- Email: [email protected]
Investigation of the He solubility in H2O–CO2 bearing silicate liquids at moderate pressure: a new experimental method
Earth and Planetary Science Letters 181 (2000) 595^604 www.elsevier.com/locate/epsl
Investigation of the He solubility in H2O^CO2 bearing silicate liquids at moderate pressure: a new experimental method A. Paonita a , G. Gigli b , D. Gozzi b , P.M. Nuccio a;c , R. Trigila d a
Dipartimento di Chimica e Fisica della Terra ed Applicazione alle georisorse ed ai rischi naturali (CFTA), Universita© di Palermo, Via Archira¢ 36, 90123 Palermo, Italy b Dipartimento di Chimica, Universita© La Sapienza, P.le Aldo Moro, 5, I-00185 Roma, Italy c Istituto di Geochimica dei Fluidi, INGV, Via Ugo La Malfa, 153, 90123 Palermo, Italy d Dipartimento di Scienze della Terra, Universita© La Sapienza, P.le Aldo Moro, 5, I-00185 Roma, Italy Received 2 July 1999; received in revised form 1 March 2000; accepted 4 July 2000
Abstract We have designed the first available experimental method capable to investigate the solubility of inert gases in H2 O^ CO2 bearing silicate melts in a large range of pressures. The method overcomes the difficulties imposed by the physical state of volatiles at room conditions. Experiments were done by using an internally heated pressure vessel, where sealed capsules containing the sample are introduced. The peculiarity of the method consists in the capability of loading, in accurately known proportions (even lower than ppm), volatiles in a gaseous state at room conditions. Gas is loaded as a weighed amount of a gas-bearing glass, which was previously prepared by using the same gas as a pressure medium of a superliquidus experimental run. By analyzing the gas concentration in the prepared glass, the glass itself can be used as a gas source. We applied this method in order to investigate the helium solubility in silicate melts having dissolved volatile mixtures mostly composed of H2 O and CO2 , in a range of pressure suitable to provide useful information regarding pre-eruptive magma degassing. Our results indicate that helium solubility is strongly affected by the H2 O content in silicate liquid, displaying an increase by about a factor three as a consequence of the addition of 3 wt% H2 O to the melt. Taking into account the physical mechanism of He dissolution, the sites of the silicate melt structure, where noble gases can be accommodated, drastically increase by adding modest aliquots of H2 O. The influence of dissolved CO2 on helium solubility is not certain, although few signs would suggest an opposite effect with respect to H2 O, at least in basalt melts. On the basis of the obtained results, we could reasonably expect that the helium behavior during the degassing of H2 O^CO2 bearing magmas is sensibly different with respect to a H2 O^CO2 free silicate liquid. Therefore, the effect of the major volatiles on noble gas solubility in magmas should be taken into account in the investigation of several geological processes, such as magma genesis, ascent and degassing, or mantle evolution. ß 2000 Elsevier Science B.V. All rights reserved. Keywords: helium; solubility; silicate melts; pressure; experimental studies
1. Introduction The investigation of a wide range of Earth evolution processes (i.e. mantle degassing and atmos-
pheric evolution, magma genesis and migration) must also take into account the noble gas solubility in natural magmas. In magmatic systems, noble gases are always minor or trace constituents of
0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 2 1 5 - 6
EPSL 5570 5-9-00
596
A. Paonita et al. / Earth and Planetary Science Letters 181 (2000) 595^604
Table 1 Composition of the starting materials, expressed as wt% Rhyolite Basalt
SiO2
TiO2
Al2 O3
FeO
Fe2 O3
MnO
MgO
CaO
Na2 O
K2 O
73.43 48.49
0.11 0.77
13.42 12.5
1.67 6.01
0.03 5.41
0.07 0.21
0.01 8.64
0.86 12.48
4.07 2.2
4.95 2.08
Duplicate samples (PVL10, VL168/1) from Vulcano Island were used [34].
