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Sampling of volcanic gases — The role of noble-gas measurements: A case study of Vulcano, south Italy
Chemical Geology, 49 (1985) 329--338
329
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
SAMPLING OF VOLCANIC GASES - - THE ROLE OF NOBLE-GAS MEASUREMENTS: A CASE STUDY OF VULCANO, SOUTH ITALY EMANUEL
MAZOR
The Frank W. Considine Professorial Chair in Hydrological Research, Geo-Isotope Group, The Weizmann Institute of Science, 76 100 Rehovot (Israel) (Accepted for publication July 16, 1984)
Abstract Mazor, E., 1985. Sampling of volcanic gases -- The Role of noble-gas measurements: a case study of Vulcano, south Italy. In: Y. Kitano (Guest-Editor), Water--Rock Interaction. Chem. Geol., 49: 329--338. Collection of volcanic gases, during a workshop at Vulcano, south Italy, revealed that the atmospheric noble gases are sensitive monitors to air contamination, whereas indigenous deep-seated compounds such as He, CO2, HC1 and H2S are insensitive to such contaminations. Fumaroles at the Vulcano crater contained high concentrations of He, 1.3 • 10 -3 cm ~ STP/g H20 , and He/Ne equaled 1000 in the least contaminated sample (compared to 0.2 in air-saturated water). 4°Ar/36Ar was up to 692, radiogenic'4°Ar was 11 • 10 -4 cm 3 STP/g H20 and radiogenic 4He/4°Ar equaled 1.2. These characteristics resemble those of steam separated from boiling geothermal fluids. Occurrences of vigorous bubbling (drowned fumaroles) were studied at the Vulcano beach and at the adjacent Hippopotamus Pond. He/Ne was 20--40, indicating the presence of some radiogenic He, accompanied by no significant radiogenic At. The ratio of atmospheric noble gases to dry gas was ~ 1 0 times higher in the drowned fumaroles at the beach than in the crater fumaroles. The water of the Hippopotamus Pond contained 9690 mg 1-1 CI-, 11,800 mg 1-1 SO~- and the pH, measured in the laboratory, was 1--2. These characteristics indicate that the fumaroles loose reactive gases (CO2, H2S , HC1) while passing shallow water.
1. Noble gases in groundwater and in geothermal f l u i d s - - A brief outline
1.1. A tmospheric input The atmosphere is a well-mixed reservoir of known concentrations of noble gases with specific isotopic compositions. Water infiltrating into the ground dissolves and carries down noble gases in concentrations d e p e n d e n t on: the ambient temperature; partial pressure -- controlled by the altitude; and salinity -- seawater dissolves 70% of the
0009-2541/85/$03.30
amounts dissolved by fresh water. Thus, one can calculate the initial noble-gas concentration in groundwater in any particular system with an accuracy of + 10% (Mazor, 1977; Herzberg and Mazor, 1979). The atmospheric Ne, At, Kr and Xe may in each case be identified b y their isotopic compositions. On closer inspection, various modes of noble gas take-up have been observed in the aerated zone: retardation of the infiltrating water and equilibration with soft-air at average annual temperatures (Herzberg and Mazor, 1979), rapid infiltration documenting the
© 1985 Elsevier Science Publishers B.V.
330
noble-gas intake temperature of the rainy season (Sugisaki: 1969), karstic recharge accompanied by excess air that is syphoned into the recharge water (Mazor et al., 1983) and cases of air bubble entrapment (Heaton, and Vogel, 1981). In the last two cases the Ne excess is significant b u t that of the most soluble noble gas, Xe, is almost negligible. Analyses of Ne, Ar, Kr and Xe together indicate which mode of recharge through the aerated zone occurred.
1.2. Retention o f atmospheric noble gases in the saturated zone o f groundwater systems
(Mazor, 1972, 1976, 1977, 1979; Mazor et al., 1974). Values of (0.5--50) • 10 -s cm 3 STP He/cm 3 H20 are c o m m o n , as compared with the atmospheric input of only 5 • 10 -8 cm 3 STP He/cm 3 H20 (e.g., Fig. 2).
1.4.
(He/Ar)radiogenic
Interestingly, the (He/Ar)radiogenic ratio in a variety of warm geothermal waters, at different locations, varies in a rather narrow range of 0.5--17, with a pronounced peak around 2--5, as seen in Fig. 3.
1.5. Partioning into steam In the saturated zone no further changes occur and the atmospheric noble gases are retained (Mazor, 1972), as demonstrated in an example in Fig. 1.
1.3. Radiogenic and deep.seated input
Groundwater, thus, acts in the saturated zone as a closed system in which noble gases are locked, due to the hydrostatic pressure. However, in geothermal and volcanic systems part of the water is occasionally converted I
4He and 4°Ar a r e formed by the radioactive
disintegration of U, Th and 4°K, present in rocks on the p p m level. These radiogenic noble gases enter groundwater, more so in old water (long time of water--rock contact) and elevated temperatures (higher diffusitivity) ]
~
I
I
~
_
I
I
I
IOOOC -1,d IOOO
~,
GABBRO / ~ GABSRO I
FU)
ASW 22"C[0
~OLOMBAIA
2
o f
I
I
I
I
n
I
I Ne ~10"7
Ar zlO"4
I
I
Kr zlO "8
Xe ~t0"9
Fig. I. Noble-gas data of four cold (20--23°C) nonkarstic springs in Israel. Data from Herzberg and Mazor (1979), normalized to sealevel. Air-saturated water at 22°C is taken as representing the local recharge water (dashed line). The close agreements indicate that the atmospheric noble gases were retained in the saturated zone.
Fig. 2. Noble gases in a transect through the Larderello (south Italy) geothermal field (Mazor, 1979). Colombaia 2 is an old highly exploited well, significantly depleted in its atmospheric Ne, Ar, Kr and Xe, as compared with air-saturated water at 10°C ( A S W 10°C), representing the local recharge water. Colombala 2 seems, thus, to produce residual liquids. In contrast, the newer well Gabbro 6, is highly enriched in atmospheric noble gases, indicating production from a steam cap. He is present in all wells in high excess over A S W I0°C, indicating it is radiogenic. This He is positively correlated to the atmospheric noble gases.
331 10
Mexico, only up to 3% of the fluid was so far turned into steam and that a Raleigh process dominates in this system (Mazor and Truesdell, 1981).
Z
1.6. Correlation of radiogenic and atmospheric noble gases in geothermal systems
m
0 5 I0 15 20 (4He/ 4OAr) radiogenic Fig. 3. Frequency histogram of (4He/4°Ar)mdiogenic in 68 samples of warm waters and geothermal fluids from various localities. A pronounced peak around 2--5 is seen (in preparation). 1500 -cl
0i4
ME
1000 - -
A positive correlation is observed in several geothermal systems between the radiogenic and atmospheric noble gases (Figs. 2 and 4). This indicates that the meteoric water is recharged into the deep parts of geothermal systems, where it gets mixed with the radiogenic He and Ar as well as CO2, H2 and HC1, and various dissolved ions. Any partitioning due to boiling happens subsequently, acting on all gases alike and, thus, maintaining positive correlations.
