Publication n° 119 of the International Association of Hydrological Sciences Proceedings of the Grenoble Symposium, August 1975

OHE HELIUM ISOTOEBS ES THERMAL .FLUIDS

Boris G. Polak, Vladimir I. Kononov Geological Institute of the USSR Academy of Sciences, aoseow, USSR Igor N. Tolstikhin Institute of Precambrian of the USSR Academy of Sciences, Leningrad, USSR Boris A. Mamyrin, Leonid V. Khabarin Physico-Technical institute of the USSR Academy of Sciences, Leningrad, USSR Abstract

ïhe isotopic composition of helium in thermal fluids (Ku­ril Islands, Kamchatka, Iceland, Caucasus, etc.; and in gases from the tectonically stable areas of Europe and Asia are compared.

The possioiiity of using che%e/%e ratio as isotopic cri­térium for determination of the genesis of che geological ob­jects is the result ofs QiJ the significant variety (four or­ders of magnitude) of this ratio in helium from different na­ture abyssal sources and (ii) the absence of sample contami­nation with atmospheric helium aue to its dissipation into space.

The relations between the helium isotopic ratio with che tectonic and seismic activity and terrestrial heat flow are discussed. The possible applications of helium isotopes dis­tribution in thermal fluids for solving some of the geologi­cal problems are considered.

On compare la composition isotope du hélium dans les flu­ides cnermaux des régions mobiles de la Terre (Kouriles, Kam­chatka, Islande, region uu Baikal, Transcaucasie, etc.) et dans les gaz des régions tectoniques stables de l'Europe et de l'Asie. La possibilité d'usage du rapport ̂ He/^He en qua­lité du critère pour determination de la genèse des objets gé­ologiques esc déterminée par ia variété de ce rapport en hé­lium des différentes sources (en quatre ordres) ainsi que par le fait de sa dissipation, ce que explique l'absence de la contamination des échantillons par le helium atmosphérique dans les conditions superficielles. , *

On discute les relations entre le rapport -TIe/Tïe et l'ac­tivité tectonique et sismique et le flux thermique abyssal. On considère la possibilité de l'application des données con­cernant la distribution des isotopes du helium dans les flui­des thermaux pour la solution des problèmes géologiques.

It was found in 1969 that the isotopic ratio of ^He/^e in natural gases varies in the wide range from 10~u to 'iu-5 (Mamyrin et al., iy6yj. Later this observation was supported by numerous isotopic analyses of helium from various rocks and minerals (Tolstikhin et al., 19V4-, Mamyrin et al., 1974-, Kryiov ec ai., 19'/4, Tolstikhin a. Drubets&oi, 1975J a&d i?°m

18

natural gases and waters (.Kamensiy et al., Iy7i, Tolstifchin et al., 1^72, Jiononov et al., 1974). The sum total or the ex­perimental data illustrating the range of the ̂ He/Tte ratio in natural objects is given in fig.1.

The analysis of the available data in the light of modern cosmochemical and geochemical concepts led us to conclusion that the main processes providing the isotopic composition of terrestrial helium are the following: (i) capture of primor­dial helium(3He/^He = 3.10-4) by the Earth during its accre­tion; (ii) the continuous generation of radiogenic helium (^He/%e = 10 ) in the Earth's matter due to radioactive de­cay of U and Th and nuclear reactions initiated by this pro­cess; (iii) Earth's degassing; (iv) dissipation of helium from atmosphere into space resulting in non-compensated loss of primordial helium by the Earth.

The simultaneous realization of the last three processes and differentiation of the mantle resulting in excretion of the Earth's crust should cause the variety of the isotopic composition of helium in different geospheres. In the inner parts of the Earth the magnitude of the 3He/THe ratio will gradually decrease due to the addition of radiogenic helium

' / / \ \

Pig.1. Hystogramms of the %e/4He ratio values in mountain rocks and minerals (1) and natural gases (2); N - number of samples.

to primordial ones. In the Earth's crust matter that is in fact entirely degassed and has lost the primordial helium while melting from the mantle, the radiogenic helium is accu­mulated. The quantitative model of degassing and differentia­tion of the Earth satisfying the data of isotopic studies not only of natural helium but also those on distribution of other noble gases was proposed before (Tolstikhin et al., 1975).