a volatile mixture usually dominated by H2 O and/ or CO2 . Nevertheless, the existing data on noble gas solubility refer to a single species or a noble gas mixture [1^12], and their application to mixed-volatile systems is uncertain because major volatiles dissolved in melts could a¡ect noble gas solubility. In high-pressure experiments, noble gas solubility has been studied by using noble gas as a pressure medium [5,7^9,13], but this technique cannot be adopted for H2 O^CO2 ^noble gas mixtures. Moreover, the Boettcher et al. [14] method, which loads the noble gas in its gaseous state into capsules, cannot be applied for our purposes seeing that the amount of noble gas to be loaded is too light (W1034 mg) and henceforth cannot be weighed. Nevertheless, in experiments with sealed capsules containing gas mixtures, our knowledge of both the charged quantity of noble gas and its concentration in the melt, is necessary to evaluate its partial pressure in the gas phase. Moreover, helium is di¤cult to con¢ne and could partially escape from the crimped capsule prior to sealing.
In order to investigate He solubility in H2 O^ CO2 -bearing magmas, we have developed a simple experimental technique for the preparation of sealed capsules containing rock powder as well as accurately known proportions of H2 O, CO2 and noble gas. We have used this method to study the helium solubility in basaltic and rhyolitic melts equilibrated with a £uid phase of H2 O^ CO2 ^He. 2. Experimental and analytical procedures 2.1. Preparation of a He-bearing glass A rhyolitic obsidian and trachy^basaltic scoriaceous lava, collected at Vulcano Island (Aeolian Islands, Italy), was used as starting material (Table 1). Grains 2^3 mm in size of each rock were accurately degassed for about 10 h at subsolidus temperature in a high vacuum line, while we monitored volatile loss by using a quadrupole
Table 2 Experimental conditions and results for He starting silicates
Rhyolite
Basalt
Sample
T (³C)
P (MPa)
t (h)
grain size (Wm)
Dissolved He (molar fraction)
Kh (MPa)
s1r a s1r b s1h s3 s2r s2h rip1 rip2 rip3 rip4
1090 1090 1090 1130 1180 1180 1180 1180 1180 1180
119 119 119 160 119 119 137 137 137 137
32 32 32 30 32 32 25 25 25 25
W1000 W1000 6 500 W1000 W1000 W1000 W1000 W1000 W1000 6 100
0.0195 0.0156 0.0123 0.0203 0.0089 0.0103 0.0099 0.0101 0.0114 0.0065
7123 8904 11303 9624 15440 13273 16282 15871 14151 24522
Kh = f/x, where f is the He fugacity in gas and x is the molar fraction of He in melt, calculated on 8-oxygen basis [35]. Grain size refers to grains used for the analysis. Powder loaded in IHPV had a grain size less than 10 Wm. Analyses of samples s1r and s2r were performed at 10 K/min heating rate. He content in s1r (Fig. 2) was computed both by integration of the total area in (sample s1ra), and by excluding the second peak (sample s1rb). Temperature and pressure were accurate to þ 5³C and þ 4 MPa respectively.
