A2aSTUTZ, G. C. and BERNARD, A. J. (eds.), ... Rapp. Tecrk, Ist. Int, Vulcan., C.N.R.. Catania. , 1974, Petrology of Some Volcanic. Rocks Series of the Aeolian ...
Genesis and Evolution of the Fumaroles of Vulcano (Aeolian Islands, Italy): a Geochemical Model M. CARAPEZZA P. M. NUCCIO M. VALENZA
Istituto di Mineralogia, Petrografia e Geochimica, Universit~ Palermo, Italy Istituto di Geochimica dei Fluidi, CNR, Palermo, Italy.
ABSTRACT A geochemical model explaining the presence of fumaroles having different gas composition and temperature at the top of the crater and along the northeastern coast of Valcano island is proposeck A pressurized biphase (liquid-vapor) reservoir at the depth of about 2 kin is hypothesized. Energy and mass balance sheets control P-T conditions in the system. PT must vary along a boiling curve of brine as liquid is present. The C02 content in the steam is governed by the thermodynamic properties of the fluids in the Hc-NaCI-CO~ system. On the assumption that oxygen fugacity in the system is between the HM-FMQ oxygen buffers, observed SOJH~S, COz/CO, CO/CH,, ratios in the fumarolic gases at the Fossa crater appear in equilibrium with a temperature higher than that observed, such as may exist at depth. The more reduced gas phases present on the sea-side may result from re-equilibrium processes in shallower aquifers, The suggested model would help in monitoring changes in volcanic activity by analyzing fumarolic gases. INTRODUCTION
Vulcano is one of t h e s e v e n islands of t h e Aeolian archipelago in t h e s o u t h e r n T y r r e n i a n sea n e a r t h e n o r t h Sicilian continental slope. According to r e c e n t h y p o t e s e s (CAPUTO et aI., 1972; BARBERI et al., 1974; KELLER, 1974) such islands would constitute an island arc structure with calc-alkaline a n d shoshonitic rock associations. Volcanological evidences a n d historical r e p o r t s from 183 B.C. to t h e p r e s e n t day, show t h a t Vulcano has b e e n one of t h e m o s t active volcanoes in t h e M e d i t e r r a n e a n area. T h e l a t e s t eruption took place
Bull. Volcanol., Vol. 44-3, 1981
a t the G r a n C r a t e r e in 1888 a n d contin u e d until 1890. T h e s e t of p h e n o m e n a a c c o m p a n y i n g this e r u p t i o n l e d MERCALLI (1891) to define it with t h e t e r m vulcanian. Since then, this t e r m h a s b e e n u s e d b y volcanologists all over t h e world to characterize explosive eruptions during which semifluid blocks of c o n s i d e r a b l e size a r e h u r l e d out of the volcano c r a t e r up to fairly g r e a t distances. T h e Vulcano active volcanic cone - 2000 m high a b o v e t h e sea floor a n d 391 m a b o v e t h e sea level - is p l a c e d a t t h e c e n t e r of a caldera. T h i s cone is called L a Fossa or Gran Cratere. Since t h e l a s t eruption in 1888 Vulcano has c h a n g e d into a typical state of fumarolic activity of various intensity. According to BERGEAT (1899) a n d KELLER (1970), t h e fumarolic activity would b e localized on a N - S fracture zone o n which t h e volcanoes L a Fossa, the Faraglione a n d Vulcanello are aligned. A t p r e s e n t , t h e m a i n m a n i f e s t a tions in t h e a r e a are: a ) T h e fi, m s r o l e s of G r a n C r a t e r e t h a t a r e l o c a t e d on t h e inside slopes of t h e crater, on t h e n o r t h e r n a n d w e s t e r n sectors of t h e rim, on t h e r i m o u t e r S - S E u p p e r slope, a n d on t h e n o r t h e r n side of t h e cone a t i n t e r m e d i a t e heights. T h e i r t e m p e r a t u r e varies f r o m 98 ° to m o r e t h a n 300°C. b) T h e f u m a r o l e s of Baia di Levante which e x t e n d from the m i d d l e of t h e Vulcano-Vulcanello isthmus. B o t h t h e f u m a r o l e s on t h e shore a n d t h e s u b m a r i n e fumarolic field a r e s c a t t e r e d in a n a r e a definitely e l o n g a t e d in t h e N-S direction (HONNOREZ et al., 1973).
548
M.
