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Analysis of volcanic samples by a parameter method established for correcting PIXE measurements
Nuclear Instruments North-Holland
and Methods
in Physics
Research
ANALYSIS OF VOLCANIC SAMPLES BY a PARAMETER FOR CORRECTING PIXE MEASUREMENTS P. ALOUPOGIANNIS and G. WEBER ‘)*
ls2), J.P. TOUTAIN
277
B49 (1990) 277-282
3), G. ROBAYE
METHOD
ESTABLISHED
I), I. ROELANDTS
4), J.P. QUISEFIT
2’
” Institut de Physique Nuclhaire Expkimeniale, Vniversit6 de Li?ge B15. Sart Tilman, B 4GOOLiPge, Belgium ” Laboratoire de Physic+Chimie de I’Atmosphsre, (unite associke au CNRS no. 717). VniversitP Paris 7, 2 Place Jussieu, 75005 Paris, France. ” Osservatorio Vesuviano, Via Manzoni 249 80123, Napoli, Italy ” Institut de Gkologie, VniversitP de Li2ge, Lisge, Belgium
The cooling down of volcanic gases produces deposits of condensed mineral phases (sublimates, incrustations) at the volcano-atmosphere interface. The analysis of these deposits presents a high volcanological interest. The procedure of fractional condensation, used to obtain the samples, gives generally very small amounts of materials and this explains the scarcity of suitable analytical methods. Efforts to prepare samples for XRF or AAS analysis failed because of the presence of insoluble chemicals and because of the necessity to avoid heating the material. PIXE analysis proved to be successful by using intermediate thickness samples
and the new (Yparameter correction method for matrix effects. The analytical procedure is described and the results and their volcanological significance are given.
1. Introduction In two previous papers [1,2], we established a new method to correct PIXE data for matrix effects. The merit of this method lies in avoiding hypotheses about sample composition. This point constitutes the main advantage of this method compared with the others [3-51. All the necessary information to calculate the corrections is obtained experimentally. The method was tested by using international reference materials: geological, biological and ore samples [6]. We shall present the results of an application of this method concerning volcanic sublimates and incrustations. Sublimates are obtained by selective condensation of volcanic emanations in silica tubes. The amount of material obtained is small, for which our method using small intermediate thickness targets, is well suited. The surface emission of the volcanic gaseous phase leads to the deposit of condensed mineral phases around fumarolic vents. They mainly consist of various salts of numerous metallic elements (Na, K, Cu, Zn. As, Se, Fe, Si, Al, Ca, etc.). The presence of these elements in the high temperature volcanic fluid is in connection with selective distillation phenomena occurring at magma level [7,8] leading to their enrichment in condensed * Research associate search (Belgium). 0168-583X/90/$03.50 (North-Holland)
of the National
Fund
for Scientific
0 Elsevier Science Publishers
B.V
Re-
phases [9]. Hence the study of metallic element chemistry of volcanic condensed phases should provide useful information about the conditions controlling their distillation, their migration toward the surface and their deposit. Nevertheless, until now, most of the studies performed have dealt only with mineralogical and geochemical aspects of these deposits [lo-121. Indeed, few quantitative chemical data have been published, mainly due to the small amount of material usually available, and to the complex matrix of samples making most of the analytical techniques ineffective [13]. Two series of samples have been analysed according to our method: sublimates and incrustations. Sublimates are mineral phases condensed in a silica tube that canalize the gaseous phase escaping from the fumarole. This new sampling technique enables metallic elements to deposit during the cooling down of the gaseous phase in the tube, without perturbations due to the atmosphere (oxygen contamination, reactions with the wall-rock). Moreover, measurements performed in the tube during sampling gave the temperature of condensation of each mineral phase [12-141. Incrustations are minerals collected in natural deposit conditions. Our samples were collected around high temperature vents or in cracks occurring on the roof of lava tubes on the following volcanoes: Etna (Italy), Ardoukoba (Djibouti), Erta-Ale (Ethiopia) and Momotombo (Nicaragua). V. GEOLOGICAL/ARCHAEOLOGICAL
APPLICATIONS
P. Aloupogiannis et al. / AnaJwis of volcanic samples
278 2. Analytical procedure
Volcanic
The energy loss A E of the excitating proton beam is determined by the use of Rutherford scattering on a secondary carbon target and it is used together with a separate X-ray transmission measurement to calculate a so-called (Y parameter. This parameter allows one to calculate two matrix correction factors taking into account the variation of X-ray production cross section with the proton energy and the absorption of the produced X-rays on their way toward the Si(Li) detector. The theory of this new correction method can be found in our previous papers [1,2].
condensed
phases
Jy---(q
. 0 +
I 0
1
2
3
d
m Cmg/cm
5
3. Experimental procedure Fig. 2. Variation
The studied sublimates were collected during the S/08/85 eruption of Piton de la Foumaise volcano (Reunion Island, Indian Ocean) [9,13]. The sampling duration was 4 days. After sampling, the silica tube was cut into nine sections, 3 to 10 cm long. Sublimates deposited on the internal walls were collected mechanically. The sampling quantity was about 5 to 25 mg of material. for each section. The sample preparation technique has been fully described elsewhere [15]. It consists of a filtration on a nuclepore filter of a suspension in cyclohexane of a small amount of the powdered volcanic material. Two targets were prepared for each sample.
of the LLD, as a function masses.
of the superficial
The description of the method of measurement of the proton energy loss can be found elsewhere [l]. Fig 1 schematically shows the experimental setup. The X-rays produced by irradiating multielement pellets with a proton beam [6] are used to obtain the transmission coefficients through the samples.