EPSL 5570 5-9-00
A. Paonita et al. / Earth and Planetary Science Letters 181 (2000) 595^604
mass spectrometer. About 100 mg of degassed and ¢nely powdered rock was loaded into Au75%^Pd25% alloy capsules (20 mm l, 3 mm d, 0.15 mm wall thickness) to prevent Fe2 loss. Unsealed capsules were pressurized at about 100 MPa and a superliquidus temperature in the internally heated pressure vessel (IHPV) apparatus, using He as a pressure medium in a Haskel two stage pneumatically driven gas booster. Experiments lasted 24^36 h and were terminated by an almost isobaric (2^4 MPa pressure increase) quench, with a rate of temperature decrease of 400³C/min. The He concentration of the obtained
597
He-bearing glass was subsequently determined by quadrupole mass spectrometry (QMS) (see Section 2.4). The temperature was measured with a Pt/Pt^10%Rh thermocouple in contact with the capsule, which had an accuracy of þ 5³C. The pressure was monitored by a Manganin Cell with an accuracy of þ 4 MPa. Run conditions for each sample are listed in Table 2. 2.2. Experiments with H2 O^CO2 ^He mixture Weighed amounts (on the order of 100 mg) of the degassed rock-powder were loaded into the
Table 3 Experimental conditions and results for H2 O+CO2 +He starting silicates
Rhyolite R20L R20M R20H R15L R15H R10L R10M R10H Basalt B20L B20M B20H B15M B15H B10L B10M
Total H2 O a (wt%)
CO2 liq.b H2 O CO2 gasb liq.b (wt%) (wt%) (molar fraction)
He liq.c QMS
(MPa) (h)
Total CO2 a (wt%)
(molar fraction 105 )
Kh Hee He gasd balance (molar (MPa) fraction 104 )
1140 1140 1140 1130 1130 1130 1130 1130
215 215 215 165 165 112 112 112
40 40 40 24 24 30 30 30
0.000 0.806 2.216 0.000 2.342 0.000 1.743 3.669
7.470 5.566 4.405 5.937 3.316 5.310 4.495 3.566
0.0E+00 3.0E-02 4.8E-02 0.0E+00 3.7E-02 0.0E+00 1.4E-02 2.1E-02
5.96 4.57 3.42 5.05 2.58 3.92 2.90 2.18
0.000 0.247 0.466 0.000 0.542 0.000 0.295 0.500
( þ 0.00) ( þ 0.05) ( þ 0.06) ( þ 0.00) ( þ 0.04) ( þ 0.00) ( þ 0.02) ( þ 0.02)
8.49 5.83 4.11 4.19 2.93 2.57 0.937 0.764
7.1 7.1 6.5 6.7 7.7 6.8 3.6 3.5
( þ 1.13) ( þ 1.01) ( þ 0.51) ( þ 1.46) ( þ 0.52) ( þ 0.83) ( þ 0.20) ( þ 0.12)
1788 2604 3401 2642 4327 2977 4297 5148
( þ 320) ( þ 413) ( þ 317) ( þ 616) ( þ 344) ( þ 407) ( þ 299) ( þ 232)
1160 1160 1160 1150 1150 1160 1160
215 215 215 165 165 112 112
40 40 40 24 24 30 30
0.000 0.669 2.255 0.819 1.801 0.000 0.508
6.213 4.825 3.878 4.262 2.986 4.436 3.241
0.0E+00 1.8E-02 2.9E-02 1.0E-02 1.6E-02 0.0E+00 3.8E-03
5.14 3.98 2.90 3.30 2.32 3.32 2.60
0.000 0.231 0.462 0.252 0.502 0.000 0.236
( þ 0.00) ( þ 0.04) ( þ 0.03) ( þ 0.03) ( þ 0.04) ( þ 0.00) ( þ 0.03)
3.07 2.57 1.98 0.855 1.37 3.82 2.16
5.1 ( þ 0.98) 5.2 ( þ 0.76) 5.1 ( þ 0.34) 3.0 ( þ 0.33) 6.5 ( þ 0.47) 15 ( þ 1.91) 12 ( þ 1.55)
3592 4353 5496 5754 7743 4395 6149
( þ 784) ( þ 709) ( þ 451) ( þ 715) ( þ 660) ( þ 626) ( þ 879)
T
P
(³C)
t
a
Uncertainties in the total loaded H2 O are within 2^3 wt% and they derive from a weighing error of about 0.02 mg. Since CO2 is loaded as Ag2 CO3 , the same weighting error gives a negligible e¡ect on the total loaded CO2 . b H2 O and CO2 dissolved in liquid (H2 O liq. and CO2 liq.) and CO2 in gas phase (CO2 gas) were recalculated by the already tested model of Papale [23], on the basis of their loaded amounts. Showed uncertainties in CO2 gas derive from propagation of a 5% error in dissolved H2 O (the average error in dissolved H2 O, obtained by testing the Papale model for our melt compositions [36]). The given error in total H2 O propagates sharply lower uncertainties in CO2 gas. A 10% error in dissolved CO2 will have a negligible e¡ect on the value of CO2 gas. c Molar fraction of He in glass (He liq.) after quenching, measured by QMS. Uncertainties are about 10%. d Molar fraction of He in gas phase (He gas), derived by mass balance between melt and gas phase. Showed uncertainties (12% average value) derive from the propagation, in mass balance, of the error given for the dissolved H2 O; instead, the error in dissolved CO2 had a negligible e¡ect. Uncertainties deriving from errors in the measurement of the dissolved He in the sample or in the He-bearing glass used for loading, are lower or similar. e PHe = Kh HeUxHe where PHe is the partial pressure of He in gas mixture and xHe is its molar fraction in melt, calculated on 8-oxygen basis of non-volatile species [35]. On the basis of very low He concentration in gas phase, we assume an ideal behavior of He in the H2 O^CO2 mixture; therefore Kh was computed by using He partial pressure in vapor. Showed uncertainties (12% average value) derive from the error in He gas.
