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Isotopes of the light noble gases in mineral waters in the eastern part of the Balkan peninsula, Bulgaria
Isotopes of the light noble gases in mineral waters in the eastern part of the Balkan peninsula, Bulgaria N. B. PIPEROV,’ 1. L. KAMENSKY,~ and 1. N. TOLSTIKWN~ ‘Institute of Genera1 and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113,Bulgaria 2Geological Institute. Kola Science Center of the Russian Academy of Sciences. Apatity I84200, Russia
Abstract-The ~~~c~~~rat~o~ and isotopic composition for the light noble gases in spontaneu~s gas from mineral waters in the eastern part of Balkan peninsufa are presented in this paper. The isotopic ratio 3He/4He in dissolved gases varies within the range 0.15- 1.1 X IO-’ and the concentration of helium in these gases is between 0.002 and 3.14 vol% It is determined that 86.7-98.8, 0.49.2, and
ranged from approximately 10m8in the PO River valley up to IO-* in the southern part of Italy, in Sicily, and in the Eolian Islands ( POLYAK et al., 1979a; HOOKER et al., 1985a; SANO et al., 1989). The distribution of helium isotopes in North Sea gas fields and groundwaters of the Rhine rift is another example for this thesis (HOOKER et al., 1985b). A substantial advance has been achieved in the interpretation of the isotopic and elemental ratios of other noble gases in natural gas mixtures (fCAMENSK?’ et al., 1976; NAGAO et al., t9X f; VERKHOVSKY et al., L983; M~~~RI~ and TOLSTIKHfh’,1984). The present study is aimed at the ~nt~~retation of the results obtained by measuring the isotopic composition and content of helium and other light noble gases (neon and argon) in some mineral waters of Balkan peninsula-another link of the Alpine erogenic belt. Recent information about the helium content ofsome Bulgarian mineral waters as well as a few data for neon and argon isotopes may be found elsewhere ( PENC~IEVet al., 1964, 1970; PENTCHEF’F et al., 1966: Kot.Ev et al., 1984).
As a rule. the mineral-water sites in the region are controlled by tectonically active zones and very often the sources are at the intersections of faults ofdifferentdirections (Fig. 1). The sites ofmineral water studied are related ta seven tectonic zones: ” (/irtrl/) separates the Rhodope mountain 1) The Munr.sa “.WIIIW massif (south ) of the Sredna Gora zone (north). It is one of the main tectonic hnes in the Balkan peninsula, seismically active, and well dehned by a complex of geophysical features. 21 The S/rtctnrcjir~f~ lid is the western boundary of the Rhodope massif. This cluster of faults of submeridional direction is seis-
1890
N. B. Piperov, 1. L. Kamensky, and I. N. Tolstikhin
.
Heat ?Iow
dansity,
kth
mW/m*
00
- 100
(---J
70-60
n
60-70
sites et nw%wDIwotas Dischsrgr, I/r > 30
FIG. 1. Schematic map of the network of deep faults, the thermal flow density, and the most important sites of minemi water in the eastern part of Balkan peninsula. The data were compiled from the publications of K~STADINOV et al. ( 1973), BOJADGIEVA et al. ( 1989), and PETROVet al. ( 1970). Abbreviations as in Table 1.
3)
4)
5)
6)
7)
mically active, intersects the Moho which is located there at a depth of 45 km, and penetrates the upper mantle. The Rhodope muss$ includes the Rila, Pirin, and Rhodope mountains. It is characterized by a block structure with a dense network of faults. The Mesta fault zone demarcates the Pirin mountain and the West Rhodope mountains. This graben is similar to the neighbou~ng Struma fault line, but the faults are undeveloped in length (80- 100 km only) and depth ( 15-20 km). The Truns-Bulkan.fuult zone is a system of subparallel deep faults which probably cut all of the crust (ca. 40 km) and serve as the northern border of the Sredna Gora zone. The Burros s~~l~ij~orj~4~is composed of upper Cretaceous scdiments, effusives, and pyroclastites dipping in the middle and east of the zone to the Black Sea. The mineral water sites are associated with Senonian andesites and tufa along the structural dislocations in the north of the synclinorium. The Strand=~a-~S~kar region is situated to the south of Burgas synchnorium. Paleozoic granites and Paleogene plutons are widespread as well as Mesozoic (Jurassic) sediments and metamorphites. Only a few dislocation and only two sites of mineral water are known in the region. The For+Balkan region is a transitional zone between the Stara Planina (Balkan) mountain and the Mysian plate and consists of subparallel anticlines and synciines (of Mesozoic age) formed by a system of subparallel dislocations. The natural discharge of mineral water is relatively rare.
A brief summary of information for the tested sources is given in Table 1.The data for mineral-water sites and for the regions described are compiled from SHTEREV( 1964). PETROV et ai. (1970), and DACHEV(1988).
METHODS
AND CHEMICAL, AND ISOTOPIC ANALYSES
OF SAMPLING,
ELEMENTAL
Spontaneous gas was collected in glass pipettes (bulbs) with two stop-cocks using a funnel as a trap for the bubbles. The funnel, pipette, and tubing were previously filled with water from the source and this water was later displaced by the gas. An exception was the sampling of 3.5.BB, when gas was collected in the pipette directly from the gas separator valve of the casing tube after an extended period of flushing the pipette and tubing with gas. The water samples for degassing (6.2.SK and 7.1.VV) were collected in 2.5 L glass bottles entirely filled with water by sinking into the source (captage). The bottle was hermetically closed ensuring absence of any air bubble. A special “soft” system of sealing was designed for the purpose. In the laboratory the bottle was put in a water bath and heated up to 80°C. The evolved gas was coilected in a previously evacuated gas pipette until the pressure rose up to 400 hPa (ca. 300 Torr). The gas sample so prepared (except Na, COZ, and noble gases dissolved in the water) also contains significant amounts of COZ obtained by thermal destruction of HCO;. For this reason, onIy the ratios between the noble gases (and nitrogen) in these two samples (6.2.SK and 7. I .VV) may be taken into consideration, with some stipulation. but not the absolute concentrations. The pipette, containing the gas sample, was connected to the evacuated analytical line where a small amount of gas was let into a single-collector mass spectrometer (MS IO) for general analysis, after freezing the water vapour in a trap cooled by melting ether (T
= -116°C). Another portion of gas (5-50 mL) was used for a more precise determination of the noble gases. This amount was measured volu-
Noble gases inmineral waters in Bulgaria
1891
TABLE 1. Some data about the studied mineral waters and their sites
Site
Geological enviro~ent, collector,
Site
T.-C M' g/l
and type of source
code 1. Maritsa fault
Predominant ions
Gas typeX
zone
l.l.Mr Merichleri
Seismo-tectonicknot, trachyandesite (Pg,),drill hole (300 m)
34
4.8
SO4 HC03 Cl / Na
1.2.Kh Kharmanlf
Granite and gneiss, drill hole (442 m)
2.3
4.3
SO4 Cl / Na
N
2. Struma
fault
N-C
line
2.1.SB Sapareva Banya
Granitogneiss(Pt), drill hole (200 ml
97
0.67
SO4 HC03 / Na
N
2.2.Bg Blagoevgrad
Gneiss (Pt). drill hole (168 m)
54
0.82
SO4 HC03 / Na
N
2.3.Sm Simitli
Gneiss fPt), drill hole (422 m)
62
0.57
SO4 HC03 /
Na
N
2.4.K-z Kozhukh
A site at the foot of a trachyandesite subvolcanicstock fNI).Gneiss (Ptf, 76
2.35
NC03 / Na
C
C-30) 62 spot
1.0
WC03 SO4 / Na
N
Granite, drill hole (500 ml
53
0.31
HC03 SO4 / Na
N
drill hole (500 m) A swamp with several hot spring spots on the bottom. Gneiss (Pt)
2.5.Mk Marikostinovo 3. Rhodope massive 3. l.GB
Guliina Banya
Gneiss, source
41
0.3
UC03 SO4 / Na
N
3.3. vg
Velingrad
Granite [Pz), drill hole (450 m)
84
0.7
SO4 HC03 / Na
N
3.4. Mi
Mikhalkovo
Gneisses with water-bearingmarble layers (Pt), drill hole (70 m)
27
4.04
HC03 SO4 / Na Ca
c
Gneiss (Pt), drill hole (436 ml
74
1.8
HC03 SO4 / Na Ca
C-N
Dislocation in gneisses, spots of ancient travertine.Drill hole (26 m)
24
1.5
SO4 HC03 / Na
Gratized gneiss (Pt). Source
32
0.92
HC03 SO4 / Na Ca
N
61
0.50
HC03 / Na
N
49
2.0
HC03 SO4 / Na Ca
25
0.54
HC03 / Na
3.2. Og Ognyanovo
3.5. BB
Bedenski Banf
3.6.Nr Narechen baths 3.7.Bn Banite 4. Trans-Balkan
fault
zone
4.1.PB Pave1 Banya
Granite iPz), drill hole
4.2.51 Sliven baths
Limestones ITrzI.silicate basement. Drill hole (373 m)
5. Burgas
N-C
synclinorium
5.1.Md Medovo 6. Strandzha-Sakar 6.1.Tr Troyanovo I
Andesite
(Crs,),
drill
hole
Sediments (Ng,,Q) with lignite coal
6.2.SK Stefan Karadzhovo Dislocationbetween marbles with sericite schists (Mz) and diorite porfirite (Pg). Captage 7. Fore-Balkan 7.1.vv Voneshta voda
N
region
layers. Silicate basement (granite?). Drill hole
Note:
N
Rn 0.13 &i
35
CH
20
1.5
HC03 / Ca Mg
N-C
13
1.3
HC03 SO4 Cl / Ca Na
N (H2S)
region
Dislocation in quarzitizitesandstone. Captage
* Total mineralization. ' N = N2, C = CQ2, CH = CH4
metrically and then,usingcoldtraps (liquid nitrogen) and heated (6OO"C)metallic calcium, the active components wereremoved. The noble gases were collected from the volume of the apparatus by a Sprengel mercury pump, put into a modified McLeod gauge and measured as the total mixture. The concentrations of helium and argon in this noble gas mixture as well as the isotopic composition of argon were determined with the same mass spectrometer. The error of "Ar,36Ar. and 38Arpeaksiswithin thelimits of I,3, and
5%. respectively. Next, the argon was removed by activated charcoal at -195°C (liquid nitrogen) and the peak of helium (4He) (and “Ne) was recorded. The fraction for the isotopic analysis of helium and neon was prepared from a 50-200 mL gas sample by adsorption ofthe chemically active components and heavy inert gases on activated charcoal cooled by liquid nitrogen. The rest, which couldnotbesorbed, was collected by a Sprengel pump in a glass ampoule which was sealed off.
