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Volcanic emission of radionuclides and magma dynamics
Earth and Planetary Science Letters, 76 (1985/86) 185-192
185
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
[61
Volcanic emission of radionuclides and magma dynamics G. L a m b e r t , M.F. Le Cloarec, B. A r d o u i n a n d J.C. Le R o u l l e y Centre Des Faibles Radioactivitbs, Laboratoire mixte C N R S / C E A , Avenue de la Terrasse, 91190 Gif - sur- Yvette (France)
Received January 11, 1985; revised version received August 20, 1985 21°Pb, 21°Bi and 21°Po, the last decay products of the 23Su series, are highly enriched in volcanic plumes, relative to the m a g m a composition. Moreover this enrichment varies over time and from volcano to volcano. A model is proposed to describe 8 years of measurements of Mt. Etna gaseous emissions. The lead and bismuth coefficients of partition between gaseous and condensed phases in the magma are determined by comparing their concentrations in lava flows and condensated volatiles. In the case of volatile radionuclides, an escaping time is calculated which appears to be related to the volcanic activity. Finally, it is shown that that m a g m a which is degassing can already be partly degassed; it should be considered as a mixture of a few to 50% of deep non-degassed m a g m a with a well degassed superficial magma cell.
1. Introduction In preceding papers [1-5] we have shown that hot magmatic gases are very rich, not only in 222Rn, which is gaseous even at low temperatures, but also in its decay products such as 21°pb, 2a°Bi, and 21°po. Compounds of these decay products are volatile at the magma high temperature of about 1000°C and are present as aerosols in a cooled volcanic plume. Although in a deep nondegassed magma the radioactive equilibrium can be assumed between 2 2 6 R a (half-life = 1600 years) and all its decay products, it was observed in every volcanic plume studied that the activity ratios 21°po/21°Pb and 21°Bi/zl°Pb were very different from this radioactive equilibrium, reaching commonly 10-100 (instead of 1). The activity ratios vary not only from one volcano to another but also in time, i.e. days, for a given volcano. It is clear that the magma temperature is the main factor controlling the emission of volatile compounds. However, taking into account the tremendous mass of a magmatic reservoir, its temperature is essentially constant (except maybe at the very surface). This does not account for the rapid variations observed. Another important parameter is the chemical composition of the magma which determines the relative abundances of different metallic compounds, the volatilities of which are extremely different. However, even from 0012-821X/85/$03.30
~'~ 1985 Elsevier Science Publishers B.V.
this point of view, it may be admitted that the chemical composition of a large magmatic reservoir changes rather slowly and cannot account for daily, or sometimes hourly observed variations. The aim of this paper is to build a model for the emission of magmatic gases in agreement with 2~°Pb, 21°Bi, and 2]°po measurements made in the plume of Mt. Etna during seven field studies from June 1976 to September 1984, and more especially the variations observed of their activity ratios.
2. Outgassing model It may be admitted that the chemical composition of those aerosols produced by bubble bursting or splashing at the magma surface should not be very different from that of the magma. By contrast the aerosol enrichment of 21°Bi, and 21°po relative to 21°Pb is attributed to their emission as vapour, followed by gas-to-particle conversion in the cool plume. One basic question is therefore to know whether 21°pb, and more generally lead, is also introduced into the volcanic aerosols through this last process or by magma spattering. A very high lead enrichment factor (EF) relative to the crustal composition was found in 1974 by Zoller et al. [6] at the South Pole and attributed to the highly volatile halides and sulphide of this element. In effect EF between 10 and 1000 were measured in volcanic emissions by Cadle et al. [7];
186 Mroz and Zoller [8], Lepel et al. [9] and more particularly an EF of 300 in Mt.Etna plume by Buat-Menard and Arnold [10]. Therefore, most of the lead present in volcanic aerosols can be considered as emitted in gaseous state. We consider first a deep non-degassed magma assumed to be in radioactive equilibrium, which means that the activity of the long-lived 226Ra is just balancing the radioactive decay of all its daughters whose half-lives are short in comparison. As there is usually no 226Ra in the plume, this nuclide can be considered as non-volatile and is assumed to stay within the magma. Measurements of 226Ra and 21°pb in samples of fresh lavas from Mt. Etna show [5] that this last nuclide is practically in radioactive equilibrium with 226Ra, in spite of the existence of gaseous 222Rn as an intermediary between them. Therefore, the outgassing of 222Rn is generally prevented during time periods as long as several 2~°Pb halflives, i.e. at least half a century. This could be due to the fact that short half-lived nuclides (as 3.8-d 222 Rn) decay during the run of gas bubbles through the upper layers of the magma. However, we have already pointed out that volcanic aerosols are very enriched in 5-d 2~°Bi which has an half-life long enough to be degassed. Thus it seems to be clear that the outgassing of volatile materials from Mr. Etna cannot be accounted for by a continuous process occurring in the entire magmatic reservoir, but by short events relative to a small superficial cell of magma for which the escape time should be short. This outgassing cell is considerably smaller than the main reservoir, so that its content in gases and volatile materials is limited and gases and vapours should be emitted b y ' a volume of magma whose composition is that of the deep non-degassed magma, but is lacking in its volatile components. In particular, we cannot assume in this cell 226Ra is in radioactive equilibrium with its daughters. Therefore the emission of volatile materials should depend not only on their partitioning between condensed and gaseous phases, but also on the turnover of the superficial m a g m a bringing new non-degassed materials to the surface. The outgassing model proposed here assumes therefore the following steps: (1) A gas emission occurs when a deep non-degassed magma cell is brought by convective circu-
lation to the top level of the magmatic reservoir where it is more or less mixed with the surface magma. The proportion of deep magma in the surface cell will be called /~. (2) Gases and vapours of volatile materials are partitioned from the condensed m a g m a in a proportion e, an emanation coefficient which characterizes each element and depends on the physical and chemical properties of the magma, e is assumed to be constant in a given volcano and equal for the different isotopes of a given element. As there is no 226Ra in the plume, this nuclide can be considered as non-volatile and is assumed to stay in the magma (eRa = 0). On the other hand, the radon is likely to be entirely released by gas emissions (eRn = 1). The following decay products, :l°Pb, 21°Bi, and 21°po, are partly degassed, proportionally to their atomic concentrations with characteristic coefficients epb, eBi, and CVo. (3) Even though the gas emission is short, it is not instantaneous. Thus we have to take into account a radioactive decay during the span of time 0 between the gaseous phase production within the cell, its emission into the atmosphere and its immediate sampling. Obviously this outgassing time 0 does not play any role in the case of stable nuclides of lead and bismuth. In fact, it will be shown later that 0 depends mainly on convection processes in the surface magma. 3. Lead and bismuth partitioning 21°po was analyzed by Bennett et al. [11] in lavas freshly emitted from Mt. St. Helens. Similar studies were undertaken on Mt. Etna by Le Cloarec et al. [5] whose results are shown in Table 1. In every case it may be seen that there is practically no more 21°po in fresh lava samples, meaning that polonium is completely degassed. We can therefore assume a good approximation is e po = 1. This result is in good agreement with Le Guern et al. [4] that the condensation temperature at which 21°po sublimates is only 300-350°C, when the m a g m a temperature is about 1000°C. In April 1983 it was possible for the first time to sample simultaneously flowing lavas and the gases they emitted in order to measure radionuclides. Since March 22, 1983, a lava flow occurred on the southern side of Mt. Etna through clefts to an effusive vent at the altitude of 2200 m.