EPSL 5570 5-9-00
598
A. Paonita et al. / Earth and Planetary Science Letters 181 (2000) 595^604
Au75%^Pd25% capsules and, subsequently, known quantities of H2 O, as deionized liquid, and CO2 , as silver carbonate, were added. A weighed amount of the previously prepared Hebearing glass (as grains of W1 mm size) was also loaded. The capsules were welded shut in order to con¢ne the gas mixture and they were pressurized by argon in the previously described IHPV apparatus; the run duration was 24^40 h at 1130^ 1160³C and a rapid quench was performed (see above). To check gas loss during experiments, capsules were weighed before and after each run by a high precision Sartorius microbalance (0.001 mg precision). Detailed run conditions are given in Table 3. 2.3. Attainment of equilibrium Several workers [3,4,7] put into evidence the importance of grain and sample size on the equilibration time. Small grain size implies short di¡usive walks of the gas atom within the melt, because gas always ¢lls grain interstices before melting. In this work, we have estimated that 24 h were more than enough for the attainment of equilibrium. This estimation was made possible by using the di¡usion coe¤cient (D) for He in basaltic [4] or in rhyolitic melts (extrapolated from [15]), as well as a critical evaluation of the run's duration and equilibrium time reported in literature at several temperatures and melt compositions ([4,5,7,9], for noble gases and [16^21] for H2 O and CO2 ). 2.4. He analysis QMS was employed for the He analysis of the run products coming from both He and H2 O^ CO2 ^He experiments. The apparatus [22] consists of a furnace including a graphite crucible, a tantalum resistance and a Pt^Rh thermocouple, connected to a high vacuum extraction line and a quadrupole mass spectrometer. Quenched glasses were crushed and bubble-free grains (size W1 mm) were selected by microscopic inspection. Prepared samples were inserted (from 5 to 40 mg for He and H2 O^CO2 ^He bearing glasses respectively) into a manipulator connected to the pre-
Fig. 1. He molar fractions analyzed in the same sample. Cases 1^3 concern analyses of bubble-free grains of W1 mm size. Data agree within the given error bar, with dispersion from the mean value (dashed line) within 10%. In case 4, a bubble-free grain was powdered to about 10 Wm, causing signi¢cant He loss.