E (9
1.7. A model of noble gases in geothermal systems
05
5oo 5~ (D
OI2
D3
T
o
X7
I
I
I
40
80
120
Ne (10-8 cm3/cm 3 dry gas) Fig. 4. Radiogenic He--atmospheric Ne in fumaroles (o) and in drowned fumaroles (o), Lassen Volcanic National Park, California, U.S.A. The positive correlation indicates that meteoric water, carrying the atmospheric noble gases, enters the geothermal system deep, below boiling depth. There it is mixed with the deep-seated He and other compounds. Subsequent partitioning due to boiling acts on all gases alike, maintaining the positive correlation.
into steam. The gases pass efficiently into the steam phase (Fig. 2): over 98% of the initial noble gases passes into the steam when 3% of the fluid is turned into steam by a Raleigh process (continuous removal of products) or 30% of the fluid has to be transformed into steam if 98% gas transfer is to be achieved by a single-stage distillation (Mazor and Truesdell, 1981). On these grounds and C1 concentrations it was possible, for example, to determine that in steam wells at Cerro Prieto,
Putting all these observations together, a general model is derived: The geothermal fluid, containing both the atmospheric and deep-seated components {radiogenic He and Ar, CO2, H2S and d~ssolved ions), ascends and due to pressure reduction begins to boil, producing steam. The gases are transferred preferentially into the steam, which emerges as a fumarole, characterized by high gas/water ratios and relatively low noble gas/dry gas values. Occasionally the steam emerges through a water body and occurs as vigorous bubblings in a spring or pond, forming a "drowned" fumarole. In the last case CO2, H2S and HC1 react with the water and rocks, and as a result: (a) the fumarole bubbles are stripped of much of their CO2, H2S and HCI, reflected in relatively high noble gas/dry gas ratios; and(b) the water through which the fumaroles pass gets enriched in bicarbonate and/or sulfuric acid and chloride. The residual geothermal fluid issues as boiling springs, highly depleted in the noble gases. Cases with less than 5% of the original atmospheric noble-gas concentrations have been
332
observed in Yellowstone National Park, Wyoming, U.S.A. (Mazor and Fournier, 1973) and Cerro Prieto, Mexico (Mazor and Truesdell, 1981). Warm springs, occurring in geothermal areas, often contain intermediate noble-gas concentrations, indicating mixing of residual geothermal fluids (nearly devoid of noble gases) with shallow groundwater (saturated with atmospheric noble gases). In the last cases the noble-gas concentrations reflect the mixing proportions. 1.8. Volcanic emanations
Volcanic emanations fit partially into this model as meteoric water or seawater and geothermal fluids are important, or even dominant, components of m a n y volcanic systems. The role of the noble gases in the study of volcanic gases is three-fold: (1) evaluating the efficiency and purity of sampling methods; (2) identifying meteoric components; and, possibly; (3) identifying magmatic components.
2. Techniques A sample of water, gas or a mixture of the two (5--10 cm 3) is sealed to the system (Fig. 5). The line is p u m p e d overnight, Ti getters are heated to 950°C and the charcoal trap to 275°C. The sample ampoule is opened to the cold trap at -140°C until the pressure no longer drops (Datametrix ® diaphragm manometer). This m i n i m u m pressure is taken to calManometer
J~ sample'r'l'
H.V. I
H.V. [ ~ J ~ ~--I I-J small Ar
culate the a m o u n t of non-condensables (N2, 02, CH4 and noble gases). The trap is changed to dry ice-alcohol (-80°C) and the new pressure is applied to calculate the amount of dry condensables (CO2, H2S and HCL mainly), after subtracting the pressure at -140°C. Gases are then expanded to the getters' section (for ~ 1 min.). If the sample contained water the cold trap is defrozen to liberate trapped gases, mainly Xe, frozen again and expanded a second time, followed by a third extraction. The Ti getter is heated to 950°C until pressure no longer drops and then the second Ti getter is operated until pressure becomes stable. At this stage only noble gases are left. Double-ionized 4°Ar masks 2°Ne and therefore these gases are separated on an activated charcoal trap immersed in liquid nitrogen (~ 1 hr). He and Ne, that are not adsorbed, are measured in the mass spectrometer, followed by atmospheric standards, pipetted-in from a reservoir. Mass 40 (residual Ar) is measured and a correction for double-ionized Ar may be needed for 2°Ne and the same is true for mass 44, that may influence 22Ne. In most cases these corrections are negligible. Ar is c o m m o n l y too abundant, causing the mass spectrometer to be non-linear. Hence, Ar is measured on a small volume ( ~ ~ of the sample), followed by a standard. The charcoal trap is then warmed to +80°C and Kr and Xe are measured, followed by standards. Standards are added to the sample ("on top") and are also measured without it ("alone") in order to check whether the sample is measured in the linear region of the mass spectrometer. If the first causes a smaller signal addition, the mass spectrometer is nonlinear and correction curves have to be applied. 2.1. Sample collection
Ti
Ti charcoal
Fig. 5. Noble-gas separation and purification line (explanations in the text).
Samples were collected during a workshop, organized by the International Association of Volcanology and Chemistry of the Earth's Interior (I.A.V.C.E.I.) at the Island of Vul-
333 cano, south Italy, in September 1982. The noble-gas samples were collected in 10-cm 3 tubes, made of He-tight glass, with two springloaded stopcocks at the end, greased with Apiezon-N ® and evacuated and leak-tested in the laboratory. The tube was connected on one end to the set-up of the Pisa group t h a t collected the fumarolic gases through Dewar tubes, cooled by ether (boiling of ether: 30°C). The other end of the noble-gas tube was dipped into ~ 2 cm water. The fumarolic gases were flushed through the collection tube for 15 min. to assure saturation of the fluid in the trap and to flush all air out. The stopcock near the trap was closed first, to ensure a reproducible (atmospheric) sampling pressure. Three samples were collected in this way at fumarole F-1 and one at F-5 (Fig. 6). F5 F|
Fig. 6. Sampling sites on the Island of Vulcano, south Italy. For comparison, samples were collected at F-5 also w i t h o u t a condenser -- once by connection to the Dewar tubes of one of the Japanese teams and once via the concentric silica tube of the French team. These samples were clogged with yellow sulfur and were extremely " d i r t y " , resulting in poor mass spectrometric data. The condenser seems, thus, essential as it removes most H2S and HC1, in addition to the removal of water vapour. This obstacle was removed in later laboratory experiments by the addition of a tube with concentrated NaOH solution (outgassed) t h a t
takes up a significant portion of the CO2 and H2S prior to the exposure to the Ti getters. 3. Fumaroles at the Vulcano crater
3.1. Air contamination The data (Table I) reveal that the triplicate samples of fumarole F-5 had practically the same concentrations of dry condensable gases (CO2 + HC1 + H2S), non-condensable gases (CH4 + N2 + noble gases) and He. However, huge variations are observed in the concentration of the atmospheric noble gases, e.g. the Ne concentration varied from 1 • 10 -8 to 42 • 10 -8 cm3/cm 3 dry gas. Similarly, the 4°Ar/36Ar ratio varied from 692 to 292 (TableII). This implies t h a t compounds which are indigneous to the fumarolic gases and are low in air are n o t sensitive air contamination monitors. On the other hand, atmospheric Ne, Ar, Kr and Xe are very sensitive contamination monitors. It is thus, highly recommended that: (a) Much care is given to proper connections of the various parts of the gas collection systems and much care is given to adequate flushing and proper storage. (b) Triplicate samples are to be measured. If all agree, the values obtained may be regarded as non-contaminated. If values differ, the sample with the lowest concentrations of the atmospheric noble gases should be regarded as least contaminated. (c) Noble gases should be collected along with the other gases as a check for contamination by atmospheric compounds, mainly 02 and N2. In the case of the F-5 samples, number 5A is the least contaminated and it will be used in the following discussion.