1892
N. B. Piperov,
I. L. Kamensky,
and I. N. Tolstikhin TABLE 2.
All procedures described were carried out at the Institute of General and Inorganic Chemistry. Bulgarian Academy of Sciences, and were checked by special tests using air. The isotopic composition of helium and neon, as well as the precise of the Ne/He ratios were determined at the Geological Institute, Kola Science Center of the Russian Academy of Sciences, measurements
using a MI 120 I mass spectrometer. Prior to analysis, the ion source was tuned to maximum intensity of the ion current of 3He+, reaching a sensitivity of 5 x 10-jA/Pa and resolution I300 (50% valley definition). The background ofthe analyzer and vacuum system as well as the contribution of the Hi + HD+ peak, which does not exceed IO-) of the ‘He+ peak height. practically did not affect the precision of measurements ofthe 3He/4He ratio. The variations did not exceed 5, 12. and 25% for ratios of the order IO-(‘. 10m7, and 2 X lo-', respectively. After adjustment and tuning, the ‘He/“He ratio in a reference sample t,artificial mixture of atmospheric helium and neon) was determined. Then the ampoule with sample helium (+ Ne) was broken in a special vacuum device and the ‘He+ and 4He+ peaks of the sample helium were recorded, followed by a new run of the standard. The next step, after tuning the source for measurement of the isotopic composition of neon, was the checking of the ion currents of “Ne . “Ne . and ‘*Ne in the standard (twice) and in the sample studied. The precision of the isotopic analysis is shown in Table 4. The 4He and “Ne peak heights for the samples. sealed for isotopic
analysis. were determined with an inaccuracy of 5% and thus the 2oNe/4He ratio was evaluated with an error of -CIO%,
RESULTS AND DISCUSSION The results from the general gas analysis are given in Table 2. The most abundant species is nitrogen. Almost all samples contain small amounts of CO*; in three cases it is the main component. Methane is predominant in only one gas sample.
The general
analysis
(vol.%)
c02
CH4
Site N2
D2
NG
code
l.l.Mr 1.2.Kh
71.6 95.7
0.60 -
26.3 0.24
0.01 0.04
1.47 3.80
2.1.SB x 2.2.Bg 2.3.Sm 2.4. Kz 2.5.Mk
89.7 97.2 97.7 5.1 92.8
0.16 -
3.30 0.46 0.06 94.0 5.58
3.71 0.54 0.59 0.61 0.26
2.49 1.71 1.60 0.095 1.77
3.1.CB ' 3.2.og 3.3.vg 3.4.Mi 3.5.BB 3.6.Nr 3.7.Bn
98.2 98.5 96.9 5.5 21.1 94.5 92.4
-
0.01 0.32 94.2 78.2 3.38 5.50
0.20 0.09 1.09
1.42 1.36 1.76 0.094 0.50 1.88 1.96
4.1.PB 4.2.Sl
96.6 64.0
-
1.78 34.4
0.24 0.05 0.20 0.08 0.02
5.l.Md '
98.3
0.10
6.1.Tr ' 6.2.SK'
7.48 16.8
0.10 1.18
2.06 (81.819
7.1.wgX
44.3
3.86
(50.8)'
90.1 0.06
1.50 1.42 1.15 0.11 0.25 0.95
Note: NC - total concentration of the noble gases _l
below detection limits _ degassing of the water at 8O'C
x
Helium
other components are also detected
in
samples
The helium content in the studied sources (Table 3) varies within a broad range between 0.002 and 3. I4 vol%, and the results given in Table 3 are in quite good agreement with the recent data (PENCHEV et al., 1964). No evidence for any conformity of the relationship between the chemical composition (Tables 1 and 2) and helium content was found. It can be noted, however, that low helium concentrations were a characteristic feature of sources in which the gas contains predominantly COa (2.4.Kz, 3.4.Mi) and a maximum when Nz is the main gas component ( I .2.Kh, 1.1 .Mr). This observation is already noted elsewhere (e.g., PETROV et al., 1963; PENCHEV et al., 1970). The 3He/4He ratio changes within the range 0.15 X 10e6I. I X 10mh. The data for neon (Tables 3 and 4), which has predominantly an atmospheric origin, offered a possibility to evaluate the contribution of helium from the mantle (m), crust (c), and atmosphere (a) to the helium sample (s) (KAMENSKY et al., 1976), presuming that the 3He/4He ratios are R, = 1.2 X 10m5, R, = 2 X IO-‘, and R, = 1.4 X 10-6, respectively and 4He, = 0.286 X “Nee ( TOLSTIKHIN, 1986). The first value (R,) is close to the maximum 3He/4He ratio known from the mineral waters in the zone of Alpine-type orogeny (MATVEEVA et al., 1978; POLYAK et al., 1979a). This value is considered to be a characteristic feature for the Earth’s upper mantle based on the isotopic composition of helium in the rocks and hydrothermal systems of the ocean floor (LUPTON, 1983; MAMYRIN and TOLSTIKHIN, 1984).
2.1.58: H2 0.62, H2S 0.01 and C2H6 0.02. 3.1.GB: H2 0.10. 5.l.Md:
H2 0.37.
6.1.Tr:
H2 0.05, C2H6 0.04 and C2H4 0.01.
7.l.W:
H2S 0.01.
The corrosion of is a possible source JONASSON, 1977).
the of
drilling hydrogen
hole steel casing (DYCK and
As seen in Table 3, the contribution of atmospheric 4He to helium in the mineral waters studied is negligible-it does not exceed I % (sample 5. I .Md is the only exception). Naturally, the least amount of atmospheric helium is observed in sources with maximum helium content: i.l.Mr, 1.2.Kh, 4.2.Sl. This is confirmed also by the 4He/20Ne ratio (Table 3). The contribution of mantle helium is considerably more significant than that ofatmospheric helium: it varies between 0.4 and 9.2%. The gases richest in mantle helium have also high 3He/4He ratios (4.2.S1,6.2.SK, 2.4.Kz). More than 85% of the “He is radiogenic and has originated from the crust. The contribution of the main reservoirs in the balance of ‘He is considerably different from that in the case of 4He. Data in Table 3 show that the main source of the light isotope of helium is the mantle. As a rule, not less than 80% of 3He is genetically related with this source (for 5.1.Md, 19.2%
Noble gases in mineral waters in Bulgaria TABLE 3. tribution
Total content and isotopic composition crustal and atmospheric of the mantle, gas evolved from mineral waters
4He
Contribution
3He/4He
Site
4
vol.% code
He/20Ne
r xlOD
(He tot)
4
of
I893 of helium. Conhelium to the
the main stocks,
He
Atm. Mantle
%
3He Crust
Atm. Mantle
Crust
0.004 0.008
3.4 1.2
96.6
0.01
98.8
0.06
94.9 84.7
5. 1 15.2
200 460 550 66 41
0.14 0.06 0.05 0.4 0.7
1.7 2.0 3.1 7.3 5.4
92.2 97.9 96.9 92.3 93.9
0.85 0.3 0.17 0.6 1.4
88.7 92.3 93.0 97.3 95.3
10.4 7.4 6.8 2.1 3.3
0.735
0.19 0. 16 0.15 0.37 0.57 0.19 0.41
100 56 110 950 520 990 1100
0.28 0.51 0.3 0.03 0.06 0.03 0.03
1.4 1.2 1.0 2.9 4.6 1.4 3.2
98.2 98.3 98.7 97.1 95.3 98.6 96.8
2.0 4.4 2.8 0.1 0.1 0.2 0.1
88.4 90.0 80.0 94.0 96.8 88.4 93.6
9.6 5.6 17.2 5.9 3.1 11.4 6.3
0.153 0.631
0.20 1.1
320 1900
0.09 0.01
1.4 9.2
98.5 90.8
0.6 0.01
84.0 - 100
15.4 -0.0
0.0032
0.25
1.1.W 1.2.Kh
1.09 3.14
0.43 0.17
6400 3500
2. l.SB 2.2. Bg 2.3. Sm 2.4. Kz 2.5.Nk
0.183 0.342 0.319 0.0022 0.070
0.23 0.26 0.40 0.90 0.68
3. l.GB 3.2. og 3.3. vg 3.4. Hi 3. 5. BB 3.6.Nr 3. 7. Bn
0.080 0.055 0.120 0.049 0.085 0.788
4.1.PB 4.2.Sl 5. l.Md 6. l.Tr 6.2. SK
0.022 0.0064’
7.1. w
0.157
l
2.2
12.9
0.84 1.08
300 36
0.09 0.80
0.70
290
0.10
0.4
86.7
19.2
8.8
6.8 8.7
93. 1 90.5
0.15 1.0
97.1 96.7
2.8 2.3
5.6
94.3
0.20
96.0
3.8
72
Note: * - degassing
of
site
water
at 8O’C.