evacuated line, where they remained at least 1 h in order to remove adsorbed gas. Two types of procedures were adopted: (a) the sample was let to drop from the manipulator while the crucible was already at the temperature necessary for complete silicate melting; or (b) the drop of the sample was done into a crucible which had a temperature of about 400³C and the He release was accomplished by increasing the temperature at 10³C/min. In both cases, the 4 He signal was continuously acquired in the multiple ion detection (MID) mode, obtaining a intensity vs. time curve, and the signal was recorded until the blank value was again reached. The integral of the curve was converted into He atoms by calibration with known air volumes. The error of analysis was within þ 10% (see Fig. 1). The two analytical procedures, for a bubble-free sample, are in accordance within the error bar (Table 2: samples s2r and s2h). According to Roselieb et al. [7], the He signal obtained for a bubble-containing sample by procedure b, showed a peak at high temperature (2 in Fig. 2), which was attributed to the decrepitude of the gas bubbles. 2.5. Gas inclusions in glasses The run products, microscopically inspected,
EPSL 5570 5-9-00
A. Paonita et al. / Earth and Planetary Science Letters 181 (2000) 595^604
599
pared to the total amount of dissolved helium (W1038 mol). Consequently, we generally used large grains for the He analyses, which were performed within a few hours after opening the capsules too. Anyway, future experiments will minimize problems due to bubble formation by ¢rst equilibrating melts at low He pressure and then raising that pressure to its ¢nal value, so as to cause the bubble collapse. 2.6. Helium loss during the procedure Fig. 2. QMS spectra of the released He from a basaltic glass (sample s1r). Heating rate 10 K/min. According to [7], we attribute the peak (b) to decrepitude of gas bubbles, which become mobile at temperature above glass transition, while peak (a) represents the dissolved gas. He content was computed by integration of the displayed area.
were found to be completely glassy. The He-bearing glasses showed dispersed gas bubbles (W100 Wm): following Roselieb et al. [7], it can be interpreted that these bubbles are not produced by exsolution during quenching, but rather represent primary bubbles originated in grain boundaries and cavities during melting of the powdered starting material. Such bubbles posed a severe problem in the evaluation of the actual dissolved gas. The powdering of glass below 100 Wm, which allows to entirely eliminate the gas inclusions [7], causes relevant He loss (see Section 2.6). In order to overcome this problem, we selected grains of He-bearing glass having a size of W1000 Wm. As concluded from optical inspection, grains of this size were generally bubble-free or contained bubbles 6 10 Wm. On the contrary, H2 O^CO2 ^He-bearing glass exhibits large semi-spherical bubbles in contact with the wall of the capsule. This suggests that the coalescence of bubbles has occurred because of the low viscosity of the liquid which is linked to the presence of dissolved water. Similarly to the experimental results of Blank et al. [21], we found rare entrapped vesicles smaller than 10 Wm, and therefore we neglected them as their helium contribution was insigni¢cant (W10313 mol) com-
Helium loss may potentially occur during several stages of the experimental procedure. Bubblefree grains of the He-bearing glass were analyzed both in their entirety and after their powdering, so as to reveal a signi¢cant loss of dissolved He during powdering (Fig. 1). The He loss which occurred from the grains during our procedure, could be roughly calculated by considering the He di¡usion coe¤cient (extrapolated from [15]) as well as the spherical shape of the grains. Seeing that we used grains approximately 1 mm in size, which were prepared 1 h or less before the analysis, we were able to compute that the helium loss from the He-bearing grains was within 5%. In preparing the capsules containing H2 O^ CO2 ^He, we made W1 mm grains of He-bearing glass just before their loading, selecting the bubble-free ones. The entire procedure for each sample lasted at most 2 h, implying that the computed He loss before the capsule sealing was estimated at less than 10% by using the previously mentioned calculation. Considering the larger size of grains of the H2 O^CO2 ^He containing glass (see Section 2.5), we were able to perform the He analyses within a few hours after the capsules were opened. 2.7. Determination of the He solubility in H2 O^CO2 bearing melts The aliquots of H2 O and CO2 which were loaded into capsules (gTH2 O e gTCO2 ), were converted in weight percentages by using the relations:
EPSL 5570 5-9-00
600