3.2. Radiogenic helium The concentration of 1.3 • 10 -a cm 3 STP He/g H20 (Table III) is 5 orders of magnitude higher than the concentration in air-saturated water at 15°C or seawater at 5°C, which is around 4.5 • 10 -s cm 3 STP He/g H20.
334
This excess He is radiogenic. Excess of radiogenic He in thermal waters that remained closed systems (no steam formation) is in the range of (0.5--50) • 10 -s cm 3 STP/cm 3 H20. The concentration in the Vulcano fumarole F-5 (Table III) is 1--2 orders of magnitude
higher. This extra excess may be due to: (a) direct outgassing from magma, (b) intensified flushing from rocks due to the elevated temperatures involved, or (c) concentration in a steam phase of a boiling geothermal fluid.
TABLEI Noble gases (in cmS/cm s dry gas,l), condensable and non-condensable gases (volume fraction) in Vulcano fumaroles .2
Fumarole
F-5
F-1
Temperature (°C)
Sample No.
Mass Dry condens- Non-condens- He able gas .4 (× 10 spectzo- able g a s . 3 (X I0 -s) metric No.
220
5A 5B 5C
758 773 778
0.60 0.64 0.66
3.0 3.3 4.1
6B
765
0.64
2.4
281
Analytical and sampling errors
Ne (× 10 -s)
Ar (×
1,050 1,030 800
1.0 3.3 41.2
720
2.2
-8)
+7.0%
-+5.4%
Kz
Xe
(X 10 -9 )
( X 1 0 -1 0)
15 30 230
1.3 5.3 --
1.4 1.1 --
22
2.0
3.6
-+7.2%
±6.0%
10 -6)
-+8.6%
, i Dry gas, i.e.without water vapour, determined by the pressure in the separation line at --80°C. ,2 Collected via Dewar tubes and ether condenser of the Pisa group. "3 Mainly C O $ + H2S + HCI, determined by pressure drop between --80° and --140°C. The zest being mainly water vapour, at the time of sampling. ,4 Mainly C H 4 + N 2 and noble gases, determined by residual pressure at --140°C.
T A B L E II Radiogenic noble-gas concentrations (in c m S / c m 3 dry gas) and ratios in V u l c a n o fumaroles Fumarole
Sample No.
4°Ar/S~Ar
4°Aratm. (X 10 -~)
4°Arrad. (*) (X I0-')
(He/Ar)rad.
He/Ne
He/Aratm.
F-5
5A 5B 5C
692 424 292
6.4 21 230
8.6 9.0 --
1.2 1.1 --
1,050 310 20
1.0 0.5 0.003
,40
. .c _ Arradiogem
TABLE
40
Armeasured -
40
AXam~osphe~ic -_
40
Armeasuzed [1 - 295.51(4°Ar/36Ar)measa.ed].
Ill
Noble gases* (in c m s S T P / g H 2 0 ) in the least contaminated sample of fumarole F-5 and in air-saturated water (ASW)
F-5 ASW, 15°C
He
Ne
Arama.
Kr
Xe
(x 10-')
(x I0 -s)
(× 10-4)
(× I0-')
(× i0-')
130,000 4.8
124 20
7.9 3.5
16.1 8.2
2.0 1.1
* F - 5 was f o u n d b y H. S h i n o h a r a and S. Matsuo (pens. c o m m u n . , 1983) t o c o n t a i n 7.5 vol.% d r y gas a n d 92.5 vol.% w a t e r at t h e t i m e t h e noble-gas s a m p l e s w e r e c o l l e c t e d ( S e p t e m b e r 1982). The c o r r e s p o n d i n g values obt a i n e d b y J. Hirabayashi (pens. c o m m u n . , 1983) w e r e 9.5 and 90.5 vol.%. We, thus, a d o p t e d t h e values o f 91 vol.% H20 a n d 9 vol.% d r y gas. F r o m this t h e g a s / w a t e r ratio was calculated: 9× 22400/91x
18=124cm
3STPdrygas/gH20
The values in this table w e r e o b t a i n e d b y m u l t i p l y i n g t h e values o f F-5 in Table I b y 124.
335
3.3. He/Ne to He/Xe ratios The enrichment in radiogenic He may be assessed not only in absolute units (cm 3 He/ cm 3 H20) but also by comparing the ratios in F-5 to air-saturated water ratios. He/Ne is 1000 (compared to 0.2), H e / A t is 1.6 (compared to 1.1 • 10-4), He/Kr is 8000 (compared to 0.8) and He/Xe is 6000 (compared to 4.5 in air-saturated water).
3.4. 3He/4He ratio A value of (6.95 + 0.16) • 1 0 - 6 has been measured in F-5 by B. Marry ( T o k y o University, pets. commun., 1983). This is in the range.of values observed in c o m m o n island-arc volcanic gases. More measurements at various volcanoes are needed to sort o u t possible origins.
geothermal field of Larderello, south Italy (Mazor, 1979), where (Ne/Ar)atm" ratios were observed to be 2.5 times the ratio in air-saturated recharge water. Ne excess was reported also for pumice from the adjacent Lipari Island (Bochsler and Mazor, 1975). A 2°Ne/36Ar ratio of 15.8 was observed, 11 times the value in air and 4 orders of magnitude higher then the value in airequilibrated water. The isotopic compositions of these neons are all similar to the atmospheric composition. Several mechanisms for the excess Ne in the Vulcano fumarole seem possible: (1) magmatic contribution; (2) solubility fractionation effects, caused by the difference in Ne/Ar solubility ratios that are different for the recharge and underground boiling temperatures; and (3) flushing from Ne-enriched rocks, such as the pumice described in the previous paragraph.
3.5. Radiogenic argon 4 . D r o w n e d f u m a r o l e s at t h e V u l c a n o b e a c h
The measured 4°Ar/36Ar ratios and the measured 4°At concentration have been used to calculate the excess of 4°At in samples 5A and 5B of fumarole F-5 (Table III). The corresponding values are 10.7 • 10 -4 and 11.1 • 10 -4 cm 3 STP/g H20. Thus, the value of 11 • 10 -4 cm 3 STP/g H20 is taken as representative for fumarole F-5. The corresponding value for F-1 is 8.4 • 10 -4 cm 3 STP/g H20.
3.6 (4He/4°Ar )radiogenic (4He/4°Ar)radiogenic is 1.3" 10-3/11 • 10 -4 = 1.2, well in the range of c o m m o n thermal and geothermal water, discussed previously (Fig. 3).