Results of analysis of samples 2.4. Kz, 2.5.Mk and 3.4.Mi, compared with results obtained 10 years ago, showed that ratio helium-3/ helium-4 remains constant within the precision mentioned in the text.
only),
and in some sources (4.2.S1, 2.4.Q 6.1 .Tr) 3He, is more than 97% abundant. The contribution of 3He from the crust is within the range -0- 17.2%, while that of atmospheric origin is less than 5%. In sample 5.1 .Md only the infiltrated atmospheric gases are represented widely: 3He, is 72% and 4He, is 12.9%. A correlation between the thermal upflow and the 3He/ 4He ratio is established on systematics of a large number (more than 400) of natural fluids ( POLYAK et al., 1979b; POLYAK and TOLSTIKHIN, 1985). In the case of this investigation, however, the relationship mentioned seems to be confirmed for only half the samples (Fig. 2). A similar discrepancy between 3He/4He ratios and the thermal flow for some spots in western Europe was observed by OXBURGH and O’NIONS ( 1987). This may be expected, as both helium and the heat flow reflect magmatic activity, but they are affected in a different way by different factors such as recent tectonic movements, paleoclimate, hydrogeological conditions, etc. Hence, the correlation between 3He/4He ratios and the thermal flow may be revealed clearly only on the basis of a large number of measurements, including data from different regions ranging from continental margins, where new crust is now forming and tectonic-magmatic activity is extremely high, to ancient (Precambrian) plates, where this activity abated at more than 1 Ga.
Neon The bulk of the neon in the gases seems to be infiltrated (dissolved) atmospheric neon. The isotopic composition of this component in most ofthe samples is close to atmospheric, although a trend towards the enrichment of neon with light isotopes is observed, e.g., ( 20Ne/22Ne), z (*‘Ne/**Ne),,, (cf. Table 4). Two explanations of this *‘Ne excess are possible: either mass fractionation of atmospheric neon, or addition of “light” mantle neon (MARTY, 1989; HIYAGON et al., 1992). The last case may be checked by comparing 20Ne,X with an unambiguous mantle component: ‘He. The 20Ne,,/3He ratios for the gases investigated are of the order 10-1000 which is lo*-lo4 times greater than these ratios in MORBs and OIBs. Hence, if a contribution of *‘NeeX from the mantle is proposed, then this neon would have to be hugely enriched (relatively to 3He), which we consider to be highly improbable. Another explanation, i.e., fractionation of air neon, was suggested by NAGAO et al. ( 1979, 198 1) for thermal waters in Japan. Several arguments support a similar interpretation of *‘NeCXin the Bulgarian mineral waters: the 40Ar/36Ar ratios show almost atmospheric values and the “Ne/**Ne ratios vs. *‘Ne/‘*Ne fit the mass fractionation line well (Fig. 3).
N. B. Piperov, I. L. Kamensky. and I. N. Tolstikhin
1894
TABLE 4. Content
and isotopic composition of neon and argon
Ne Site VOL. code
20
Ne/22Ne
f0.2 9.91 kO.04
1.89
9.73
9.9
40Ar/36Ar
vol.%
t* 5)
21Ne/22Ne
PPm
1.1.w 1.2.K.h
Ar
0.0296 0.0301
to.002 $0.0006
0.38 0.66
361 337
to.02 f0.05 20.02 kO.06 TO.05
0.0291 0.0299 0.0293 0.0290 0.0292
~0.0002 +O. 0007 +o. 0002 -+o.0006 +O. 0005
2.31
19
9.80 9.99 10.01 9.79 9.91
1.37 1.28 0.093 1.70
305 301 306 303 295
3.l.GB 3.2.Og 3.3.vg 3.4.M
8.5 10.8 12 a.57
10.16 10.08 9.86 9.95
20.03 f0.03 to.06 to.05
3.5.BB 3.6.Nr 3.7.Bn
1.8 8.0 7.6
9.82 f0.08
0.0297 0.0295 0.0292 0.0297 0.0293 0.0292 0.0284
+O. 0003 +O. 0002 to.0005 to.0006 +o. 001 +O. 0002 to. 0007
1.34 1.30 1.64 0.045 0.41 1.09 1.22
295 292 293 303 305 295 296
4.l.PB 4.2.51
5.2 3.4
10.30 20.07 10.27 20.07
0.0297 0.0301
+O. 0006 to. 0005
1.35 0.79
298 321
5.1.Md
16.0
9.84 f0.03
0.0291
+0.0002
1.15
294
2.1.SB 2.2.Bg 2.3.Sm 2.4. Kz 2.5.Mk
10.1 8.1 6.4 0.37
9.83 fO.05 9.70 f0.03
6.1.Tr 6.2.SK
0.8 2.0,
9.83 f0.03 10.03 TO.02
0.0292 0.0294
+o. 0002 fO.0002
0.091 0.24 *
292 296
7.l.W
6.0'
9.89 fO.03
0.0293
+o. 0002
0.79 *
292
9.800
0.0290
0.934
295.5
Air
18.2
Note: * - degassing
of water at
8O’C.
Argon
3Hr/4He
c
__--
i
,
/
/-
I
The content
I-
, , ., ,
/ /@
---
of argon in the samples
studied ranges from
0.045 to 2.31 vol%, exceeding in several cases the concentration ofthis gas in the atmosphere (Table 41, which is typical for N-bearing mineral waters (FLORENSKY, 1956). The
“Na
I
22N.
I
1
Mk 0
Bn 0
I
SB
CB
0
0
OKti
i
--II-d
LO
I
60
80
9.40 100
q, mW/m2 FIG. 2. Correlation between the heat flow (q) and the isotopic composition of helium from the sites of mineral water. Data for the heat flow are taken from the schematic map, Fig. 1. Abbrevjations as in Table I.
0.027
0.028
0.029
0.030
0.031
21 NI,~‘N~ FIG. 3. Isotopic ratios “Ne/*%e vs. Z’Ne/2ZNe in the gases of the sites investigated. It can be seen that within the accuracy of measurements the results plot along the mass fmctionation line of neon. Abbreviations as in Table i.
Noble gases in mineral waters
highest value obtained is for 2. I .SB and reffects the high temperature of this water (97°C). According to its isotopic composition, argon differs substantially from that of atmospheric argon in only three cases ( 1.I .Mr, 1.2.Kh, and 4.2.5 1) where an excess of radiogenic “‘Ar has been detected amounting to IS. 1, 12.3, and 7.9% of the argon content, respectively. The 4He/40Ar, values for I. 1.Mr and 4.2.Sl are 16 and 10, respectively-typical for regions which have been involved in Alpine orogeny. The ratio 4He/40Ar, = 38 ( 1.2.Kh) is relatively high and normally observed in gases from ancient Precambrian plates (TOLSTIKHIN, 1986). It must be noted that these three sites show a minimum content of atmospheric helium (Table 3). In the rest of the samples the 40Ar/36Ar ratio is close to 295.6 (air) or shows a slight excess of radiogenic 40Ar, in some cases.
in Bulgaria
1895
the ratios between at least two noble gases of atmospheric origin in the sample with the same ratio in the air as well as in water equilibrating with the atmosphere. Table 5 and Figures 4 and 5 suggest the conclusion that the ( Ar/Ne), ratio in the waters of the sites studied is within the range of the values for air (5 19) and dissolved air ( 18 10). Thus, the observed deviations from the values of (Ar/Ne), can be explained by single stage degassing of underground waters (ANUFRIEV et al., 1976). which should determine also the ratios of other components of atmospheric origin. A similar approach may be applied to the Ar/N2 ratio, presented also in Table 5 and Fig. 4. The range of values for this ratio, 0.0 12 (air) and 0.027 (dissolved air), provides evidence for the atmospheric origin of N2 in most of the sites. A substantial excess of N2 with regard to the single stage degassing model ofdissolved atmospheric gases has been found only in sampIes 1.1.Mr and 1.2.Kh, and a similar assumption may be made also for 3.4.Mi. Revealing the origin of C-containing components is more complicated. It is known that neither the concentration, nor the isotopic composition of carbon offer an unambiguous
Origin of the Main Components
In order to understand the gas fractionation during the dissolution and degassing processes, it is useful to compare
TABLE 5. Some ratios between the main components (nitrogen, carbon compounds - cf. Table 2) and the noble gases
Site code
Xl0
3 He/20Ne
Ara/N2
Ara/Ne -3
X103
Xl0
4.3 6.1
27 6.0
4
ZC/“Ne x10
-3
150 0.31
x/3He x10
-7
5.6 0.05
l.l.Mr 1.2.Kh
1.6 0.58
2. I.SB 2.2. Bg 2.3. sm 2.4. KZ 2.5.Mk
2.3 1.7 2.0 2.5 0.89
26 14 13 18 18
0.46 1.2 2.2 0.59 0.28
7.7 1.4 1.1 2900 3.4
3.1.GB 3.2. og 3.3. vg 3.4. Mi 3.5. BB 3.6. Nr 3.7. Bn
1.6 1.2 1.4 0.79 2.3 1.4 1.6
14 13 17 8.2 19 12 13
0.19 0.09 0.16 3.5 2.9 1.9 4.4
0.27 0.09 1.3 1800 490 4.8 8.0
4. l.PB 4.2. Sl
2.6 2.1
14 11
5. l.Md
0.72
12
0.006
6. l.Tr 6.2.SK
1.1 1.2
12 14
2.5 0.39
7.1.w
1.3
18
2.0
-*
.‘.*
Air
0.519
11.96
0.0044
0.02
4.5
1.81
26.8
0.0034
3.4
AS%O
0.64 21
4.2 110 0.014 1300 _*
Note: xc = co2 + cH4 _* - degassing ASW. ^ -
air
of
site
saturated
water water
at fat
8O’C. T = lO’C1.
