Geothermal gas compositions in yellowstone National Park, USA

Journal of Volcanology and Geothermal Research, 51 (1992) 79-93

79

Elsevier Science Publishers B.V., Amsterdam

Geothermal gas compositions in Yellowstone National Park, USA D.S. Sheppard a, A.H. Truesdellb and C.J. Janikb "DSIR Chemistry, Petone, New Zealand bu.s. GeologicalSurvey, Menlo Park, CA 94025, USA (Received July 17, 1991; revised and accepted October 27, 1991 )

ABSTRACT Sheppard, D.S., Truesdell, A.H. and Janik, C.J., 1992. Geothermal gas compositions in Yellowstone Park, USA. J. Volcanol. Geotherm. Res., 51: 79-93. Gas samples collected between 1974 and 1986 have been analysed for the ten major components. Samples have been collected almost exclusively from the tops of pools, which has degraded the value of the data, and limited inter-comparisons to the relatively insoluble components, Ar, N2, CH4, H2 and He. A general gas distribution pattern in the park, in terms of these components, shows the major heat source(s) to underlie the Gibbon and Mud Volcano areas with all other geothermal areas having gas compositions consistent with a general north-south water flow. Shoshone Basin gases show a large range of compositions and these are analysed in detail. The patterns conform to that which would be expected from an east-west flow or fluid with progressive boiling and subsequent dilution.

Introduction Since 1974, samples of geothermal gases from the thermal features at Yellowstone National Park have been collected and analysed at the U.S. Geological Survey at Menlo Park; few of these analyses have been published. Sampling and analytical techniques have evolved over this period so that the quality of the data has improved since the inception (by AHT) of the sampling programme.

Geological environment The structure and the geothermal features in the Park result from a series of high-volume, catastrophic volcanic ash eruptions and collapses. The eruption centres have been migratCorrespondence to: D.S. Sheppard, DSIR Chemistry, Private Bag, Petone, New Zealand.

ing progressively to the northeast. The sequence of events and details of the structures are eloquently portrayed in Smith and Christiansen ( 1980): it suffices here to show, in Figure 1, the location of the thermal areas, the faulting patterns and the collapse caldera within which the bulk of the features are located. The most recent review of the chemistry and structure of the hydrothermal system is by Fournier (1989). The most active areas are close to the caldera margins, suggesting that structural features may control these locations. Seismic and gravity data suggest that the caldera is underlain by a batholithic structure at about 6 km depth (Lehman et al., 1982), which provides the heat source driving the geothermal systems. Fournier and Pitt (1985) have suggested that the transition from brittle fracture to quasi-plastic flow takes places at a depth of 3 to 4 km beneath the Yellowstone caldera and that this is likely to to be the maximum

0377-0273/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

80

Fig. 1. Map of Yellowstone National Park, Wyoming, USA. Thermal areas are indicated by the solid marks. The caldera rim location is approximate only. Faulting patterns shown are generalised from Smith and Christiansen (1980).

depth of circulation of hydrostatically pressured fluid. Previous surveys

Hague ( 1911 ) and Allen and Day (1935) have analysed gases from most of the geothermally active areas in the park. More recently, gases have been analysed by Mazor and Wasserburg ( 1965 ), Gunter and Musgrave (1966), White et al. (1971), Mazor and Fournier (1973), Thompson and Hutchinson ( 1981 ), Welhan ( 1981 ), and Truesdell and Thompson ( 1982 ). Welhan ( 1981 ) presents many analyses, but the absence of units for the acid gas component limits the usefulness of the data. Mazor determined only the noble gases, while the others analysed for some or all of what are usually regarded as the ten major components of geothermal steam: H20, CO2, H2S, NH3, He, H2, Ar, 02, N2 and CH4. In many cases the analytical techniques used were not sensitive enough to detect some of these components. In

D.S. SHEPPARD ET AL.

general, the samples taken cannot be expected to accurately represent the steam supplying the features sampled, because of the sampling technique used. The sampling technique and its effect on the resultant gas composition will be discussed later in this paper. The main conclusion reached in early studies, that air-saturated meteoric waters are a dominant gas source, while probably correct, was based on analyses of air-contaminated samples collected mainly from pools. Samples taken from non-pool sources such as research wells (Mazor and Fournier, 1973) or fumaroles and "frying pans" (this paper), show much smaller influence of atmospheric gases. The most recent papers on Yellowstone gases were a series on noble gas isotope studies by the Berkeley RARGAgroup (see Kennedy et al., 1985, 1987, 1988) in which they commented on the problems in obtaining representative samples. Their studies refined earlier hydrological models of the Yellowstone system. They postulated that the compositions of their entire suite of samples from Yellowstone was explainable by invoking three basic source components: magmatic, crustal and atmospheric, each having a characteristic noble gas isotopic compositional range. They were able to correlate high 3He compositions with high 4°Ar and dilutions with lower 4°Ar crustal gases (all relative to 4He). In addition, they found atmospheric gases in all samples taken from spring and pools. Truesdell and Kennedy (1986) and Fournier (1989) presented general hydrologic models of the Yellowstone geothermal system based upon the earlier models of Truesdell and Fournier (1976) and Truesdell et al. (1977), and helium isotope studies. The general model is one of a deep, extensive, homogeneous 350°C aquifer within the caldera, recharged with cold waters from mountains to the north and northwest. This water is heated within the caldera, and flows to the west and south to supply the western geyser basins, Shoshone and Boundary Creek areas (see Fig. 1 ), and north