3.7. Ne excess Excess Ne, as compared to atmospheric Ar, is observed in fumarole F-5. The ratio of (Ne/ Ar)atm. is 16 • 10 -4 (data from Table III), i.e. 2.9 times the value in air-saturated water at the assumed recharge temperature of 15°C. A similar Ne excess has been reported for the
4.1. The Hippopotamus Pond On the beach, 2 km north of the Vulcano crater with its active fumaroles (Fig. 6), a 15 × 25-m p o n d serves visitors who bath in the mineralized water and cover themselves with sulfur smelling mud. This vivid picture brought up the name H i p p o p o t a m u s Pond. It originated by ejection of steam and water from an abandoned borehole (M. Martini, pers. commun., 1.982) and from time to time it is cleaned of accumulating mud. The depth of the water is ~ 4 0 cm. The water is lukewarm. At several points vigorous bubbling occurs, locally raising the water temperature to 49°C or more. The composition of the water in the p o n d (Table IV) is highly acid and high in SO~- and CI-. Thus, H2S and HCI, which have significant abundances in the Vulcano fumaroles, are stripped from the drowned fumaroles by interaction with the water and H2S is oxidized in the pond. CO2 is most probably also transferred to the water, but due to the high acidity it does not form
336 TABLE IV Composition *~ (in mg 1-1) of the Hippopotamus Pond water Laboratory pH 1.5
SO~ -(* 2) 11,600
C19,690
Na ÷ 5,250
K÷
C a 2+
M g 2÷
730
810
750
,1 Geochemical Laboratory, Israel Geological Survey, Jerusalem. The sum o f anions (in meq 1-~) exceeds that of the reported cations, the difference being balanced by H + (low pH). ,2 SO~- includes all sulfur species. TABLE V Noble gases (in c m 3 S T P / e m 3 H20) dissolved in the Hippopotamus Pond water
Sample No.
Mass spectrometric No.
He (× 10 -8)
Ne (X 10 -8)
Ar (× 10 -4)
Kr (× 10 -8)
1C 1D
769 784
22 9
10.0 14.5
1.38 1.56
2.71 2.63
2.1 3.0
20
4.5
8.2
1'1.2
ASW, 15°C
4.7
HCO] but remains as a gas and is mostly expelled into the air. The concentration of the atmospheric noble gases in two samples of the pond water are low (Table V), reflecting the elevated salinity of the water. He is only slightly higher than in air-saturated water and He/Xe ratios are progressively lower than in air-saturated water, by factors of ½ --¼.
4.2. Drowned fumaroles Gases in the vigorously issuing bubbles were sampled with an inversed funnel at the Hippopotamus Pond, in the sand on the 20 m away beach and off shore (Table V£). The following observations emerged:
4.2.1. Reproducibility and purity. Reasonable reproducibility is observed in the data of duplicate samples in Table VI. This reveals that no significant contamination by air occurred during sampling. 4.2.2. Radiogenic He is observed, He/Ne being 20--50, as compared to 0.2 in air-satu-
Xe (× 10 -9)
40Ar/3,Ar
281 295.5
rated water. This is, however, less than the He/Ne ratio of 1000 observed in fumarole F-5 of the Vulcano crater.
4.2.3. Radiogenic Ar is insignificant, the minute amounts that may be present are probably masked by the relatively large amounts of atmospheric Ar present. 4.2.4. Atmospheric noble gas/dry gas ratios are 2--10 times higher in the drowned fumaroles as compared to the least contaminated sample of fumarole F-5 (Table VI). This fact reflects, most probably, removal of CO2, H2S and HC1 by interaction with rocks and water, as indicated by the composition of the water in the Hippopotamus Pond.
5. Conclusions The set-up of fumarolic activity at the Island of Vulcano lends itself ideally to a comparative study of fumaroles and drowned fumaroles, revealing the processes of gas removal while passing through water. The atmospheric
337
O0
O0
O0
0
O0
O0
O0
0
0 ~X
0
o
o
o o
o o 0~
0
,~ 0
338
noble gases serve as natural tracers in these systems. Atmospheric noble gases are observed to be most sensitive tracers to air contamination of samples collected at active volcanoes. Their concentration in the original fluid (cm 3 noble gas/g fluid) may distinguish possible origins from total or partial evaporation of groundwater or seawater, or entrapment of free air. Radiogenic He and Ar, excess Ne and 3He/ 4He ratio seem to be possible keys to magmatic contributions vs. flushing from country rocks.
Acknowledgements It is a pleasure to thank Dr. Egizio Corazza, for his constant help and marvelous hospitality during the workshop on volcanic gases he organized at Vulcano during September 1982, on behalf of I.A.V.C.E.I. The participants of the workshop are thanked for inspiring discussions. The general model of noble-gas partitioning in geothermal systems was often discussed with Dr. A.H. Truesdell, U.S. Geological Survey, Menlo Park, California. Mrs. E. Negreanu and Mr. M. Feld are warmly thanked for their share in the mass spectrometric measurements.
References Bochsler, P. and Mazor, E., 1975. Excess of atmospheric neon in pumice from the Island of Lipari. Nature (London), 257 : 474--475. Heaton, T.H.E. and Vogel, J.C., 1981. "Excess air" in groundwater. J. Hydrol., 50: 201--216. Herzberg, O. and Mazor, E., 1979. Hydrological appli-
cations of noble gases and temperature measurements in underground water systems: examples from Israel. J. Hydrol., 41: 217--231. Hirabayashi, J., 1983. Report of I.A.V.C.E.I. held workshop on volcanic gases at Vulcano, 1983. Preprint, 6 pp. Mazor, E., 1972. Paleotemperatures and other hydrological parameters deduced from noble gases dissolved in groundwaters: Jordan Rift Valley, Israel. Geochim. Cosmochim. Acta, 36: 1321--1336. Mazor, E., 1976. Atmospheric and radiogenic noble gases in thermal waters -- their potential application to prospection and steam production studies. 2nd U.N. Geotherm. Syrup., San Francisco, Calif., May 1975, pp. 793--802. Mazor, E., 1977. Geothermal tracing with atmospheric and radiogenic noble gases. Geothermics, 5: 21--26. Mazor, E., 1979. Noble gases in a section across the vapor dominated geothermal field in Larderello, Italy. Pure Appl. Geophys., 117: 262--275. 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 Truesdell, A.H., 1981. Dynamics of a geothermal field traced by noble gases: Cerro Prieto, Mexico. 3rd U.S.D.O.E. (U.S. Dep. Energy) --C.F.E.M. (Com. Fed. Electr. M4x.) Symp., San Francisco, Calif., March 1981, on Cerro Prieto Geothermal Field, Baja California, pp. 163--173. Mazor, E., Verhagen, B.Th. and Negreanu, E., 1974. Hot springs o f the igneous terrain of Swaziland: their noble gases, hydrogen, oxygen and carbon isotopes and dissolved ions. Syrup. on Isotope Techniques in Groundwater Hydrology, Vol. 2. Int. At. Energy Agency (I.A.E.A.), Vienna, pp. 29--47. Mazor, E., Van Everdingen, R.O. and Krouse, H.R., 1983. Noble-gas evidence for geothermal activity in a karstic terrain: Rocky Mountains, Canada. Geochim. Cosmochim. Acta, 47 : 1111--1115. Shinohara, H. and Matsuo, S., 1983. Rep. I.A.V.C.E.I. (Int. Assoc. Volcanol. Chem. Earth's Inter.) Workshop on Volcanic Gases at Vulcano, 1982. Preprint, 10 pp. Sugisaki, R., 1969. Measurement of effective flow velocity of groundwater by means of dissolved gases. Am. J. Sci., 259: 144--153.
