icelandic crust. Chem. Geol. 88, 53-67. KURZ M. D., MEYER P. S., and SIGURDSSONH. (1985) Helium isotope systematics within the neo-volcanic zones of Iceland. Earth Planet. Sci. Lett. 74, 29 l-305. MARTYB. and JAMBONA. ( 1987) C/‘He in volatile fluxes from the solid Earth: Implications for carbon geodynamics Earth Planet. Sci. Lett. 83, 16-26. PALMASONG. and SRMUNDSONK. ( 1974) Iceland in relation to the Mid-Atlantic Ridge. Ann. Rev. Earth Planet. Sci. 2, 25-50. POLAKB. G., KONONOVI., TOLSTIKHINI. N., MAMYRINB. A., and KHABARINL. ( 1976) The helium isotopes in thermal fluids. In Thermal and Chemical Problems of Thermal Waters: Intl. Assoc. Hydrol. Sci. Pub/ I 19, pp. 17-33. POREDAR. J. ( 1983) Helium, neon, water and carbon in volcanic rocks and gases. Ph.D. dissertation, University of California, San Diego. POREDAR. J. and ARN~RSSONS. ( 1992) Helium isotopes in Icelandic geothermal systems: II. Helium-heat relationships. Geochim. Cosmochim. Acta 57, 4229-4235 (this issue). POREDAR. J., SCHILLINGJ-G., and CRAIGH. ( 1980) 3He/4He variations along the Reykjanes Ridge. Eos 61, 1158. CRAIG

POREDAR. J., SCHILLINGJ-G., and CRAIG H. ( 1986) Helium and hydrogen isotopes in ocean ridge basalts north and south of Iceland. Earth Planet. Sci. Lett. 78, I-1 7. POREDAR. J., JEFFREYA. W., KAPLANI. R., and CRAIGH. ( 1988) Magmatic helium in subduction zone natural gases. Chem. Geol. 71, 198-210. RISONW. and CRAIGH. ( 1983) Helium isotopes and mantle volatiles in Loihi Seamount and Hawaiian Island basalts and xenoliths. Earth Planet. Sci. Lett. 66, 407-426. S/EMUNDSSON K. ( 1986) Subaerial volcanism in the western North Atlantic. In Geology of North America, Volume M: The Western North Atlantic Region (ed. B. E. TUCHOLKEand P. R. VOGT), Chap. 5, pp. 69-86. Geol. Sot. Amer. SANO Y., URABEA., WAKITAH., CHIBAH., and SAKAIH. ( 1985) Chemical and isotopic compositions of gases in geothermal fluids in Iceland. Geochem. J. 19, 135-148. SCHILLINCJ-G. ( 1973) Icelandic mantle plume. Nature 246, 141143. SCHILLINGJ-G. (1986) Geochemical and isotopic variations along the Mid-Atlantic Ridge axis from 79”N to 0”N. In Geology of North America, Volume M: The Western North Atlantic Region (ed. P. R. VOGT and B. E. TUCHOLKE).Chap. 9, pp. 137-156. Geol. Sot. Amer. SCHILLING J-G., MEYERP. S., and KINGSLEYR. H. ( 1982) Evolution of the Icelandic hotspot. Nature 296, 3 13-320. SCHOELLM. ( 1983) Genetic characterization of natural gases. AAPG Bull. 67, 2225-2238. WELHANJ. A. ( 198 1) Carbon and hydrogen gases in hydrothermal systems: The search for a mantle source. Ph.D. dissertation, Univ. of California, San Diego. WELHANJ. A. and CRAIGH. ( 1983) Methane, hydrogen and helium in hydrothermal fluids. In Hydrothermal Processes at Seafloor Spreading Centers (ed. P. A. RONA et al.), Vol. 12, pp. 39 l-409. Plenum Press. ZINDLER A., HART S. R., FREY F. A., and JAKOBSSON S. P. ( 1979) Nd and Sr isotopic ratios and rare earth element abundances in Reykjanes Peninsula basalts: Evidence for mantle heterogeneity beneath Iceland. Earth Planet. Sci. Lett. 45, 249-262.