Fractionation of metals in volcanic emissions

Earth and Planetary Science Letters, 88 (1988) 284-288 Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands

284

[6]

Fractionation of metals in volcanic emissions M. Pennisi, M . F . L e Cloarec, G. L a m b e r t a n d J.C. L e R o u l l e y Centre des Faibles Radioactivitds, Laboratoire mixte CNRS/CEA, Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex (France) Received May 8, 1987; revised version received February 25, 1988 The enrichment of some metals (A1, Mg, Na, K, Cu, Zn, Cd) in volcanic gases is measured by an emanation coefficient c x relating the amount of the element x in lavas and in aerosols. % is determined in the volcanic emissions of Mount Etna (Sicily), with regard to that of 21°pb. The accuracy of the results is limited by the geochemical behavior of c o m m o n lead compared to that of 21°Pb. Concerning the most volatile species, it appears that practically all the volcanic aerosols are produced by evaporation followed by gas-to-particle conversion and the spattered fraction appears to be negligible.

1. Introduction

Since the early work by Zoller et al. [1] and Maenhaut and Zoller [2] on trace metals at the South pole, it has been recognized that the volcanic aerosols are considerably enriched in volatile metals [3-6]. Chemical analyses of volcanic aerosols are often expressed by an "enrichment factor" (E.F.) relative to the average crustal composition which is found to vary by as much as two orders of magnitude, not only from one volcano to another one, but from time to time for a given volcano. In fact, the E.F. are not directly related to the gas emission processes in volcanoes. For this reason, Gill et al. [7] and Lambert et al. [8] preferred to characterize each element by an emanation coefficient ~, defined as a partition between gases and magma: (

--

C i - Cf Ci

_

property of the metal per se, but it is related to the volatility and proportions of its different compounds, mainly halides and sulfides, existing in the degassing m a g m a and to its physical characteristics such as temperature, viscosity and pressure prevailing at the very place where the gas phase is produced. F r o m a consideration of disequilibrium within a radioactive series, Lambert et al. [8] determined the emanation coefficients of a few elements in the emissions of Mount Etna (Sicily): CRn ~ ~Po ~ 1

(in agreement with Benett et al. [9] and Gill et al. [7]) and: EBi = 0.46,

and ¢Pb = 0.015

It is possible to extend these studies to other metals by using the following very simple conservation equation [8]:

_

where C i is the initial concentration of the m a g m a and Ce its concentration after degassing. ¢ is equal to unity for materials normally in a gaseous state at the magma temperature, and to zero for substances which are not volatilized at all. The difficulty is to quantify these ~ for most of the other substances, which have intermediate properties. In fact, this emanation coefficient is not a 0012-821X/88/$03.50

e'x/¢ Pb

( X/Pb)gas

© 1988 Elsevier Science Publishers B.V.

(X/Pb),.va

-

(1 - c x ) / ( 1

CPb

- cp.)

(1 -- %)

This can be made by comparing the amounts of a given metal simultaneously in lavas and in aerosols, owing to chemical analyses of their sampies. This is the object of this paper. However,