CARAPEZZA
-
P.M. NUCCIO
c) T h e thermal springs along: the plain of Vulcano Porto. A geochemical study of the waters of the wells in the plain of Vulcano Porto, and geoelectric investigations carried but in the two-year period 1968-70 (MARTINI et al., 1970) showed the presence of a surface aquifer of a relatively ~imple geometry, sloping northwards and containg mainly sea water. Over this aquifer, on a limited area in the southern part of the p|nin; at the back of Gran Cratere, there is a soft water aquifer. Two further hot aquifers, at heights of - 90 and -229 m respectively, were found by
- M.
VALENZA
driUlngs carried out by AGIP in 1956. These drillings indicated clearly a discontinuous permeability of the various levels. A difficult circulation of fluids between higher and lower levels is thus to be expected. The geochemical study of the waters has shown anomalous contents of borates, sulphates, fluorides etc. and mainly of C O s aligned approximately along a N-S direction (MARTINI and TONANI, 1970). This alignement (Fig. i) is subparallel to a fracture crossing Gran Cratere and Acque Calde in the direction of VulcaneUo (KELLER, 1980).
FiG. 1 - Distributionof ~,mArolic activityon the island of Vulcano. The fi,mRrolesare aligned mainly along a NNW-SSE directioncrossingthe crater and are also developed along a secondary E-W direction.
GENESIS AND EVOLUTION OF THE FUMAROLES OF VULCANO
After an increase in temperature (from 200 ° to 300°C) in the fumaroles of Gran Cratere towards the end of 1977, both geochemical and geophysical surveillance were greatly intensified during 1978 and 1979. O n the basis of the data acquired during this period of surveillance and of those available in the literature, a geochemical model capable of explaining the genesis and evolution of fumarolic manifestations in the island of Vulcano has been proposed.
GEOCHEMISTRY OF FUMAROLIC GASES Chemical Evolution
The first analysis of fumarolic gases of Vulcano reported in the literature was made by St. Claire Deville in 1855. Since then these fumaroles have extensively been studied (FOUQUI}E, 1865; DE FIORE, 1922; SICARDI, 1941, 1955; MARTINI and TONANI, 1970; LE GUERN, 1972; CIONI et al., 1978; LE GUERN et al., 1979). The composition of the gases analyzed in the time span from 1923 to 1979 is given in Table 1 (1). Notwithstanding the different analytical methods used by the various authors, a remarkable feature can persistently be observed, that is, the correlation between the maximum temperature and the gas/ H20 ratio of the fumaroles of Gran Cratere. T h e gas/H20 ratio increases as the maximum temperature of fumaroles (1) It is to be noted that, due to differences in analytical and sampling methods, analyses are not comparable. This is particularly relevant for H2S and SO2 according to the reaction (SIcARDI, I941): 4H¢S~-2S02 = 382÷41-I~0
Therefore analytical methods for H2S and SO2 are required either to treat samples in the field properly, e.g. by using specific absorbents (SICARDI, 1956), or to analyse them on sampling, e.g. field gas-chromatography (LE GUERNet al., 1979).
549
increases going from 7 vol% at the temperature of 186°C to 22.5% at the temperature of 303°(]. Then, at higher temperatures, the ratio decreases reaching a value of 3.5]96.5 at the temperature of 580°(:] (Table 1). Moreover the composition of non condensable gases, e.g., C02/SO2 % H2S ratio, changes from the temperatures range 186%300°C to 580°C. This may be due to the presence of two different sources of gas, one prevailing at lower and the other at higher temperatures. T h e existence of an aquifer beneath the Grail Cratere of Vulcano that is distinct from the aquifer in the surroundings was proposed by MARTINI and TONANI (1970). According to these authors the water ultim a t e | y originates from marine water. The composition of fumarolic gases puts some restrictions to the characteristics of the deep aquifer. The NaC1 content in the gas phase is a function of the NaC1 dissolved in the liquid as well as of the temperature (Fig. 2). On the other hand the HC1 contents in the gas phase depend on the contents of C1- and on the pH of the solution as well as on the pKa according to the following relation (ELLIS and MAHON, 1977): log PHCl = 1% log mcl- -- pH -- pKa It should be noted that: the chloride molar concentration mci. is defined by the salinity of the aquifer; pka decreases as the temperature increases; p H is a function of both temperature and salinity, and finally the partial pressure of HC1 (P~cl) depends on temperature and salinity. In the fumarolic gases collected at the temperature of 303°C, the NaC1 content was 10 ppm; the content of HC1 was 0.8%
(~).
All these data are consistent with an aquifer containing 5-15% of NaC1 with a pH value of 3.5-4 at a temperature 320340°C (Fig. 2). (2)NaCI in the fumarolic gases has been determined by atomic absorption spectrophotometry from the sodium contentin the condensate using the standard additionsmethod.