4. Analytical considerations Two opposite problems have been encountered in the measurement of S and Cl in our samples: (a) In the case of silica tube sublimates the PIXE spectra showed very high Cl peaks with a small S component at their left side. In these conditions we could just estimate an upper limit of 2% for S concentrations. (b) In the case of incrustations, the very high S peaks allowed no Cl estimation. Our PIXE measurements have been performed by using intermediate thickness samples. Their superficial mass varied between 0.5 and 3.5 mg/cm2. The total matrix correction factor FC which depends on the detected element and on the sample thickness has an influence on the value of the detection limits LLD, (in ppm and for 1 PC). We have: LLD, = 3( B,)0~5FC,/0.001R,m.
I
Fig. 1. Experimental
setup for simultaneous energy loss measurements.
1
PIXE and proton
where B, is the number of counts in the background below the peak of element i. R, is the sensitivity in counts per ug and PC, and m is the superficial mass in mg/cm2. Fig. 2 illustrates the typical trend of this variation for 3 elements: Cr, Cu, and Fe, and shows
P. Aloupogiannis et al. / Ana!vsis
of volcanicsamples
219
Table 1 Analytical results for sublimates from Piton de la Foumaise (silica tubes). Concentrations are expressed in ppm (except for Cl, K, and Cu in %). The concentration values with an * are from NAA for comparison. S was generally present below 2% (see text). Co and As were often visible but below detection limit of about 30 ppm Sample
Element Cl
RI
WI
5356.5
K[I] 14
Cr +1.5
70+ 35
Fe
Cu
2OOk 100
0.05 +0.006
PI
Zn
Se
Rb
Pb
Tl
< 20
Cl2
3OOk 80 340* 410+ 50 4001 750+ 80 650* 14OOk250 1460* 1300&230 1210* 15OOk250 1120* 13OOk230 1110* 138Ok240 1580*
~65
-=z60
~65
i 60
1* R2
5917.2
14.6f1.5
R3
67k8.2
21
R4
64*7.8
22.4f2.3
R5
69k8.5
23.352.4
18+13
R6
73k8.9
23
18*
R7
71+8.7
22.4*2.3
17+
R8
67k8.2
22
32f13
that
as
k2.3
k2.2
4
120 + 30
0.05 *0.005
< 20
(15
100 f
50
0.21 kO.03
~15
110+
15
1
160?
25
0.98+0.10
< 20
3
120+
15
1.0350.09
< 20
3
250+
50
0.81+0.08
110* 12
4705
50
1.6350.15
30f15
as LLD, are concerned, the best is situated between 2 and 3.5 mg/cm2.
far
thickness
k2.2
19*
* 0.04
< 20 22+14
112 2* <12 1* 21* 3 39* 15Ok 20 110* 37Ok 50 388* 8005100 968* 38OOk500 4080*
70+
20
23Ok
30
150f
15
81Ok
75
830+
90
93Ok
80
400&150 ~65
~60
3500+550 426Ok
45
7600 k 800
target
5. Results Tables 1 and 2 give the results relative to the two series of volcanic condensates. The silica tube sublimates from the first series (Rl to R8) have also been analysed by using neutron activation (Laboratoire Pierre Sue, Paris). In these samples, only two elements (Se and Rb) detectable both by PIXE and NAA are present at concentrations above detection limits. The results of NAA measurements are given in table 1 for comparison. The agreement is very good.
6. Discussion 6.1. The silica tube sublimates from Piton de la Fournaise X-ray diffraction and SEM (scanning electron microscopy) show that the sublimates in the tubes mainly consist of Na, K and Cu chlorides deposited between 850 and 1OO’C. and a mixed K-Si fluoride deposited between 550 and 100°C [9,13]. Geochemical study of the sublimates, based on our PIXE measurements, shows that the elements have two different behaviours. Either they are enriched in the low temperature zone (Rb, Se, Cu. Pb, Tl) (fig. 3a), or they show little or no variation
Fig. 3. (a) Evolution of the concentrations of Cu. Pb and Tl with temperature. (b) Evolution of the concentrations of Cl and K with temperature.
V. GEOLOGICAL/ARCHAEOLOGICAL
APPLICATIONS
k2.1
11.9
f2.2
f1.7
f1.9
f1.9
f2.7
~~2.4
20.6
18.7
21.9
lb.7
18.6
17.8
26.1
23.2
21.1
4.54+
8.86 f 0.9
20.5 i 2.1
19.3 k2.0
22.8 k 2.3
23.3 * 2.4
20.3 +2.1
18.2+0.8
22.6 f2.4 22.bk 2.3
20.4 f 2.1
20.6 f2.1
17.8 + 1.8
EA 78/3
EA 78/4
EA 83/l
EA 83,‘2
EA 79/l
EA 85/2
EA 85/3
78/l
78/2
78/3
78/4
EA
AR
AR
AR
AR
AR
AR
20.4 f 2.1
86
MO
21.7 i 2.2
ER 57
21.4 f 2.2
78/b
15.7
14.0
+l.b
zt1.4
jz1.0
+1.1
11.1
22.0 f 2.2
78/5
10.2
+1.1
10.7
+1.0
10.2
19.7 k 2.0
0.4
20.5 i 2.1
EA 85/l
f2.2
fl.b
15.7
_
13.3 k 1.35
EA 78,‘2
80/l
t2.0
K PI
19.8
PI
22.6+2.3
S
Element
EA 78/l
Samples
for Aphtitalites.