17 1.2 0.50 4800 12 1.4 1.0 8.1 530 170 2.5 1.8 6.6 5.2 2.3 520 _*
“1000
N. B. Piperov,
1896
1. L. Kamensky,
Mk
/ ‘kd‘ Kh
1
2
5
III
20
50 N2 vol.
----tB og Bn
too %
FE. 4. Plot of the argon concentration vs. Nj for the gases investigated. The solid line represents the Ar/N, ratio for air; the dashed line is this ratio in air-saturated water (at 10°C). Abbreviations as in Table 1.
answer to the question of its origin, nor do they reveal the presence of a juvenile component of carbon ( KONONOVand POLYAK, 1982; PRASOLOVand TOLSTIKHIN, 1987; O’NIONS and OXBURGH, 1988). In order to overcome these difficulties it has been proposed to consider the ZC13He ratio where ZC and ‘He are carbon content (CO2 + C,,H,) and (juvenile) 3He content, respectively, in a natural gas (POLYAK et al., 1976). Recent studies (MARTY and JAMBON,1987; O’NIONS and OXBURGH, 1988). however, definitely show that C/He ratios should be used with caution. After release of the juvenile carbon and helium from the mantle into the exosphere. their behaviour is considerably different. Helium dissipates from the atmosphere into the space. while carbon is bonded to the Earth’s crust, predominantly in the form of carbonates. even with possible recycling into the mantle (MARTY and JAMBON, 1987). Hence, it should be expected. that the crustal C13He ratio is always higher than this ratio in the mantle. Analysing some mantle derivatives (MORE%, uftramafic xenoliths) MARTY and JAMBON( 1987) propose that C/3He = 2 X IO9 for the upper mantle. For the sources on the continental crust. however, this ratio ranges widely: from 10sto 1Ol4 (O’NIONS and OXBURGH, 1988). The results given in this paper (Table 5) show SC/3He ratios of the order of IO’ or even lower. Thus, some assumptions are possible: ( I ) ratios higher than IO9 imply crustal contribution, as it was shown above, but (2) ratios lower than this value may indicate that either mantle CO2 is bonded immediately in the crust, OJ subcontinental mantle avoids recycling and the C/3He ratio in this reservoir is lower than that in the bulk asthenosphere. The case with the samples studied is somewhat complicated. The formation of the mineral waters occurs at a shallow depth (~10 km) in the crust. It is well known that Na+ and HCO: are specific ions for these underground fluids when
and I. N. Tolstikhin
they are formed even in a silicate environment. It is supposed that CO2 takes an active part in this process (e.g., CADEK and SULCEK, 1965): NaAlSi30e(albite) + Hz0 + CO* -+ Na-montmorillonite + Si02 + Na+ t HCO;. When the influx of deep-seated COz’ is too small, it may be entirely exhausted and only dissolved atmospheric gases could be detected in the gas evolved spontaneously from the mineral water. Hence, the original (deep seated) CC/3He ratio may be changed significantly in the gas analysed. For one of the sites (2.3.Sm: 0.3 19% He, containing 3.1% He,, and ZC/3He = OS X 10’ in the spontaneous gas), we have some more information. The concentration of HCO, in the water is 108.3 mgfL (PETROV et at., 1970). i.e., 1.8 X 10p3M, and 100 mL spontaneous gas were evolved from ca. 300 L of mineral water. Taking into account the helium solubility in water ( T = 63”C), the gas concentrations given in Tables 2 and 3, and including the HCO; content in the Z;C, one may calculate that ZC/3He zz 10 lo in this source and, hence, the prevalence of crustal CO2 over a mantle component seems very probable. The COI-containing gases from the sites 3.4.Mi and 3.5.BB. with C02/3He ratios of the order of 10’. are associated with the marble complex of the West Rhodope, and COz is considered to be metamorphogenic ( PETROVet al., 1970). The highest value C02/‘He = 4.8 X IO’” is obtained for a postvolcanic exhalation of nearly# pure CO, (2.4.K~). probabty also of crustal (metamorphic) origin ( PETROVet al., f 970). The ZC/3He ratio of 5.2 X IO9 for sample 6.1.Tr is due to the high CH, content, which is assumed to be entirely biogenic (coal). The difficulties in revealing the present mantle-to-exosphere C-flux are confirmed also by other authors (MARTY et al., 1989).
SB
vol. %
2 -
Yd
1 -
.‘Mk
Nc vol.
4.
FIG. 5. Plot of the argon concentration vs. neon. The solid and dashed lines fit the Ar/Ne ratios in air and in air-saturated lO’C), respectively. Abbreviations as in Table I
water (at
1897
Noble gases in mineral waters in Bulgaria CONCLUSIONS
Kamchatka (according to He, Ne. Ar, and C isotopes). Gwkhi682-695 (in Russian). KANEOKA1. (1985) Noble-gas state in the Earth’s interior-some constraints on the present state. Chem. Geol. 52, 75-95. KOLEVE., KUZMANOVL., MARINOVB., TSANOVTs., and YANITSKY I. ( 1984) On the helium content and the isotope composition of some underground waters in Bulgaria. Spis. hzd,,. grol. drzch 45, 3 18-328 (in Russian). KONONOVV. 1. and POLYAKB. G. (1982) The problem of determining juvenile component in modern hydrothermal systems. Geokhimiya, 163- 177 ( in Russian). KOSTADINOV V.. KOZHUKHAROVD., BONCHEVE.. KARAGYULEVA J., SAVOVS., ZAGORCHEVI.. and DABOVSKITS. ( 1973) Geo/oxli Map ofBz&ariu. I : 1 000 000. Geol. Inst. Bulg. Acad. Sci.. Sofia. LUPTONJ. E. (1983) Terrestrial inert gases: Isotope traces studies and clues to primordial components in the mantle. .4nn. Ro’. b.Yzrth Pluwt. Sci. 11, 371-414. LUPTONJ. E. and CRAIG H. ( 1981) A major helium-3 source at 15”s on the East Pacific Rise. Screncc 214, 13- 18. MAMYRINB. A. and TOLS~IKHINI. N. ( 1984) ffeliztm f.sotopes in Nutzzrc: Dev~1opment.s in Geochrmistr!~. 3. Elsevier. MARTYB. ( 1989) Neon and xenon isotopes in MORB: Implications for the earth-atmosphere evolution. Earth P/awl. Sc,i. Lctt. 94, m&a,
1) The isotope measurements of the helium from mineral waters in the eastern part of the Balkan peninsula lead to the conclusion that all main reservoirs-mantle, crust, and atmosphere-contribute to the isotopic composition of helium from the sites investigated. The most significant part of 4He is produced in the Earth’s crust (>85% ), the mantle component is 0.4-9%, and atmospheric helium is usually less than 1%. The main part of 3He (>80%) in the investigated objects is a mantle derivative. 2) In most of the sites the isotopic compositions of argon and neon are close to those in the atmosphere. In some samples neon isotopes reveal a trend to mass fractionation and argon in some cases is enriched in radiogenic 40Ar. 3) Nitrogen in the gas of most samples is entirely atmospheric. The X/‘He ratios, although of the order of IO’ in half of the cases, cannot be considered unambiguously as evidence of mantle-derived C02.
45-56.
ric,knoM/~~d~~m/,s--The authors are thankful to the anonymous reviewers for their constructive criticism of the manuscript as well as for their useful remarks.
Editoriul
lzundling:
D. E. Fisher REFERENCES
ANUFRIEVG. S., KAMENSKY1. L., and PAVLOVV. P. ( 1976) Anomalous isotopic composition of neon in hydrothermal sources of recent volcanism zones. lIokl. Akad. rzauk SSSR 231, 1454-1457 (in Russian). BOJAIXXVA K., PETROVP.. GASHAROVS., and VELINOVT. ( 1989) Geothermal field in Bulgaria. In GeothermalAtlas qj’Eurupe (ed. E. HUR~IGet al.). Surface Heat Flow Density, 1 : 2 500 000, Sheet I8 Sofiya. VEB Hermann Haack Geographische-Kartographische Anstalt. Gotha. ~‘ADEKJ. and SULCEKZ. ( 1965) Origin of Na-bicarbonate waters. In G~oche~n. Ce\ko.~/ov. (Sbornik praci, 1. Geochem. Konf., Ostrava). 229-236. CLARKEW. B., BEG M. A.. and CRAIG H. ( 1969) Excess ‘He in the sea: Evidence for terrestrial primordial helium. Earth Planet. Sci. L.nr. 6, 2 13-220. CRAIG H. and L~JPTONJ.
E. (1976) Primordial neon, helium and hydrogen in oceanic basalts. Earfh Planet. Sci. Left. 31, 369-385. DACHEVKH. ( 1988) Strzrctzwe c!/‘the Earth Crust in Bzrlgariu. Tekhnika, Sofia. (in Bulgarian). DYCK W. and JONASSON1. R. ( 1977) The nature and behaviour of gases in natural waters. Water Res. II, 705-7 11. FLORENSKYK. P. ( 1956) On the noble gas-to-nitrogen ratios in natural gases. Geokhimzva, 33-41 (in Russian). HIYAGON H., OZIMA M., MARTY B., ZASHU S., and SAKAIH. (1992) Noble gases in submarine glasses from mid-oceanic ridges and Loihi seamount: Constraints on early history ofthe Earth. Geoclzim. C’o.smot,him. Acta 56, 130 I- 13 16. HOOKERP. J.. BERTRAMIR., LOMBARDIS., O’NIONS R. K., and OXBURGHE. R. ( 1985a) Helium-3 anomalies and crust-mantle interaction in Italy. Gwchim. Cosmochim. Acta 49, 2505-2513. HOOKERP. J., O’NIONSR. K., and OXBLJRGH E. R. ( 198513)Helium isotopes in North Sea gas fields and the Rhine rift. Nature 318, 273-275. JENKINSW. J., RONAP. A., and EDMONDJ. M. (1980) Excess 3He in deep water over the Mid-Atlantic Ridge at 26”N: Evidence of hydrothermal activity. Earth Planet. Sci. Lett. 49, 39-44. KAMENSKY1. L., LOBKOVV. A., PRASOLOVE. M., BESKROVNYI N. S.. KUDRYAVTSEVAE. U., ANUFRIEVG. S., and PAVLOV V. P. ( 1976) Components of the Earth’s upper mantle in gases of
MARTYB. and JAMBONA. ( 1987) C/‘He in volatile fluxes from the solid Earth: Implications for carbon geodynamics. Earth Planet. Sci. Lest. 83, 16-26.
MARTYB.. JAMBONA.. and SANOY. ( 1989) Helium isotopes and carbon dioxide in volcanic gases of Japan. C’llrr??.G&. 76, 2540. MA~VEEVAE. S., TOLSI.IKHINI. N., and YAKU~SENIV. P. (1978) Helium isotopes criterion for origin of the gases and for neotectogenesis zones revelation. Caucasus as example. Gcwkhimz~n, 307317 (in Russian). MAZORE. ( 1977) Geothermal tracing with atmospheric and radiogenie noble gases. Geothrrmzcs 5, 2 l-36. NAGAO K., TAKAOKAN., and MATSUBAY.~SHI 0. (1979) Isotopic anomalies of the rare gases in the Nigorikava geothermal area, Hokkaido. Japan. Earth Planet. S(,i. Lctt. 44, 82-90. NACAO K., TAKAOKAN.. and MA~SLJBAYASHI 0. ( 1981) Rare gas isotopic compositions in natural gases ofJapan. Earth P/awt. ,S?i. Lrtt
53, 175-188.