GEOTHERMAL GAS COMPOSITION IN YELLOWSTONE NATIONAL PARK, USA

81

towards Mammoth Springs. The fumarolic areas in the park result from steam rising off ascending and hence boiling waters. Fournier ( 1989 ) suggested that variations in liquid and gas composition correlate with the presence or absence of permeable rhyolite flows within relatively impermeable ash-flow tufts. In most areas of the park, waters boil and lose steam and gases, a n d / o r mix with cold water, before reaching the surface. Regional trends in the water chemistry show increasing dilution by cold waters from Norris, through the Lower and Upper Geyser Basins, to Shoshone. Dilution by cold water inhibits the boiling of ascending hot waters, especially at Shoshone. The waters of the Upper and Lower Geyser Basins are likely to be the result of mixing of deep fluids with cold infiltrating groundwaters. The Norris Geyser Basin contains both of these water types. White et al. (1988) postulated a hot water upflow north of the Norris Basin supplying hot water as far north as Mammoth. This hypothesis is supported by evidence from radium isotope concentrations in waters along the Norris/Mammoth Corridor (Jordan and Turekian, 1990). The northeastern features are underlain by vapour-dominated systems, which are characterized by the acid-sulphate features at Mud Volcano. The purpose of this paper is to address whether gas compositions can be used to reveal which deep and shallow processes affect the fluids at different locations in the system and to review previously reported gas data.

and the exchange of gases between water and vapour. Rising gas bubbles are collected in an inverted water-filled funnel and are conducted via tubes to either an evacuated flask or a flask of cold water. Such a technique generates two problems: (1) Components of the gas mixtures are fractionated with respect to each other due to their differing solubilities in the liquid water phase (s) in the pool or in the collecting train. The most soluble components (NH3, H2S, CO2) will be reduced relative to the less soluble components unless the pool water itself has higher concentrations of dissolved gases than water at equilibrium with the steam. In addition, water vapour will be removed from the gas stream. (2) All water in contact with air tends to become saturated with atmospheric gases (N2, 02, Ar) and a gas mixture passing through this liquid will entrain to some extent these dissolved gases. The reliance on water-displacement collection methods and pool sources has resulted in a bias in the types of samples taken, since fumaroles and other steam-heated features have tended to be ignored because there was no liquid water available. These latter types of sample sources are now regarded as being able to give the most representative samples of the subterranean gases (Sheppard and Giggenbach, 1985; Kennedy et al., 1988). An investigation of the effect on gas composition of depth of sampling is described later in this paper.

Samplingtechnique

Sample preservation and analysis

Sample collection

Samples are generally collected as the gases, and stored and analysed as such, or collected into NaOH solution to absorb the dominant acid gases. The gas phase is analysed by gas chromatography, although ORSAT apparatus has been used by some investigators. The condensed phase is generally analysed by wet chemical methods, for CO2, H2S, and (rarely) NH3. The caustic collection method has the

The prevalent method used to sample the gases from thermal features in Yellowstone National Park has been some variation on a water displacement method. This has resulted in unfortunate consequences because practitioners typically failed to allow for the significant effects of the solubility of gases in water

82

D.S. SHEPPARD

advantage of concentrating the non-condensable gases, enabling the determination of minor components on standard equipment. A secondary advantage of the method is that the larger total amount of gas collected serves to lessen collection errors caused by steam condensation in the collection tubes and the entrainment of small quantities of air or liquid water. All the samples analysed at USGS were collected into caustic soda solution, and most of the later samples were analysed on the gas chromatographs described by Sheppard and Truesdell ( 1985 ). The condensed components were analysed using methods described by Sheppard and Giggenbach ( 1985 ), except that H2S was determined as sulphate by ion chromatography after oxidation of the sample with hydrogen peroxide. Prior USGS analyses used a similar GC procedure: CO2 was determined by pressure change measurement following the addition of phosphoric acid to carbonate precipitated by adding strontium, and H2S w a s determined gravimetrically after oxidation and subsequent precipitation as BaSO4. Analytical method errors render CO2 analyses of samples collected in 1974 and 1975 unreliable. The results up to 1986 are presented in data listings which are available on request from the first author; some data are listed in Tables 1 and 2. Units are mole fraction expressed as percent, on a dry gas basis. Mole fraction ex-

ET AL.

pressed as percent is also used to express the concentration of gas in the steam collected, as Xg. The data listings contain the analyses of some 193 USGS samples, and 102 from other investigators.