Proceeding from the above concepts one should expect that primordial helium will be best preserved in the internal parts of the Earth where the matter is less degassed and dif­ferentiated as compared to the external strata of the Earth.

Indeed, the helium with%e/*Ee.10~-' was found in the ultra-mafic xenolites (Tolstikhin et al., 1974), contained within igneous rocks and considered by their composition similar to the matter of the upper mantle. The same isotopic composition of helium was found in different young volcanic rocks which are assumed to be of the mantle origin (Mamyrin et al., 1974; Krylov et al., 197*)» On the contrary, the ancient, often me-tamorphized rocks of the Earth's crust contain the helium with the low isotopic ratio having the order of magnitude as 10"8 (Tolstikhin a. Drubetskoi, 1975)

Excreting from the mantle and crust rocks, the helium passes into the composition of underground waters and gases and then to atmosphere. The isotopic composition of modern atmospheric helium formed during the geological evolution, is characterized by the value 'He/^He = 1.4 . 10-6 (Mamyrin et al., 1970). Due to the continuous dissipation of helium into space, this being a unique geochemical peculiarity of this gas, the possible contamination of natural gases with atmospheric helium is some orders of magnitude less than with some other components of atmosphere. Therefore, during the discharge and sampling of natural gases, the isotopic compo­sition of their helium is practically not distorted by addi­tion of the atmospheric helium.

Indeed, the helium which was obtained from gas deposits within ancient stable platforms has the same "radiogenic" isotopic composition as the gases from rocks of these regions (Kamensky et al., 1971, Voronov, et al., 1974, Tolstikhin a. Drubetskoi, 1975). There is a certain tendency of changing the •'He/Tie values in natural gases of the geological struc­tures of different age, i.e. in younger plates it is some­what higher (Table 1;. The reasons of this tendency are still obscure. For example, one can believe that it is the result of (i) a greater enrichment of underground gases with radio­genic helium in the regions with more ancient (and thick) crust, and (ii) a greater loss of primordial helium in the same regions. But at the same time it has been established (Polak, Smirnov, 1968) that the age of tectonic activity is inversely proportional to the value of terrestrial heat flow which can be considered as an indicator of intensity of abys­sal (mantle) processes. Therefore, it is very interesting to consider the isotopic composition of helium from thermal flu­ids duscharged in tectonically mobile belts of the Earth whe­re there are the most favourable conditions for the outflow of abyssal matter and energy from the deep interiors onto the Earth's surface.

20

Table 1. The He/^He ratio (in 10~6) in gas deposits (after

Voronov et al., 1974) and terrestrial heat flow q (in 10~e

cal/cm2 sec) in platform areas (after Makarenko et al., 1969)

Age of basement

Number of J deposits -f-

'He/ % e 10

max.

Precambrian 44 2.35 0.87 _£ • 5

1.0

Hercynian 85 6.21 3.3 11.0 1.3

The earliest attempts undertaken in this direction led us to the discovery of very high values of the 3He/*Be ratio in gases of the active volcanoes and hydrothermae of the Kuril-Kamchatka region (Mamyrin et al., 1972). Later, the same and still higher values of the ratio were obtained in gases of thermal and mineral springs and recent hydrothermal systems of Iceland (Kononov et al., 1974). The vailable data allow us to elucidate the peculiarities of distribution of values of

the ̂ He/Tïe isotopie ratio in thermal fluids from different parts of the Earth's modern mobile belts.

With this aim in view let us compare the values of the % e / He ratio in thermal fluids from the following areas: 1) the Caucasus region which is a typical element of the Alpine folding belt named Tethys; 2) the Kuril-Kamchatka regions presenting one of the links of the Pacific island arc chain; 3) Iceland, being a unique landing expression of the mid-oce­anic rift system; 4) the Baikal region of recent intraconti-nental rifting.