187 The lava flowed out freely from this vent and was therefore easily accessible for sampling. Hot gases at temperatures between 906 a n d 928°C were also collected by c o n d e n s a t i o n techniques [12] using chloroform as a cooler. The chemical treatment of condensed gases has already been described [13] and consists essentially of dissolving a sulfur precipitate. Then p o l o n i u m a n d b i s m u t h are plated respectively on a silver a n d copper disk with the aim of measuring their alpha or beta radioactivity. A n elemental analysis of both metals is performed on another part of the solution by atomic absorption spectrometry using a Perkin Elmer 2380. Solutions were 1% HC1; flameless atom±sat±on was used together with the method of s t a n d a r d addition. The same m e a s u r e m e n t s were made on lava samples after dissolving them in a mixture of H F a n d HCIO 4 at a moderate temperature [5]. In this case however, owing to the complexity of the matrix of lava solutions, and to the very weak c o n c e n t r a t i o n of stable bismuth, hydride generation technique a n d flame atom±sat±on were used for its determination. All the results are shown in T a b l e 1. D u r i n g two months, from April to June 1983, we also collected aerosols of 2mpb, 2mBi and 21°po emitted from the lava flow, according to the techniques described later. Almost c o n t i n u o u s m e a s u r e m e n t s have shown that no significant change occurred within this period in the emitted
gases [5]. We infer from this observation that the results o b t a i n e d from our irregular sampling of lavas and condensed gases are well representative of the whole eruption. It may be seen in Table 1 that the average value of the c o n c e n t r a t i o n ratio of b i s m u t h and lead in gases and in the degassed m a g m a is: ( B i / P b ) g ~ / ( B i / P b ) ~ v a = 57. In the case of stable nuclides the partition between gaseous a n d condensed phases is only d e t e r m i n e d by the e m a n a t i o n coefficients E. We have therefore: (Bi/Pb)ga,,
(Bi/E pb
(Bi/Pb)lava
(1 - t , ~ ) / ( 1 - eph )
= 57
We already m e n t i o n e d that the 2mpb activity in lavas is very close to that of 226Ra, which indicates that the lead e m a n a t i o n coefficient is of the order of a few percent. Equation (1) gives:
~Bi
I[Pb
- - - 5 7
- -
1 -
1
IEBi
-
-
Cpb
-A
or: ui
£Pb
A +
57
A +1
TABLE 1
21°pb, 21°Bi, 21°po, lead and bismuth in condensed gases and lavas collected at the effusive vents from Mt. Etna Date
2mPb (dpm/g)
Pb (tag/g)
2mPb/Pb (dpm/lag Pb)
Bi (lag/g)
2mPo (dpm/g)
(210po) (21°pb)
20/04/83 20/04/83
4.8 ± 1.0 3.7 ± 0.8
11.64 ± 1.54 13.13 _+1.63
0.41 ± 0.14 0,28 ± 0.10
4.07 + 0.67 3,68 ± 0A5
15.0 _+6.6 12.8 ± 4.1
3.1 3.4
Mean value
4.3_+0.9
12.39± 1.60
0.35 +0.12
3,88 +_0.56
t3.9+5.4
3.2
20/04/83 8/05/83 26/05/83 2/06/83 18/06/83
4.6 ± 0.4 5.0 ± 0.4 5.8 + 0.5 5.0 + 0.4 4,8 _+0,4
12.49 _+2.13 11.72 _+1.37 n.d. n.d. 12.27 ± 2.11
0.37 ± 0.09 0.43 ± 0.09 0,39 ± 0.10
n.d. n.d. 0.052 + 0.009 0.085 _+0.015 0.064 + 0.011
0.7 0.1 0 0 0.5
0.15 0,02 0 0 0.1
Mean value
5.0_+0.5
12.2 _+1.87
0.40_+0.10
0.067_+0.017
0.03
0.05
Condensed gases
Lavas
n.d. = not determined.
(1)
188
According to Le Cloarec et al. [5], we can assume that Epb < 0.05. This leads to:
4. Measurements of 2t°pb, 2t°Bi and 21°pb in the Etna plume
A < 3 and 15
The volatile compounds of 2t°pb, 2~°Bi and 2t°pb outgassed from the hot magma are very rapidly condensed as aerosols as soon as the
<
EBi/(Pb < 57
TABLE 2 (a) Activity ratios in aerosols emitted from the main craters of Mt E m a and dynamical parameters deduced
Place
Date
(21°Bi/21°pb)
0
(21°po/2mpb)
(days) N.E.
06/76
4.5_+ 3.9_+ 7 _+ 4.3+ 5.3_+ 8.3_+
06/81
8
0.6 0,5 0,7 0,4 0,7 1,4
16 17 11 16 14 10
l h l _+ 8.9_+ 12.2_+ 11.8+ 8.4+ 10.8+
_+ 8
10
39.5_+27
19 + 1 1 11.5_+11
4 8
S.E.
06/80 06/81
Central
03/81
Bocca N u o v a
06/80 06/81
2.8_+ 2 12.6+4
04/83
f 6 . 5 ± 1.2 ~,3.9_+ 0.4
12 17
25 + 6 1 1 . 5 + 2.4
18/09/84
25 23 20 20 21 21 16 23 22 23 20 21 22 21 22 22 24 24 23 15 13 19 19 16 26 22 22 18 21
2 2 3 3 3 3 5 2 3 3 3 3 3 3 3 3 2 2 3 6 6 4 4 5 1 3 3 4 3
29 + 22 _+ 15 _+ 13.5+ 37 + 35 _+ 25 ± 39 _+ 26 ± 38 _+ 39 + 31 _+ 42 + 33 _+ 30 + 30 +_ 28 + 22 _+ 23.5+ 29 _+ 19 _+ 29 _+ 23 _+ 25 ± 31 _+ 26 + 30 + 25 ± 32 -+
22/09/84 23/09/84
29/09/84 27/09/84
28/09/84
04/10/84
1 . 3 + 0,4
_+ +_ + _+ + _+ + _+ +_ _+ + + + _+ + _+ _+ _+ _+ + _+ + + + _+ _+ + -+ +
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 1 1 1 2 2 2 2 1 2 1 1
33 1 7
20 11
2.8 2.1 2.9 2.5 1.9 2.8
_+ 9 + 8
6.3_+ 2.1 30 69
+ 5 +21
5 4 3 2.7 8 7 5 8 5 8 8 6 8 6 6 6 6 4 4.5 6 4 6 5 5 6 5 6 5 6
T (days)
(%)
27 20 31 29 18 27
5 1 9 6 2 8
162
51
62 28
25 9
11
0
109
42 97
82 28
29 5
104 71 43 38 148 136 83 162 89 155 161 114 182 125 108 108 99 71 78 102 57 103 75 83 115 89 108 184 119
40 29 18 16 51 48 32 55 35 53 54 42 59 45 41 41 38 29 31 37 22 38 29 32 43 35 41 32 43
189 (b) Activity ratios in aerosols emitted from adventive craters Date
(21°Bi/21°pb)
(21°po/21°pb)
Yellow Vent
06/76
Blowing up Vent Hot Hole Spatter Cone 1300 m
06/78 06/78
2.6 10.8 <1 1.5 3 <1
0.9 0.7 0.6 0.6 8.8 3
03/81
17.3
1.2
0 = delay between the gaseous phase partitioning and its subsequent emission into the atmosphere. ~"= time required to yield measured 21°Po in degassing m a g m a by means of 2u)pb decay. = Percentage of deep m a g m a in the degassing cell.