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Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
SAMPLING OF VOLCANIC GASES - - THE ROLE OF NOBLE-GAS MEASUREMENTS: A CASE STUDY OF VULCANO, SOUTH ITALY EMANUEL
MAZOR
The Frank W. Considine Professorial Chair in Hydrological Research, Geo-Isotope Group, The Weizmann Institute of Science, 76 100 Rehovot (Israel) (Accepted for publication July 16, 1984)
Abstract Mazor, E., 1985. Sampling of volcanic gases -- The Role of noble-gas measurements: a case study of Vulcano, south Italy. In: Y. Kitano (Guest-Editor), Water--Rock Interaction. Chem. Geol., 49: 329--338. Collection of volcanic gases, during a workshop at Vulcano, south Italy, revealed that the atmospheric noble gases are sensitive monitors to air contamination, whereas indigenous deep-seated compounds such as He, CO2, HC1 and H2S are insensitive to such contaminations. Fumaroles at the Vulcano crater contained high concentrations of He, 1.3 • 10 -3 cm ~ STP/g H20 , and He/Ne equaled 1000 in the least contaminated sample (compared to 0.2 in air-saturated water). 4°Ar/36Ar was up to 692, radiogenic'4°Ar was 11 • 10 -4 cm 3 STP/g H20 and radiogenic 4He/4°Ar equaled 1.2. These characteristics resemble those of steam separated from boiling geothermal fluids. Occurrences of vigorous bubbling (drowned fumaroles) were studied at the Vulcano beach and at the adjacent Hippopotamus Pond. He/Ne was 20--40, indicating the presence of some radiogenic He, accompanied by no significant radiogenic At. The ratio of atmospheric noble gases to dry gas was ~ 1 0 times higher in the drowned fumaroles at the beach than in the crater fumaroles. The water of the Hippopotamus Pond contained 9690 mg 1-1 CI-, 11,800 mg 1-1 SO~- and the pH, measured in the laboratory, was 1--2. These characteristics indicate that the fumaroles loose reactive gases (CO2, H2S , HC1) while passing shallow water.
1. Noble gases in groundwater and in geothermal f l u i d s - - A brief outline
1.1. A tmospheric input The atmosphere is a well-mixed reservoir of known concentrations of noble gases with specific isotopic compositions. Water infiltrating into the ground dissolves and carries down noble gases in concentrations d e p e n d e n t on: the ambient temperature; partial pressure -- controlled by the altitude; and salinity -- seawater dissolves 70% of the
0009-2541/85/$03.30
amounts dissolved by fresh water. Thus, one can calculate the initial noble-gas concentration in groundwater in any particular system with an accuracy of + 10% (Mazor, 1977; Herzberg and Mazor, 1979). The atmospheric Ne, At, Kr and Xe may in each case be identified b y their isotopic compositions. On closer inspection, various modes of noble gas take-up have been observed in the aerated zone: retardation of the infiltrating water and equilibration with soft-air at average annual temperatures (Herzberg and Mazor, 1979), rapid infiltration documenting the
© 1985 Elsevier Science Publishers B.V.
330
noble-gas intake temperature of the rainy season (Sugisaki: 1969), karstic recharge accompanied by excess air that is syphoned into the recharge water (Mazor et al., 1983) and cases of air bubble entrapment (Heaton, and Vogel, 1981). In the last two cases the Ne excess is significant b u t that of the most soluble noble gas, Xe, is almost negligible. Analyses of Ne, Ar, Kr and Xe together indicate which mode of recharge through the aerated zone occurred.
1.2. Retention o f atmospheric noble gases in the saturated zone o f groundwater systems
(Mazor, 1972, 1976, 1977, 1979; Mazor et al., 1974). Values of (0.5--50) • 10 -s cm 3 STP He/cm 3 H20 are c o m m o n , as compared with the atmospheric input of only 5 • 10 -8 cm 3 STP He/cm 3 H20 (e.g., Fig. 2).
1.4.
(He/Ar)radiogenic
Interestingly, the (He/Ar)radiogenic ratio in a variety of warm geothermal waters, at different locations, varies in a rather narrow range of 0.5--17, with a pronounced peak around 2--5, as seen in Fig. 3.
1.5. Partioning into steam In the saturated zone no further changes occur and the atmospheric noble gases are retained (Mazor, 1972), as demonstrated in an example in Fig. 1.
1.3. Radiogenic and deep.seated input
Groundwater, thus, acts in the saturated zone as a closed system in which noble gases are locked, due to the hydrostatic pressure. However, in geothermal and volcanic systems part of the water is occasionally converted I
4He and 4°Ar a r e formed by the radioactive
disintegration of U, Th and 4°K, present in rocks on the p p m level. These radiogenic noble gases enter groundwater, more so in old water (long time of water--rock contact) and elevated temperatures (higher diffusitivity) ]
~
I
I
~
_
I
I
I
IOOOC -1,d IOOO
~,
GABBRO / ~ GABSRO I
FU)
ASW 22"C[0
~OLOMBAIA
2
o f
I
I
I
I
n
I
I Ne ~10"7
Ar zlO"4
I
I
Kr zlO "8
Xe ~t0"9
Fig. I. Noble-gas data of four cold (20--23°C) nonkarstic springs in Israel. Data from Herzberg and Mazor (1979), normalized to sealevel. Air-saturated water at 22°C is taken as representing the local recharge water (dashed line). The close agreements indicate that the atmospheric noble gases were retained in the saturated zone.
Fig. 2. Noble gases in a transect through the Larderello (south Italy) geothermal field (Mazor, 1979). Colombaia 2 is an old highly exploited well, significantly depleted in its atmospheric Ne, Ar, Kr and Xe, as compared with air-saturated water at 10°C ( A S W 10°C), representing the local recharge water. Colombala 2 seems, thus, to produce residual liquids. In contrast, the newer well Gabbro 6, is highly enriched in atmospheric noble gases, indicating production from a steam cap. He is present in all wells in high excess over A S W I0°C, indicating it is radiogenic. This He is positively correlated to the atmospheric noble gases.
331 10
Mexico, only up to 3% of the fluid was so far turned into steam and that a Raleigh process dominates in this system (Mazor and Truesdell, 1981).
Z
1.6. Correlation of radiogenic and atmospheric noble gases in geothermal systems
m
0 5 I0 15 20 (4He/ 4OAr) radiogenic Fig. 3. Frequency histogram of (4He/4°Ar)mdiogenic in 68 samples of warm waters and geothermal fluids from various localities. A pronounced peak around 2--5 is seen (in preparation). 1500 -cl
0i4
ME
1000 - -
A positive correlation is observed in several geothermal systems between the radiogenic and atmospheric noble gases (Figs. 2 and 4). This indicates that the meteoric water is recharged into the deep parts of geothermal systems, where it gets mixed with the radiogenic He and Ar as well as CO2, H2 and HC1, and various dissolved ions. Any partitioning due to boiling happens subsequently, acting on all gases alike and, thus, maintaining positive correlations.