In the Caucasus the isotopic composition of helium has been studied in thermal and mineral springs of myogeosyncline orogen of the Mayor Caucasus range and eugeosyncline of the Minor Caucasus as well as in natural gases from the sedimen­tary cover of some localities of the Kurinskaya depression and Indolo-Kuban foredeep. In the Mayor Caucasus the object under study was the springs, in the Baksan canyon in the envi­rons of the Elbrus volcano, where the weak solfajaric activi­ty is still being manifested. The values of the ̂ He/4He ratio in gases of this area where the thickness of the Earth's _g crust is about 60 km, were found to lie within (1.6-6.8).10 . In the Minor Caucasus the well known carbon-dioxide thermae of the Quaternary volcanic Armenian upland were investigated, the thickness of its crust being ahout 30-35 km. In these fi thermae the studied parameter ranges within (4-8.8;.10-°. The values obtained Doth in the Mayor and the Minor Caucasus were much higher than the "atmospheric" value, and, the more so, than those typical of the radiogenic helium of the Earth crust rocks. On the contrary, the natural gases from the se­dimentary cover of the abovementioned deep depressions were found to contain helium with the low isotopic ratio lying

"Kuril-Kamchatka region the objects under study we­re both the thermal fluids (hot springs, geysers, steam jets,

21

volcanic gases and the gases of the regional fissure-bedded water-head systems, as well as some gas manifestations of the obscure origin having no evident relation both in the first and the latter. Altogether 39 objects were investigated and 85 determinations of the helium isotopic composition were ma­de (Tolstikhin et al., 1972; Kamensky et al., 1975). Tae ma­jority of investigated objects are located on the Kamchatka peninsula, pne being on the Paramushir Island (the Northern Kuril Island), three on the Iturup Island and three on the Kunashir Island (the Southern Kuril Island).

The obtained results (fig.2, table 2) indicate that the value of the ?He/THe ratio in thermal fluids of this region has a stable maximum at the level of 10~5 or somewhat higher, both in hydrothermae and gases of active volcanoes. This has been not only in the zone of modern volcanic activity on the Eastern Kamchatka and the Kuril Islands, but also in the Me­dian massif of the Central Kamchatka, where the Quaternary activity was also manifested locally (especially northward of the investigated area), and where at present the magnitude of the terrestrial conductive heat flow is higher than anywhere else on the Kamchatka (Smirnov et al., 197*)« It is very inte­resting that other kinds of gas manifestations both of the Eastern and Central Kamchatka are characterized by higher va­lues of 3He/*He. On the contrary, the underground gases in the west of Kamchatka where there is no geothermal activity and the crust has a normal continental section (Markov et al., 1969) are characterized by the -̂ He/̂ He ratios which are lower than the atmospheric one. With the only exception explained possibly by the fracture tectonics of concrete locality, all the values of the *He/*He ratio in gases of the Western Kam­chatka lie in the range of (0.09-1.2) ,10~b. The slump of the ^HeAHe values from east to west is well evident on the sec­tion across the strike of the island arc (fig.2).

In Iceland the isotopic composition of helium in the ga­ses of hydrothermae and volcanoes was studied and the highest of the known *He/THe ratios were found (up to 33.10""6). The total number of the objects of sampling is 60, where 77 deter­minations were made (Kononov et al., 197*)• The sampling of control objects carried out during different years demonstra­ted that the *He/4-He ratio in the same objects is practically constant. The disposition of sampled objects and distribution of the obtained values of the ration studied are shown in fig.3.

The study of the isotopic composition of natural helium was one of the kinds of researches fulfilled by the Soviet geodynamic expedition in Iceland to elucidate its relation­ship with structure of the mid-oceanic ridge, on the axis of which Iceland is disposed. The island is crossed by the Median zone of numerous linear ruptures and recent volcanic activity. Many researchers (for instance, Thorarinsson, 1965, Saemundsson, 1973) believe that this zone is a landing expres­sion of the rift valley of the submarine ridge. It seemed that the fluids with maximal value of the ^e/THe ratio must dis­charge just in this zone. But the zone of the maximal values of this 3He/ *He ratio and the Median zone proved to coincide only in the southern part of the island, whereas in the north­ern part the maximal -?He/*He values are displaced to the west and tend towards an ancient branches of the Median zone, the

22

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Fig.2. Distribution of values of the 3He/4He ratio in gases

of Kamchatka. A - disposition of sampling points of the 3He/4He in: Th F - thermal fluids, KEG - gases of undergro­

und waters of regional distribution, ? - gas manifestations

of obscure genesis; B - change of values of the He/Tie ra­

tio in the studied gas manifestations across the stretching

of the arc.