The effect of sampling depth To establish the effect of depth of collection within pools on the sample composition, sets of collections were made on a pool at Yellowstone National Park and in pools in three New Zealand geothermal fields. The composition of the waters from these pools, details of the sampling conditions, and results of the gas analyses are listed in Tables 1 and 2. Compositiondepth profiles relative to hydrogen are shown in Figure 2. Two collection train systems were evaluated as a part of these experiments. The USGS method uses flexible 6 mm ID polyethylene tubing to conduct the steam sample from a translucent polyethylene funnel to flask, and was used in all but the 1983 survey of Yellowstone features. The DSIR method, described in Giggenbach (1975), and Sheppard and Giggenbach (1985), uses 25 m m diameter thin wall joined titanium tube sections. The two methods approach, in different ways, the problem of minimizing the mixing of phases, and the consequent effects of the composition of the gas phase. The narrow bore tubing ensures that

TABLE1 Compositions of pool waters Pool

Jubilee Ngawha, N.Z.

Whakarewarewa, N.Z.

Opaheke, N.Z.

Horseshoe Norris, U.S.A.

pH

Li

Na

K

Rb

Cs

Mg

Ca

SiO 2

B

NHa

F

CI

ar

SO,

HCO s

7.2

10.4

909

64

0.29

0.60

1.4

10.7

151

851

160

1.4

1290

5.7

446

331

493

85

0.60

0.48

0.07

1.9

360

0.6

4.3

772

185

83

1.5

5.4

742

45

305

5.2

341

175

8.1 8.7

5.5

670

76

0.51

1.31

0.005

0.35

337

15

2.5

2.75

235

56

0.47

0.36

0.25

2.5

325

4.8

All analyses at DSIR Chemistry Division, except Horseshoe Pool, from a 1975 sample (Thompson and Yadav, 1979 ). Concentration units are mg/kg.

G E O T H E R M A L GAS C O M P O S I T I O N IN YELLOWSTONE N A T I O N A L PARK, U S A

83

TABLE2 Gas analyses of samples collected from pools Pool

T, *C

Jubilee Ngawha, N.Z.

Whakarewarewa Rotorua, N.Z.

Opaheke N.Z. Horseshoe Norris, U.S.A.

Sampling Depth (m)

Xg

CO2

H25

Nila

He

Ne xl04

H2

Ar

02

N2

CH4

46

S

18.5

92.0

0.22

0.061

0.0015

0.914

0.0064

0.0031

1.15

5.61

Dupli- ( catcs ( (

0.3

87.8

92.3

0.51

0.0070

0.0013

0.914

0.0064

0.0031

1.15

5.61

0.0042

0.0013

88

89

0.3

85.4

92.6

0.43

0.868

0.0021

0.0073

0.77

5.61

(0.2)

57.5

87.5

2.59

I <.00044

0.0016

3.1

0.874

0.201

<.00099

8.42

0.42

0.2

45.0

86.3

2.88

<.00044

0.0016

4.1

0.863

0.210

<.00098

9.24

0.46

0.8

76.6

89.7

2.97

<.00034

0.0013

2.7

0.776

0.140

<.00073

6.06

0.33

1.4

63.6

88.0

3.75

<.00051

0.0015

2.6

0.783

0.163

<.00095

6.87

0.39

2.0

21.4

90.7

4.22

<.00034

0.0009

1.6

0.477

0.104

<.00065

4.20

0.26

(0.2)

41.4

6.6

1.45

0.034

0 . 0 0 1 7 4 27.3

2.22

1.70

<.0~0

0.2

12.9

49.9

0.62

0.020

0 . 0 0 2 3 8 44.9

1.43

1.17

1.5

45.9

97.6

1.42

0.0018

0.0012

0.00037

0.024

0.00123

0.842

0.091

(S)

183

98.1

0.89

0.0024

0.0009

0.00035

0.023

0.00034

0.837

0.066

(1.5)

22.7

98.4

0.84

<.00022

0.0007

0.00(X38

0.019

<.00005

0.646

0.056

79.4

8.60

40.4

6.49

-

?