285 new experimental evidence requires that the above mentioned value of cpb will be reconsidered. All the results presented further obviously depend on the coefficient ~Pb, because only lead can be taken as a reference in the equation of conservation. In effect Cpo is of the order of 1, and £ai is known with a large uncertainty due to peculiar processes. In effect it was shown that, along a lava flow, the bedding rocks are heated at temperatures high enough to let 21°Bi, continuously produced within these rocks by the decay of 2mpb, be evaporated and subsequently absorbed either by the aerosols or by the surface of the lava flow [10]. 2. Sampling Two sampling campaigns were conducted on Mt. Etna in 1984 and 1985, during lava flows. In 1984 strombolian and effusive activity occurred from April to October at the S.E. Crater. In 1985, from March to July, lavas flowed along a fracture on the Southern flank at about 2500 m a.s.1. (Piccolo Rifugio). Lava samples of the order of a few kilograms were collected. Aerosols were sampled at the same time from the plume emitted by the main craters at the top of the volcano (Bocca Nuova). In effect, Le Cloarec et al. [10] have shown that the 21°po/21°pb activity ratio is untypically low in small plumes emitted from adventive vents or directly from lava flows, which are almost completely degassed. This indicates that the main gas emission occurs primarily in the superficial cell of the main craters, which is directly connected to the deep non-degassed magma. Moreover, as far as the pressure could be an important parameter of the degassing, as shown by Gerlach [11], it seems more interesting to measure the values of e relative to the main outgassing than to local superficial effects. The aerosols were collected on PoelmannSchneider blue cellulose filters, at a flow rate of 15-20 m 3 per hour according to the techniques described b y Polian and Lambert [12]. Filter activities of 21°pb were determined through those of its daughter 2~°Bi. Each sample corresponded to about 5 m 3 of gas. 3. Chemical treatments and measurements Solutions of the filters were prepared through concentrated H N O a / H F attack. The analyses of

the selected stable metals (A1, Mg, Na, K, Cu, Zn, Cd, Bi, Pb) were performed by absorption spectrometry using a Perkin-Elmer 2380. Furnace and flame atomization were used, depending on the elemental concentrations. In order to avoid matrix interferences, the standard addition method was used. For bismuth, the hydride technique was utilized. The lavas were solubilized with concentrated H N O a / H F at 1 0 0 - 1 5 0 ° C . After addition of a known amount of 2°8po, for the standardization of 21°Pb and 21°Po measurements, silica was expelled as volatile fluosilicic acid at about 100-105 o C. 2°8po and 21°po were then plated on a silver disk at 60 ° C [13]. Their activities were measured by a spectrometry. The procedure was repeated on the same lava sample several months later, in order to let 21°po to grow from 21°pb, whose activity was deduced from that of 21°Po. 4. Results The results of the chemical analyses of lavas are shown in Table 1, together with data relative to 1976 and 1983 lavas. The values for the major components A1, Mg, Na, K measured by Clochiatti and Lenoble (personal communication), agree with the results obtained for other years b y Condomines et al. [141. Concentration data for elemental determinations for filters collected in the Etna plume are summarized in Table 2. The average values of the seven blanks studied are given as well as the uncertainties resulting from the chemical treatment and analyses on the various elements. In the interpretation of the experimental resuits, a general problem consists in distinguishing between spattered and evaporated-condensed materials. For samples collected in 1984 this selection was only based on a lack of visible dusts on the filters. Lambert "et al. [15] by analysing alpha spectra of volcanic aerosols, showed that the emanation coefficient of thorium, ETh, is about 10 4 fold smaller than that of lead and therefore of the order of 1 × 10 -6. Consequently, the presence of the 1.9-year half-life 228Th on a filter can be considered as a tracer of spattered materials. The 228Th activity being negligible in sample 4, its spattered fraction can be considered as negligible as well.

286 TABLE 1 Concentration of elements in Etna lavas (in # g / g ) Date

AI

Mg

Na

K

Cu

Zn

Cd

Bi

Pb

2x°Pb

21°Pb/Pb

1976

93,000 a

29,800 a

32,200 a

15,770 a

90,400 b

35,200 b

28,300 b

16,000 b

109.2 (22.4) n.a.

0.175 (0.035) n.a.

1984

89,700 c

29,970 c

28,860 c

15,770 c

1985

90,000 ¢

37,500 c

29,000 c

16,500 ¢

129.4 (0.40) 188.4 (14.8)

99.3 (0.3) 134.6 (12.0)

0.20 (0.028) 0.11 (0.01)

0.054 (0.005) 0.067 (0.017) 0.045 (0.015) 0.047 (0.008)

9.22 (1.02) 12.20 (1.87) 6.18 (0.4) 9.94 (1.27)

5.0 (0.25) 5.0 (0.25) 3.63 (0.04) 3.75 (0.04)

0.54

1983

129.7 (0.88) n.a.