A. Paonita et al. / Earth and Planetary Science Letters 181 (2000) 595^604
wt%H2 O T
gTH2 O
wt%TCO2
gTH2 O 100; pgTCO2 ps
gTCO2 100 gTH2 O gTCO2 ps
1
where ps is the weight of loaded rock. A corresponding wt% of dissolved volatiles was obtained through calculation using a proper thermodynamic model of solubility of H2 O^CO2 mixtures in silicate melts [23] at experimental pressure and temperature. The result obtained by subtracting the dissolved volatile from the loaded one furnished us the amount of H2 O and CO2 existing in the gas phase. Consequently, the CO2 molar fraction in vapor was computed by the relation : yCO2
gCO2 =MWCO2 gCO2 =MWCO2 gH2 O =MW H2 O
2
where g and MW are the weight of volatile in gas phase and its molecular weight, respectively. The weight and helium concentration of Hebearing glass permitted us to know the total number of He moles existing in a capsule. Moreover, the number of He mol dissolved in melt (mHe ) was subsequently obtained by analyzing the dissolved He (xHe ) in the H2 O^CO2 bearing melt: mHe mH2 O mCO2 MxHe
The amount of helium loaded was decided in order to obtain its molar fraction of about 0.001^ 0.0005 in the gas phase; this implied the use of 1^ 4 mg of the He-bearing glass. So, the H2 O^CO2 silicate system was not signi¢cantly perturbed by the presence of a small amount of He, and we were able to study the He partitioning under physico-chemical conditions very similar to those of the majority of natural magmatic systems. We also evaluated the possible amount of helium due to the presence of air in the sealed capsule, so concluding that it could be neglected (atmospheric He was at least 104 times lower than the quantity loaded by He bearing glass). 3. Results and discussion In the absence of other volatiles, He solubility sharply increases from basaltic to rhyolitic melts (Fig. 3), in agreement with the solubility model of Carroll and Stolper [9] which is based on the concept of ionic porosity. A sharp increase of the He solubility, with respect to anhydrous melts, is caused by the presence of dissolved H2 O (Figs. 4 and 5). Nevertheless, it seems that a further increase of dissolved H2 O from 3 up to 6 wt%
3
where mH2 O and mCO2 are the amount (mol) of the two volatiles in the melt, while M is the total amount loaded (mol) of silicate rock. The result obtained by subtracting the dissolved from the loaded amount furnished us the He (mol) existing in gas phase at equilibrium ; therefore He molar fraction (yHe ) in vapor was computed using the equation : Y He
mvHe mvCO2
mvH2 O
where mv represents the amount in vapor phase (mol). Then, the value of the Henry constant was obtained by knowing both yHe and xHe , (see Table 3).
Fig. 3. Dissolved He vs. He fugacity for rhyolitic and basaltic melt. Molar fraction of He in melt is computed on 8-oxygen basis (see Table 3), while He fugacity is obtained by using a Redlich^Kwong equation of state with (a) and (b) parameters calculated by the He critical constants given in Weast et al. [37]. Lines represent solubility predicted by the ionic porosity model [9].
EPSL 5570 5-9-00
A. Paonita et al. / Earth and Planetary Science Letters 181 (2000) 595^604
(PH2 O from 100^200 MPa) causes a decline in the initial rate of increment of the He solubility, both in rhyolitic and basaltic melts. Taking into account the physical mechanism of He dissolution ([23] and reference therein), the sites where noble gases can be accommodated drastically increase by adding modest quantities of H2 O to a melt, reaching a maximum value almost unmodi¢ed by the further addition of H2 O. This result could depend on the solubility mechanism of H2 O above 2^3 wt% in melt: in such conditions, dissolved molecular H2 O becomes signi¢cant and it occupies the free spaces in the silicate structure ([19,24] and their reference, [25,26]). Moreover, when CO2 is present in the gas phase (Figs. 4 and 5), the He solubility appears to decline in accordance with the decrease of dissolved H2 O from 6 to 2^3 wt% due to the CO2 partitioning into vapor at constant Ptot . Nevertheless, a lower He solubility is exhibited by an H2 O^ CO2 -bearing basalt with respect to an H2 O-bearing basalt having a similar content of dissolved H2 O (see Fig. 5). This ¢nding suggest that the dissolved CO2 would have an opposite e¡ect with respect to H2 O on He solubility, although the very low CO2 concentration in the melt likely makes its ultimate e¡ect di¤cult to detect. How-
Fig. 4. He solubility (as Kh , see Table 3) vs. dissolved water in a rhyolitic liquid which contains H2 O or H2 O and CO2 . Composition of gas phase is approximately indicated as CO2 molar fraction (XCO2 g). Samples with the same composition of gas phase, have been performed at di¡erent total pressures (see Table 3).