(Momotombo).
results
MO
and
Ale),
2
Analytical
Table
2700i
2900f
14OOf
1600-t
1040+
1100*
1020f
53ooi1450
15OOf
~800
8OOi
1370+
750f
205Oi
1400*
930f
3OOOi
90f13
750
800
400
440
285
300
300
400
250
375
9
6
-z 20
< 20
3oi
75ilO
58+
49f
37+
< 18
4
8
7
54+7
<17
18k2
17+2
2553
115
<14
114
<17
< 20
< 17
<15
18+2
23k3
20+2
21*3
<15
<17
5
3
9
5
26+3
~15
Cr
(except
< 20
‘z 20
6
25f
< 20
~18
360 i 50
110+15
4Ok
8Oill
68klO
87k12
86+12
64k
3b+
9
in ppm
100*14
b3k
< 20
V
9
8
250 + 35
62f
< 35
441
651
58f
180*25
250 i 40
210
560
8
200+30
55?
365 k 50
Ti
are expressed
400
260
820
450
concentrations
1bOOf
Ca
The
44-t
100+11
19f
2
5
5
325 43*
6 4
SO+
5
9
9
46+
811
87+10
84f
7
130
10400 + 1050
60
40
410*
bOO+
30
270t
30
70
650+
280+
50
470f
2030 + 200
1970+200
2400 f 250
1250+
17bOi175
62i
1OlOf
100
20
760+
-z 20
~10
~10
cl0
~10
< 20
to.2
1.17*0.1
0.55 + 0.06
2.4
0.77 * 0.07
io.5
0.91 f 0.1
2.54 f 0.3
4.05 f 0.4
4.95 + 0.5
5.4
5.29 f 0.5
3.91 f0.4
1.12kO.l
30
b0
20
70
45
85
75
2900 f 350
2601
1040+110
560+
1280f135
122Oi130
1040+110
150f
1010+110
1000*110
660f
1420+150
400+
710f
0.06
0.59 k
525
3500+380
2400+260
4950;
4750+500
Zn
are as follows:
0.61 + 0.06
3.18kO.3
1.87kO.2
11.65il.l
2.77 k 0.3
21i61.14+0.1
< 20
< 20
< 20
i
c 20 c 20
20
7400 + 720
i
~20
labels
Cu @l
sample
< 20
Ni
The
4900 * 500
1800+180
470*
50
100
x7*10 8
9
15
970+
in 8).
2910+285
Fe
Cu
69i
115*15
83t
100*11
120+
117+13
110*12
Mll
for S, K. and
1570+280
30+5
< 20
< 20
112
-z 12
-z 12
<12
50 100
150 14001250
810+
540 -I_ 100
57OilOO
blOi110
580f
530 f 100
270+
17ooi310
< 12
310
1500 f 270
790 + 140
1750*
<12
145 730+130
x10*
<12
-x 12
22k4
x 12
< 12
1200+235
185Ok330
2140+375
16800*1600
275+
173Ok
1500f
1480+
1680+
16005
1450f
270+
1180-t
1200+
430*
25
160
150
140
160
150
140
25
110
120
45
90 750
95oi
70
8200+
730i
730+
19000~1800
12000+1100
70
ER (Ert
11200f1050
Pb
(Ardoukoba),
1850+320
Rb
AR
25+4
30f5
44+8
112
Se
EA (Etna),
P. Aloupogiannis et al. / Ana!vsis
with increasing temperature (Cl, K) (fig. 3b). It can be noted that among the first group, Tl exhibits the clearest trend as a function of decreasing temperature. In spite of its obvious volatile behaviour during magmatic processes [9], only two examples of high amounts of Tl in gaseous samples are known. Hence, a very rich Tl gaseous phase should be a significant characteristic of Piton de la Fournaise. According to SEM evidence, Tl is believed to be transported as a volatile chloride and condensed as a mixed Tl-K solid chloride. A volatile behaviour of lower intensity is observed, for lead, according to the decreasing Pb concentrations from R6 to R8 samples (fig. 3a). Cl and K concentrations are not highly temperature-dependent (fig. 3b). Since these elements have been present in the gaseous phase in very large quantities, they should be at saturation point in the system and decreasing temperature should have no effect on their condensation. A significant result compared with previous studies lies in the lack or low concentration of both S bearing minerals and constituents of the sulfides usually found in sublimates (S. Fe, Zn, As, Cr, Co). These mineralogical assemblages and chemical compositions are interpreted as the result of an early condensation of S as solid sulfides species in the central cone and in the natural lava tunnels. The above-mentioned elements should therefore be fractionated from the gaseous phase and integrated into the solid sulfides [9].