23

Table 2. The JEe/ He ratio (Jin 10~6) in gases of the Kuril-Kamchatka region (according to Tolstikhin et al., 1972; Kamensky et al., 1975 and authors)

! Type of eas^ {manifesta-; t i o n

J Thermal • f l u i d s

{Regional {bedded wa-' t e r - h e a d ;sys terns

,'Surface gas ', mani fes ta ­i t ions of {obscure {origin

A r e a s {

Kur i l { Eas te rn I s l a n d s { Kamchatka

! ! ! I N ;min ;max; N'minj max

( t i l l 1 I î t !

1 1 ; 7-2 - )4 .0 ;48 ;2 .8 ;11 .3

- ; - ; - I 1; 4*9

_ ; _ ; _ • ^-'{1.43Î8.9

Cen t ra l ! Western { Kamchatka! Kamchatka !

i { I • ! ! If [min {max;ïf{ min {max {

! Î I t ! 1

i i l l j ; 6 {4.6 {1D.2-; - J - {

- { - { - { 9{0.09 { 3.8{

6 ! 0 . 7 4 ! 4 . 4 ! - j - ! - j

N number of data

existence of which is proposed by some researchers. This bra­nch. has a rather good junction with the submarine ridge Koln-beinsey which is a north continuation of the Mid-Oceanic rid­ge, whereas the present active north branch of the Median zo­ne lies somewhat eastwards due to the supposed displacement along the transform fracture zone (Saemundsson, 1973).

At the same time the zonality of the general gas composi­tion of the Icelandic thermal fluids is in a very good accor­dance with the structural pattern of the island (Kononov, Po-lak, 1974)« The Median zone is marked by hydrogenous thermae of the "Icelandic" type (see Table 3) and some other geoche-mical peculiarities (Vinogradov et al., 1974). , ^

The main general feature in distribution, of the ''He/ He ratio in Iceland is the "mantle" order of their magnitudes on the whole area of the island; the minimal of the values recor­ded is 7»10™°, This feature distinguishes sharply Iceland from the volcanlcally active Kamchatka link of the Pacific is­land arcs and from other studied parts of the tectonically mo­bile belts. The feature appears to be peculiar to the oceanic structures. In this connection one would remind that Clarke and his co-authors (Clarke et al., 1969; Jenkins et al., 1972) discovered an excess of 3He in sea water is some points of the Pacific and Atlantic Oceans, while Fisher (1973) and Dy-mond and Hogan (1973) studying young basalts of the oceanic floor found heavy noble gases there in proportion typical of

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carboniferous chondrites, and interpreted this phenomena as an indication of presence of primordial gases in the Earth. That is why it is interesting to compare the data obtained for the oceanic rift with the results of analogous investiga­tions carried out in the Baikalian intracontinental rift zone.

The Baikal region, the basement of which was consolidated during the Baikalian epoch, is the Siberian platform fragment re-activated by the processes of Cenozoic rifting. This reac­tivation was expressed in displacement of blocks along the newly appeared and old rejuvenated zones and with the forma­tion of superimposed depressions of the rift type. The lar­gest pf them is a depression of the Baikal lake, its depth be­ing 19*0 m. The depressions were filled with sedimentary vol-canogenic rocks. Volcanic activity was manifested in the re­gion untill recently. This has been confirmed by some little volcanic cones preserved in the southern part of the rift zo­ne. At present the Baikal region is characterized by high seismic activity, higher terrestrial heat flow and abundance of thermal and cold springs of different composition.