601
Fig. 5. He solubility vs. dissolved water in a basaltic liquid which contains H2 O or H2 O and CO2 . Samples (a) and (b) have the same aliquot of dissolved H2 O, but (a) is CO2 -free and (b) contains 100 ppm of dissolved CO2 . Therefore, they would suggest the He solubility somewhat decreases by adding CO2 to basalt melt.
ever, only further He solubility data, concerning the CO2 ^He-silicate system, could con¢rm this conclusion. Analogous to the described H2 O in£uence on the helium solubility, we argue that this volatile very probably a¡ects the solubility of all the noble gases. The design of our experimental method obviously makes it possible to investigate solubility of any inert gases in H2 O^CO2 bearing melts. Moreover, given the lower di¡usivity of heavier noble gases with respect to helium [4,8,27], the noble gas bearing glass could be powdered before analysis or loading, in order to avoid the microscopic check of grains for bubble absence. Our data show the helium solubility in H2 O rich melts can also be one order of magnitude higher than that of CO2 [23] in the same melt (i.e. a 5 wt% H2 O basalt with respect to a dry basalt). By using the fractionation equations of Jambon et al. [3], we can roughly evaluate how such di¡erences of solubility modify the He behavior throughout magma degassing (Fig. 6). Although this is only a preliminary computation, nevertheless predictions at low melt vesicularity should result nearly indicative. In fact, more accurate results should be achieved taking into account the variation of the noble gas solubility
EPSL 5570 5-9-00
602
A. Paonita et al. / Earth and Planetary Science Letters 181 (2000) 595^604
Fig. 6. Predicted variations of the CO2 /He ratio in basalt as a function of magma vesicularity. Calculations for dry melt were performed by using equations of Jambon et al. [3] for closed degassing system. Because the small CO2 e¡ect on He solubility, the same curve can be practically assumed for a CO2 -rich melt. Curve for 5 wt% H2 O bearing magma has been calculated by using, in the same equation, the CO2 solubility for the H2 O^CO2 bearing basalt [23] and the He solubility from this work. The initial CO2 /He ratio was within the range for MORB basalts.
caused by the degassing of H2 O from the magma. During a gas fractionation from an H2 O-rich magma, He will be scarcely released from the melt until the less soluble CO2 is almost entirely degassed. Therefore, a higher decrease of the CO2 /He ratio during the degassing can be computed for a H2 O rich melt compared to a dry melt (Fig. 6). On the other hand, the He solubility of a CO2 rich dry melt is probably similar to one found in the system He-silicate melt, which implies that the helium solubility is 2^3 times higher than that of CO2 . Hence, CO2 -rich dry magmas exhibit a less variable CO2 /He ratio (Fig. 6), which decreases at more advanced degassing stages, when dissolved CO2 ¢nally approaches very low contents and then He can be largely degassed from the silicate melt. Therefore, during protracted degassing, CO2 -rich anhydrous melts retain the dissolved helium more e¤ciently than H2 O-rich magmas. The evidence resulting from the discussion above, is that more helium generally remains in a natural H2 O-bearing magma when it degasses
its vapor phase. Therefore, the residual fraction of helium in natural melts, which is computed by the magma vesicularity [28], could be strongly underestimated using the solubility data of He-melt systems. Finally, the He solubility in a silicate melt is also used when estimating the ascent of magma towards the surface and in evaluating volcanic hazard [29]; then, the increase of He solubility, linked to the dissolved water, a¡ects such estimations too. In particular, the computed value of magma ascent would become up to four times higher when considering the e¡ect of H2 O. In the view of the latter, an underestimation of volcanic hazard could derive when using the He solubility of the He-melt system. 