6.2. The incrustations All samples consist mainly of aphtitalite, which is a mixed Na-K sulfate commonly regarded as (K.Na), Na(SO,),, in admixture with various minor contents of thenardite (Na,SO,). Samples occur mostly as high size dendritic crystals and generally show yellowish to bluish tints. Table 2 shows that these samples are nearly exclusively constituted by volatile elements (S, K. Cu. Zn, Rb, Pb). On the contrary, refractory elements of metasomatic origin (Ca, Ti, V, Cr. Mn, Ni) are scarcely occurring, or lacking in the condensates. This shows that the cation input from the wall-rock, which is a common mechanism acting during the deposit of incrustations [lo], is not operating at the high temperature conditions allowing the growth of most of the aphtitalites. and that they clearly derive from gas-solid reactions. Nevertheless. these samples are undoubtedly of secondary origin, in accordance with their growth location on the field. Indeed, they cover the external part of the high temperature vents (fumaroles or cracks in lava tunnels), and thus they grow in highly oxidized conditions. This observation corroborates the previous studies dealing with aphtitalite genesis [16,17], and thermochemical simulations showing that Na and K are conveyed by the gaseous phase as gaseous chloride
ofc~olcamc samples
281
species, and deposited as solid chlorides or sulfates, respectively, in reduced or oxidized conditions [11,13.14]. The chemical composition of the sublimates also confirms this interpretation: the main minor and trace elements (Cu, Zn. Rb, Pb) are believed to be mobilized in the high temperature gaseous phase mainly as gaseous halides [13.14]. One can see that the samples collected at the same place and at the same time are roughly homogeneous with regard to their volatile element chemical composition (except copper which exhibits strong variations, even in samples of same origin). Sample AR l/78 is an exception with respect to other AR samples: it shows a low content of volatile elements and a high content of refractory ones. This indicates a probable participation of the wall-rock to the growth of sample AR l/78, obviously through acid alteration mechanisms. The high level of variability of copper in samples of the same origin could be due to its ability to incorporate the aphtitalite lattice [16], which certainly strongly varies with small fluctuations in the temperature conditions during the sublimate deposit [13].
7. Conclusions
We have just shown an example of the use of the (Y parameter correction method to study intermediate thickness volcanic samples. For sublimates deposited in silica tubes, PIXE gave useful results while classical methods such as XRF and AA were useless because of the small amount of available material. The concentration measurement of elements such as Cu. Pb and Tl is very important for volcanologic studies. As a matter of fact, the analysis of metallic elements in condensed volcanic incrustations and sublimates provides important information about magmatic and degassing processes: the mineralogical and geochemical composition of sublimates from Piton de la Fournaise indicates considerable fractionation phenomena occurring before the surface emission of the gaseous phase; volatile and refractory element composition of aphtitalites enables one to go further into details of the condensation mechanisms occurring in high temperature fumarolic vents.
Special acknowledgement is due to J.L. Cheminee and H. Delorme from French Volcanological Observatories (IPG, Paris) for collecting most of the samples and to A. Person (University Paris 6) for helpful collaboration. We are indebted to the Institut Interuniversitaire des Sciences Nucleaires (Belgium) and the Centre National de la Recherche Scientifique (France) for their financial support. V. GEOLOGICAL/ARCHAEOLOGICAL
APPLICATIONS
282
P. Aloupogiannis et al. / Analvsis of volcanic samples
References [l] P. Aloupogiannis, G. Robaye. I. Roelandts, G. Weber, J.M. Delbrouck-Habaru. J.P. Quisefit, Nucl. Instr. and Meth. B14 (1986) 297. [2] P. Aloupogiannis, G. Robaye. I. Roelandts and G. Weber, Nucl. Instr. and Meth. B22 (1987) 72. [3] J.L. Campbell, J.A. Cookson and H. Paul, Nucl. Instr. and Meth. 212 (1983) 427. (41 E. Clayton, D.D. Cohen. P. Duerden, Nucl. Instr. and Meth. 180 (1981) 541. [5] G. Lagarde. J. Cailleret and C. Heitz, Nucl. Instr. and Meth. 205 (1983) 545. [6] P. Aloupogiannis. G. Robaye, I. Roelandts. G. Weber. J.M. Delbrouck-Habaru, submitted to X-Ray Spectrometry. [7] K.B. Krauskopf, Econom. Geol. 52 (1957) 786. [8] R.B. Symonds. WI. Rose, M.H. Reed, F.E. Lichte, D.L. Finnegan Geochim. Cosmochim. Acta 51 (1987) 2083.
[9] J.P. Toutain, P. Aloupogiannis, H. Delorme. A. Person, P. Blanc and G. Robaye, J. Volcanol. Geotherm. Res. (in press). [lo] R.E. Stoiber and W.I. Rose. Geochim. Cosmochim. Acta. 38 (1974) 495. [ll] J.J. Naughton, V.A. Greenberg and R. Goguel, J. Volcanol. Geotherm. Res. 1 (1976) 149. [12] A. Bernard. Ph.D. Thesis, Universitt Libre de Bntxelles, Belgium (1985). [13] J.P. Toutain. Ph.D. Thesis, UniversitC Paris 6 (1987). [14] J.P. Quisefit. J.P. Toutain, G. Bergametti, M. Javoy, B. Cheynet and A. Person. submitted to Geochim. Cosmochim. Acta. [15] P. Aloupogiannis, Analusis 15 (1987) 347. [16] J.G. Angus and C.R. Davis, Mineral, Mag. 40 (1976) 481. [17] N. Oskarsson, J. Volcanol. Geotherm. Res.. 10 (1981) 93.