0.4 0.59 0.38

( ) denotes lo; n.a. = not analysed. a M. Lenoble (personal communication); b from Condomines et al. [14]; c R. Clochiatti (personal communication).

In the case of sample PR9 the 2 2 8 T h activity is not negligible, but its measurement was made 8 months after its sampling. Therefore we cannot rule out the possibility that at least one part of this 2Z8Th was produced on the filter by decay of 228Ra, which is practically in radioactive equilibrium with 228Th in lavas [9,14], and whose one unknown proportion could be due to volatilization and gas-to-particle conversion. Consequently no quantitative correction was possible. However, by assuming that all 228Th measured only results from spattering, we can evaluate the maximum part of spattering in this sample, by referring to

the lava composition, normalized to 228Th. This maximum spattered fraction (MSF) is also reported in Table 2. In effect, the calculated MSF are larger than the amounts actually measured for A1 and Mg. In contrast, it may be observed that, even in this extreme case, the spattered fractions remain negligible for volatile metals Cd, Bi and Pb and small for Cu and Zn. 5. Emanation coefficient of lead In their initial work lated

an emanation

Lambert

coefficient

e t al. [8] c a l c u (Pb o f 1.5 X 1 0 - 2 .

TABLE 2 Concentration of elements on filters in the Etna plume (in ~g/filter) Date

Sample

AI

1985/04

PR9

1364.00 (309.9)

MSF a

2934

1222

340.0 (151.3) < I.d. (340A) < l.d. (359.0) < 1.d. (402.94) 153.0 (343.1)

74.6 (10.2) < 1.d. (31.0) n.a.

1985/05

4

1984/09

23

1984/09

26

1984/09

27

1984/09

28 Blank

1500.0 (280.0)

Mg

372.0 (21.2)

Na

997.4 (14.0)

952.1 (85.7)

945

538

108.6 (30.4)

358.4 (3.6) 584.5 (111.0) 1797.3 (130.0) 1244.0 (113.0) 1168.6 (119.0)

143.0 (23.0)

82.0 (91.0)

n.a.

K

Cu

Zn

Cd

Bi

Pb

21°Pb (dpm/ filter)

21°pb/Pb (dpm/ # g Pb)

34.2 (3.28)

15.1 (1.21)

1.48 (0.13)

0.94 (0.10)

5.97 (0.54)

5.0 (0.25)

0.84

6.4

4.4

0.004

0.002

0.32

0.12

27.8 (1.67) 1.42 (3.37) 16.7 (3.77) 5.38 (3.38) 6.94 (3.45)

1.43 (0.13) 1.23 (0.095) 4.9 (0.18) 2.9 (0.13) 2.55 (0.21)

1.10 (0.13) 0.75 (0.026) 1.63 (0.059) 1.47 (0.17) 0.93 (0.052)

9.42 (0.40) 8.52 (1.28) 24.0 (1.06) 9.7 (0.60) 10.6 (0.55)

7.5

0.80

6.4

0.75

25.1

1.05

11.1

1.14

8.8

0.83

6.0 (3.0)

0.034 (0.006)

0.0094 (0.0007)

0.33 (0.55)

1188.0 43.9 (127.0) (2.94) 933.5 36.1 (80.0) (0.9) 3741.4 142.9 (176.0) (2.22) 1831.1 66.9 (109.0) (1.24) 1473.6 67.8 (1013.0) (0.91) 41.0 (34.0)

0.79 (0.24)

( ) denotes lo; n.a. = not analysed; < I.d. = below detection limit. a MSF = m a x i m u m part of spattered fraction on PR9 filter.

< I.d.