Compositional units, except for Xs, are tool% dry gas. Xs is tool% gas/( gas + steam ). S-Surface; ( ) USGS narrow bore plastic tubing method, otherwise DSIR method of collection (see text); GC analyses on N.Z. samples by W.F. Giggenbach.

both the condensed and gaseous phases flow with equal velocity and reach the sampling bottle in the correct proportion. The use of the plastic should result in smaller effects due to external temperature variations, due to the low thermal conductivity of the material. The rigid metal tubes allow free drainage of pool water, although this is a mixed blessing because condensed steam and dissolved gases can also be lost. It is usually possible, however, to arrange the tubes so that the last segments are sloped downwards towards the collection flask, allowing collection of most of the condensed steam. Large compositional differences can exist between samples collected by the two methods, whether deep or shallow in source (Table 2; Fig. 2). For samples collected near the surface, higher gas contents were found using narrow bore plastic tubing compared to the results obtained using wide bore titanium tubing; the reverse is also true for the one comparison made at depth. An explanation for these observations has not occurred to us. The variation in total gas content of the

steam, Xg, with depth of sample collection is complex, as shown by detailed profile from the Whakarewarewa site. Individual components exhibit a more readily explainable behaviour, in terms of sources and solubilities. Large changes are observed in the more soluble components ( H 2 S and NH3) due presumably to dissolution in or exsolution from pool water. However, where the narrow bore tubing was used, these changes are quite small. In all cases, atmospheric gas concentrations increase towards the surface, due either to the scrubbing of these gases from the water or to the solution of CO2 in the pool water. This latter mechanism is supported by the observations that in the acid (Yellowstone) pool sampled, where CO2 absorption should be minimal, these effects are small. The effects of removal or addition of water as a component can be negated by using gas ratios, and hydrogen is chosen for these comparisons because it is reliably analysed and has a very low solubility. The composition-depth effects are emphasized in this presentation; that

84

D.S. SHEPPARD

is, the decreasing concentration of atmospheric component gases with depth, the increasing concentration of soluble gases with Xg SURFACE

E T AL.

depth, particularly in the alkaline Whakarewarewa pool, and the constancy of the ratios

CO2

H2S

NH3

¢

0

o

0

.......

e

@

10

E

v -r" I-0_ iii E)

20

I

0

2:~0 410 60 8O I mol °/o I~gas/steam • gas

i

L

H2

H SURFACE

I

200 400 6 0 0 8 0 0

L

I

I

I

I

I

10 20 30 4 0 0204 moI °/o dry gas

4;

Ar

I

I

0608

CH4

0

O-. . . . . .

6

0

F • e o e 0

1'0

/

J

2"0

(a)

0 0.01 0.02

10

2b

~

Ngawha Yellowstone

Whakarewarewa Opaheke Narrow bore tube

tx

;

2.ss

I

#o ~'odoso

mOPJod r y g a s

Fig. 2. (a). Composition-depth profiles for gas samples taken from pools. The sampled pools were: at Ngawha Springs, Northland, New Zealand--Jubilee Pool; at Whakarewarewa, Rotorua, New Zealand--Pool (no. 358?) near Roto-a-Tamaheke; at Opaheke Springs, New Zealand--Bath Pool; at Norris Basin, Yellowstone National park, USA--Horseshoe Pool. Most samples were taken with 25 mm tubing, but some, for comparative purposes, with narrow bore (6 mm ) tubing. See text for details. (b). Compositions relative to hydrogen, plotted against depth. Otherwise as for (a).

GEOTHERMAL GAS COMPOSITION IN YELLOWSTONE NATIONAL PARK, USA

85

SURFACE --4D ~l~x.1)

I

1"0 (::

-r I-(:L LLI

..

C3 2"0

I

I

5O

Xg/H2

I

IO0

I

I

I

150

5°C02~ °

5

I

10

H2S/H2

SURFACE

.o?

1"0 E

! --°4~

212

t3 2'0

o

(b)

I

I

I

2

4

6

l

I

I

I

I

I

8 10

NH3/H2

He/H 2

Ar/H2

I

2

I

4

1

6

Nz/H2

l

I

I

8

2

4

8 10

C HJH2

Fig. 2. (Continued).

of the sparingly soluble gases, CH4 and He, to H2, with depth. The relationship between samples taken from the surfaces of pools and the composition of the steam being injected into those pools is best understood by comparing those compo-

nents least dependent upon the processes taking place within the pools. The relatively insoluble, un-reactive and non-atmospheric components He, H2, and CH4 are the components least affected. Two conclusions can be drawn as a conse-

86

D.S. SHEPPARD ET AL

quence of these investigations. The first is that investigators should avoid taking samples from pools. Fumaroles and frying pans (areas of sizzling emanations of steam) are far superior locations for sampling representative subterranean steam. The second conclusion is that if pools must be sampled, then the sampling should be done at the depth of the steam injection point using narrow bore tubing, not by trapping bubbles at the surface.