550
M. CARAPEZZA
- P.IVL
- M.
NUCCIO
VALF_2qZA
TABLE 1 - Analyses of t51rnerolic gases. The composition is expressed in vol. % except for H2, CH4 and CO that are in ppm. Ana]ysi S N.
1
T fumaroI.~C 480 H20 Dry 9ases
2
3
4
5
8
7
8
9
10
~1
12
13
]4
15
16
17
18
237
]0¢
100
170
138
ZlO
]86
I~5
28]
280
250
98
220
220
98
305
98
96.5
99.5
93
8t.9:
80.23
80.60
78
77.8
3.5
0.9
7
18.1
19.77
19.40
22
22.2
93.9
CO2
55,2
59.6
62,4
94,3
86.0
9Z.3
88.9
84
99.48
93,77
93,81
95.71
95.74
90.1
502
21.2
18.9
18.2
--
6.3
3,0
4.5
--
5.50
5.2 a
4.58
4.38
--
4.59
3.18
0.]3
3.5
--
H2S
20,0
17.3
17.0
¢.2
5.5
2.2
3.9
0.6
0.01
--
0.15
0.17
3.21
2.75
--
3.12
2.1
3.3
HC1
1.7
1.5
0.2
0.2
0.6
N2
1.9
2.7
2,4
1.5
2.3
2.3
2.1
12
0~91
n.d.
0.60
0.65
0.95
0.104
1.36
0.41
0.8
0.7
3.2
0.08
n.d.
0.03
0:03
0.08
0-02
0,14
0.01]
0.1
0.06
H2
22
9500
CH 4
--
800
CO
3 Crater
Shore
Crater
Crater
95.3
95.2
3.9
02+Ar
L~Cat~OR
91.8
Cra~e~ Crater Crater
40 8~B0 5300
--
I000
20
Crater
ShoPe
CPater
Shore
1SO
Crater
Shore
T ~Oax °C
SBD
"170
210
186
185
28~
293
293
Made by
(15
(15
(Z)
(35
(45
(5)
(65
(7)
303 (S)
year,
1923
1937
1951
1989
]970
1978
1978
1978
1979
a Total sulphu~ expressed as SOz b Below the detection l i m i t (2ppm) (1) Stcardt,1941; (2) Sicardt,1955; (3) Hartini snd Tonani.1970; (4) Le Guern,1972; 1979,
Water with salinity ranging from a few thousand p p m up to 20-25% is the source of steam in m a n y geothermal fields. T h e thermal manifestations of the plain of Vulcano Porto as well as the flzmaroles of Gran Cratere are both related at depth to the same source of heat. T h e high temperature fumaroles of Gran Cratere can be considered as natural wells tapping from the vapor zone. T h e n the chemical composition of the film,rolic gases will reflect the physicoChemical features on the liquid-vapor deep interface (ELLIS and MAHON, 1977). Furthermore, considering that the 98% or the fumarolic gases is formed of H20 and COD and taking into account that the estimated content of NaC1 of the deep aquifer ranges from 5 to 15 wt%, the conditions governing the whole system will be defined b y the thermodynamic properties of the H20-NaCIC02 system.
The System H~O-CO2-NaCI and Its Application to the Fluids of Vulcano It is reasonable to assume that N a + and Cl- are the dominant ions in solution in
(5) This wopk; (6) Cteni et a1.,1978; (75 Le ~uern et al..
the deep aquifer of Vulcano. O n the other hand, the presence of small a m o u n t of other species such as K +, M g +~, C a +~, S O ~ - etc., is unlikely to change substan-
- 10.7724 ii
•
TL-~--_-E--_2:-_-=:-_2~
Z~ . . . . . . . . . . . . . . . . . . . . . . . . .
30
2'5
~o
J I
45 ~r
I~G. 2 - Distribution of NaCl between liquid and gas as a function of temperature. Data from ELLIS and M A H O N (1977). The a-b interval in the log K values, relatively to a vapor containing 10 p p m of NaCl and a liquid cont~in;ng 5-15% NaCl, suggests an equilibrium temperature in the biphase system of 320-
340oC.