and Methods
in Physics
Research
ANALYSIS OF VOLCANIC SAMPLES BY a PARAMETER FOR CORRECTING PIXE MEASUREMENTS P. ALOUPOGIANNIS and G. WEBER ‘)*
ls2), J.P. TOUTAIN
277
B49 (1990) 277-282
3), G. ROBAYE
METHOD
ESTABLISHED
I), I. ROELANDTS
4), J.P. QUISEFIT
2’
” Institut de Physique Nuclhaire Expkimeniale, Vniversit6 de Li?ge B15. Sart Tilman, B 4GOOLiPge, Belgium ” Laboratoire de Physic+Chimie de I’Atmosphsre, (unite associke au CNRS no. 717). VniversitP Paris 7, 2 Place Jussieu, 75005 Paris, France. ” Osservatorio Vesuviano, Via Manzoni 249 80123, Napoli, Italy ” Institut de Gkologie, VniversitP de Li2ge, Lisge, Belgium
The cooling down of volcanic gases produces deposits of condensed mineral phases (sublimates, incrustations) at the volcano-atmosphere interface. The analysis of these deposits presents a high volcanological interest. The procedure of fractional condensation, used to obtain the samples, gives generally very small amounts of materials and this explains the scarcity of suitable analytical methods. Efforts to prepare samples for XRF or AAS analysis failed because of the presence of insoluble chemicals and because of the necessity to avoid heating the material. PIXE analysis proved to be successful by using intermediate thickness samples
and the new (Yparameter correction method for matrix effects. The analytical procedure is described and the results and their volcanological significance are given.
1. Introduction In two previous papers [1,2], we established a new method to correct PIXE data for matrix effects. The merit of this method lies in avoiding hypotheses about sample composition. This point constitutes the main advantage of this method compared with the others [3-51. All the necessary information to calculate the corrections is obtained experimentally. The method was tested by using international reference materials: geological, biological and ore samples [6]. We shall present the results of an application of this method concerning volcanic sublimates and incrustations. Sublimates are obtained by selective condensation of volcanic emanations in silica tubes. The amount of material obtained is small, for which our method using small intermediate thickness targets, is well suited. The surface emission of the volcanic gaseous phase leads to the deposit of condensed mineral phases around fumarolic vents. They mainly consist of various salts of numerous metallic elements (Na, K, Cu, Zn. As, Se, Fe, Si, Al, Ca, etc.). The presence of these elements in the high temperature volcanic fluid is in connection with selective distillation phenomena occurring at magma level [7,8] leading to their enrichment in condensed * Research associate search (Belgium). 0168-583X/90/$03.50 (North-Holland)
of the National
Fund
for Scientific
0 Elsevier Science Publishers
B.V
Re-
phases [9]. Hence the study of metallic element chemistry of volcanic condensed phases should provide useful information about the conditions controlling their distillation, their migration toward the surface and their deposit. Nevertheless, until now, most of the studies performed have dealt only with mineralogical and geochemical aspects of these deposits [lo-121. Indeed, few quantitative chemical data have been published, mainly due to the small amount of material usually available, and to the complex matrix of samples making most of the analytical techniques ineffective [13]. Two series of samples have been analysed according to our method: sublimates and incrustations. Sublimates are mineral phases condensed in a silica tube that canalize the gaseous phase escaping from the fumarole. This new sampling technique enables metallic elements to deposit during the cooling down of the gaseous phase in the tube, without perturbations due to the atmosphere (oxygen contamination, reactions with the wall-rock). Moreover, measurements performed in the tube during sampling gave the temperature of condensation of each mineral phase [12-141. Incrustations are minerals collected in natural deposit conditions. Our samples were collected around high temperature vents or in cracks occurring on the roof of lava tubes on the following volcanoes: Etna (Italy), Ardoukoba (Djibouti), Erta-Ale (Ethiopia) and Momotombo (Nicaragua). V. GEOLOGICAL/ARCHAEOLOGICAL
APPLICATIONS
P. Aloupogiannis et al. / AnaJwis of volcanic samples
278 2. Analytical procedure
Volcanic
The energy loss A E of the excitating proton beam is determined by the use of Rutherford scattering on a secondary carbon target and it is used together with a separate X-ray transmission measurement to calculate a so-called (Y parameter. This parameter allows one to calculate two matrix correction factors taking into account the variation of X-ray production cross section with the proton energy and the absorption of the produced X-rays on their way toward the Si(Li) detector. The theory of this new correction method can be found in our previous papers [1,2].
condensed
phases
Jy---(q
. 0 +
I 0
1
2
3
d
m Cmg/cm
5
3. Experimental procedure Fig. 2. Variation
The studied sublimates were collected during the S/08/85 eruption of Piton de la Foumaise volcano (Reunion Island, Indian Ocean) [9,13]. The sampling duration was 4 days. After sampling, the silica tube was cut into nine sections, 3 to 10 cm long. Sublimates deposited on the internal walls were collected mechanically. The sampling quantity was about 5 to 25 mg of material. for each section. The sample preparation technique has been fully described elsewhere [15]. It consists of a filtration on a nuclepore filter of a suspension in cyclohexane of a small amount of the powdered volcanic material. Two targets were prepared for each sample.
of the LLD, as a function masses.
of the superficial
The description of the method of measurement of the proton energy loss can be found elsewhere [l]. Fig 1 schematically shows the experimental setup. The X-rays produced by irradiating multielement pellets with a proton beam [6] are used to obtain the transmission coefficients through the samples.