-

287

This determination was based on the assumption that no fractionation could occur between 21°pb and common lead. A significant physical isotopic fractionation can obviously be ruled out. However, in a m a g m a 21°pb atoms may not necessarily be associated with the stable isotopes of lead, since these atoms are produced by the radioactive decay of 226Ra (and its short-lived daughters); they are likely associated with the same chemical species as radium, whose the volatility can be different from that of lead compounds. Indeed, it can be observed in Table 2 that the specific activity of 21°pb is close to 1 d p m / / t g Pb in Mt. Etna aerosols, when it is close to 0.4 in the corresponding lavas (see Table 1). This observation agrees with unpublished measurements of one of us (Le Cloarec) who measured specific activities between 0.4 and 1.5 dpm//~g Pb in sublimates and condensates sampled from a hot vent of Mr. Etna. Values of ~ different for 21°Pb and common lead could lead to values of Epb in the range of 0.6 X 10 -2 to 1.5 X 10 -2. In this paper, all the calculations were made with an intermediate value of 1% for ePb, and an uncertainty of a factor two. This is the main cause of uncertainty in the evaluation of the other c. 6. Emanation coefficients of metals

The elemental concentrations of the different filters are not directly comparable because the sample volumes as well as the dilution of the volcanic gases are different. In contrast, the values of the emanation coefficients, deduced from Tables 1 and 2 and shown in Table 3, should be directly comparable.

It appears that the emanation coefficient c corresponding to the less volatile metals A1, Mg, N a and to a lesser extent K, is very similar in sample 4 collected in 1985, for which the spattering was negligible, to those of 1984, which were assumed to be free of spattering. The agreement between the 1984 and 1985 sampies suggests that we can adopt the following average figures: cK = 6 × ~Na = 2 CAI =

10 - 4

X 10 - 4

0.2

X 10 - 4

EMg = 0 . 2 X 10 - 4

These four metals are major components of lavas, and despite their relatively low volatilities, they represent a large proportion of the volcanic aerosols. For Bi, Cd and Zn, the mean values of E seem to be systematically lower in 1984 than in 1985: 0.12 versus 0.22 for Bi (instead of 0.46 as initially measured by Lambert et al. [8]); 0.065 versus 0.155 for Cd; 3.3 X 10 - 4 v e r s u s 18 x 10 -4 for Zn. For this latter metal, there is also a possibility that gaseous compounds were not retained by the filters. This variability could indicate that the composition of the superficial degassing cell was different between these years since the emanation coefficient is related to the volatility of the metal halides and sulfides present in the degassing magma. Also it should be noted that in 1984 the sampling was performed at the end of the eruption, whereas in 1985 it took place near the beginning of the eruptive period. One could reasonably

TABLE 3 Emanation coefficients obtained by chemical analysis of lavas and filters ( x 10-4) . Date

Sample

A1

Mg

Na

K

Cu

Zn

Cd

Bi

1985/04 1985/05 1984/09 1984/09 1984/09 1984/09

PR9 4 23 26 27 28

n.d 0.4 n.d. 0.05 0.2 0.1

n.d. 0.2 n.d. n.a. n.a. 0.2

1.3 1.5 1.6 2.8 2.4

7.7 4.3 6.2 7.5 5.5

26 25 20 29 33 31

14 22 1.1 4.4 3.5 4.1

1900 1200 430 600 850 700

2500 2000 1100 900 1700 1100

0.19

0.2

1.8

6.0

27.3

8.1

950

1550

Average

n.a. = not analysed;- = not calculated due to high spattered fraction contribution; n.d. = not determined.

288

suggest that these metals are combined in two or more volatile compounds whose proportions vary from one eruption to another. On the contrary ecu seems to remain approximately constant in 1984 and 1985, at about 27 X 10e4, meaning that Cu was likely expelled in another chemical form than the other metals. More experiments are necessary to check all these assumptions. However, as a first approximation, and taking into account the uncertainty of a factor 2 in the f evaluations, the following figures can be tentatively adopted: eai = 2 x 10-l lzcd = 1 x 10-l Ecu = 3 x 10-3.