So far, the very first data on the •'ze/ He values have been obtained in gases of nitrogenous thermae, carbon-dioxide cold springs and methane bedded waters of the superimposed de­pressions. The maximal values of the studied ratio (2.6-8.9). 10~b were found in three gas manifestations of the very south­western part of the rift, namely the Tunka depression, the values of this order of magnitude characterizing all the types of waters. In two nitrogenous thermae located on the bank of the Baikal lake the 3He/*He ratio was found to be relatively low (0.42-0.61)„10_6, but these data require checking up by means of sampling other gas manifestations of this region. In two carbon-dioxide mineral springs of the Transbaikalian which are 500-550 km from the rift axis, this value is found to lie in the range (0.55-1.82).10~6.

The^available data testify to a rather wide range of the 5He and Tie concentrations in natural gases (fig.4). However, within the regions having the similar tectonic regime the ra­tio SHeAle is fairly constant. Thus, the studied ratio acqui­res the meaning of a geologo-historical parameter reflecting the summary results of an influx of helium from the mantle and its generation in the Earth's crust. The role of each of the two last processes depends on the intensity of degassing and differentiation of the mantle (i.e. on the activity of abys­sal processes) and on the peculiarities of the structure and formation of the Earth's crust in a certain region.

The other independent parameter reflecting the history and character of geological development of different regions is known to be the magnitude of a feerrestrial heat fbw (Polak, Smirnov, 1966, 1968). It is reasonable to expect the correla­tion between these paramètres. The juxtaposition of the avai­lable data confiras this supposition (fig.5). la tectonically stable areas the 3He/*He ratio increases with the rise of ter­restrial heat flow values (see also Table 1). In tectonically mobile belts the both paramètres are found to have the signi­ficant variations which are generally in accordance with each other and characterize different structural units of the same region.

The last regularity is especially best pronounced in the

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most "ripe" parts of tectonically mobile belts where during a relatively long time the volcanic activity was localized in the same zones. The Caucasus region of the Alpine folding is a good example. Here the maximal values of both paramètres characterize the structures of the Mayor- and Minor Caucasus, whereas their minimal values have been observed in the Kurin-skaya depression and the western outskirts of the Indolo-Ku-ban fore-deep.

In the tectonically less "ripe" island arcs this relation­ship is less distinct. For example, on the Kamchatka these zo­nes of minimal values of both paramètres coincide with one another distinguishing the West-Kamchatka rear-deep, but the

maximal -'He/ He values had been observed not only in the zone of maximal values of the terrestrial heat flow (Central Kam­chatka) but also in the eastern volcanically active zone whe­re at present the terrestrial heat flow is relatively lower.

It is known, however, that in the areas of powerful volca­nic and hydrothermal activity the total outflow of abyssal energy that can serve as an indicator of intensity of abyssal processes, is expressed by not only a terrestrial conductive heat flow. It is determined to a considerable degree (and so­metimes greater) by the convective outflow of the Earth's ene­rgy by mean of magmatic (mantle) melts and fluids (Polak, 1966 Horai, Uyeda, 1969). When correlating the ^He/^He values not with conductive heat flow but with total heat losses, the di­vergence similar to the above one will fee lower or disappear at all.

The data on the Baikal region are not sufficient yet for reliable conclusions about the relation of the parameters compared. The available materials seem to indicate that the ^e/^He ratio varies signifficantly along the rift axis. It is difficult to coordinate them with the terrestrial heat flow field that is of a typically "rift" character here (Lubiaova, Shelyagin. 1969). At the same time worth attention is that the high ?He/%e values coincide with the localities of the latest volcanic activity. The relatively low 5He/%e values observed in the Transbaikalian at the significant distance from the rift axis correspond to the views on the heat flow values in such regions.

Unlike the above regions, the variations of the helium iso-topic composition of Icelandic thermal fluids lie in the li­mits of one order of magnitude of 'He/^He. Nevertheless, the same general relation between this ratio and terrestrial heat flow can be recognized in distribution of the ̂ He/*He values; the minimal values of both paramètres in the most ancient parts coincide. At the same time the level of magnitude of the ^He/*He ratio distinguishes significantly Iceland among the other studied parts of the mobile belts, although the regio­nal maxima of terrestrial heat flow are approximately equal in all the regions.