4. Analytical considerations Two opposite problems have been encountered in the measurement of S and Cl in our samples: (a) In the case of silica tube sublimates the PIXE spectra showed very high Cl peaks with a small S component at their left side. In these conditions we could just estimate an upper limit of 2% for S concentrations. (b) In the case of incrustations, the very high S peaks allowed no Cl estimation. Our PIXE measurements have been performed by using intermediate thickness samples. Their superficial mass varied between 0.5 and 3.5 mg/cm2. The total matrix correction factor FC which depends on the detected element and on the sample thickness has an influence on the value of the detection limits LLD, (in ppm and for 1 PC). We have: LLD, = 3( B,)0~5FC,/0.001R,m.
I
Fig. 1. Experimental
setup for simultaneous energy loss measurements.
1
PIXE and proton
where B, is the number of counts in the background below the peak of element i. R, is the sensitivity in counts per ug and PC, and m is the superficial mass in mg/cm2. Fig. 2 illustrates the typical trend of this variation for 3 elements: Cr, Cu, and Fe, and shows
P. Aloupogiannis et al. / Ana!vsis
of volcanicsamples
219
Table 1 Analytical results for sublimates from Piton de la Foumaise (silica tubes). Concentrations are expressed in ppm (except for Cl, K, and Cu in %). The concentration values with an * are from NAA for comparison. S was generally present below 2% (see text). Co and As were often visible but below detection limit of about 30 ppm Sample
Element Cl
RI
WI
5356.5
K[I] 14
Cr +1.5
70+ 35
Fe
Cu
2OOk 100
0.05 +0.006
PI
Zn
Se
Rb
Pb
Tl
< 20
Cl2
3OOk 80 340* 410+ 50 4001 750+ 80 650* 14OOk250 1460* 1300&230 1210* 15OOk250 1120* 13OOk230 1110* 138Ok240 1580*
~65
-=z60
~65
i 60
1* R2
5917.2
14.6f1.5
R3
67k8.2
21
R4
64*7.8
22.4f2.3
R5
69k8.5
23.352.4
18+13
R6
73k8.9
23
18*
R7
71+8.7
22.4*2.3
17+
R8
67k8.2
22
32f13
that
as
k2.3
k2.2
4
120 + 30
0.05 *0.005
< 20
(15
100 f
50
0.21 kO.03
~15
110+
15
1
160?
25
0.98+0.10
< 20
3
120+
15
1.0350.09
< 20
3
250+
50
0.81+0.08
110* 12
4705
50
1.6350.15
30f15
as LLD, are concerned, the best is situated between 2 and 3.5 mg/cm2.
far
thickness
k2.2
19*
* 0.04
< 20 22+14
112 2* <12 1* 21* 3 39* 15Ok 20 110* 37Ok 50 388* 8005100 968* 38OOk500 4080*
70+
20
23Ok
30
150f
15
81Ok
75
830+
90
93Ok
80
400&150 ~65
~60
3500+550 426Ok
45
7600 k 800
target
5. Results Tables 1 and 2 give the results relative to the two series of volcanic condensates. The silica tube sublimates from the first series (Rl to R8) have also been analysed by using neutron activation (Laboratoire Pierre Sue, Paris). In these samples, only two elements (Se and Rb) detectable both by PIXE and NAA are present at concentrations above detection limits. The results of NAA measurements are given in table 1 for comparison. The agreement is very good.
6. Discussion 6.1. The silica tube sublimates from Piton de la Fournaise X-ray diffraction and SEM (scanning electron microscopy) show that the sublimates in the tubes mainly consist of Na, K and Cu chlorides deposited between 850 and 1OO’C. and a mixed K-Si fluoride deposited between 550 and 100°C [9,13]. Geochemical study of the sublimates, based on our PIXE measurements, shows that the elements have two different behaviours. Either they are enriched in the low temperature zone (Rb, Se, Cu. Pb, Tl) (fig. 3a), or they show little or no variation
Fig. 3. (a) Evolution of the concentrations of Cu. Pb and Tl with temperature. (b) Evolution of the concentrations of Cl and K with temperature.
V. GEOLOGICAL/ARCHAEOLOGICAL
APPLICATIONS
k2.1
11.9
f2.2
f1.7
f1.9
f1.9
f2.7
~~2.4
20.6
18.7
21.9
lb.7
18.6
17.8
26.1
23.2
21.1
4.54+
8.86 f 0.9
20.5 i 2.1
19.3 k2.0
22.8 k 2.3
23.3 * 2.4
20.3 +2.1
18.2+0.8
22.6 f2.4 22.bk 2.3
20.4 f 2.1
20.6 f2.1
17.8 + 1.8
EA 78/3
EA 78/4
EA 83/l
EA 83,‘2
EA 79/l
EA 85/2
EA 85/3
78/l
78/2
78/3
78/4
EA
AR
AR
AR
AR
AR
AR
20.4 f 2.1
86
MO
21.7 i 2.2
ER 57
21.4 f 2.2
78/b
15.7
14.0
+l.b
zt1.4
jz1.0
+1.1
11.1
22.0 f 2.2
78/5
10.2
+1.1
10.7
+1.0
10.2
19.7 k 2.0
0.4
20.5 i 2.1
EA 85/l
f2.2
fl.b
15.7
_
13.3 k 1.35
EA 78,‘2
80/l
t2.0
K PI
19.8
PI
22.6+2.3
S
Element
EA 78/l
Samples
for Aphtitalites.