GENESIS
AND
EVOLUTION
OF THE
tially the behaviour of the actual system relative to the ternary system (KEEVIL 1942; HOLLAND, 1967). With respect to the H20-CO2 system, data on the fluids composition are known up to 400~C and 3500 bars (TODHEIDE and FRANK, 1963; TAKENOUCHTand KENNEDY, 1964 and 1965; EUGSTER and SKIPPEN, 1967). H20 and CO2 are completely miscible only above the critical temperature. Below the critical temperature, solubility of C02 in water decreases progressively as the pressure increases and has a minimum which, according to TAKENOUCHI and KENNEDY (1964) is characterized by T = 265°C, P ~ 2,150 bars, CO~. = 31% tool. 100 0.;
140
180
220
FUMAROLES
551
OF VULCANO
The H.~O-NaC1 system is also well known (OLANDER et al., 1950; SOURIRAJAN, 1962) and has been thoroughly revised by HOLLAND (1967), whom the reader is referred to. In the system HzONaC1 particular significance is to be given to the fact that the critica} temperature of water increases with the rise in concentration of NaCl. The presence of NaC1 in H20 has an effect in contrast with that which can be observed in the system H20CO2. The more striking effect is the so called salting-out effect, which is a consequence of the decrease in the solubility of COs in H20 when the concentration in NaC1 increases. Through simple measurements of the Henry's law constant we see, 260
300
340
T~
380
20
40
~'~\
,'-10%NaCI
60
80
100 ~.
~.
120
140
160
H~O'~6cYoCO~'e-Pstm.
\
, ,
180 i~,~ -- 3%,C0~
FaG. 3 - Boiling curves of water, water + 10% of NaCt, water + 6% of CO~ at 306~'C,water + 10% NaC1, and 1-2-3 wt% of C02 at 350°C respectively.
552
M.
CARAPEZZA
- P.M.
for example, that the solubility of CO s in HsO is almost twice as great as that found in water with 2 M of NaCI. Other contrasting properties due to the simultaneous presence of CO s and NaC1 in H~O are reported in ELLIS and G O L D I N G (1963), ELLIS (1967), MAI2NIN (1974), SCHUBERT and STRAUSS (1979). A simplified description of the Vulcano system in terms of temperature, pressure and composition, can be given if based on some suitable boiling curve in the ternary system H20-NaC1-CO s. However, we have to consider, first, that in such system there are degassing phenomena on regional scale involving that the gas contents of underground waters may be related to magmatic activity. This fact is suggestive of a liquid phase with dissolved gases changing in contents with temperature in some as yet undefined way. The boiling curve is likely dependent on the gas contents; however, details are hardly predictable. In Fig. 3 the boiling curves of pure water, and of 10% NaC1 are shown, as well as the boiling curves of a water containing 6% COs at 306°C (SUTTONand MCNABB, 1977), and the boiling curves of 10% NaC1 water solutions containing respectively 1, 2, 3 wt% of COs in a liquid at 350°C. Each
NUCCIO
- M.
VALENZA
of these groups of curves has been drawn using preselected couples of temperature values (186oC and 350°(3) and pressure. For a given temperature, at the boiling point
P~t = P.~o + Pco~ Pn o has been obtained from the boiling eurv~ of 10 wt% NaC1 solutions (the estimated range of salinity is 5-15 wt%, as mentioned previously). Pco at 186°C has been obtained taking into a~eount that the content of COs in the vapor phase (vol. 7%, No. 8 in Table 1) is the same as that found in fumarolic gases having the same 7 temperature. T h a t is, Pco~ = 93 P~o, where PH20 is the boiling pressure of the 10% NaCI solution at 186°C. Pco° at 350°C has been obtained from the 4quation: =
K(NaCl 10%)
PcO2 (350oc) •x where K is the Henry's constant (ELLIS and GOLDING, 1963) and x is the molar fraction of C09 relatively to the selected solutions containing 1-2-3 wt% of COs and 10% of NaC1. These three couples of points have been defined in the P, T plane and the
/
#
32
-= •= '
/ 24
/¢.;/
16
-
r
.,
0 ~
140
1SO
2~
260
3130
340
"C
~
Fro. 4 - C O s contents in the vapor phase as a fimction of the temperature for different C O s contents in the liquid (1.5-2-3 wt%). The plotted data correspond to the analyses N. 8, 10, 11, 14, 17 reported in Table 1. The inferred trend is sub-parallel to the 1.5'2% curves, but shifted of 3040°C. This suggest that the vapor separated at depth from a liquid boiling at a temperature of 3040°C higher than the m a x i m u m temperature of the fumaroles on the surface.