OXBURGHE. R. and O’NIONSR. K. ( 1987) Helium loss. tectonics. and the terrestrial heat budget. Sc~iwzce 237, I583- 1588. O’NIONS R. K. and OXBURG E. R. (1988) Helium volatile fluxes and the development of continental crust. Eurth Plurwt. J?i. Lctt. 90,33
l-347.
P~NCHEVN. P.. JORDANOVN., and KoCtn%A L. (1964) On the helium and argon content of the spontaneous gas of some Bulgarian mineral sources. Compt. Rcrzd. Ac,ud. Bz&. Sci 17, 133-l 36 (in Russian ) PENCHEVN. P., PIPEROVN. B., and LILOV P. Y. (1970) Helium and argon in the spontaneous gas evolved from carbon dioxidebearing mineral springs. l~v. Otdrl. I\hrnz.nauki. LIZ&. Acud. &i. 3, 319-324 (in Bulgarian). PEN~CHEFFN. P., PIPEROVN. B., and LILOVP. ( 1966) Sur certains problPmes relatifs $ la composition isotopique du &on et de I’argon dans les gaz naturels. Compt. Rend Acad. Bzdg. Sci. 19, 603-605. PETROVP. S., PENCHEVAE. N., and PAVLOVAV. N. (1963) Geochemical study on the natural gases from thermal sources in the valley of Struma river. I:!>. Geoi inst.. Bzd,,. .4c,ad. Sci. 12, 245% 256 (in Bulgarian). PE~ROVP., MARTINOVS., LIMONADOV K., and STRAKAYu. ( 1970) Hydrogeological
Investigations
on I&
,lfinerul
U’utcw in Bz&aria.
Tekhnika, Sofia (in Bulgarian). POLYAKB. G. and TOLST~KHIN I. N. (1985) Isotopic composition of the Earth’s helium and the problem of the motive forces of tectogenesis. Chem. Gcol. 52, 9-33. POLYAKB. G.. KONONOV V. I., TOLSTIKHIN I. N.. MAMYRIN B.A., and KHABARIN L. V. ( 1976) The helium isotopes in thermal fluids. ln Thermal and ChtJmical Problems oj’T/zermul M &txs: fntl. ~.s.coc~. Hydrol. Sci. Pzrhl. 119, 15-29.
1898
N. B. Piperov. I. L. Kamensky, and 1. N. Tolstikhin
POLYAKB. G. et al. (1979a) He and Ar isotopic composition in fluids of Alpo-Apeninnes region and its relation to the volcanism. Dokl. Akad. nauk SSSR 247, 1220- 1225 (in Russian ). POLYAKB. G.. TOLSTIKHINI. N., and YAKLJTSENI V. P. (1979b) Isotopic composition of helium and the thermal flow-geochemical and geophysical aspect of the tectogenesis. Gmtektoniku, 3-23 (in Russian).
PRASOLOVE. M. and TOLSTIKHIN1. N. ( 1987) Juvenile gases He, COz, CH,: Their relations and contribution to the fluids of Earth’s crust. Geokhimiyu 1406 14 14 ( in Russian ). SANO Yu., WAKITA H., ITALIANOF., and NUCCIOM. P. (1989)
Helium isotopes and tectonics in southern Italy. Geophys. Rex Ld(. 16, 51 I-514. SHTEREVK. (1964) The Mineral Waters in Bulgaria. Nauka i Izkustvo, Sofia (in Bulgarian). TOLSTIKHINI. N. ( 1986) I.so@e Geochemistry qf Helium, Argon, and the Rare Gases. Nauka, Leningrad (in Russian). TORGERSENT., CLARKEW. B., and HABERMEHLM. A. ( 1987) Hehum isotopic evidence for recent subcrustal volcanism in eastern Australia. Geophys. Rex Letl. 14, I2 I5- 1218. VERKHOVSKYA. B., YURGINAE. K., and SHUKOLYUKOVYv. A. ( 1983) Element and isotope ratios of juvenile noble gases. Geokhimi.va. 1559-l 576 (in Russian).
Abstract-The ~~~c~~~rat~o~ and isotopic composition for the light noble gases in spontaneu~s gas from mineral waters in the eastern part of Balkan peninsufa are presented in this paper. The isotopic ratio 3He/4He in dissolved gases varies within the range 0.15- 1.1 X IO-’ and the concentration of helium in these gases is between 0.002 and 3.14 vol% It is determined that 86.7-98.8, 0.49.2, and
ranged from approximately 10m8in the PO River valley up to IO-* in the southern part of Italy, in Sicily, and in the Eolian Islands ( POLYAK et al., 1979a; HOOKER et al., 1985a; SANO et al., 1989). The distribution of helium isotopes in North Sea gas fields and groundwaters of the Rhine rift is another example for this thesis (HOOKER et al., 1985b). A substantial advance has been achieved in the interpretation of the isotopic and elemental ratios of other noble gases in natural gas mixtures (fCAMENSK?’ et al., 1976; NAGAO et al., t9X f; VERKHOVSKY et al., L983; M~~~RI~ and TOLSTIKHfh’,1984). The present study is aimed at the ~nt~~retation of the results obtained by measuring the isotopic composition and content of helium and other light noble gases (neon and argon) in some mineral waters of Balkan peninsula-another link of the Alpine erogenic belt. Recent information about the helium content ofsome Bulgarian mineral waters as well as a few data for neon and argon isotopes may be found elsewhere ( PENC~IEVet al., 1964, 1970; PENTCHEF’F et al., 1966: Kot.Ev et al., 1984).
As a rule. the mineral-water sites in the region are controlled by tectonically active zones and very often the sources are at the intersections of faults ofdifferentdirections (Fig. 1). The sites ofmineral water studied are related ta seven tectonic zones: ” (/irtrl/) separates the Rhodope mountain 1) The Munr.sa “.WIIIW massif (south ) of the Sredna Gora zone (north). It is one of the main tectonic hnes in the Balkan peninsula, seismically active, and well dehned by a complex of geophysical features. 21 The S/rtctnrcjir~f~ lid is the western boundary of the Rhodope massif. This cluster of faults of submeridional direction is seis-
1890
N. B. Piperov, 1. L. Kamensky, and I. N. Tolstikhin
.
Heat ?Iow
dansity,
kth
mW/m*
00
- 100
(---J
70-60
n
60-70
sites et nw%wDIwotas Dischsrgr, I/r > 30
FIG. 1. Schematic map of the network of deep faults, the thermal flow density, and the most important sites of minemi water in the eastern part of Balkan peninsula. The data were compiled from the publications of K~STADINOV et al. ( 1973), BOJADGIEVA et al. ( 1989), and PETROVet al. ( 1970). Abbreviations as in Table 1.
3)
4)
5)
6)
7)
mically active, intersects the Moho which is located there at a depth of 45 km, and penetrates the upper mantle. The Rhodope muss$ includes the Rila, Pirin, and Rhodope mountains. It is characterized by a block structure with a dense network of faults. The Mesta fault zone demarcates the Pirin mountain and the West Rhodope mountains. This graben is similar to the neighbou~ng Struma fault line, but the faults are undeveloped in length (80- 100 km only) and depth ( 15-20 km). The Truns-Bulkan.fuult zone is a system of subparallel deep faults which probably cut all of the crust (ca. 40 km) and serve as the northern border of the Sredna Gora zone. The Burros s~~l~ij~orj~4~is composed of upper Cretaceous scdiments, effusives, and pyroclastites dipping in the middle and east of the zone to the Black Sea. The mineral water sites are associated with Senonian andesites and tufa along the structural dislocations in the north of the synclinorium. The Strand=~a-~S~kar region is situated to the south of Burgas synchnorium. Paleozoic granites and Paleogene plutons are widespread as well as Mesozoic (Jurassic) sediments and metamorphites. Only a few dislocation and only two sites of mineral water are known in the region. The For+Balkan region is a transitional zone between the Stara Planina (Balkan) mountain and the Mysian plate and consists of subparallel anticlines and synciines (of Mesozoic age) formed by a system of subparallel dislocations. The natural discharge of mineral water is relatively rare.
A brief summary of information for the tested sources is given in Table 1.The data for mineral-water sites and for the regions described are compiled from SHTEREV( 1964). PETROV et ai. (1970), and DACHEV(1988).