stone National Park has low H2 content in relation to C O 2 , the major diluting gas, when compared with other geothermal systems in the world. The total gas content of the steam, Xg, also tends to be very low, by world standards. Sample sources at temperatures below boiling ( < 92.5 °C ) will have elevated gas contents as a consequence of lower water vapour pressures and must also be suspected as having undergone processes such as condensation and interaction with cold waters. Trends and groupings based upon the location of the sample source are discernable in Figure 3. Samples from four areas dominate along the H2-He axis, these being the Norris and Gibbon basins and the Mud Volcano and Crater Hills areas (see Fig. 1 ). A significant number of samples from both

Discussion of data

A triangular plot of He-H2-CH, is shown in Figure 3. The scales of this plot have been manipulated to give a good spread of data on the figure, but the reader should be aware of the large factors involved. Steam from YellowH2

S9

Gibbon

Thumb ,Potts ~ West Heart Lal~ Shoshone

41PBoundary Creek

Norris

O Upper Geyser Basin @ Lower Geyser Basin I Gibbon Basin-including Norris Barn terrace rl Mud Volcano I1 Crater Hills /~ Nocth Eastern Sources Mammoth

4(

50 \ Shoshone

Mud Volcano Crater Hills

~

7c 20

100 He

50

60 I

-

-7o

8o

CH4

West Thumb,Heart Lake

Fig. 3. Plot of relative He, H2 and CH4, includingdata from partial analysesand from Welhan ( 1981 ) for outlyingareas.

87

GEOTHERMAL GAS COMPOSITION IN YELLOWSTONE NATIONAL PARK, USA

N21100

( )-Corrected for atr contaminatK~n (~)-Severe air contamination le high 02

-'-Air

? 7 8

o

0

/

o

(

~

ASW (10"C)

(~l(

O

20

9

/ 10 He

. I(~

20

3~)

40

5~)

6~)

70

80

90

Ar

Fig. 4. Plot of He, N2and Ar. ASW is air-saturated water compositionat the temperatures indicated. Symbolsare for areas, as for Figure 3. Bracketed symbols have been corrected for air contamination, as described in the text. Circled symbolsindicate those samplesbadly air contaminated,i.e., with 02 > 0.1 tool%. the Norris and Gibbon areas also contain relatively high CH4. A number of samples from the Upper and Lower Geyser Basins form an intermediate group between high relative H2 and CH4. Samples from Shoshone show a large range in composition from high to low methane, and this is the only area to do so. The bulk of the remaining samples on the figure, from West Thumb, Heart Lake, and Lower Geyser Basin, tend to have relatively high CH4 and low H2 values. High He gases have been sampled in a CO2 spring at Terrace, in a fumarole on Roaring Mountain (both near Gibbon), and from Mammoth Springs (Welhan, 1981 ). Only two analyses of samples from the "northeast" area (Fig. 5 ) are usable; one from east of Mount Washburn (Welhan, 1981 ) had high H2 and low He, while an incomplete anal-

ysis of a sample from Ponuntpa Springs (sample YJ-81-45, with non-condensable gas compositions in tool%, of He 0.014, H2 2.39, 02 14.4, N2 71.6, CH4 13.2), has relatively high CH4 and low He. Partial analyses for a few samples from the shallow research wells drilled in the late 1960's tend to show low relative H2. A plot of the components He-Ar-N2 along with the compositional ratios for air-saturated water (ASW) at two temperatures is shown in Figure 4. The N2 and He apices and the ASW point are believed to represent "magmatic" (N2); "crustal" (He), and meteoric (ASW) end members, an admittedly simplified interpretation after observations of gas compositions from many New Zealand geothermal and volcanic gases. The meteoric attribution is obvious while long residence times and/or deep

88

D.S. SHEPPARD ET AL.

YELLOWSTONE

NATIONAL

)

" h

.,.,=/"

•

2-5

•

5-10 samples

• ....~ 0 •

N

)tortheest~'."r

Oib~ O O - " - / ~l

km

i"~'"'X,x --~

(.)

•(.~j..lerr~ce

PARK

/

~.

Crater ~ \ . . . . .