E (9
1.7. A model of noble gases in geothermal systems
05
5oo 5~ (D
OI2
D3
T
o
X7
I
I
I
40
80
120
Ne (10-8 cm3/cm 3 dry gas) Fig. 4. Radiogenic He--atmospheric Ne in fumaroles (o) and in drowned fumaroles (o), Lassen Volcanic National Park, California, U.S.A. The positive correlation indicates that meteoric water, carrying the atmospheric noble gases, enters the geothermal system deep, below boiling depth. There it is mixed with the deep-seated He and other compounds. Subsequent partitioning due to boiling acts on all gases alike, maintaining the positive correlation.
into steam. The gases pass efficiently into the steam phase (Fig. 2): over 98% of the initial noble gases passes into the steam when 3% of the fluid is turned into steam by a Raleigh process (continuous removal of products) or 30% of the fluid has to be transformed into steam if 98% gas transfer is to be achieved by a single-stage distillation (Mazor and Truesdell, 1981). On these grounds and C1 concentrations it was possible, for example, to determine that in steam wells at Cerro Prieto,
Putting all these observations together, a general model is derived: The geothermal fluid, containing both the atmospheric and deep-seated components {radiogenic He and Ar, CO2, H2S and d~ssolved ions), ascends and due to pressure reduction begins to boil, producing steam. The gases are transferred preferentially into the steam, which emerges as a fumarole, characterized by high gas/water ratios and relatively low noble gas/dry gas values. Occasionally the steam emerges through a water body and occurs as vigorous bubblings in a spring or pond, forming a "drowned" fumarole. In the last case CO2, H2S and HC1 react with the water and rocks, and as a result: (a) the fumarole bubbles are stripped of much of their CO2, H2S and HCI, reflected in relatively high noble gas/dry gas ratios; and(b) the water through which the fumaroles pass gets enriched in bicarbonate and/or sulfuric acid and chloride. The residual geothermal fluid issues as boiling springs, highly depleted in the noble gases. Cases with less than 5% of the original atmospheric noble-gas concentrations have been
332
observed in Yellowstone National Park, Wyoming, U.S.A. (Mazor and Fournier, 1973) and Cerro Prieto, Mexico (Mazor and Truesdell, 1981). Warm springs, occurring in geothermal areas, often contain intermediate noble-gas concentrations, indicating mixing of residual geothermal fluids (nearly devoid of noble gases) with shallow groundwater (saturated with atmospheric noble gases). In the last cases the noble-gas concentrations reflect the mixing proportions. 1.8. Volcanic emanations
Volcanic emanations fit partially into this model as meteoric water or seawater and geothermal fluids are important, or even dominant, components of m a n y volcanic systems. The role of the noble gases in the study of volcanic gases is three-fold: (1) evaluating the efficiency and purity of sampling methods; (2) identifying meteoric components; and, possibly; (3) identifying magmatic components.
2. Techniques A sample of water, gas or a mixture of the two (5--10 cm 3) is sealed to the system (Fig. 5). The line is p u m p e d overnight, Ti getters are heated to 950°C and the charcoal trap to 275°C. The sample ampoule is opened to the cold trap at -140°C until the pressure no longer drops (Datametrix ® diaphragm manometer). This m i n i m u m pressure is taken to calManometer
J~ sample'r'l'
H.V. I
H.V. [ ~ J ~ ~--I I-J small Ar
culate the a m o u n t of non-condensables (N2, 02, CH4 and noble gases). The trap is changed to dry ice-alcohol (-80°C) and the new pressure is applied to calculate the amount of dry condensables (CO2, H2S and HCL mainly), after subtracting the pressure at -140°C. Gases are then expanded to the getters' section (for ~ 1 min.). If the sample contained water the cold trap is defrozen to liberate trapped gases, mainly Xe, frozen again and expanded a second time, followed by a third extraction. The Ti getter is heated to 950°C until pressure no longer drops and then the second Ti getter is operated until pressure becomes stable. At this stage only noble gases are left. Double-ionized 4°Ar masks 2°Ne and therefore these gases are separated on an activated charcoal trap immersed in liquid nitrogen (~ 1 hr). He and Ne, that are not adsorbed, are measured in the mass spectrometer, followed by atmospheric standards, pipetted-in from a reservoir. Mass 40 (residual Ar) is measured and a correction for double-ionized Ar may be needed for 2°Ne and the same is true for mass 44, that may influence 22Ne. In most cases these corrections are negligible. Ar is c o m m o n l y too abundant, causing the mass spectrometer to be non-linear. Hence, Ar is measured on a small volume ( ~ ~ of the sample), followed by a standard. The charcoal trap is then warmed to +80°C and Kr and Xe are measured, followed by standards. Standards are added to the sample ("on top") and are also measured without it ("alone") in order to check whether the sample is measured in the linear region of the mass spectrometer. If the first causes a smaller signal addition, the mass spectrometer is nonlinear and correction curves have to be applied. 2.1. Sample collection
Ti
Ti charcoal
Fig. 5. Noble-gas separation and purification line (explanations in the text).
Samples were collected during a workshop, organized by the International Association of Volcanology and Chemistry of the Earth's Interior (I.A.V.C.E.I.) at the Island of Vul-
333 cano, south Italy, in September 1982. The noble-gas samples were collected in 10-cm 3 tubes, made of He-tight glass, with two springloaded stopcocks at the end, greased with Apiezon-N ® and evacuated and leak-tested in the laboratory. The tube was connected on one end to the set-up of the Pisa group t h a t collected the fumarolic gases through Dewar tubes, cooled by ether (boiling of ether: 30°C). The other end of the noble-gas tube was dipped into ~ 2 cm water. The fumarolic gases were flushed through the collection tube for 15 min. to assure saturation of the fluid in the trap and to flush all air out. The stopcock near the trap was closed first, to ensure a reproducible (atmospheric) sampling pressure. Three samples were collected in this way at fumarole F-1 and one at F-5 (Fig. 6). F5 F|
Fig. 6. Sampling sites on the Island of Vulcano, south Italy. For comparison, samples were collected at F-5 also w i t h o u t a condenser -- once by connection to the Dewar tubes of one of the Japanese teams and once via the concentric silica tube of the French team. These samples were clogged with yellow sulfur and were extremely " d i r t y " , resulting in poor mass spectrometric data. The condenser seems, thus, essential as it removes most H2S and HC1, in addition to the removal of water vapour. This obstacle was removed in later laboratory experiments by the addition of a tube with concentrated NaOH solution (outgassed) t h a t
takes up a significant portion of the CO2 and H2S prior to the exposure to the Ti getters. 3. Fumaroles at the Vulcano crater
3.1. Air contamination The data (Table I) reveal that the triplicate samples of fumarole F-5 had practically the same concentrations of dry condensable gases (CO2 + HC1 + H2S), non-condensable gases (CH4 + N2 + noble gases) and He. However, huge variations are observed in the concentration of the atmospheric noble gases, e.g. the Ne concentration varied from 1 • 10 -8 to 42 • 10 -8 cm3/cm 3 dry gas. Similarly, the 4°Ar/36Ar ratio varied from 692 to 292 (TableII). This implies t h a t compounds which are indigneous to the fumarolic gases and are low in air are n o t sensitive air contamination monitors. On the other hand, atmospheric Ne, Ar, Kr and Xe are very sensitive contamination monitors. It is thus, highly recommended that: (a) Much care is given to proper connections of the various parts of the gas collection systems and much care is given to adequate flushing and proper storage. (b) Triplicate samples are to be measured. If all agree, the values obtained may be regarded as non-contaminated. If values differ, the sample with the lowest concentrations of the atmospheric noble gases should be regarded as least contaminated. (c) Noble gases should be collected along with the other gases as a check for contamination by atmospheric compounds, mainly 02 and N2. In the case of the F-5 samples, number 5A is the least contaminated and it will be used in the following discussion.