(Momotombo).
results
MO
and
Ale),
2
Analytical
Table
2700i
2900f
14OOf
1600-t
1040+
1100*
1020f
53ooi1450
15OOf
~800
8OOi
1370+
750f
205Oi
1400*
930f
3OOOi
90f13
750
800
400
440
285
300
300
400
250
375
9
6
-z 20
< 20
3oi
75ilO
58+
49f
37+
< 18
4
8
7
54+7
<17
18k2
17+2
2553
115
<14
114
<17
< 20
< 17
<15
18+2
23k3
20+2
21*3
<15
<17
5
3
9
5
26+3
~15
Cr
(except
< 20
‘z 20
6
25f
< 20
~18
360 i 50
110+15
4Ok
8Oill
68klO
87k12
86+12
64k
3b+
9
in ppm
100*14
b3k
< 20
V
9
8
250 + 35
62f
< 35
441
651
58f
180*25
250 i 40
210
560
8
200+30
55?
365 k 50
Ti
are expressed
400
260
820
450
concentrations
1bOOf
Ca
The
44-t
100+11
19f
2
5
5
325 43*
6 4
SO+
5
9
9
46+
811
87+10
84f
7
130
10400 + 1050
60
40
410*
bOO+
30
270t
30
70
650+
280+
50
470f
2030 + 200
1970+200
2400 f 250
1250+
17bOi175
62i
1OlOf
100
20
760+
-z 20
~10
~10
cl0
~10
< 20
to.2
1.17*0.1
0.55 + 0.06
2.4
0.77 * 0.07
io.5
0.91 f 0.1
2.54 f 0.3
4.05 f 0.4
4.95 + 0.5
5.4
5.29 f 0.5
3.91 f0.4
1.12kO.l
30
b0
20
70
45
85
75
2900 f 350
2601
1040+110
560+
1280f135
122Oi130
1040+110
150f
1010+110
1000*110
660f
1420+150
400+
710f
0.06
0.59 k
525
3500+380
2400+260
4950;
4750+500
Zn
are as follows:
0.61 + 0.06
3.18kO.3
1.87kO.2
11.65il.l
2.77 k 0.3
21i61.14+0.1
< 20
< 20
< 20
i
c 20 c 20
20
7400 + 720
i
~20
labels
Cu @l
sample
< 20
Ni
The
4900 * 500
1800+180
470*
50
100
x7*10 8
9
15
970+
in 8).
2910+285
Fe
Cu
69i
115*15
83t
100*11
120+
117+13
110*12
Mll
for S, K. and
1570+280
30+5
< 20
< 20
112
-z 12
-z 12
<12
50 100
150 14001250
810+
540 -I_ 100
57OilOO
blOi110
580f
530 f 100
270+
17ooi310
< 12
310
1500 f 270
790 + 140
1750*
<12
145 730+130
x10*
<12
-x 12
22k4
x 12
< 12
1200+235
185Ok330
2140+375
16800*1600
275+
173Ok
1500f
1480+
1680+
16005
1450f
270+
1180-t
1200+
430*
25
160
150
140
160
150
140
25
110
120
45
90 750
95oi
70
8200+
730i
730+
19000~1800
12000+1100
70
ER (Ert
11200f1050
Pb
(Ardoukoba),
1850+320
Rb
AR
25+4
30f5
44+8
112
Se
EA (Etna),
P. Aloupogiannis et al. / Ana!vsis
with increasing temperature (Cl, K) (fig. 3b). It can be noted that among the first group, Tl exhibits the clearest trend as a function of decreasing temperature. In spite of its obvious volatile behaviour during magmatic processes [9], only two examples of high amounts of Tl in gaseous samples are known. Hence, a very rich Tl gaseous phase should be a significant characteristic of Piton de la Fournaise. According to SEM evidence, Tl is believed to be transported as a volatile chloride and condensed as a mixed Tl-K solid chloride. A volatile behaviour of lower intensity is observed, for lead, according to the decreasing Pb concentrations from R6 to R8 samples (fig. 3a). Cl and K concentrations are not highly temperature-dependent (fig. 3b). Since these elements have been present in the gaseous phase in very large quantities, they should be at saturation point in the system and decreasing temperature should have no effect on their condensation. A significant result compared with previous studies lies in the lack or low concentration of both S bearing minerals and constituents of the sulfides usually found in sublimates (S. Fe, Zn, As, Cr, Co). These mineralogical assemblages and chemical compositions are interpreted as the result of an early condensation of S as solid sulfides species in the central cone and in the natural lava tunnels. The above-mentioned elements should therefore be fractionated from the gaseous phase and integrated into the solid sulfides [9].