553
GENESIS AND EVOLUTION OF THE FUMAROLES OF VULCANO
most suitable boiling curve has been drawn through each couple of points. From the boiling curves of Fig. 3, the CO~ content in the vapor has been obtained as a function of the vaporization temperature (Fig. 4). In Fig. 4 are also shown the CO 2 contents in the gases collected in a high temperature fi~marole of Gran Cratere during several months, as the temperature of the fumarole increased. The trend at the CO 2 contents in the gases of the high temperature htmaroles is subparallel and is shifted of 20°40°C with respect to the curves of the CO 2 contents in the vapor phase of a boiling liquid with 1.5-2 wt% of C02 and 10 wt% NaC1. That is, boiling 10% NaC1 solutions with 1.5-2% COD over the temperature range from 180 ~ to 320 °- 340°C, separate a vapor phase that has the same composition as that observed in the temperature range of 180°-300°C at the vent. It is relevant the fact that a temperature of 330°C was computed already on the basis of pre]im~nary isotopic 5 L~O data of the couple H20-CO2 sampled at Gran Cratere, if fractionation factors are considered relatively to a water containing about 10-20 wt% NaCl in solution (LONOINELLI et al., in preparation). The CO 2 contents in the fumaroles of Gran Cratere are shown in Fig. 5. The almost constant CO 2 contents in the fumaroles in spite of the differences in temperature, indicate very similar physico-chemical conditions for the separation of the vapor phase feeding all the fumaroles of Gran Cratere and having temperatures higher than 100°C. At lower temperatures, extensive condensation phenomena determine new conditions of gasdiquid distribution. It is worth noting that the curves in Fig. 5 can be used to determine the temperature of the deep aquifer as based on the CO2 content in fumarolic gases.
Distribution of the Sulphur-bearing Molecular Species The considerations made for CO2 a r e valid also for SOu and H2S in as much as their distribution between liquid and
vapor is controlled by the Henry's law. S02 and HaS occur in hydrothermal systems in considerable concentrations (RAFAL'SKrY et al., 1969). In the range of 300-350°(3 the Henry's constants for these gases are quite comparable and are about one order of magnitude lower than the corresponding values for CO 2. Therefore, the gas composition of the Gran Cratere fumaroles showing a differentiated enrichment of CO 2 with respect to SO 2 and H2S could well be explained in terms of partition of these gases between liquid and vapor. The distribution of elements having more than one oxidation state, appears to be a basic feature :distinguishing the gases of the fumaroles of Gran Cratere from those of the beach. In fact, at the crater, S02 predominates over H2S and S~ while CH4 is practically absent, whereas in the fumaroles at the beach H2S becomes the predominant sulphuretted species; S02 is below observable limits; CH 4 appears in traces, and the H2/CO2 ratio increases. The gases of the beach area appear to be in a much more reduced state than those of the crater.
~e
18
8
°~o
~
~
2~o
~o
~o
~;o ~c ~o
FiG. 5 - CO~ contents in the fumarolic gases of the crater at different temperatures. COs of fumarolic gases can be used as a geothermometer by means of the selected curves shown in Fig. 4. LE GUERN(personal communication) (open circles) and CIoNI et al, (1978) (solid circles) found almost constant COs contents in fumaroles of the crater having different temperatures. This suggests that the same gas feeds them_ The 20-40°C shift between the m s , m u m temperature of the fumaroles and the temperature of the corresponding CO2 values on the curve (dotted area) indicates that the vapor can reach the surface with a cooling of only 2040°C.
554
M.
GARAPEZZA
- P.M.
In our system considered at the equilibrium, the fundsmental reaction governing the concentration ratios among the sulphuretted species is the following: H2S+2H20 = SO2+3H 2 hence, log /so~.
IH~S
(1)
= log R =
= log k 1 -}- 2log/H20 - 310g [H2
(la)
NUCCI0 - M . V A L E N Z A
log R s o ~ s =
(HM)
= 16.77
log Rs~/~S =
(FMQ) =
9.32
(3)
As the concentration of H20 varies from about 80% (at the crater) to over 99% (shore) (Table 1), log fHo0 will be between --0.1 and 0, thus this t e / m in equation 3 is of limited importance. From eq. 3 we see that~ for a given temperature, the SO~H2S ratio depends on the oxygen fugacity. However, assuming that the total mass of volatiles is small compared to the mass of volcanic rocks through which the gases pass, the solid will control the oxygen fugacity (SATO and WHITE, 1966; CARMICHAEL et al., 1974). In this case [o~ can reasonably vary in a range between the values defined by the two buffers HM (Hematite-Magnetite) and FMQ (FayaliteMagnetite-Quartz) according to the following equations (EUGSTER and WONES, 1962)
(FMQ) log/os = 9.0
24.634 × 10s T (4) 25.738 x l0 s T
1.17 x 104
log/~o (7b)
T
log R(SO~/H~S) = log KI -- 3log K~ --
(HM) log/o~ = 13.966
1.01 x 104 T --log/I~O(7a)
(2)
If the values of [H. obtained from (2) are used in (la) we ~aave
log [~ho + 2 log [o2
(6)
giving values of f~ comprised between the HM and FMQ bffffers. Making explicit in equation 3 the values of K1, I~ and f~ as a function of T and using for fc~ the varues relative to the equations 4, g and 6 we have:
fH2 must also satisfy the dissociation equilibrium of H20, i.e.