volcanic gases are cooled. The method of measurement, detailed elsewhere [3], consists of filtering the cold mixture of air and volcanic gases through a cellulose fiber filter and then measuring its alpha and beta radioactive decays. The beta counting efficiency is measured by reference to a 4°K source. The alpha counting efficiency is determined by comparing the alpha and beta activities of shortlived radon daughter sampled in the atmosphere to their theoretical ratio. The figures obtained can be checked by comparing the alpha and beta counting rates of filters sampled at a time long enough to let 21°Bi and 21°po be in radioactive equilibrium with their long-lived precursor 21°pb. Small corrections are sometimes necessary to discard the artificial radioactivity of atmospheric aerosols. The results obtained on Mt. Etna are listed in Table 2. The most important errors mentioned in this table are due to the uncertainties in radioactive countings. The alpha background is negligible which enables long counting times if necessary. On the other hand the determination of the counting efficiency cannot be truly accurate: it is of the order of 19-21%. However, the value utilized for this efficiency is obviously the same regardless of the measurement: 2~°po, or 2~°pb measured through the amount of 21°Po in radioactive equilibrium (which is obtained about 1.5 years after the sampling). In the case of beta counting, the efficiency is well known, but the background is about 0.5 counts per minute. This is negligible for 21°Bi measurements but not for those of 21°pb. Moreover, every time that the 2~°pb activity per m 3 of air is low, we have to take into account a
possible contribution of the artificial beta radioactivity, generally between 0.05 and 0.1 d p m / m 3 of air. All these error causes have been taken into account in Table 2 where they represent generally 5 25%.
5. Theoretical interpretation To simplify, let us consider first the outgassing of a non-degassed magma, in radioactive equilibrium, which can be written by neglecting the 222Rn short-lived daughters: R~ NRa = ~. Rn NRn = ~ Pb N p b = ~ Bi NBi = )k Po Npo
(2) where N is the atom number of a nuclide per gram of magma and X its radioactive decay constant. The composition of the gaseous phase, in the degassing cell, at the very moment of its partition from the magma is quite different: 226Ra remains in the magma (eRa = 0) and all the atoms of 222Rn are in the gaseous phase (~Rn = 1). However, the number of atoms NRn is about 2000 times smaller than Npb and can be neglected in what follows. During the transit of the gas through the degassing superficial cell, its composition changes versus time, according to the radioactivity laws. For the three successive radionuclides considered, the classical equations of radioactivity give, for 21°pb: N a ( t ) = U o , e x,,
190
for
2lOBi:
N*(r)=*
e pxl1+ i
1
2
hi&l x2 - Xl
No*--
r e
-X,1
(4)
for *“‘PO:
~,~2Wl,
N3(f)= (X3-X,)(X,-X,) A, - A*
e
A,&I
A2 +
i
In fact. it is no longer possible to assume that the degassing magma which is already partly degassed is still in radioactive equilibrium. Let the ratio of the ““PO initial activity in the degassing activity be u = cell to its equilibrium with the preceding notations where X ‘j,,N,,,/X ‘I,,Nr,, u < 1. Equation (3) becomes:
-A,/
No*- x,-x,
-h21
e
210 p. =(--+-
The number of atoms of ““Pb, ““Bi, and ““‘PO per gram of magma which are in the gaseous phase are initially: N,,l
=
NPhcPh
42
=
NRI~B,
413
=
NPocP<,
=
NPu
where Nph, N,, and NPc, are given by equation (2). The corresponding values of X are: h, = hPh = 9 hz=X8,=0.14d-‘; X,=h,,,=5x x 10m5d-‘; lO_“d-1 It is clear that h,, can be neglected relative to h “, and XPor and X r. relative to X,,. Moreover, we have already mentioned that the delay 0 between the gaseous phase partitioning and its subsequent emission into the atmosphere is on the order of a few days (to let 210Bi reach the free atmosphere). Consequently both exp( - h,,8) and exp( -h,,,(3) are very close to 1. With these approximations, the activities of the three radionuclides at time 8 are respectively: (2’0Pb)o = X,,N,,(e)
= ~,,NP&rh
(‘“‘Bi) B= x B,N,, ( 19) = h rh N,# Ph + (ht~N”ica, (2’“Po)~ = h,,,Npo(~)
- hp,Nr+p,) =
~P,>(NP,>~P,>
e x’s’ +
N,,c
H, )
In the case of degassing magma initially in radioactive equilibrium the activity ratios measured in the volcanic plume at time B should be therefore:
i *“‘Pb i ,,
cPO
xP
cPh
XB,CP,
81
A similar correction is not necessary for equation (4) in the case of *“‘Bi whose activity in volcanic aerosols is significantly smaller than that of ““‘PO (see Table 2). Moreover, the ““Bi half-life is close to 13,thus enabling 2’0Pb, always present in the degassed magma, to compensate for the losses of ““Bi. Owing to the range of values of .zB1/ePh, and it is possible to simplify the preceding c I’<> /‘Ph. equations:
=a-+
6 PO EPh
(7)
It may be seen in Table 2 that the highest value of the *“j PO/ 2’0Pb activity ratio ever measured in the Etna plume was 69. This figure is therefore the nearest to a magma in radioactive equilibrium. We assume that this magma was, in fact, in radioactive equilibrium, which means that u = 1. The validity of this assumption is supported by the numerical results obtained. In Table 2, the corresponding value of the activity ratio 2’0Bi/2’0Pb is 12.6. We have therefore, besides equation (1). the following two equations, in which 0 has the same value:
191 where t[pb, l[Bi and e x p ( - X s i 0 ) are unknown. We have admitted that %0 = 1. We find that EBi = 0.46 and ~Pb = 0.015. These values can be considered as known with an accuracy of 3% in the case of polonium, and 20% for bismuth and lead and are in agreement with the results shown in Table 1. The escape time 0 is then determined in each case from equation (6), assuming that the coefficients ~R~ and l[pb are constant; in effect, these coefficients depend almost exclusively on the temperature and, in a smaller way, on the chemical composition of the magma which can be considered as constant over decades in a volcanic body. It is worthy to note that 0 is calculated by taking a log: this results in an accuracy generally better than 30%.