\

"I

/

) \

f

samples

~ 10 samples revers high He high H 2 low CH~ high CHt. low H 2

Fig. 5. Arealdistribution of relative gas concentrationsin the YellowstonePark basins. The size of the symbols is proportional to the number of samples. circulation are thought to provide the means for He accumulation. High relative N2 contents are found in non basaltic volcanic vents and the "hotter" geothermal systems. Obvious atmospheric air contamination (shown by 02 in the analysis) has been corrected for by subtracting N2 and Ar in their air ratios to 02. This does not fully compensate for dissolved air because of the different solubilities, but does allow for air contamination during sampling, and is considered to be adequate, particularly as dissolved air molecular ratios approach the ratios of air as temperature increases. Most plotted analyses cluster around a trendline between ASW and the He apex. Few samples show the nitrogen enrichment shown

in volcanic vents or geothermal systems closely associated with volcanoes, except for the samples taken in the Shoshone Basin. A 1986 resampling at Shoshone failed to yield any Narich samples which casts doubt on the earlier analyses. Most of the samples taken elsewhere in the 1983 survey, from pools or from frying pans and fumaroles, plot on the predominant trend line but near the He apex. Figures 3 and 4 illustrate the geographic groupings in the relative gas concentrations between the basins. The dominant feature is the largely coincident high He and high Ha set of samples from Gibbon, Mud Volcano and Crater Hills and from parts of Norris and the Upper and Lower Geyser Basins. A widespread high CH4-1ow H2 field (Fig. 5) includes the Gibbon and Geyser Basins, West Thumb, Shoshone, Heart Lake and Boundary Creek areas. A number of samples from Shoshone show a range to high CH4 and low H2. The distribution pattern shown in Figure 5 correlates with the (3He/4He) m,x distribution patterns shown in Kennedy et al. (1985) in that high He from the Northern basins correlates with the high 3He/4He found in Gibbon and Mud Volcano. The similar pattern of high H2-1ow CH4 includes some Shoshone samples, and so correlates with an unexpectedly high (3He/4He) maximum found there. Elevated 3He occurs where magmatie volatiles reach the surface. Subsequent fractionation by boiling and mixing with crustal He will disguise this signature to some extent so where it persists it can be considered to be due to very limited boiling, (Kennedy et al., 1987 ). Truesdell and Thompson ( 1982 ) consider that the Shoshone Basin system is a largely non-boiled but cold water-diluted system and this may also account for the slightly elevated 3He/4He. The question becomes one of explaining the distribution patterns observed within the system for He, H2 and CH4, and attempting to address the problem of how many heat sources exist for the system. Helium is perhaps the most easily interpret-

GEOTHERMAL GAS COMPOSITION IN YELLOWSTONE NATIONAL PARK, USA

able gas because it is non reactive, its sources are known, and isotopic evidence assists in the interpretation. The evidence suggests that high He gases surface mostly in the northern arc. High He/H2, He/CH4, He/N2 or He/Ar have not been fund elsewhere in the park, except at Mammoth Springs and Roaring Mountain. The correlation with high 3He/4He suggests that these basins are the most directly connected with the degassing magma (s) presumed to be the heat source (s). The high 3He/4He noted at Shoshone does not correlate with high He, which would support the hypothesis that the Shoshone system derives from a deep hot water diluted with cold water which has not degassed significantly by boiling, rather than the result of a separate degassing source suggesting by Lehman et al. (1982) and Kennedy et al. (1985). A temperature dependant equilibrium can exist between Hz and CH4,e.g.: CH4 + 2HEO = CO2 + 4H2

( 1)

with the products being favoured with increasing temperature. Mineral phases can also affect the equilibrium. Both H2 and CH4 are present in magmatic gases, especially H2 with abundances of up to 10%. High H2 and perhaps low CH4 with high He in the northern arc could be evidence for a degassing magma body close at hand. Relatively high CH4 would be favoured in reaction (1) at lower temperatures, i.e., in a fluid cooled by boiling or dilution. CH4 can also be incorporated in gases as they pass through organic-containing sediments, (Love and Good, 1970; Fournier, 1989 ) and, particularly in the east and north of the park, may not have equilibrated with the other gases. The Hot Springs Basin has not been adequately sampled for gases. Geophysical evidence (e.g., Lehman et al., 1982) is interpreted to indicate that a primary heat source is located under the basin. Welhan ( 1981 ) lists samples from alongside the Yellowstone River and one partial analysis is available of non-

89

condensible gases from Ponuntpa Springs within the Basin. Kennedy et al. ( 1985 ) report a relatively low 3He/4He ratio, and the available gas analyses indicate low He and H2, and

high CH4. These observations do not support this area as overlying a magmatic heat flow zone, nor is there any convincing (e.g. H2 and CH4trends) that would link either the northeastern sources with the northern-arc gases. Shoshone Basin