3.2. Radiogenic helium The concentration of 1.3 • 10 -a cm 3 STP He/g H20 (Table III) is 5 orders of magnitude higher than the concentration in air-saturated water at 15°C or seawater at 5°C, which is around 4.5 • 10 -s cm 3 STP He/g H20.
334
This excess He is radiogenic. Excess of radiogenic He in thermal waters that remained closed systems (no steam formation) is in the range of (0.5--50) • 10 -s cm 3 STP/cm 3 H20. The concentration in the Vulcano fumarole F-5 (Table III) is 1--2 orders of magnitude
higher. This extra excess may be due to: (a) direct outgassing from magma, (b) intensified flushing from rocks due to the elevated temperatures involved, or (c) concentration in a steam phase of a boiling geothermal fluid.
TABLEI Noble gases (in cmS/cm s dry gas,l), condensable and non-condensable gases (volume fraction) in Vulcano fumaroles .2
Fumarole
F-5
F-1
Temperature (°C)
Sample No.
Mass Dry condens- Non-condens- He able gas .4 (× 10 spectzo- able g a s . 3 (X I0 -s) metric No.
220
5A 5B 5C
758 773 778
0.60 0.64 0.66
3.0 3.3 4.1
6B
765
0.64
2.4
281
Analytical and sampling errors
Ne (× 10 -s)
Ar (×
1,050 1,030 800
1.0 3.3 41.2
720
2.2
-8)
+7.0%
-+5.4%
Kz
Xe
(X 10 -9 )
( X 1 0 -1 0)
15 30 230
1.3 5.3 --
1.4 1.1 --
22
2.0
3.6
-+7.2%
±6.0%
10 -6)
-+8.6%
, i Dry gas, i.e.without water vapour, determined by the pressure in the separation line at --80°C. ,2 Collected via Dewar tubes and ether condenser of the Pisa group. "3 Mainly C O $ + H2S + HCI, determined by pressure drop between --80° and --140°C. The zest being mainly water vapour, at the time of sampling. ,4 Mainly C H 4 + N 2 and noble gases, determined by residual pressure at --140°C.
T A B L E II Radiogenic noble-gas concentrations (in c m S / c m 3 dry gas) and ratios in V u l c a n o fumaroles Fumarole
Sample No.
4°Ar/S~Ar
4°Aratm. (X 10 -~)
4°Arrad. (*) (X I0-')
(He/Ar)rad.
He/Ne
He/Aratm.
F-5
5A 5B 5C
692 424 292
6.4 21 230
8.6 9.0 --
1.2 1.1 --
1,050 310 20
1.0 0.5 0.003
,40
. .c _ Arradiogem
TABLE
40
Armeasured -
40
AXam~osphe~ic -_
40
Armeasuzed [1 - 295.51(4°Ar/36Ar)measa.ed].
Ill
Noble gases* (in c m s S T P / g H 2 0 ) in the least contaminated sample of fumarole F-5 and in air-saturated water (ASW)
F-5 ASW, 15°C
He
Ne
Arama.
Kr
Xe
(x 10-')
(x I0 -s)
(× 10-4)
(× I0-')
(× i0-')
130,000 4.8
124 20
7.9 3.5
16.1 8.2
2.0 1.1
* F - 5 was f o u n d b y H. S h i n o h a r a and S. Matsuo (pens. c o m m u n . , 1983) t o c o n t a i n 7.5 vol.% d r y gas a n d 92.5 vol.% w a t e r at t h e t i m e t h e noble-gas s a m p l e s w e r e c o l l e c t e d ( S e p t e m b e r 1982). The c o r r e s p o n d i n g values obt a i n e d b y J. Hirabayashi (pens. c o m m u n . , 1983) w e r e 9.5 and 90.5 vol.%. We, thus, a d o p t e d t h e values o f 91 vol.% H20 a n d 9 vol.% d r y gas. F r o m this t h e g a s / w a t e r ratio was calculated: 9× 22400/91x
18=124cm
3STPdrygas/gH20
The values in this table w e r e o b t a i n e d b y m u l t i p l y i n g t h e values o f F-5 in Table I b y 124.
335
3.3. He/Ne to He/Xe ratios The enrichment in radiogenic He may be assessed not only in absolute units (cm 3 He/ cm 3 H20) but also by comparing the ratios in F-5 to air-saturated water ratios. He/Ne is 1000 (compared to 0.2), H e / A t is 1.6 (compared to 1.1 • 10-4), He/Kr is 8000 (compared to 0.8) and He/Xe is 6000 (compared to 4.5 in air-saturated water).
3.4. 3He/4He ratio A value of (6.95 + 0.16) • 1 0 - 6 has been measured in F-5 by B. Marry ( T o k y o University, pets. commun., 1983). This is in the range.of values observed in c o m m o n island-arc volcanic gases. More measurements at various volcanoes are needed to sort o u t possible origins.
geothermal field of Larderello, south Italy (Mazor, 1979), where (Ne/Ar)atm" ratios were observed to be 2.5 times the ratio in air-saturated recharge water. Ne excess was reported also for pumice from the adjacent Lipari Island (Bochsler and Mazor, 1975). A 2°Ne/36Ar ratio of 15.8 was observed, 11 times the value in air and 4 orders of magnitude higher then the value in airequilibrated water. The isotopic compositions of these neons are all similar to the atmospheric composition. Several mechanisms for the excess Ne in the Vulcano fumarole seem possible: (1) magmatic contribution; (2) solubility fractionation effects, caused by the difference in Ne/Ar solubility ratios that are different for the recharge and underground boiling temperatures; and (3) flushing from Ne-enriched rocks, such as the pumice described in the previous paragraph.
3.5. Radiogenic argon 4 . D r o w n e d f u m a r o l e s at t h e V u l c a n o b e a c h
The measured 4°Ar/36Ar ratios and the measured 4°At concentration have been used to calculate the excess of 4°At in samples 5A and 5B of fumarole F-5 (Table III). The corresponding values are 10.7 • 10 -4 and 11.1 • 10 -4 cm 3 STP/g H20. Thus, the value of 11 • 10 -4 cm 3 STP/g H20 is taken as representative for fumarole F-5. The corresponding value for F-1 is 8.4 • 10 -4 cm 3 STP/g H20.
3.6 (4He/4°Ar )radiogenic (4He/4°Ar)radiogenic is 1.3" 10-3/11 • 10 -4 = 1.2, well in the range of c o m m o n thermal and geothermal water, discussed previously (Fig. 3).