6. Discussion and conclusion The values of 0 reported in Table 2 are between 1 and 17 days with one exceptional value of 33 days. This strong variability could indicate that gas emissions result essentially from convective movements in the superficial magma cell rather than from quiet and reproducible rising of bubbles through a motionless magma. ' It is worthwhile noting that as fJ'r a s ' 0 is the mean degassing time of a magma cell, it is equal to the ratio of its volume to the daily output. For instance, in June 1976 Lambert et al. [1] measured in the Etna plume a 2mPo activity of 2.5 × 10 .3 Ci per ton of SO 2. An output of 3.7 × 10 3 t SO 2 per day, as determined by Haulet et al. [14] corresponds to 9.2 Ci of 21°Po per day. The 21°pb activity of the magma indicated in Table 1 is 5 d p m / g and the activity ratio shown in Table 2 for June 1976 is 11 in the plume, which corresponds in our model to a 21°po/21°pb activity ratio in the m a g m a of 11/69, or 0.8 d p m of 21°Po per gram of magma. The daily output of 21°Po is therefore emitted from 26 × 1012 g of magma. The escape time was at this moment about 13 days. The mass of the degassing cell is therefore estimated to be 3 × 1014 g, or 0.13 km 3 with a density of 2.5. The high emanation coefficient of 21°Po results in a rapid depletion of the magma. In effect, lava flows, as well as their gaseous emissions, have shown a very small 21°po/2mPb activity ratio (Table 1). In Table 2, we report other measures obr
tained in adventive craters, which show also atypically low values. In both cases these low 21°p0 activities are attributed to the short passage of time since the lava outgassing, which did not enable 2~°po to be produced by decay of 21°Pb. In contrast to this peculiar case, the general presence of 21°P0 in the degassing cell results from either a renewed increase of this nuclide by 2~°Pb radioactive decay, or a partial mixing with the deep non-degassing magma. In the absence of such a mixing, we incidate in Table 2 the time ~- which would be necessary to yield the amount of 2~°Po actually present in the degassing magma by 2mpb decay. ~" is given by the simple equation:
o=l-e
x.or.
Assuming that O is the actual time of 2]°po production, it is now possible to calculate the corresponding value of the percentage ~ of deep magma in the degassing cell: g=(1-e
x""~) - (1 - e-X,,,°)
It may be observed in Table 2 that i1 ranges from 0 to 100%. However, the outgassing from one specific crater at a given period seems to be wellcharacterised by this mixing rate. Very small figures were obtained during short eruptions such as June 1978, March 1981, and April 1983, which could indicate the superficial origin of the involved magma. On the other hand, the very long lava emission from May to September 1984 corresponded during these last months to high values on the order of 40%, with short escape times of 2 - 3 days. Such a result is not surprising: after a 5-month emission of lava, the proportion of deep magma contributing to the outgassing is necessarily important. The results obtained in September 1984 give important collaborative evidence. The value of the mixing rate/z varies during this month between 17 and 55%, a variability which exceeds by far the errors of measurement. This shows that the deep magma injected into the so-called degassing cell is poorly mixed. Consequently, the hypothesis that the emanation coefficients ~ are constant is probably a first approximation, and will have to be somewhat revised in future works. In conclusion, it is clear that the gaseous emissions from an active volcano are very often characteristic of superficial magmatic cells already more
192 o r less d e g a s s e d in t h e p a s t . I n c o n t r a s t , a f t e r a l o n g l a v a e m i s s i o n it a p p e a r s t h a t t h e c o m p o s i t i o n o f t h e d e g a s s i n g m a g m a is close t o t h a t o f t h e d e e p n o n - d e g a s s e d m a g m a , T h i s s h o u l d r e s u l t t h e n in a particularly high contribution of the most volatile m a t e r i a l s to t h e v o l c a n i c e m i s s i o n . In case of a cataclysmic volcanic explosion the gas composition should correspond to that of a d e e p n o n - d e g a s s e d m a g m a a n d n o t to t h a t o f a s u p e r f i c i a l m a g m a cell. T h i s o b s e r v a t i o n a g r e e s w i t h t h e m e a s u r e m e n t s m a d e in t h e c l o u d o f M o u n t A u g u s t i n e w h i c h w a s s i g n i f i c a n t l y e n r i c h e d in v o l a t i l e m a t e r i a l s [9]. By m e a s u r i n g 21°pb, 21°Bi, a n d 21°p0, it is p o s s i b l e to d i s t i n g u i s h b e t w e e n t h e compositions of gases emitted by a quiet volcanic a c t i v i t y a n d o f t h o s e w h i c h w o u l d b e e m i t t e d in c a t a c l y s m i c e x p l o s i o n s . It s h o u l d t h e r e f o r e b e int e r e s t i n g to s t u d y to w h a t e x t e n t t h e m o d e l p r o p o s e d i n t h i s p a p e r for M t . E t n a c o u l d b e a p p l i e d t o o t h e r v o l c a n o e s f o r w h i c h a risk o f e x p l o s i o n exists.
References 1 G. Lambert, P. Bristeau and G. Polian, Emission and enrichments of radon daughters from Etna volcano magma, Geophys. Res. Len. 3, 724, 1976. 2 G. Lambert, P. Bristeau, F. Le Guern, G. Polian and J.-C. Sabroux, Caracterisation des gaz magmatiques et fumerolliens par leurs aerosols radioactifs, C.R. Acad. Sci. 288. 743-746, 1979. 3 G. Polian and G. Lambert, Radon daughters and sulfur output from Erebus volcano, Antarctica, J. Volcanol. Geotherm. Res. 6, 125-137, 1979.