The gases from the Shoshone Geyser Basin were sampled in 1981, and again in 1986. Good sample location control, reasonably extensive sampling, consistent analytical technique and a study (by Truesdell and Thompson, 1982) of water chemistry in the basin, all aid in the interpretation of gas compositions. The increase in CO2/H2S from east to west across the basin, noted in Truesdell and Thompson (1982) is confirmed, in a gross scale, while the (CO2/H2S)/other gases pattern noted by them was not confirmed. The reliance on taking samples from pools makes interpretation of their results more difficult due to the differing solubilities and scavenging of atmospheric components, as discussed above. Ratios of sparingly soluble species presented in Figure 6a and b show high H2 in the east, a high He tongue in the centre, and high N2 in the west and centre of the basin (Fig. 6d). The strike of these contours is generally subparallel to the surface faulting (Truesdell and Thompson, 1982). Most samples were taken from sources aligned in this direction (NESW) and this observation may be an artifact of the pattern of sample sites. The model for the subsurface hydrology of the basin illustrated in Truesdell and Thompson (1982) provides an adequate explanation for the observed gas ratio distributions. The model is of liquid upflow from the northeast intersecting near vertical faults at progressively shallower depths to the west, with boil-

90

D.S. SHEPPARD

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Fig. 6. Contours of relative gas compositions in Shoshone Basin springs. (a) H2/CH4 and faulting pattern (Truesdell and Thompson, 1982). (b) H2/He.

91

GEOTHERMALGASCOMPOSITIONIN YELLOWSTONENATIONALPARK,USA

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92

ing at depth giving steam dominated fracture feeds in the northeast. In the western faults, the up-flowing fluid is diluted with descending meteoric waters causing the high N2/He ratios seen in the west and central upflow zones. The high CHa/H2 in the west may be due to background methane picked up as the fluids flow through glacial sediments after removal of most Hz and CH4 by earlier boiling. The early boiling in the east would preferentially remove the more insoluble gases, (He, H2, CH4, N2) resulting in higher concentrations of these in the gases from the eastern sources.

General model A general hydrologic model of the Yellowstone Geothermal system can be inferred from the data as analysed in this report. A deep meteorically derived flow of water from the mountains in the north (Truesdell and Kennedy, 1986; Fournier, 1989) is heated as it passes below the Gibbon Basin, Mud Volcano and Crater Hills areas. Some fluid rises to the surface in these areas, some is diverted north along fault systems from Norris to M a m m o t h (White et al., 1988 ), while much of the heated deep fluid continues south to mix with percolating meteoric waters to varying degrees and emerging at the surface where structural features, such as the caldera rim fault system, allow.

Conclusions ( 1 ) The large data set is degraded because most samples were taken from pools rather than fumaroles. Such samples are not representative of the steam supplying the gases to these sources and are therefore not able to be fully utilized. (2) Sparingly soluble gases, particularly He, Hz and c n 4 show areal changes in their ratios over the park. High He and H2 sources are limited to a northern arc of basins, including Gibbon, Mud Volcano and Crater Hills and also

D.S. SHEPPARD ET AL.

parts of the Norris, Lower and Upper Geyser Basins. (3) The remaining sampled basins show lower He and higher CH4, which is taken to indicate that they are outflows from the northern arc of basins, modified in transit. (4) There seems to be no corroboration of a northeastern (Hot Springs Basin) major heat source (suggested by Truesdell and Fournier, 1976 and Truesdell et al., 1977 ) although sample coverage in this area is very poor. However, this observation may result from a lack of surface expression over the area rather that the absence of a heat source. (5) An anomalous spread in H2/CH4 in Shoshone basin can be explained by progressive boiling across the basin from a deep feed. (6) An additional sampling survey will be required to fill in the gaps in coverage and extend the number of adequate samples to cover the system. This should enable more detailed analysis of all components as a test of the model presented here.

Acknowledgements We acknowledge the helpful comments of Dick Glover, Colin Downes, Bill Evans and Bob Fournier. The field work assistance of Terrie Winnett, Mark Wheeler, Ann Brown and Mac Kennedy is also acknowledged with thanks. Thanks also to the staff at Yellowstone National Park for their assistance and cooperation.

References Allen, E.T. and Day, A.L., 1935. Hot springs of Yellowstone National Park. Carnegie Inst. Washington, Publ. 466. Fournier, R.O., 1989. Geochemistry and dynamics of the Yellowstone National Park hydrothermal system. Annu. Rev. Earth Planet. Sci., 17:13-53. Fournier, R.O. and Pitt, A.M., 1985. The Yellowstone magmatic-hydrothermal system. In: C. Stone (Editor), Trans. Geotherm. Counc. Int. Syrup. Geotherm. Energy, Int. Vol., pp. 319-327. Giggenbach, W.F., 1975. A simple method for the collec-