H20 = H2 + ~- 02
26.8 x l0s T
log/o2 = 13
(GF)
(5)
Moreover, for geothermal fields, TONANI (1973) suggested the following relation
(GF)
log RsodH2S = = 15.32
1.34 × 104 T
lOg/H20 (7c)
Graphs of log R vs. 1IT based on equations 7 a-c are shown in Figure 6. Individual values of log R as observed in the fumaroles of Vulcano (Table 1) are also reported in Fig. 6. Figure 6 shows very clearly that the observed high-temperature R-values fall in the field between the two log R lines as calculated from equations 7a and 7b. This indicates that the [ O 2 values of the gaseous phase are reasonably close to those expected for an acid magma, and then, that the SO~/HsS ratio reflects an equilibrium value in the gaseous phase in which /p~iS fixed from the outside. The observed esent log R and T values for the crateric gas, on the other hand, fall outside the field defined by the lines obtained from eq. 7a and 7b. More precisely, they are shifted towards more positive values of log R, hence of higher ./o2, than those calculated on the basis of eq. 7a for the same temperature. T h e observed present R values in the crater fumaroles, however,
555
GENESIS A N D EVOLUTION OF T H E FUMAROLES O F V U L C A N O
canRot be explained simply out of oxidation by admixture of atmospheric oxygen. In fact, the N 2 content, computed as air, would supply a quantity of O 2 which would not justify the observed a m o u n t of SO s. E v e n a complicate process assuming t h a t the oxygen fugacity is controlled by sinks such as e.g. Magnetite-Hematite, would still result in exceedingly low f 02, which involves H2S to be the dominant species for T = 300°C. Therefore, even if it can be admitted that the composition of the gases m a y be modified b y tiquid-vapour partition phe-
+10
too *
Wo
=
III
, s?o, ,
S;; dominant /
"
UY/Y" ,
%
t"c
rl2=
cc
/
H2S d o m i n a n t
-10
ANALY$1S IN TAILL| 1
• V 4)' i
[L2.~I [s.8 } (7) (TT,]2)
• •
2'o
nomena, it is probable t h a t the R values as observed at the crater ftunaroles reflect higher-temperature equilibrium conditions. CARIVflCHAEL et al. (1974) report examples of gas compositions reflecting higher equilibrium temperatures t h a t are interpreted by quenching phenomena. T h e fact that the R values at the crater are independent of the temperature of the fumaroles would m e a n that the equilibrium according to eq. 7 is not attained, and suggests that, as for the CO2 content, there are no significant variations in the gas composition as long as there are not condensation phenomena. I n the case of the shore fumaroles, the deep gases, that, probably, are the same as those of the crater, undergo continuous solid-liquid-gas reactions in passing through the shallowest aquifers. T h e [c~ values will follow a pattern close to t h a t given by eq. 6 and the gases, being able to stay in the aquifers for relatively long periods, will tend to show values of R according to eq. 7c. This would explain the predominance of H2S in the beach fumaroles.
Distribution of Carbon Species
[i4,~51 07)
~o
-~ lo 4°K
FIo. 6 - Variations in the SO2/H2S ratio in the fumarolic gases as a function of T. The stTaight lines GF, HM and FMQ show the variation of this ratio as a function of the temperature for values of [ 0 x which vary according to the equations proposed for geothermal fields and to those relative to the hematite-magnetite and magnetite-fayaIite-quartz buffers. The R value voicanic gases in equilibrium with baSalts, andesite and dacite magmas fall in the dashed area. The plotted data are referred to the Gran Crate-re gases. These gases show a log Rso~]H~s between 1 and 2, which does not seem m change with the temperature of the fumaroles, suggesting higher equilibrium temperatures. The gases of the fumaroles of the shore area having a SO~ content below the detection limit (2 ppm), seem to have a tendency to riequilibrate at lower temperature in the shallower aquifers.