4 F. Le Guern, J.-C. Le Roulley and G. Lambert, Condensation du polonium dans les gaz volcaniques, C.R. Acad. Sci. 294, 887 890, 1982. 5 M.-F. Le Cloarec, G. Lambert, F. Le Guern and B. Ardouin. Echanges de materiaux volatils entre phases solide, liquide et gazeuse au cours de 1'6ruption de I'Etna de 1983, C.R. Acad. Sci. 298, 805-808, 1984. 6 W.H. Zoller, E.S. Gladney and R.A. Duce, Atmospheric concentrations and sources of trace metals at the South Pole, Science 183, 198-200, 1974. 7 R.D. Cadle, A.L. Lazrus, B.J. Huebert, L.E. Heidt. W.I. Rose, Jr., D.C. Woods, R.L. Chuan, R.E. Stoiber, D.B. Smith and R.A. Zielinski, Atmospheric implications of studies of central American volcanic eruption clouds, J. Geophys. Res. 84, 6961-6968, 1979. 8 E. Mroz and W.H. Zoller. Composition of atmospheric particulate matter from the eruption of Heimaey, Iceland, Science 190, 461 464, 1975. 9 E.A. Lepel, K.M. Stefansson and W.H. Zoller, The enrichment of volatile elements in the atmosphere by volcanic activity: Augustine volcano 1976, J. Geophys. Res. 83. 6213-6220, 1978. 10 P. Buat-Menard and M. Arnold, The heavy metal chemistry of atmospheric particulate matter emitted by Mr. Etna volcano, Geophys. Res. Lett. 5, 245 248, 1978. 11 J.J. Bennett, S. Krishnaswami, K.K. Turekian, M.G. Melson and C.A. Hopson, The uranium and thorium decay series nuclides in Mount St. Helens effusives, Earth Planet. Sci. Len. 60, 60-69, 1982. 12 R. Cioni and E. Gorazza, Medium-temperature fumarolic gas sampling, Bull. Volcanol. 44, 23 29, 1981. 13 M.-F. Le Cloarec, G. Lambert. J.-C. Le Roulley and B. Ardouin, Long-lived radon daughters in Mount St. Helens emissions: a size estimation of the magmatic reservoir, J. Volcanol. Geotherm. Res. (in press). 14 R. Haulet, P. Zettvoog and J.-C. Sabroux, Sulfur dioxyde discharge from Mount Ema, Nature 268, 715, 1977.
185
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
[61
Volcanic emission of radionuclides and magma dynamics G. L a m b e r t , M.F. Le Cloarec, B. A r d o u i n a n d J.C. Le R o u l l e y Centre Des Faibles Radioactivitbs, Laboratoire mixte C N R S / C E A , Avenue de la Terrasse, 91190 Gif - sur- Yvette (France)
Received January 11, 1985; revised version received August 20, 1985 21°Pb, 21°Bi and 21°Po, the last decay products of the 23Su series, are highly enriched in volcanic plumes, relative to the m a g m a composition. Moreover this enrichment varies over time and from volcano to volcano. A model is proposed to describe 8 years of measurements of Mt. Etna gaseous emissions. The lead and bismuth coefficients of partition between gaseous and condensed phases in the magma are determined by comparing their concentrations in lava flows and condensated volatiles. In the case of volatile radionuclides, an escaping time is calculated which appears to be related to the volcanic activity. Finally, it is shown that that m a g m a which is degassing can already be partly degassed; it should be considered as a mixture of a few to 50% of deep non-degassed m a g m a with a well degassed superficial magma cell.
1. Introduction In preceding papers [1-5] we have shown that hot magmatic gases are very rich, not only in 222Rn, which is gaseous even at low temperatures, but also in its decay products such as 21°pb, 2a°Bi, and 21°po. Compounds of these decay products are volatile at the magma high temperature of about 1000°C and are present as aerosols in a cooled volcanic plume. Although in a deep nondegassed magma the radioactive equilibrium can be assumed between 2 2 6 R a (half-life = 1600 years) and all its decay products, it was observed in every volcanic plume studied that the activity ratios 21°po/21°Pb and 21°Bi/zl°Pb were very different from this radioactive equilibrium, reaching commonly 10-100 (instead of 1). The activity ratios vary not only from one volcano to another but also in time, i.e. days, for a given volcano. It is clear that the magma temperature is the main factor controlling the emission of volatile compounds. However, taking into account the tremendous mass of a magmatic reservoir, its temperature is essentially constant (except maybe at the very surface). This does not account for the rapid variations observed. Another important parameter is the chemical composition of the magma which determines the relative abundances of different metallic compounds, the volatilities of which are extremely different. However, even from 0012-821X/85/$03.30
~'~ 1985 Elsevier Science Publishers B.V.
this point of view, it may be admitted that the chemical composition of a large magmatic reservoir changes rather slowly and cannot account for daily, or sometimes hourly observed variations. The aim of this paper is to build a model for the emission of magmatic gases in agreement with 2~°Pb, 21°Bi, and 2]°po measurements made in the plume of Mt. Etna during seven field studies from June 1976 to September 1984, and more especially the variations observed of their activity ratios.
2. Outgassing model It may be admitted that the chemical composition of those aerosols produced by bubble bursting or splashing at the magma surface should not be very different from that of the magma. By contrast the aerosol enrichment of 21°Bi, and 21°po relative to 21°Pb is attributed to their emission as vapour, followed by gas-to-particle conversion in the cool plume. One basic question is therefore to know whether 21°pb, and more generally lead, is also introduced into the volcanic aerosols through this last process or by magma spattering. A very high lead enrichment factor (EF) relative to the crustal composition was found in 1974 by Zoller et al. [6] at the South Pole and attributed to the highly volatile halides and sulphide of this element. In effect EF between 10 and 1000 were measured in volcanic emissions by Cadle et al. [7];
186 Mroz and Zoller [8], Lepel et al. [9] and more particularly an EF of 300 in Mt.Etna plume by Buat-Menard and Arnold [10]. Therefore, most of the lead present in volcanic aerosols can be considered as emitted in gaseous state. We consider first a deep non-degassed magma assumed to be in radioactive equilibrium, which means that the activity of the long-lived 226Ra is just balancing the radioactive decay of all its daughters whose half-lives are short in comparison. As there is usually no 226Ra in the plume, this nuclide can be considered as non-volatile and is assumed to stay within the magma. Measurements of 226Ra and 21°pb in samples of fresh lavas from Mt. Etna show [5] that this last nuclide is practically in radioactive equilibrium with 226Ra, in spite of the existence of gaseous 222Rn as an intermediary between them. Therefore, the outgassing of 222Rn is generally prevented during time periods as long as several 2~°Pb halflives, i.e. at least half a century. This could be due to the fact that short half-lived nuclides (as 3.8-d 222 Rn) decay during the run of gas bubbles through the upper layers of the magma. However, we have already pointed out that volcanic aerosols are very enriched in 5-d 2~°Bi which has an half-life long enough to be degassed. Thus it seems to be clear that the outgassing of volatile materials from Mr. Etna cannot be accounted for by a continuous process occurring in the entire magmatic reservoir, but by short events relative to a small superficial cell of magma for which the escape time should be short. This outgassing cell is considerably smaller than the main reservoir, so that its content in gases and volatile materials is limited and gases and vapours should be emitted b y ' a volume of magma whose composition is that of the deep non-degassed magma, but is lacking in its volatile components. In particular, we cannot assume in this cell 226Ra is in radioactive equilibrium with its daughters. Therefore the emission of volatile materials should depend not only on their partitioning between condensed and gaseous phases, but also on the turnover of the superficial m a g m a bringing new non-degassed materials to the surface. The outgassing model proposed here assumes therefore the following steps: (1) A gas emission occurs when a deep non-degassed magma cell is brought by convective circu-
lation to the top level of the magmatic reservoir where it is more or less mixed with the surface magma. The proportion of deep magma in the surface cell will be called /~. (2) Gases and vapours of volatile materials are partitioned from the condensed m a g m a in a proportion e, an emanation coefficient which characterizes each element and depends on the physical and chemical properties of the magma, e is assumed to be constant in a given volcano and equal for the different isotopes of a given element. As there is no 226Ra in the plume, this nuclide can be considered as non-volatile and is assumed to stay in the magma (eRa = 0). On the other hand, the radon is likely to be entirely released by gas emissions (eRn = 1). The following decay products, :l°Pb, 21°Bi, and 21°po, are partly degassed, proportionally to their atomic concentrations with characteristic coefficients epb, eBi, and CVo. (3) Even though the gas emission is short, it is not instantaneous. Thus we have to take into account a radioactive decay during the span of time 0 between the gaseous phase production within the cell, its emission into the atmosphere and its immediate sampling. Obviously this outgassing time 0 does not play any role in the case of stable nuclides of lead and bismuth. In fact, it will be shown later that 0 depends mainly on convection processes in the surface magma. 3. Lead and bismuth partitioning 21°po was analyzed by Bennett et al. [11] in lavas freshly emitted from Mt. St. Helens. Similar studies were undertaken on Mt. Etna by Le Cloarec et al. [5] whose results are shown in Table 1. In every case it may be seen that there is practically no more 21°po in fresh lava samples, meaning that polonium is completely degassed. We can therefore assume a good approximation is e po = 1. This result is in good agreement with Le Guern et al. [4] that the condensation temperature at which 21°po sublimates is only 300-350°C, when the m a g m a temperature is about 1000°C. In April 1983 it was possible for the first time to sample simultaneously flowing lavas and the gases they emitted in order to measure radionuclides. Since March 22, 1983, a lava flow occurred on the southern side of Mt. Etna through clefts to an effusive vent at the altitude of 2200 m.