7. Conclusion This study has shown that the emanation coefficients for the radon decay products 210Pb, 210Bi and 210Po measured in the volcanic emissions from Mount Etna can be extended to other non-radiogenie metals. However, the accuracy of the results is limited by the uncertainty in the geochemical behavior of common lead and 210Pb. Therefore the values indicated in this work are only given with an uncertainty of a factor two. Concerning the most volatile species, it appears that practically all the volcanic aerosols are produced by evaporation followed by gas-to-particle conversion. The spattered fraction appears to be negligible. In this case, no correction is needed, and the observed variations of their emanation coefficient are likely to be related to changes in magmatic gases. In contrast, for the poorly volatile metals, it would be better to differentiate between spattered and evaporated materials. Acknowledgements The authors are grateful to all people having participated to this hard work: sampling on the rims of the craters, and to T.M. Gerlach and C.C. Patterson for their comments. This work was supported by the “Programme Interdisciplinaire de Recherche pour la Prevision et la Surveillance des

eruptions Volcaniques” C.N.R. (Italy).

du C.N.R.S

and

by the

References 1 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. 2 W. Maenhaut and W.H. Zoller, Determination of the chemical composition of the south pole aerosol by instrumental neutron activation analysis, J. Radioanal. Chem. 37, 637-650, 1977. 3 P. Buat-Menard and M. Arnold, The heavy metal chemistry of atmospheric particulate matter emitted by Mt. Etna volcano, Geophys. Res. Lett. 5, 245-248, 1978. 4 E.A. Lepel, K.M. Stefansson and W.H. Zoller, The emichment of volatile elements in the atmosphere by volcanic activity: Augustine volcano 1976. J. Geophys. Res. 83 (C12), 6213-6220, 1978. 5 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. 6 J.M. Phelan, D.L. Finnegan, D.S. Ballantine, W.H. Zoller, M.A. Hart and J.L. Moyers, Airborne aerosol measurements in the quiescent plume of Mount St. Helens: September 1980, Geophys. Res. Lett. 9, 1093-1096, 1982 7 J. Gill, R. Williams and K. Bruland, Eruption of basalt and andesite lava degasses 222Rn and 210Po, Geophys. Res. Lett. 12, 1, 17-20, 1985. 8 G. Lambert, M.F. Le Cloarec, B. Ardouin and J.C. Le Roulley, Volcanic emission of radionuclides and magma dynamics, Earth Planet. Sci. Lett. 76, 185-192, 1985/86. 9 J.T. Bennett, S. Krishnaswami, K.K. Turekian, W.G. Melson and CA. Hopson, The uranium and thorium decay series nuclides in Mt. St. Helens effusives, Earth Planet. Sci. Lett. 60, 61-69, 1982. 10 M.F. Le Cloarec, G. Lambert, F. Le Guem and B. Ardouin, Echanges de materiaux volatils entre phases solide, liquide et gazeuse au tours de l’eruption de l’Etna de 1983, CR. Acad. Sci. Paris, Ser. II 298, 805-808, 1984 11 T.M. Gerlach, Exsolution of H,O, CO,, and S during eruptive episodes at Kilauea volcano, Hawaii, J. Geophys. Res. 91, 12,177-12,185, 1986. 12 G. Polian and G. Lambert, Radon daughters and sulfur output from Erebus volcano, Antarctica, J. Volcanol. Geotherm. Res. 6, 125-137, 1979. 13 S.C. Black, Low level polonium and radiolead analysis, Health Phys. 7, 87, 1961. 14 M. Condomines, R. Bouchez, J.L. Ma, J.C. Tanguy, J. Amosse and M. Piboule, Short-lived radioactive disequilibria and magma dynamics in Etna volcano, Nature 325, 607-609, 1987. 15 G. Lambert, M.F. Le Cloarec, J.C. Le Roulley, B. Ardouin, Y. Yokoyama and H.V. Nguyen, Radioactive disequilibria in volcanic emissions, Hawaii Symposium on How Volcanoes Work, Hilo, Hawaii, 1987.