METHODS
AND CHEMICAL, AND ISOTOPIC ANALYSES
OF SAMPLING,
ELEMENTAL
Spontaneous gas was collected in glass pipettes (bulbs) with two stop-cocks using a funnel as a trap for the bubbles. The funnel, pipette, and tubing were previously filled with water from the source and this water was later displaced by the gas. An exception was the sampling of 3.5.BB, when gas was collected in the pipette directly from the gas separator valve of the casing tube after an extended period of flushing the pipette and tubing with gas. The water samples for degassing (6.2.SK and 7.1.VV) were collected in 2.5 L glass bottles entirely filled with water by sinking into the source (captage). The bottle was hermetically closed ensuring absence of any air bubble. A special “soft” system of sealing was designed for the purpose. In the laboratory the bottle was put in a water bath and heated up to 80°C. The evolved gas was coilected in a previously evacuated gas pipette until the pressure rose up to 400 hPa (ca. 300 Torr). The gas sample so prepared (except Na, COZ, and noble gases dissolved in the water) also contains significant amounts of COZ obtained by thermal destruction of HCO;. For this reason, onIy the ratios between the noble gases (and nitrogen) in these two samples (6.2.SK and 7. I .VV) may be taken into consideration, with some stipulation. but not the absolute concentrations. The pipette, containing the gas sample, was connected to the evacuated analytical line where a small amount of gas was let into a single-collector mass spectrometer (MS IO) for general analysis, after freezing the water vapour in a trap cooled by melting ether (T
= -116°C). Another portion of gas (5-50 mL) was used for a more precise determination of the noble gases. This amount was measured volu-
Noble gases inmineral waters in Bulgaria
1891
TABLE 1. Some data about the studied mineral waters and their sites
Site
Geological enviro~ent, collector,
Site
T.-C M' g/l
and type of source
code 1. Maritsa fault
Predominant ions
Gas typeX
zone
l.l.Mr Merichleri
Seismo-tectonicknot, trachyandesite (Pg,),drill hole (300 m)
34
4.8
SO4 HC03 Cl / Na
1.2.Kh Kharmanlf
Granite and gneiss, drill hole (442 m)
2.3
4.3
SO4 Cl / Na
N
2. Struma
fault
N-C
line
2.1.SB Sapareva Banya
Granitogneiss(Pt), drill hole (200 ml
97
0.67
SO4 HC03 / Na
N
2.2.Bg Blagoevgrad
Gneiss (Pt). drill hole (168 m)
54
0.82
SO4 HC03 / Na
N
2.3.Sm Simitli
Gneiss fPt), drill hole (422 m)
62
0.57
SO4 HC03 /
Na
N
2.4.K-z Kozhukh
A site at the foot of a trachyandesite subvolcanicstock fNI).Gneiss (Ptf, 76
2.35
NC03 / Na
C
C-30) 62 spot
1.0
WC03 SO4 / Na
N
Granite, drill hole (500 ml
53
0.31
HC03 SO4 / Na
N
drill hole (500 m) A swamp with several hot spring spots on the bottom. Gneiss (Pt)
2.5.Mk Marikostinovo 3. Rhodope massive 3. l.GB
Guliina Banya
Gneiss, source
41
0.3
UC03 SO4 / Na
N
3.3. vg
Velingrad
Granite [Pz), drill hole (450 m)
84
0.7
SO4 HC03 / Na
N
3.4. Mi
Mikhalkovo
Gneisses with water-bearingmarble layers (Pt), drill hole (70 m)
27
4.04
HC03 SO4 / Na Ca
c
Gneiss (Pt), drill hole (436 ml
74
1.8
HC03 SO4 / Na Ca
C-N
Dislocation in gneisses, spots of ancient travertine.Drill hole (26 m)
24
1.5
SO4 HC03 / Na
Gratized gneiss (Pt). Source
32
0.92
HC03 SO4 / Na Ca
N
61
0.50
HC03 / Na
N
49
2.0
HC03 SO4 / Na Ca
25
0.54
HC03 / Na
3.2. Og Ognyanovo
3.5. BB
Bedenski Banf
3.6.Nr Narechen baths 3.7.Bn Banite 4. Trans-Balkan
fault
zone
4.1.PB Pave1 Banya
Granite iPz), drill hole
4.2.51 Sliven baths
Limestones ITrzI.silicate basement. Drill hole (373 m)
5. Burgas
N-C
synclinorium
5.1.Md Medovo 6. Strandzha-Sakar 6.1.Tr Troyanovo I
Andesite
(Crs,),
drill
hole
Sediments (Ng,,Q) with lignite coal
6.2.SK Stefan Karadzhovo Dislocationbetween marbles with sericite schists (Mz) and diorite porfirite (Pg). Captage 7. Fore-Balkan 7.1.vv Voneshta voda
N
region
layers. Silicate basement (granite?). Drill hole
Note:
N
Rn 0.13 &i
35
CH
20
1.5
HC03 / Ca Mg
N-C
13
1.3
HC03 SO4 Cl / Ca Na
N (H2S)
region
Dislocation in quarzitizitesandstone. Captage
* Total mineralization. ' N = N2, C = CQ2, CH = CH4
metrically and then,usingcoldtraps (liquid nitrogen) and heated (6OO"C)metallic calcium, the active components wereremoved. The noble gases were collected from the volume of the apparatus by a Sprengel mercury pump, put into a modified McLeod gauge and measured as the total mixture. The concentrations of helium and argon in this noble gas mixture as well as the isotopic composition of argon were determined with the same mass spectrometer. The error of "Ar,36Ar. and 38Arpeaksiswithin thelimits of I,3, and
5%. respectively. Next, the argon was removed by activated charcoal at -195°C (liquid nitrogen) and the peak of helium (4He) (and “Ne) was recorded. The fraction for the isotopic analysis of helium and neon was prepared from a 50-200 mL gas sample by adsorption ofthe chemically active components and heavy inert gases on activated charcoal cooled by liquid nitrogen. The rest, which couldnotbesorbed, was collected by a Sprengel pump in a glass ampoule which was sealed off.
1892
N. B. Piperov,
I. L. Kamensky,
and I. N. Tolstikhin TABLE 2.
All procedures described were carried out at the Institute of General and Inorganic Chemistry. Bulgarian Academy of Sciences, and were checked by special tests using air. The isotopic composition of helium and neon, as well as the precise of the Ne/He ratios were determined at the Geological Institute, Kola Science Center of the Russian Academy of Sciences, measurements
using a MI 120 I mass spectrometer. Prior to analysis, the ion source was tuned to maximum intensity of the ion current of 3He+, reaching a sensitivity of 5 x 10-jA/Pa and resolution I300 (50% valley definition). The background ofthe analyzer and vacuum system as well as the contribution of the Hi + HD+ peak, which does not exceed IO-) of the ‘He+ peak height. practically did not affect the precision of measurements ofthe 3He/4He ratio. The variations did not exceed 5, 12. and 25% for ratios of the order IO-(‘. 10m7, and 2 X lo-', respectively. After adjustment and tuning, the ‘He/“He ratio in a reference sample t,artificial mixture of atmospheric helium and neon) was determined. Then the ampoule with sample helium (+ Ne) was broken in a special vacuum device and the ‘He+ and 4He+ peaks of the sample helium were recorded, followed by a new run of the standard. The next step, after tuning the source for measurement of the isotopic composition of neon, was the checking of the ion currents of “Ne . “Ne . and ‘*Ne in the standard (twice) and in the sample studied. The precision of the isotopic analysis is shown in Table 4. The 4He and “Ne peak heights for the samples. sealed for isotopic
analysis. were determined with an inaccuracy of 5% and thus the 2oNe/4He ratio was evaluated with an error of -CIO%,
RESULTS AND DISCUSSION The results from the general gas analysis are given in Table 2. The most abundant species is nitrogen. Almost all samples contain small amounts of CO*; in three cases it is the main component. Methane is predominant in only one gas sample.
The general
analysis
(vol.%)
c02
CH4
Site N2
D2
NG
code
l.l.Mr 1.2.Kh
71.6 95.7
0.60 -
26.3 0.24
0.01 0.04
1.47 3.80
2.1.SB x 2.2.Bg 2.3.Sm 2.4. Kz 2.5.Mk
89.7 97.2 97.7 5.1 92.8
0.16 -
3.30 0.46 0.06 94.0 5.58
3.71 0.54 0.59 0.61 0.26
2.49 1.71 1.60 0.095 1.77
3.1.CB ' 3.2.og 3.3.vg 3.4.Mi 3.5.BB 3.6.Nr 3.7.Bn
98.2 98.5 96.9 5.5 21.1 94.5 92.4
-
0.01 0.32 94.2 78.2 3.38 5.50
0.20 0.09 1.09
1.42 1.36 1.76 0.094 0.50 1.88 1.96
4.1.PB 4.2.Sl
96.6 64.0
-
1.78 34.4
0.24 0.05 0.20 0.08 0.02
5.l.Md '
98.3
0.10
6.1.Tr ' 6.2.SK'
7.48 16.8
0.10 1.18
2.06 (81.819
7.1.wgX
44.3
3.86
(50.8)'
90.1 0.06
1.50 1.42 1.15 0.11 0.25 0.95
Note: NC - total concentration of the noble gases _l
below detection limits _ degassing of the water at 8O'C
x
Helium
other components are also detected
in
samples
The helium content in the studied sources (Table 3) varies within a broad range between 0.002 and 3. I4 vol%, and the results given in Table 3 are in quite good agreement with the recent data (PENCHEV et al., 1964). No evidence for any conformity of the relationship between the chemical composition (Tables 1 and 2) and helium content was found. It can be noted, however, that low helium concentrations were a characteristic feature of sources in which the gas contains predominantly COa (2.4.Kz, 3.4.Mi) and a maximum when Nz is the main gas component ( I .2.Kh, 1.1 .Mr). This observation is already noted elsewhere (e.g., PETROV et al., 1963; PENCHEV et al., 1970). The 3He/4He ratio changes within the range 0.15 X 10e6I. I X 10mh. The data for neon (Tables 3 and 4), which has predominantly an atmospheric origin, offered a possibility to evaluate the contribution of helium from the mantle (m), crust (c), and atmosphere (a) to the helium sample (s) (KAMENSKY et al., 1976), presuming that the 3He/4He ratios are R, = 1.2 X 10m5, R, = 2 X IO-‘, and R, = 1.4 X 10-6, respectively and 4He, = 0.286 X “Nee ( TOLSTIKHIN, 1986). The first value (R,) is close to the maximum 3He/4He ratio known from the mineral waters in the zone of Alpine-type orogeny (MATVEEVA et al., 1978; POLYAK et al., 1979a). This value is considered to be a characteristic feature for the Earth’s upper mantle based on the isotopic composition of helium in the rocks and hydrothermal systems of the ocean floor (LUPTON, 1983; MAMYRIN and TOLSTIKHIN, 1984).
2.1.58: H2 0.62, H2S 0.01 and C2H6 0.02. 3.1.GB: H2 0.10. 5.l.Md:
H2 0.37.
6.1.Tr:
H2 0.05, C2H6 0.04 and C2H4 0.01.
7.l.W:
H2S 0.01.