T h e m o s t c o m m o n species a m o n g those containing carbon atoms is CO2; other species, of which there are traces, are CO and CH~. T h e first reaction to be considered is the following: CH 4 + 2 0 2
= CO 2 + 2 H 2 0
(9)
F r o m (9) we have: log Rco2/cH4 = log K9 -{- 2 log/o2 -- 2 log/H20
(10)
For log fn 0 and log fo~, the same observations as for the sulphuretted specms are to be made. T h e following equations define, in the log R-lIT plane, the range of variation of the C 0 2 / C H 4 ratio: .
2
o
556
M.
CARAPEZZA
- P.M,
NUCCIO - M. VALENZA
7268 (HM) log Rco2/c~, = 2 6 . 8 3 - - ~ T
(10a)
(HM)
9787 log R = 24,74 -- ~
(12a)
9476 (FMQ) log Rco2/cH4 = 16.9 - - - - T
(10b)
(FMQ)
log R = 17.29
11443 T
(12b)
11600
(GF) log Rco2/cH4 = 24.9
T
(10c)
These three equations are shown in Fig. 7, together with the R values of the beach gases. The following reaction: 3 CH4+~O2 2
= C0+2H90
(ii)
CO makes it possible to obtain the ratio - CH4 as a function of T. In fact from (11) we have:
13036 (12e) T These three equations are shown in Fig. 8 together with the ratio referring to the beach geses. The reaction:
(GF)
log R = 23.29
CO2 = C O +---z 02 (13) 2 allows us to obtain the CO~]CO ratio as a function of T and fo~. From eq. (13) we have log Rco2/co = log K1s + 1 l o g fO s (14)
= l°g Kll + 2
log/o2 - - 2 logf~o
(12)
from which, substituting for foo the values obtained from equations 4, 5" and 6, we have respectively: 1oo
,
~,o
,~,°,,
2518 -}-~
(HM)
log R = 2.084
(FMQ)
log R = 0.399 + ~
log RCO/CH~ =
(GF)
1966
100 I
t~c
1435 T
log R = 1.601 + 300 !
I
i
500 I I
(14a)
(14b)
(14c) !
t °C
d~l~ +10 =,
g
"Y- +10 u ¢¢
..Q O'
_,! / j
o..-o-, 2'0
Ib
~IO4.K
FIG. 7 - log Rco2tc~4 as a function of temperature. Analyses 13, 16, 18 (Table i)
"-
/ C H
2"o
4 dominant
{o
~ 10,OK
Fire 8 - log Rco/c~4 as a function of temperature. Analyses 18 (Table 1)
GENESIS AND EVOLUTION OF T H E FUMAROLES OF VULCAN0
t00 |
3OO
i
•
50O , ,
o
t °C
1¢.
C02
dominant
d"
5
;io
0 .................................
2'o
lb
+lo~°K
FIG. 9 - log Rco2/co as a function of temperatare. Analyses 11, 17, 18 (Table 1). These three equations, together with the R values observed at the fumaroles of the crater and of the beach, are shown in Fig. 9. The CO~CH4, C02/C0, and CO]CH4 ratios for the beach fumaroles do not reflect equilibrium conditions under the same oxygen fugacity value. In fact, while the CO2/CH4 and CO]CH4 ratios fall close to the HM buffer, the C02/CO ratio falls below the FMQ buffer. This behaviour can be explained by assuming that the gases retain memory of a higher temperature equilibrium, although some solid-liquidgas reactions at lower temperatures may occur during their ascent. On the other hand the ratios of the minor species over the major ones (that is CO/CO~ H2/H20 eta..) (NORDLIE, 1971) are gauges of equilibrium of quenching temperatures.