187 The lava flowed out freely from this vent and was therefore easily accessible for sampling. Hot gases at temperatures between 906 a n d 928°C were also collected by c o n d e n s a t i o n techniques [12] using chloroform as a cooler. The chemical treatment of condensed gases has already been described [13] and consists essentially of dissolving a sulfur precipitate. Then p o l o n i u m a n d b i s m u t h are plated respectively on a silver a n d copper disk with the aim of measuring their alpha or beta radioactivity. A n elemental analysis of both metals is performed on another part of the solution by atomic absorption spectrometry using a Perkin Elmer 2380. Solutions were 1% HC1; flameless atom±sat±on was used together with the method of s t a n d a r d addition. The same m e a s u r e m e n t s were made on lava samples after dissolving them in a mixture of H F a n d HCIO 4 at a moderate temperature [5]. In this case however, owing to the complexity of the matrix of lava solutions, and to the very weak c o n c e n t r a t i o n of stable bismuth, hydride generation technique a n d flame atom±sat±on were used for its determination. All the results are shown in T a b l e 1. D u r i n g two months, from April to June 1983, we also collected aerosols of 2mpb, 2mBi and 21°po emitted from the lava flow, according to the techniques described later. Almost c o n t i n u o u s m e a s u r e m e n t s have shown that no significant change occurred within this period in the emitted
gases [5]. We infer from this observation that the results o b t a i n e d from our irregular sampling of lavas and condensed gases are well representative of the whole eruption. It may be seen in Table 1 that the average value of the c o n c e n t r a t i o n ratio of b i s m u t h and lead in gases and in the degassed m a g m a is: ( B i / P b ) g ~ / ( B i / P b ) ~ v a = 57. In the case of stable nuclides the partition between gaseous a n d condensed phases is only d e t e r m i n e d by the e m a n a t i o n coefficients E. We have therefore: (Bi/Pb)ga,,
(Bi/E pb
(Bi/Pb)lava
(1 - t , ~ ) / ( 1 - eph )
= 57
We already m e n t i o n e d that the 2mpb activity in lavas is very close to that of 226Ra, which indicates that the lead e m a n a t i o n coefficient is of the order of a few percent. Equation (1) gives:
~Bi
I[Pb
- - - 5 7
- -
1 -
1
IEBi
-
-
Cpb
-A
or: ui
£Pb
A +
57
A +1
TABLE 1
21°pb, 21°Bi, 21°po, lead and bismuth in condensed gases and lavas collected at the effusive vents from Mt. Etna Date
2mPb (dpm/g)
Pb (tag/g)
2mPb/Pb (dpm/lag Pb)
Bi (lag/g)
2mPo (dpm/g)
(210po) (21°pb)
20/04/83 20/04/83
4.8 ± 1.0 3.7 ± 0.8
11.64 ± 1.54 13.13 _+1.63
0.41 ± 0.14 0,28 ± 0.10
4.07 + 0.67 3,68 ± 0A5
15.0 _+6.6 12.8 ± 4.1
3.1 3.4
Mean value
4.3_+0.9
12.39± 1.60
0.35 +0.12
3,88 +_0.56
t3.9+5.4
3.2
20/04/83 8/05/83 26/05/83 2/06/83 18/06/83
4.6 ± 0.4 5.0 ± 0.4 5.8 + 0.5 5.0 + 0.4 4,8 _+0,4
12.49 _+2.13 11.72 _+1.37 n.d. n.d. 12.27 ± 2.11
0.37 ± 0.09 0.43 ± 0.09 0,39 ± 0.10
n.d. n.d. 0.052 + 0.009 0.085 _+0.015 0.064 + 0.011
0.7 0.1 0 0 0.5
0.15 0,02 0 0 0.1
Mean value
5.0_+0.5
12.2 _+1.87
0.40_+0.10
0.067_+0.017
0.03
0.05
Condensed gases
Lavas
n.d. = not determined.
(1)
188
According to Le Cloarec et al. [5], we can assume that Epb < 0.05. This leads to:
4. Measurements of 2t°pb, 2t°Bi and 21°pb in the Etna plume
A < 3 and 15
The volatile compounds of 2t°pb, 2~°Bi and 2t°pb outgassed from the hot magma are very rapidly condensed as aerosols as soon as the
<
EBi/(Pb < 57
TABLE 2 (a) Activity ratios in aerosols emitted from the main craters of Mt E m a and dynamical parameters deduced
Place
Date
(21°Bi/21°pb)
0
(21°po/2mpb)
(days) N.E.
06/76
4.5_+ 3.9_+ 7 _+ 4.3+ 5.3_+ 8.3_+
06/81
8
0.6 0,5 0,7 0,4 0,7 1,4
16 17 11 16 14 10
l h l _+ 8.9_+ 12.2_+ 11.8+ 8.4+ 10.8+
_+ 8
10
39.5_+27
19 + 1 1 11.5_+11
4 8
S.E.