One of the possible explanations of such feature of Ice­land can be the difference in the structure and age of the Earth's crust between the mobile parts of continents and island arcs, on the one hand, and the areas of the mid-ocea­nic ridges, on the other hand. These differences have to re­sult in more intense contamination of the influx of mantle

30

helium with radiogenic one in the more ancient and thicker continental crust enriched with radioactive elements. True, in this case it is difficult to understand why in the struc­tures of the first kind no ^He/^He value,as high as the Ice­landic maximal ones have been observed (i>He/it'He=5.10~5)„ The­se structures seemed to have the zones that were permeable enough for the mantle fluids, so that they could avoid conta­mination with the crust radiogenic helium.

Another possible explanation is the assumption about the regional differences of the sRe/^Ue values in the mantle due to different degree of its degassing beneath the different structures. This assumption is supported by observed regional differences of geoenergetic affect of volcanic and hydrother-mal œtivity, the initial cause of which is considered to be the mantle degassing.

It should be taken into consideration that the both propo­sed explanations do not exclude each other and can prove valid similtaneously.

The comparative analysis of the helium isotopis composi­tion in thermal fluids from different regions led us to one more conclusion that is also important for solving the prob­lems of genesis of these fluids. It was found that there is no direct connection between the ^He/^He values and the general gas composition of thermal fluids (table 3). Evidently the he­lium with the mantle isotopic composition and the main compo­nents of hydrothermae gas are formed at different levels, he­lium being supplied from the deepest interiors. As it is shown on table 3» the high values of the 3He/*He 10~5 were found in all studied types of fluids.

The wide range of this parameter is typical both of the methane underground waters widespread in some tectonically ac­tive structures, and of the local fissure-veined systems of nitrogenous thermal and cold carbon-dioxide mineral waters. Formerly it was believed that the two last types of fluids had been formed without any participation of mantle (juvenile) emanations (Ivanov, 1960). The data on the helium isotopic composition make it necessary to revise this opinion. The sta­ble high values of the ^He/^He ratio were found in the most high temperature fluids, namely nitrogen-earbon-dioxide ther­mae of the "geyseric" type and, hydrogenous thermae of the Ice­landic type. These observations confirm the previous ideas ba­sed on the estimations of the heat potential of such fluids, i.e. that abyssal heat-carriers participated in their forma­tion (Averiev, 1966).

Indeed, it is difficult to imagine that the helium migra­tes quite alone fro» the Earth's interior to its surface. Un­doubtedly there are other volatile components of the mantle origin. In this connection it should be said that the isotopic composition of sulphur in thermal fluids of the Median zone of Iceland proved similar to the meteoritic one in all its compounds (Vinogradov et ai., 197*). This testifies to a com­pletely or, at least, partly juvenile origin of this sulphur.

Using the data on. the helium isotopic composition in hyd-drothermae an attempt could be made to estimate the possible mantle (juvenile) part of their other components. The essence of this estimation isj the ratio of As^He, where A and ̂ He are the quantities of some volatile component A (£UO, G02,etc)

31

and ^He excreted from the interior of the Earth during its existence can be compared with an analogous ratio observed in the present-day thermal fluids. The basis for such calcula­tions can be the known estimations of the mass of the Earth's hydrosphere or the estimations of the total mass of 0 accumu­lated in the Earth's crust, hydrosphere and atmosphere during geological evolution (Galimov et al., 1975)» on the one hand, and the estimations of the total mass of ?He including its loss by the Earth due to dissipation into space, on the other hand (Tolstikhin et al., 1975). Based on these ideas the rough preliminary calculations indicate to a significant dif­ference in the possible content of juvenile OO2 in various thermal manifestations. A detailed analysis of this problem is beyond the possibilities of the given paper and requires further studies.

The authors thank I.S. Lomonosov, E.S. Matveeva, V.V. Po-nomarev for some valuable samples and analyses of gases which were investigated and taken into consideration in this work.

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