The corrosion of is a possible source JONASSON, 1977).
the of
drilling hydrogen
hole steel casing (DYCK and
As seen in Table 3, the contribution of atmospheric 4He to helium in the mineral waters studied is negligible-it does not exceed I % (sample 5. I .Md is the only exception). Naturally, the least amount of atmospheric helium is observed in sources with maximum helium content: i.l.Mr, 1.2.Kh, 4.2.Sl. This is confirmed also by the 4He/20Ne ratio (Table 3). The contribution of mantle helium is considerably more significant than that ofatmospheric helium: it varies between 0.4 and 9.2%. The gases richest in mantle helium have also high 3He/4He ratios (4.2.S1,6.2.SK, 2.4.Kz). More than 85% of the “He is radiogenic and has originated from the crust. The contribution of the main reservoirs in the balance of ‘He is considerably different from that in the case of 4He. Data in Table 3 show that the main source of the light isotope of helium is the mantle. As a rule, not less than 80% of 3He is genetically related with this source (for 5.1.Md, 19.2%
Noble gases in mineral waters in Bulgaria TABLE 3. tribution
Total content and isotopic composition crustal and atmospheric of the mantle, gas evolved from mineral waters
4He
Contribution
3He/4He
Site
4
vol.% code
He/20Ne
r xlOD
(He tot)
4
of
I893 of helium. Conhelium to the
the main stocks,
He
Atm. Mantle
%
3He Crust
Atm. Mantle
Crust
0.004 0.008
3.4 1.2
96.6
0.01
98.8
0.06
94.9 84.7
5. 1 15.2
200 460 550 66 41
0.14 0.06 0.05 0.4 0.7
1.7 2.0 3.1 7.3 5.4
92.2 97.9 96.9 92.3 93.9
0.85 0.3 0.17 0.6 1.4
88.7 92.3 93.0 97.3 95.3
10.4 7.4 6.8 2.1 3.3
0.735
0.19 0. 16 0.15 0.37 0.57 0.19 0.41
100 56 110 950 520 990 1100
0.28 0.51 0.3 0.03 0.06 0.03 0.03
1.4 1.2 1.0 2.9 4.6 1.4 3.2
98.2 98.3 98.7 97.1 95.3 98.6 96.8
2.0 4.4 2.8 0.1 0.1 0.2 0.1
88.4 90.0 80.0 94.0 96.8 88.4 93.6
9.6 5.6 17.2 5.9 3.1 11.4 6.3
0.153 0.631
0.20 1.1
320 1900
0.09 0.01
1.4 9.2
98.5 90.8
0.6 0.01
84.0 - 100
15.4 -0.0
0.0032
0.25
1.1.W 1.2.Kh
1.09 3.14
0.43 0.17
6400 3500
2. l.SB 2.2. Bg 2.3. Sm 2.4. Kz 2.5.Nk
0.183 0.342 0.319 0.0022 0.070
0.23 0.26 0.40 0.90 0.68
3. l.GB 3.2. og 3.3. vg 3.4. Hi 3. 5. BB 3.6.Nr 3. 7. Bn
0.080 0.055 0.120 0.049 0.085 0.788
4.1.PB 4.2.Sl 5. l.Md 6. l.Tr 6.2. SK
0.022 0.0064’
7.1. w
0.157
l
2.2
12.9
0.84 1.08
300 36
0.09 0.80
0.70
290
0.10
0.4
86.7
19.2
8.8
6.8 8.7
93. 1 90.5
0.15 1.0
97.1 96.7
2.8 2.3
5.6
94.3
0.20
96.0
3.8
72
Note: * - degassing
of
site
water
at 8O’C.
Results of analysis of samples 2.4. Kz, 2.5.Mk and 3.4.Mi, compared with results obtained 10 years ago, showed that ratio helium-3/ helium-4 remains constant within the precision mentioned in the text.
only),
and in some sources (4.2.S1, 2.4.Q 6.1 .Tr) 3He, is more than 97% abundant. The contribution of 3He from the crust is within the range -0- 17.2%, while that of atmospheric origin is less than 5%. In sample 5.1 .Md only the infiltrated atmospheric gases are represented widely: 3He, is 72% and 4He, is 12.9%. A correlation between the thermal upflow and the 3He/ 4He ratio is established on systematics of a large number (more than 400) of natural fluids ( POLYAK et al., 1979b; POLYAK and TOLSTIKHIN, 1985). In the case of this investigation, however, the relationship mentioned seems to be confirmed for only half the samples (Fig. 2). A similar discrepancy between 3He/4He ratios and the thermal flow for some spots in western Europe was observed by OXBURGH and O’NIONS ( 1987). This may be expected, as both helium and the heat flow reflect magmatic activity, but they are affected in a different way by different factors such as recent tectonic movements, paleoclimate, hydrogeological conditions, etc. Hence, the correlation between 3He/4He ratios and the thermal flow may be revealed clearly only on the basis of a large number of measurements, including data from different regions ranging from continental margins, where new crust is now forming and tectonic-magmatic activity is extremely high, to ancient (Precambrian) plates, where this activity abated at more than 1 Ga.
Neon The bulk of the neon in the gases seems to be infiltrated (dissolved) atmospheric neon. The isotopic composition of this component in most ofthe samples is close to atmospheric, although a trend towards the enrichment of neon with light isotopes is observed, e.g., ( 20Ne/22Ne), z (*‘Ne/**Ne),,, (cf. Table 4). Two explanations of this *‘Ne excess are possible: either mass fractionation of atmospheric neon, or addition of “light” mantle neon (MARTY, 1989; HIYAGON et al., 1992). The last case may be checked by comparing 20Ne,X with an unambiguous mantle component: ‘He. The 20Ne,,/3He ratios for the gases investigated are of the order 10-1000 which is lo*-lo4 times greater than these ratios in MORBs and OIBs. Hence, if a contribution of *‘NeeX from the mantle is proposed, then this neon would have to be hugely enriched (relatively to 3He), which we consider to be highly improbable. Another explanation, i.e., fractionation of air neon, was suggested by NAGAO et al. ( 1979, 198 1) for thermal waters in Japan. Several arguments support a similar interpretation of *‘NeCXin the Bulgarian mineral waters: the 40Ar/36Ar ratios show almost atmospheric values and the “Ne/**Ne ratios vs. *‘Ne/‘*Ne fit the mass fractionation line well (Fig. 3).
N. B. Piperov, I. L. Kamensky. and I. N. Tolstikhin
1894
TABLE 4. Content
and isotopic composition of neon and argon
Ne Site VOL. code
20
Ne/22Ne
f0.2 9.91 kO.04
1.89
9.73
9.9
40Ar/36Ar
vol.%
t* 5)
21Ne/22Ne
PPm
1.1.w 1.2.K.h
Ar
0.0296 0.0301
to.002 $0.0006
0.38 0.66
361 337
to.02 f0.05 20.02 kO.06 TO.05
0.0291 0.0299 0.0293 0.0290 0.0292
~0.0002 +O. 0007 +o. 0002 -+o.0006 +O. 0005
2.31
19
9.80 9.99 10.01 9.79 9.91
1.37 1.28 0.093 1.70
305 301 306 303 295
3.l.GB 3.2.Og 3.3.vg 3.4.M
8.5 10.8 12 a.57
10.16 10.08 9.86 9.95
20.03 f0.03 to.06 to.05
3.5.BB 3.6.Nr 3.7.Bn
1.8 8.0 7.6
9.82 f0.08
0.0297 0.0295 0.0292 0.0297 0.0293 0.0292 0.0284
+O. 0003 +O. 0002 to.0005 to.0006 +o. 001 +O. 0002 to. 0007
1.34 1.30 1.64 0.045 0.41 1.09 1.22
295 292 293 303 305 295 296
4.l.PB 4.2.51
5.2 3.4
10.30 20.07 10.27 20.07
0.0297 0.0301
+O. 0006 to. 0005
1.35 0.79
298 321
5.1.Md
16.0
9.84 f0.03
0.0291
+0.0002
1.15
294
2.1.SB 2.2.Bg 2.3.Sm 2.4. Kz 2.5.Mk
10.1 8.1 6.4 0.37
9.83 fO.05 9.70 f0.03
6.1.Tr 6.2.SK
0.8 2.0,
9.83 f0.03 10.03 TO.02
0.0292 0.0294
+o. 0002 fO.0002
0.091 0.24 *
292 296
7.l.W
6.0'
9.89 fO.03
0.0293
+o. 0002
0.79 *
292
9.800
0.0290
0.934
295.5
Air
18.2
Note: * - degassing
of water at
8O’C.
Argon
3Hr/4He
c
__--
i
,
/
/-
I
The content
I-
, , ., ,
/ /@
---
of argon in the samples
studied ranges from
0.045 to 2.31 vol%, exceeding in several cases the concentration ofthis gas in the atmosphere (Table 41, which is typical for N-bearing mineral waters (FLORENSKY, 1956). The
“Na
I
22N.
I
1
Mk 0
Bn 0
I
SB
CB
0
0
OKti
i
--II-d
LO
I
60
80
9.40 100
q, mW/m2 FIG. 2. Correlation between the heat flow (q) and the isotopic composition of helium from the sites of mineral water. Data for the heat flow are taken from the schematic map, Fig. 1. Abbrevjations as in Table I.
0.027
0.028
0.029
0.030
0.031
21 NI,~‘N~ FIG. 3. Isotopic ratios “Ne/*%e vs. Z’Ne/2ZNe in the gases of the sites investigated. It can be seen that within the accuracy of measurements the results plot along the mass fmctionation line of neon. Abbreviations as in Table i.