06/80 06/81
Central
03/81
Bocca N u o v a
06/80 06/81
2.8_+ 2 12.6+4
04/83
f 6 . 5 ± 1.2 ~,3.9_+ 0.4
12 17
25 + 6 1 1 . 5 + 2.4
18/09/84
25 23 20 20 21 21 16 23 22 23 20 21 22 21 22 22 24 24 23 15 13 19 19 16 26 22 22 18 21
2 2 3 3 3 3 5 2 3 3 3 3 3 3 3 3 2 2 3 6 6 4 4 5 1 3 3 4 3
29 + 22 _+ 15 _+ 13.5+ 37 + 35 _+ 25 ± 39 _+ 26 ± 38 _+ 39 + 31 _+ 42 + 33 _+ 30 + 30 +_ 28 + 22 _+ 23.5+ 29 _+ 19 _+ 29 _+ 23 _+ 25 ± 31 _+ 26 + 30 + 25 ± 32 -+
22/09/84 23/09/84
29/09/84 27/09/84
28/09/84
04/10/84
1 . 3 + 0,4
_+ +_ + _+ + _+ + _+ +_ _+ + + + _+ + _+ _+ _+ _+ + _+ + + + _+ _+ + -+ +
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 1 1 1 2 2 2 2 1 2 1 1
33 1 7
20 11
2.8 2.1 2.9 2.5 1.9 2.8
_+ 9 + 8
6.3_+ 2.1 30 69
+ 5 +21
5 4 3 2.7 8 7 5 8 5 8 8 6 8 6 6 6 6 4 4.5 6 4 6 5 5 6 5 6 5 6
T (days)
(%)
27 20 31 29 18 27
5 1 9 6 2 8
162
51
62 28
25 9
11
0
109
42 97
82 28
29 5
104 71 43 38 148 136 83 162 89 155 161 114 182 125 108 108 99 71 78 102 57 103 75 83 115 89 108 184 119
40 29 18 16 51 48 32 55 35 53 54 42 59 45 41 41 38 29 31 37 22 38 29 32 43 35 41 32 43
189 (b) Activity ratios in aerosols emitted from adventive craters Date
(21°Bi/21°pb)
(21°po/21°pb)
Yellow Vent
06/76
Blowing up Vent Hot Hole Spatter Cone 1300 m
06/78 06/78
2.6 10.8 <1 1.5 3 <1
0.9 0.7 0.6 0.6 8.8 3
03/81
17.3
1.2
0 = delay between the gaseous phase partitioning and its subsequent emission into the atmosphere. ~"= time required to yield measured 21°Po in degassing m a g m a by means of 2u)pb decay. = Percentage of deep m a g m a in the degassing cell.
volcanic gases are cooled. The method of measurement, detailed elsewhere [3], consists of filtering the cold mixture of air and volcanic gases through a cellulose fiber filter and then measuring its alpha and beta radioactive decays. The beta counting efficiency is measured by reference to a 4°K source. The alpha counting efficiency is determined by comparing the alpha and beta activities of shortlived radon daughter sampled in the atmosphere to their theoretical ratio. The figures obtained can be checked by comparing the alpha and beta counting rates of filters sampled at a time long enough to let 21°Bi and 21°po be in radioactive equilibrium with their long-lived precursor 21°pb. Small corrections are sometimes necessary to discard the artificial radioactivity of atmospheric aerosols. The results obtained on Mt. Etna are listed in Table 2. The most important errors mentioned in this table are due to the uncertainties in radioactive countings. The alpha background is negligible which enables long counting times if necessary. On the other hand the determination of the counting efficiency cannot be truly accurate: it is of the order of 19-21%. However, the value utilized for this efficiency is obviously the same regardless of the measurement: 2~°po, or 2~°pb measured through the amount of 21°Po in radioactive equilibrium (which is obtained about 1.5 years after the sampling). In the case of beta counting, the efficiency is well known, but the background is about 0.5 counts per minute. This is negligible for 21°Bi measurements but not for those of 21°pb. Moreover, every time that the 2~°pb activity per m 3 of air is low, we have to take into account a
possible contribution of the artificial beta radioactivity, generally between 0.05 and 0.1 d p m / m 3 of air. All these error causes have been taken into account in Table 2 where they represent generally 5 25%.
5. Theoretical interpretation To simplify, let us consider first the outgassing of a non-degassed magma, in radioactive equilibrium, which can be written by neglecting the 222Rn short-lived daughters: R~ NRa = ~. Rn NRn = ~ Pb N p b = ~ Bi NBi = )k Po Npo
(2) where N is the atom number of a nuclide per gram of magma and X its radioactive decay constant. The composition of the gaseous phase, in the degassing cell, at the very moment of its partition from the magma is quite different: 226Ra remains in the magma (eRa = 0) and all the atoms of 222Rn are in the gaseous phase (~Rn = 1). However, the number of atoms NRn is about 2000 times smaller than Npb and can be neglected in what follows. During the transit of the gas through the degassing superficial cell, its composition changes versus time, according to the radioactivity laws. For the three successive radionuclides considered, the classical equations of radioactivity give, for 21°pb: N a ( t ) = U o , e x,,
190
for
2lOBi:
N*(r)=*
e pxl1+ i
1
2
hi&l x2 - Xl
No*--
r e
-X,1
(4)
for *“‘PO:
~,~2Wl,
N3(f)= (X3-X,)(X,-X,) A, - A*
e
A,&I
A2 +
i
In fact. it is no longer possible to assume that the degassing magma which is already partly degassed is still in radioactive equilibrium. Let the ratio of the ““PO initial activity in the degassing activity be u = cell to its equilibrium with the preceding notations where X ‘j,,N,,,/X ‘I,,Nr,, u < 1. Equation (3) becomes:
-A,/
No*- x,-x,
-h21
e
210 p. =(--+-
The number of atoms of ““Pb, ““Bi, and ““‘PO per gram of magma which are in the gaseous phase are initially: N,,l
=
NPhcPh
42
=
NRI~B,
413
=
NPocP<,
=
NPu
where Nph, N,, and NPc, are given by equation (2). The corresponding values of X are: h, = hPh = 9 hz=X8,=0.14d-‘; X,=h,,,=5x x 10m5d-‘; lO_“d-1 It is clear that h,, can be neglected relative to h “, and XPor and X r. relative to X,,. Moreover, we have already mentioned that the delay 0 between the gaseous phase partitioning and its subsequent emission into the atmosphere is on the order of a few days (to let 210Bi reach the free atmosphere). Consequently both exp( - h,,8) and exp( -h,,,(3) are very close to 1. With these approximations, the activities of the three radionuclides at time 8 are respectively: (2’0Pb)o = X,,N,,(e)
= ~,,NP&rh
(‘“‘Bi) B= x B,N,, ( 19) = h rh N,# Ph + (ht~N”ica, (2’“Po)~ = h,,,Npo(~)
- hp,Nr+p,) =
~P,>(NP,>~P,>
e x’s’ +
N,,c
H, )
In the case of degassing magma initially in radioactive equilibrium the activity ratios measured in the volcanic plume at time B should be therefore:
i *“‘Pb i ,,
cPO
xP
cPh
XB,CP,
81
A similar correction is not necessary for equation (4) in the case of *“‘Bi whose activity in volcanic aerosols is significantly smaller than that of ““‘PO (see Table 2). Moreover, the ““Bi half-life is close to 13,thus enabling 2’0Pb, always present in the degassed magma, to compensate for the losses of ““Bi. Owing to the range of values of .zB1/ePh, and it is possible to simplify the preceding c I’<> /‘Ph. equations:
=a-+
6 PO EPh
(7)
It may be seen in Table 2 that the highest value of the *“j PO/ 2’0Pb activity ratio ever measured in the Etna plume was 69. This figure is therefore the nearest to a magma in radioactive equilibrium. We assume that this magma was, in fact, in radioactive equilibrium, which means that u = 1. The validity of this assumption is supported by the numerical results obtained. In Table 2, the corresponding value of the activity ratio 2’0Bi/2’0Pb is 12.6. We have therefore, besides equation (1). the following two equations, in which 0 has the same value:
191 where t[pb, l[Bi and e x p ( - X s i 0 ) are unknown. We have admitted that %0 = 1. We find that EBi = 0.46 and ~Pb = 0.015. These values can be considered as known with an accuracy of 3% in the case of polonium, and 20% for bismuth and lead and are in agreement with the results shown in Table 1. The escape time 0 is then determined in each case from equation (6), assuming that the coefficients ~R~ and l[pb are constant; in effect, these coefficients depend almost exclusively on the temperature and, in a smaller way, on the chemical composition of the magma which can be considered as constant over decades in a volcanic body. It is worthy to note that 0 is calculated by taking a log: this results in an accuracy generally better than 30%.