3.7. Ne excess Excess Ne, as compared to atmospheric Ar, is observed in fumarole F-5. The ratio of (Ne/ Ar)atm. is 16 • 10 -4 (data from Table III), i.e. 2.9 times the value in air-saturated water at the assumed recharge temperature of 15°C. A similar Ne excess has been reported for the
4.1. The Hippopotamus Pond On the beach, 2 km north of the Vulcano crater with its active fumaroles (Fig. 6), a 15 × 25-m p o n d serves visitors who bath in the mineralized water and cover themselves with sulfur smelling mud. This vivid picture brought up the name H i p p o p o t a m u s Pond. It originated by ejection of steam and water from an abandoned borehole (M. Martini, pers. commun., 1.982) and from time to time it is cleaned of accumulating mud. The depth of the water is ~ 4 0 cm. The water is lukewarm. At several points vigorous bubbling occurs, locally raising the water temperature to 49°C or more. The composition of the water in the p o n d (Table IV) is highly acid and high in SO~- and CI-. Thus, H2S and HCI, which have significant abundances in the Vulcano fumaroles, are stripped from the drowned fumaroles by interaction with the water and H2S is oxidized in the pond. CO2 is most probably also transferred to the water, but due to the high acidity it does not form
336 TABLE IV Composition *~ (in mg 1-1) of the Hippopotamus Pond water Laboratory pH 1.5
SO~ -(* 2) 11,600
C19,690
Na ÷ 5,250
K÷
C a 2+
M g 2÷
730
810
750
,1 Geochemical Laboratory, Israel Geological Survey, Jerusalem. The sum o f anions (in meq 1-~) exceeds that of the reported cations, the difference being balanced by H + (low pH). ,2 SO~- includes all sulfur species. TABLE V Noble gases (in c m 3 S T P / e m 3 H20) dissolved in the Hippopotamus Pond water
Sample No.
Mass spectrometric No.
He (× 10 -8)
Ne (X 10 -8)
Ar (× 10 -4)
Kr (× 10 -8)
1C 1D
769 784
22 9
10.0 14.5
1.38 1.56
2.71 2.63
2.1 3.0
20
4.5
8.2
1'1.2
ASW, 15°C
4.7
HCO] but remains as a gas and is mostly expelled into the air. The concentration of the atmospheric noble gases in two samples of the pond water are low (Table V), reflecting the elevated salinity of the water. He is only slightly higher than in air-saturated water and He/Xe ratios are progressively lower than in air-saturated water, by factors of ½ --¼.
4.2. Drowned fumaroles Gases in the vigorously issuing bubbles were sampled with an inversed funnel at the Hippopotamus Pond, in the sand on the 20 m away beach and off shore (Table V£). The following observations emerged:
4.2.1. Reproducibility and purity. Reasonable reproducibility is observed in the data of duplicate samples in Table VI. This reveals that no significant contamination by air occurred during sampling. 4.2.2. Radiogenic He is observed, He/Ne being 20--50, as compared to 0.2 in air-satu-
Xe (× 10 -9)
40Ar/3,Ar
281 295.5
rated water. This is, however, less than the He/Ne ratio of 1000 observed in fumarole F-5 of the Vulcano crater.
4.2.3. Radiogenic Ar is insignificant, the minute amounts that may be present are probably masked by the relatively large amounts of atmospheric Ar present. 4.2.4. Atmospheric noble gas/dry gas ratios are 2--10 times higher in the drowned fumaroles as compared to the least contaminated sample of fumarole F-5 (Table VI). This fact reflects, most probably, removal of CO2, H2S and HC1 by interaction with rocks and water, as indicated by the composition of the water in the Hippopotamus Pond.
5. Conclusions The set-up of fumarolic activity at the Island of Vulcano lends itself ideally to a comparative study of fumaroles and drowned fumaroles, revealing the processes of gas removal while passing through water. The atmospheric
337
O0
O0
O0
0
O0
O0
O0
0
0 ~X
0
o
o
o o
o o 0~
0
,~ 0
338
noble gases serve as natural tracers in these systems. Atmospheric noble gases are observed to be most sensitive tracers to air contamination of samples collected at active volcanoes. Their concentration in the original fluid (cm 3 noble gas/g fluid) may distinguish possible origins from total or partial evaporation of groundwater or seawater, or entrapment of free air. Radiogenic He and Ar, excess Ne and 3He/ 4He ratio seem to be possible keys to magmatic contributions vs. flushing from country rocks.
Acknowledgements It is a pleasure to thank Dr. Egizio Corazza, for his constant help and marvelous hospitality during the workshop on volcanic gases he organized at Vulcano during September 1982, on behalf of I.A.V.C.E.I. The participants of the workshop are thanked for inspiring discussions. The general model of noble-gas partitioning in geothermal systems was often discussed with Dr. A.H. Truesdell, U.S. Geological Survey, Menlo Park, California. Mrs. E. Negreanu and Mr. M. Feld are warmly thanked for their share in the mass spectrometric measurements.
References Bochsler, P. and Mazor, E., 1975. Excess of atmospheric neon in pumice from the Island of Lipari. Nature (London), 257 : 474--475. Heaton, T.H.E. and Vogel, J.C., 1981. "Excess air" in groundwater. J. Hydrol., 50: 201--216. Herzberg, O. and Mazor, E., 1979. Hydrological appli-
cations of noble gases and temperature measurements in underground water systems: examples from Israel. J. Hydrol., 41: 217--231. Hirabayashi, J., 1983. Report of I.A.V.C.E.I. held workshop on volcanic gases at Vulcano, 1983. Preprint, 6 pp. Mazor, E., 1972. Paleotemperatures and other hydrological parameters deduced from noble gases dissolved in groundwaters: Jordan Rift Valley, Israel. Geochim. Cosmochim. Acta, 36: 1321--1336. Mazor, E., 1976. Atmospheric and radiogenic noble gases in thermal waters -- their potential application to prospection and steam production studies. 2nd U.N. Geotherm. Syrup., San Francisco, Calif., May 1975, pp. 793--802. Mazor, E., 1977. Geothermal tracing with atmospheric and radiogenic noble gases. Geothermics, 5: 21--26. Mazor, E., 1979. Noble gases in a section across the vapor dominated geothermal field in Larderello, Italy. Pure Appl. Geophys., 117: 262--275. 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 Truesdell, A.H., 1981. Dynamics of a geothermal field traced by noble gases: Cerro Prieto, Mexico. 3rd U.S.D.O.E. (U.S. Dep. Energy) --C.F.E.M. (Com. Fed. Electr. M4x.) Symp., San Francisco, Calif., March 1981, on Cerro Prieto Geothermal Field, Baja California, pp. 163--173. Mazor, E., Verhagen, B.Th. and Negreanu, E., 1974. Hot springs o f the igneous terrain of Swaziland: their noble gases, hydrogen, oxygen and carbon isotopes and dissolved ions. Syrup. on Isotope Techniques in Groundwater Hydrology, Vol. 2. Int. At. Energy Agency (I.A.E.A.), Vienna, pp. 29--47. Mazor, E., Van Everdingen, R.O. and Krouse, H.R., 1983. Noble-gas evidence for geothermal activity in a karstic terrain: Rocky Mountains, Canada. Geochim. Cosmochim. Acta, 47 : 1111--1115. Shinohara, H. and Matsuo, S., 1983. Rep. I.A.V.C.E.I. (Int. Assoc. Volcanol. Chem. Earth's Inter.) Workshop on Volcanic Gases at Vulcano, 1982. Preprint, 10 pp. Sugisaki, R., 1969. Measurement of effective flow velocity of groundwater by means of dissolved gases. Am. J. Sci., 259: 144--153.