Noble gases in mineral waters
highest value obtained is for 2. I .SB and reffects the high temperature of this water (97°C). According to its isotopic composition, argon differs substantially from that of atmospheric argon in only three cases ( 1.I .Mr, 1.2.Kh, and 4.2.5 1) where an excess of radiogenic “‘Ar has been detected amounting to IS. 1, 12.3, and 7.9% of the argon content, respectively. The 4He/40Ar, values for I. 1.Mr and 4.2.Sl are 16 and 10, respectively-typical for regions which have been involved in Alpine orogeny. The ratio 4He/40Ar, = 38 ( 1.2.Kh) is relatively high and normally observed in gases from ancient Precambrian plates (TOLSTIKHIN, 1986). It must be noted that these three sites show a minimum content of atmospheric helium (Table 3). In the rest of the samples the 40Ar/36Ar ratio is close to 295.6 (air) or shows a slight excess of radiogenic 40Ar, in some cases.
in Bulgaria
1895
the ratios between at least two noble gases of atmospheric origin in the sample with the same ratio in the air as well as in water equilibrating with the atmosphere. Table 5 and Figures 4 and 5 suggest the conclusion that the ( Ar/Ne), ratio in the waters of the sites studied is within the range of the values for air (5 19) and dissolved air ( 18 10). Thus, the observed deviations from the values of (Ar/Ne), can be explained by single stage degassing of underground waters (ANUFRIEV et al., 1976). which should determine also the ratios of other components of atmospheric origin. A similar approach may be applied to the Ar/N2 ratio, presented also in Table 5 and Fig. 4. The range of values for this ratio, 0.0 12 (air) and 0.027 (dissolved air), provides evidence for the atmospheric origin of N2 in most of the sites. A substantial excess of N2 with regard to the single stage degassing model ofdissolved atmospheric gases has been found only in sampIes 1.1.Mr and 1.2.Kh, and a similar assumption may be made also for 3.4.Mi. Revealing the origin of C-containing components is more complicated. It is known that neither the concentration, nor the isotopic composition of carbon offer an unambiguous
Origin of the Main Components
In order to understand the gas fractionation during the dissolution and degassing processes, it is useful to compare
TABLE 5. Some ratios between the main components (nitrogen, carbon compounds - cf. Table 2) and the noble gases
Site code
Xl0
3 He/20Ne
Ara/N2
Ara/Ne -3
X103
Xl0
4.3 6.1
27 6.0
4
ZC/“Ne x10
-3
150 0.31
x/3He x10
-7
5.6 0.05
l.l.Mr 1.2.Kh
1.6 0.58
2. I.SB 2.2. Bg 2.3. sm 2.4. KZ 2.5.Mk
2.3 1.7 2.0 2.5 0.89
26 14 13 18 18
0.46 1.2 2.2 0.59 0.28
7.7 1.4 1.1 2900 3.4
3.1.GB 3.2. og 3.3. vg 3.4. Mi 3.5. BB 3.6. Nr 3.7. Bn
1.6 1.2 1.4 0.79 2.3 1.4 1.6
14 13 17 8.2 19 12 13
0.19 0.09 0.16 3.5 2.9 1.9 4.4
0.27 0.09 1.3 1800 490 4.8 8.0
4. l.PB 4.2. Sl
2.6 2.1
14 11
5. l.Md
0.72
12
0.006
6. l.Tr 6.2.SK
1.1 1.2
12 14
2.5 0.39
7.1.w
1.3
18
2.0
-*
.‘.*
Air
0.519
11.96
0.0044
0.02
4.5
1.81
26.8
0.0034
3.4
AS%O
0.64 21
4.2 110 0.014 1300 _*
Note: xc = co2 + cH4 _* - degassing ASW. ^ -
air
of
site
saturated
water water
at fat
8O’C. T = lO’C1.
17 1.2 0.50 4800 12 1.4 1.0 8.1 530 170 2.5 1.8 6.6 5.2 2.3 520 _*
“1000
N. B. Piperov,
1896
1. L. Kamensky,
Mk
/ ‘kd‘ Kh
1
2
5
III
20
50 N2 vol.
----tB og Bn
too %
FE. 4. Plot of the argon concentration vs. Nj for the gases investigated. The solid line represents the Ar/N, ratio for air; the dashed line is this ratio in air-saturated water (at 10°C). Abbreviations as in Table 1.
answer to the question of its origin, nor do they reveal the presence of a juvenile component of carbon ( KONONOVand POLYAK, 1982; PRASOLOVand TOLSTIKHIN, 1987; O’NIONS and OXBURGH, 1988). In order to overcome these difficulties it has been proposed to consider the ZC13He ratio where ZC and ‘He are carbon content (CO2 + C,,H,) and (juvenile) 3He content, respectively, in a natural gas (POLYAK et al., 1976). Recent studies (MARTY and JAMBON,1987; O’NIONS and OXBURGH, 1988). however, definitely show that C/He ratios should be used with caution. After release of the juvenile carbon and helium from the mantle into the exosphere. their behaviour is considerably different. Helium dissipates from the atmosphere into the space. while carbon is bonded to the Earth’s crust, predominantly in the form of carbonates. even with possible recycling into the mantle (MARTY and JAMBON, 1987). Hence, it should be expected. that the crustal C13He ratio is always higher than this ratio in the mantle. Analysing some mantle derivatives (MORE%, uftramafic xenoliths) MARTY and JAMBON( 1987) propose that C/3He = 2 X IO9 for the upper mantle. For the sources on the continental crust. however, this ratio ranges widely: from 10sto 1Ol4 (O’NIONS and OXBURGH, 1988). The results given in this paper (Table 5) show SC/3He ratios of the order of IO’ or even lower. Thus, some assumptions are possible: ( I ) ratios higher than IO9 imply crustal contribution, as it was shown above, but (2) ratios lower than this value may indicate that either mantle CO2 is bonded immediately in the crust, OJ subcontinental mantle avoids recycling and the C/3He ratio in this reservoir is lower than that in the bulk asthenosphere. The case with the samples studied is somewhat complicated. The formation of the mineral waters occurs at a shallow depth (~10 km) in the crust. It is well known that Na+ and HCO: are specific ions for these underground fluids when
and I. N. Tolstikhin
they are formed even in a silicate environment. It is supposed that CO2 takes an active part in this process (e.g., CADEK and SULCEK, 1965): NaAlSi30e(albite) + Hz0 + CO* -+ Na-montmorillonite + Si02 + Na+ t HCO;. When the influx of deep-seated COz’ is too small, it may be entirely exhausted and only dissolved atmospheric gases could be detected in the gas evolved spontaneously from the mineral water. Hence, the original (deep seated) CC/3He ratio may be changed significantly in the gas analysed. For one of the sites (2.3.Sm: 0.3 19% He, containing 3.1% He,, and ZC/3He = OS X 10’ in the spontaneous gas), we have some more information. The concentration of HCO, in the water is 108.3 mgfL (PETROV et at., 1970). i.e., 1.8 X 10p3M, and 100 mL spontaneous gas were evolved from ca. 300 L of mineral water. Taking into account the helium solubility in water ( T = 63”C), the gas concentrations given in Tables 2 and 3, and including the HCO; content in the Z;C, one may calculate that ZC/3He zz 10 lo in this source and, hence, the prevalence of crustal CO2 over a mantle component seems very probable. The COI-containing gases from the sites 3.4.Mi and 3.5.BB. with C02/3He ratios of the order of 10’. are associated with the marble complex of the West Rhodope, and COz is considered to be metamorphogenic ( PETROVet al., 1970). The highest value C02/‘He = 4.8 X IO’” is obtained for a postvolcanic exhalation of nearly# pure CO, (2.4.K~). probabty also of crustal (metamorphic) origin ( PETROVet al., f 970). The ZC/3He ratio of 5.2 X IO9 for sample 6.1.Tr is due to the high CH, content, which is assumed to be entirely biogenic (coal). The difficulties in revealing the present mantle-to-exosphere C-flux are confirmed also by other authors (MARTY et al., 1989).
SB
vol. %
2 -
Yd
1 -
.‘Mk
Nc vol.
4.
FIG. 5. Plot of the argon concentration vs. neon. The solid and dashed lines fit the Ar/Ne ratios in air and in air-saturated lO’C), respectively. Abbreviations as in Table I
water (at
1897
Noble gases in mineral waters in Bulgaria CONCLUSIONS
Kamchatka (according to He, Ne. Ar, and C isotopes). Gwkhi682-695 (in Russian). KANEOKA1. (1985) Noble-gas state in the Earth’s interior-some constraints on the present state. Chem. Geol. 52, 75-95. KOLEVE., KUZMANOVL., MARINOVB., TSANOVTs., and YANITSKY I. ( 1984) On the helium content and the isotope composition of some underground waters in Bulgaria. Spis. hzd,,. grol. drzch 45, 3 18-328 (in Russian). KONONOVV. 1. and POLYAKB. G. (1982) The problem of determining juvenile component in modern hydrothermal systems. Geokhimiya, 163- 177 ( in Russian). KOSTADINOV V.. KOZHUKHAROVD., BONCHEVE.. KARAGYULEVA J., SAVOVS., ZAGORCHEVI.. and DABOVSKITS. ( 1973) Geo/oxli Map ofBz&ariu. I : 1 000 000. Geol. Inst. Bulg. Acad. Sci.. Sofia. LUPTONJ. E. (1983) Terrestrial inert gases: Isotope traces studies and clues to primordial components in the mantle. .4nn. Ro’. b.Yzrth Pluwt. Sci. 11, 371-414. LUPTONJ. E. and CRAIG H. ( 1981) A major helium-3 source at 15”s on the East Pacific Rise. Screncc 214, 13- 18. MAMYRINB. A. and TOLS~IKHINI. N. ( 1984) ffeliztm f.sotopes in Nutzzrc: Dev~1opment.s in Geochrmistr!~. 3. Elsevier. MARTYB. ( 1989) Neon and xenon isotopes in MORB: Implications for the earth-atmosphere evolution. Earth P/awl. Sc,i. Lctt. 94, m&a,
1) The isotope measurements of the helium from mineral waters in the eastern part of the Balkan peninsula lead to the conclusion that all main reservoirs-mantle, crust, and atmosphere-contribute to the isotopic composition of helium from the sites investigated. The most significant part of 4He is produced in the Earth’s crust (>85% ), the mantle component is 0.4-9%, and atmospheric helium is usually less than 1%. The main part of 3He (>80%) in the investigated objects is a mantle derivative. 2) In most of the sites the isotopic compositions of argon and neon are close to those in the atmosphere. In some samples neon isotopes reveal a trend to mass fractionation and argon in some cases is enriched in radiogenic 40Ar. 3) Nitrogen in the gas of most samples is entirely atmospheric. The X/‘He ratios, although of the order of IO’ in half of the cases, cannot be considered unambiguously as evidence of mantle-derived C02.
45-56.
ric,knoM/~~d~~m/,s--The authors are thankful to the anonymous reviewers for their constructive criticism of the manuscript as well as for their useful remarks.
Editoriul
lzundling:
D. E. Fisher REFERENCES
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