6. Discussion and conclusion The values of 0 reported in Table 2 are between 1 and 17 days with one exceptional value of 33 days. This strong variability could indicate that gas emissions result essentially from convective movements in the superficial magma cell rather than from quiet and reproducible rising of bubbles through a motionless magma. ' It is worthwhile noting that as fJ'r a s ' 0 is the mean degassing time of a magma cell, it is equal to the ratio of its volume to the daily output. For instance, in June 1976 Lambert et al. [1] measured in the Etna plume a 2mPo activity of 2.5 × 10 .3 Ci per ton of SO 2. An output of 3.7 × 10 3 t SO 2 per day, as determined by Haulet et al. [14] corresponds to 9.2 Ci of 21°Po per day. The 21°pb activity of the magma indicated in Table 1 is 5 d p m / g and the activity ratio shown in Table 2 for June 1976 is 11 in the plume, which corresponds in our model to a 21°po/21°pb activity ratio in the m a g m a of 11/69, or 0.8 d p m of 21°Po per gram of magma. The daily output of 21°Po is therefore emitted from 26 × 1012 g of magma. The escape time was at this moment about 13 days. The mass of the degassing cell is therefore estimated to be 3 × 1014 g, or 0.13 km 3 with a density of 2.5. The high emanation coefficient of 21°Po results in a rapid depletion of the magma. In effect, lava flows, as well as their gaseous emissions, have shown a very small 21°po/2mPb activity ratio (Table 1). In Table 2, we report other measures obr
tained in adventive craters, which show also atypically low values. In both cases these low 21°p0 activities are attributed to the short passage of time since the lava outgassing, which did not enable 2~°po to be produced by decay of 21°Pb. In contrast to this peculiar case, the general presence of 21°P0 in the degassing cell results from either a renewed increase of this nuclide by 2~°Pb radioactive decay, or a partial mixing with the deep non-degassing magma. In the absence of such a mixing, we incidate in Table 2 the time ~- which would be necessary to yield the amount of 2~°Po actually present in the degassing magma by 2mpb decay. ~" is given by the simple equation:
o=l-e
x.or.
Assuming that O is the actual time of 2]°po production, it is now possible to calculate the corresponding value of the percentage ~ of deep magma in the degassing cell: g=(1-e
x""~) - (1 - e-X,,,°)
It may be observed in Table 2 that i1 ranges from 0 to 100%. However, the outgassing from one specific crater at a given period seems to be wellcharacterised by this mixing rate. Very small figures were obtained during short eruptions such as June 1978, March 1981, and April 1983, which could indicate the superficial origin of the involved magma. On the other hand, the very long lava emission from May to September 1984 corresponded during these last months to high values on the order of 40%, with short escape times of 2 - 3 days. Such a result is not surprising: after a 5-month emission of lava, the proportion of deep magma contributing to the outgassing is necessarily important. The results obtained in September 1984 give important collaborative evidence. The value of the mixing rate/z varies during this month between 17 and 55%, a variability which exceeds by far the errors of measurement. This shows that the deep magma injected into the so-called degassing cell is poorly mixed. Consequently, the hypothesis that the emanation coefficients ~ are constant is probably a first approximation, and will have to be somewhat revised in future works. In conclusion, it is clear that the gaseous emissions from an active volcano are very often characteristic of superficial magmatic cells already more
192 o r less d e g a s s e d in t h e p a s t . I n c o n t r a s t , a f t e r a l o n g l a v a e m i s s i o n it a p p e a r s t h a t t h e c o m p o s i t i o n o f t h e d e g a s s i n g m a g m a is close t o t h a t o f t h e d e e p n o n - d e g a s s e d m a g m a , T h i s s h o u l d r e s u l t t h e n in a particularly high contribution of the most volatile m a t e r i a l s to t h e v o l c a n i c e m i s s i o n . In case of a cataclysmic volcanic explosion the gas composition should correspond to that of a d e e p n o n - d e g a s s e d m a g m a a n d n o t to t h a t o f a s u p e r f i c i a l m a g m a cell. T h i s o b s e r v a t i o n a g r e e s w i t h t h e m e a s u r e m e n t s m a d e in t h e c l o u d o f M o u n t A u g u s t i n e w h i c h w a s s i g n i f i c a n t l y e n r i c h e d in v o l a t i l e m a t e r i a l s [9]. By m e a s u r i n g 21°pb, 21°Bi, a n d 21°p0, it is p o s s i b l e to d i s t i n g u i s h b e t w e e n t h e compositions of gases emitted by a quiet volcanic a c t i v i t y a n d o f t h o s e w h i c h w o u l d b e e m i t t e d in c a t a c l y s m i c e x p l o s i o n s . It s h o u l d t h e r e f o r e b e int e r e s t i n g to s t u d y to w h a t e x t e n t t h e m o d e l p r o p o s e d i n t h i s p a p e r for M t . E t n a c o u l d b e a p p l i e d t o o t h e r v o l c a n o e s f o r w h i c h a risk o f e x p l o s i o n exists.
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