6.2. The incrustations All samples consist mainly of aphtitalite, which is a mixed Na-K sulfate commonly regarded as (K.Na), Na(SO,),, in admixture with various minor contents of thenardite (Na,SO,). Samples occur mostly as high size dendritic crystals and generally show yellowish to bluish tints. Table 2 shows that these samples are nearly exclusively constituted by volatile elements (S, K. Cu. Zn, Rb, Pb). On the contrary, refractory elements of metasomatic origin (Ca, Ti, V, Cr. Mn, Ni) are scarcely occurring, or lacking in the condensates. This shows that the cation input from the wall-rock, which is a common mechanism acting during the deposit of incrustations [lo], is not operating at the high temperature conditions allowing the growth of most of the aphtitalites. and that they clearly derive from gas-solid reactions. Nevertheless. these samples are undoubtedly of secondary origin, in accordance with their growth location on the field. Indeed, they cover the external part of the high temperature vents (fumaroles or cracks in lava tunnels), and thus they grow in highly oxidized conditions. This observation corroborates the previous studies dealing with aphtitalite genesis [16,17], and thermochemical simulations showing that Na and K are conveyed by the gaseous phase as gaseous chloride
ofc~olcamc samples
281
species, and deposited as solid chlorides or sulfates, respectively, in reduced or oxidized conditions [11,13.14]. The chemical composition of the sublimates also confirms this interpretation: the main minor and trace elements (Cu, Zn. Rb, Pb) are believed to be mobilized in the high temperature gaseous phase mainly as gaseous halides [13.14]. One can see that the samples collected at the same place and at the same time are roughly homogeneous with regard to their volatile element chemical composition (except copper which exhibits strong variations, even in samples of same origin). Sample AR l/78 is an exception with respect to other AR samples: it shows a low content of volatile elements and a high content of refractory ones. This indicates a probable participation of the wall-rock to the growth of sample AR l/78, obviously through acid alteration mechanisms. The high level of variability of copper in samples of the same origin could be due to its ability to incorporate the aphtitalite lattice [16], which certainly strongly varies with small fluctuations in the temperature conditions during the sublimate deposit [13].
7. Conclusions
We have just shown an example of the use of the (Y parameter correction method to study intermediate thickness volcanic samples. For sublimates deposited in silica tubes, PIXE gave useful results while classical methods such as XRF and AA were useless because of the small amount of available material. The concentration measurement of elements such as Cu. Pb and Tl is very important for volcanologic studies. As a matter of fact, the analysis of metallic elements in condensed volcanic incrustations and sublimates provides important information about magmatic and degassing processes: the mineralogical and geochemical composition of sublimates from Piton de la Fournaise indicates considerable fractionation phenomena occurring before the surface emission of the gaseous phase; volatile and refractory element composition of aphtitalites enables one to go further into details of the condensation mechanisms occurring in high temperature fumarolic vents.
Special acknowledgement is due to J.L. Cheminee and H. Delorme from French Volcanological Observatories (IPG, Paris) for collecting most of the samples and to A. Person (University Paris 6) for helpful collaboration. We are indebted to the Institut Interuniversitaire des Sciences Nucleaires (Belgium) and the Centre National de la Recherche Scientifique (France) for their financial support. V. GEOLOGICAL/ARCHAEOLOGICAL
APPLICATIONS
282
P. Aloupogiannis et al. / Analvsis of volcanic samples
References [l] P. Aloupogiannis, G. Robaye. I. Roelandts, G. Weber, J.M. Delbrouck-Habaru. J.P. Quisefit, Nucl. Instr. and Meth. B14 (1986) 297. [2] P. Aloupogiannis, G. Robaye. I. Roelandts and G. Weber, Nucl. Instr. and Meth. B22 (1987) 72. [3] J.L. Campbell, J.A. Cookson and H. Paul, Nucl. Instr. and Meth. 212 (1983) 427. (41 E. Clayton, D.D. Cohen. P. Duerden, Nucl. Instr. and Meth. 180 (1981) 541. [5] G. Lagarde. J. Cailleret and C. Heitz, Nucl. Instr. and Meth. 205 (1983) 545. [6] P. Aloupogiannis. G. Robaye, I. Roelandts. G. Weber. J.M. Delbrouck-Habaru, submitted to X-Ray Spectrometry. [7] K.B. Krauskopf, Econom. Geol. 52 (1957) 786. [8] R.B. Symonds. WI. Rose, M.H. Reed, F.E. Lichte, D.L. Finnegan Geochim. Cosmochim. Acta 51 (1987) 2083.
[9] J.P. Toutain, P. Aloupogiannis, H. Delorme. A. Person, P. Blanc and G. Robaye, J. Volcanol. Geotherm. Res. (in press). [lo] R.E. Stoiber and W.I. Rose. Geochim. Cosmochim. Acta. 38 (1974) 495. [ll] J.J. Naughton, V.A. Greenberg and R. Goguel, J. Volcanol. Geotherm. Res. 1 (1976) 149. [12] A. Bernard. Ph.D. Thesis, Universitt Libre de Bntxelles, Belgium (1985). [13] J.P. Toutain. Ph.D. Thesis, UniversitC Paris 6 (1987). [14] J.P. Quisefit. J.P. Toutain, G. Bergametti, M. Javoy, B. Cheynet and A. Person. submitted to Geochim. Cosmochim. Acta. [15] P. Aloupogiannis, Analusis 15 (1987) 347. [16] J.G. Angus and C.R. Davis, Mineral, Mag. 40 (1976) 481. [17] N. Oskarsson, J. Volcanol. Geotherm. Res.. 10 (1981) 93.