GEOTHERMAL GAS COMPOSITION IN YELLOWSTONE NATIONAL PARK, USA

tion and analysis of volcanic gas samples. Bull. Volcanol., 39: 132-145. Gunter, B.D., 1973. Aqueous phase-gaseous phase material balance studies of Argon and Nitrogen in hydrothermal features at Yellowstone National Park. Geochim. Cosmochim Acta, 30:1175-1189. Gunter, B.D. and Musgrave, B.C., 1966. Gas chromatographic measurements in hydrothermal emanations at Yellowstone National Park. Geochim. Cosmochim. Acta, 30:1175-1189. Hague, A., 1911. Origin of the thermal waters in the Yellowstone National Park. Bull. Geol. Soc. Am., 22: 103122. Jordan, F.C. and Turekian, K.K., 1990. Time scale ofhydrothermal water rock reactions in Yellowstone National Park based on radium isotopes and radon. J. Volcanol. Geotherm. Res., 40:169-180. Kennedy, B.M., Lynch, M.A. Reynolds, J.H. and Smith, S.P., 1985. Intensive sampling of noble gases in fluids at Yellowstone: 1. Early overview of the data; regional patterns. Geochim. Cosmochim. Acta, 49:1251-1261. Kennedy, B.M., Reynolds, J.H., Smith, S.P. and Truesdell, A.H., 1987. Helium isotopes: Lower Geyser Basin, Yellowstone National Park. J. Geophys Res., 92 (B 12 ): 12,477-12,489. Kennedy, B.M., Reynolds, J.H. and Smith, S.P., 1988. Noble gas geochemistry in thermal springs. Geochim. Cosmochim. Acta, 52: 1919-1928. Lehman, J.A., Smith, R.B., Schilly, M.M. and Braile, L.W., 1982. Upper crustal structure of the Yellowstone Caldera from seismic delay time analyses and gravity correlations. J. Geophys. Res., 87:2713-2730. Love, J.D. and Good, J.M., 1970. Hydrocarbons in thermal areas, Northwestern Wyoming. U.S. Geol. Surv., Prof. Pap. 644B. Mazor, E. and Fournier, R.O., 1973. More on noble gases in Yellowstone National Park hot waters. Geochim. Cosmochim. Acta, 37:515-525. Mazor, E. and Wasserburg, G.J., 1965. Helium, Neon, Argon, Krypton and Xenon in gas emanations from Yellowstone and Lassen Volcanic National Parks. Geochim. Cosmochim. Acta, 29: 443-454.

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Sheppard, D.S. and Giggenbach, W.F., 1985. Methods for the analysis of geothermal and volcanic waters and gases. NZ DSIR Rep. CD2364. Sheppard, D.S. and Truesdell, A.H., 1985. A GC system for the analysis of residual geothermal gases. Chromatographia, 11:681-682. Smith, R.B. and Christiansen, R.L., 1980. Yellowstone Park as a window on the earth's interior. Sci. Am., 2: 104-117. Thompson, J.M. and Hutchinson, R.A., 1981. Chemical analyses of waters from the Boundary Creek thermal area, Yellowstone National Park, Wyoming. U.S. Geol. Surv., Open File Rep. 81-1310. Thompson, J.M. and Yadav, S., 1979. Chemical analyses of waters from geysers, hot springs and pools in Yellowstone National Park, Wyoming from 1974 to 1978. U.S. Geol. Surv., Open File Rep. 79-104. Truesdell, A.H. and Fournier, R.O., 1976. Conditions in the deeper parts of the hot spring systems of Yellowstone National Park, Wyoming. U.S. Geol. Surv., Open File Rep. 76-428. Truesdell, A.H. and Kennedy, B.M., 1986. Yellowstone before (and after) RARGA. Unpublished manuscript. Truesdell, A.H., Nathenson, M. and Rye, R.O., 1977. The effects of subsurface boiling and dilution on the isotopic compositions of Yellowstone thermal waters. J. Geophys. Res., 82: 3694-3704. Truesdell, A.H. and Thompson, J.M., 1982. The geochemistry of Shoshone Geyser Basin, Yellowstone National Park. In: Thirty-third Annual Field Conference, 1982, Wyo. Geol. Assoc. Guideb. Welhan, J.A., 1981. Carbon and hydrogen gases in hydrothermal systems: the search for a mantle source. Ph.D. Thesis, Scripps Institute, University of California, San Diego, CA. White, D.E., Hutchinson, R.A. and Keith, T.E.C., 1988. The geology and remarkable thermal activity of Norris Geyser Basin, Yellowstone National Park, Wyoming. U.S. Geol. Surv., Prof. Pap. 1456. White, D.E., Muffler, L.J.P. and Truesdell, A.H., 1971. Vapor-dominated hydrothermal systems compared with hot-water systems. Econ. Geol., 66: 75-97.