PRINCIPAL CHARACTERISTICS OF THE RESERVOIR The occurrence, among Lipari andesites, of lavas containing cordierite, garnet, andalusite and sillimanite xenoliths,
557
suggests that the basement under Lipari and Vulcano is metamorphic (BERGEAT, 1910; HONNOREZ and ~.1.~.R, 1968; BARBERI et al., 1974). T h e structural model proposed by LATTER (1971), on the basis of seismological data, is qualitatively similar to the basement morphology as deduced by gravimetric and magnetic data by IACOBUCCI et al. (1977), although in this latter model the depth of the basement under Vulcano results to be higher, and is a unitary system characterized by the presence of structural highs and lows. The geothermal reservoir is probably formed of layers of pyroclastic materials, lavas and sedimentary rocks laying on a basement structural low beneath Vulcano. If the observed relatively rapid changes of temperatures and composition of the fi]mArolic gases, are taken into account, it is reasonable to assume that the reservoir is able to evolve quickly towards new equilibria of gas distribution between liquid and vapor. This implies the presence of a relatively small reservoir (formed by rocks having a good permeability). On the other hand, both hydrothermal alteration and circulation of hot waters in volcanic rocks causes an extensive solution of silica. Then, lateral migration of hot waters from a reservoir system may be hindered by processes of solution and deposition to seal off channels of flow (FACCA and TONANI, 1964; ELLrS, 1967). Similar phenomena will affect the vapor phase in which the silica solubility is a function of temperature and pressure (FOURNIER and ROWE, 1966). Therefore we can presume that, at a certain distance from the feeding center an acquiclude developed by self-sealing phenomena, giving rise to a nearly dome-shaped reservoir. An aquielude can maintain subsurface temperatures and pressures, whereas a aquifer is recheargeable by deep circulation through faults. The fluids in the reservoir are heated mainly by very hot magmatic gases rising up through the faults, although conduction through the metamorphic basement may give a small contribution to the heat.
558
M.
CARAPEZZA
- P.M.
NUCCIO
Equilibrium Conditions in the Reservoir A schematic diagram of the reservoir model is given in Fig. 10. The condition that the system is in a steady state sets the following constraints o n fluxes: Ei = E w + E c + E s
(15)
MI+M 2 = Ms+M4
(16)
If a change in one of the parameters of these two equations occurs, the system will tend towards a new steady state of equilibrium, changing the other parameters until the equations (15) and (16) are again satisfied. In such system the main changes we can expect are in E i and M l, being both parameters strictly related to the state of the magmatic body below the reservoir. Therefore if Ei and Mt increase (assuming M s and M 3 to be nearly constant, and considering that both the conductive and convective losses of energy Ec and Ew respectively, represent a small fraction of the total toss of energy the rate of vapor formation from the boiling liquid in the reservoir will increase. If the rate of discharge (E~ M4) is able to follow the rate of the vapor formation, the pressure in the reservoir will remain constant as well as the temperature, which will be buffered since the liquid phase is present. On the contrary, when the rate of discharge (E~ M4) is sm~ller than the rate of vapor formation, both pressure and temperature in the reservoir will increase following the boiling curve of the liquid_ In -
surface leakage (fumaroles etc...)
M4 • (water)
water inflow
Ew M2
1
RESERVOIR
- M.
VALENZA
this case some energy will be stored up in the rock and in the fluids of the reservoir. Furthermore, depending on the energy input, the system can become definitely unstable and :an explosion is to be expect~ ed (TONANI, 1970). On the other h~nd, the possibility that the rate of discharge does or does not follow the rate of vapor formation is related to the properties of permeability of the system.
The Pressure-cooker Model As one can see in the section (H20-CO 2NaC1) the prevailing conditions in the reservoir can be defined by the boiling curve of the liquid phase containing, at the tempera~_re of 350°C, 1.5-2% of C02 and 10%NaC1. When the m ~ m u m temperature of the fnm~rolic gases changes from 200°C up to 303°C, the CO~. content in the gases is 20% and (Fig. 5) a boiling temperature of 320-340°C can be inferred_ For this value of temperature the boiling pressure is of 130-170 arm (Fig. 3). T h e vapor phase formed in the pressurized reservoir will migrate towards the surface. Obviously the expansion of a fluid rising toward the surface is a highly complex process. As TOULMIN HI and CLARK (1967) pointed out, a semplified thermodynamic description of a fluid expansion may be given by both a and an . Normal or transcurrent faults form channels of permeability in the cap-rock, linking the reservoir with the surface through a highly convective losses (steam + gases)
E3 A SYSTEM
water - r o c k - steam
I
Ec
conductivelosses
M3 water outflow
• A MI El hot deep cumulative heat imput fluids (convective, conductive)
FIG. 10 - Schematic rapresentation of the energy-mass balance for the Vulcano system.
GENESIS AND EVOLUTION OF T H E F U M A R O L E S OF V U L C A N O
intricate and tortuous path. Under these conditions local steeper pressure gradient m a y result near the faults, and the thermal contact of the fluid with walls would prevent reversibility. Although an irreversible adiabatic expansion has a much lesser cooling effect, still it is inadequate to explain the high temperature measured at the surface. Therefore a nearly isothermal expansion must be considered in order to explain the few degree of cooling (~ 30°C). If the expansion of gases takes place along a fault sunk into the reservoir, the required energy for the isothermal expansion can be supplied by the reservoir itself. In fact, considering that our system is similar in m a n y aspects to the so called ~