4. Conclusions We propose the ¢rst available experimental method to investigate the solubility of any inert gas in H2 O^CO2 bearing magma. This method allows to experimentally reproduce chemical conditions very similar to natural ones, where inert gas is only a tracer constituent of a H2 O^CO2 dominated vapor phase. We applied this technique in order to estimate the helium solubility in a rhyolite and basalt which contained H2 O or H2 O and CO2 . As a result, a positive dependence of the helium solubility in regard to the dissolved H2 O has been demonstrated by our experiments. In particular, the He solubility appears to increase by about a factor three when adding 3 wt% of H2 O to a melt. Evidently, a modest addition of H2 O to silicate liquid modi¢es the melt network so as to create a great number of new sites, into which noble gas atoms can enter. The breaking of Si^O^Si bonds and melt depolymerization by water addition [24^26] could qualitatively explain the above e¡ect, because it would render the silicate melt closer to a sphere packing than to a structure of silica-polymer chains. Surely, more extensive information concerning the structural e¡ect of water dissolution in a silicate liquid will be needed in order to better understand this behavior. Finally, the in£uence of CO2 on helium solubility has not been clearly recognized, how-
EPSL 5570 5-9-00
A. Paonita et al. / Earth and Planetary Science Letters 181 (2000) 595^604
ever, suspicion exist that the e¡ect of CO2 can be opposite with respect to H2 O in basaltic melts. We also argued on the expected modi¢cations of helium behavior during the fractionations of a vapor phase from H2 O^CO2 bearing melts with respect to noble gas-silicate melt systems. Our preliminary computations put into evidence that the e¡ect of H2 O on helium solubility signi¢cantly in£uences the behavior of this noble gas during magma degassing. Similar results should be expected for other noble gases. The processes of gas fractionation in magmatic systems which contain H2 O and/or CO2 , concern the majority of large scale geological processes, such as geochemical cycling, mantle evolution, Earth degassing and formation of the atmosphere [7]. For these last ones, noble gases and their solubility in silicate melt provide precious insights [28,30,31]. In particular, helium is the unique tracer of the exhalation of primordial volatiles from the mantle, while noble gases generally provide secular information on their Earth reservoirs. Geological environments of magma genesis and evolution are often constrained on the basis of the initial CO2 /He ratio of the melt, which can be computed by both vesicularity and the residual CO2 /He ratio of the melt itself. The same ratio or ratios among noble gases are often used to provide the degassing extent and modalities of vesiculated MORB glasses. Moreover, noble gas fractionation has acquired great relevance in the study of pre-eruptive degassing of shallow magmatic bodies for the estimation of volcanic hazard [29]. As noble gas solubility in melts has been largely used in the study of all these processes [28^33], the geochemical impact of our results on the previous arguments becomes particularly relevant. Consequently, we hope this research may provide the tool in acquiring further experimental data on noble gas solubility in H2 O^CO2 bearing silicate melts, in order to describe magmatic systems with parameters more adequate to true natural conditions. Acknowledgements This work is part of the Ph.D. thesis of Dr.
603
A.P., supported by the European Social Fund. We appreciated useful comments and suggestions from Prof. Mike Carroll and another anonymous reviewer. We also are grateful to Eng. L. Puxeddu for his assistance during high pressure-high temperature experiments. Dr. A.P. would also like to thank Dr. Tatiana Terranova for the linguistic revisions done to this article.[FA] References [1] J.F. Shackelford, P.L. Studt, R.M. Fulrath, Solubility of gases in glass II. He, Ne, and H2 in fused silica, J. Appl. Phys. 43 (1972) 1619^1626. [2] A. Hayatsu, C.E. Waboso, The solubility of rare gases in silicate melts and implications for K^Ar dating, in: F.A. Podosek (guest Ed.), Terrestrial Noble Gases, Chem. Geol. 52 (special issue) (1985) 97^102. [3] A. Jambon, H.W. Weber, O. Braun, Solubility of He, Ne, Ar, Kr, Xe in a basalt melt in the range of 1250^1600³C: Geochemical implications, Geochim. Cosmochim. Acta 50 (1986) 401^408. [4] G. Lux, The behavior of noble gases in silicate liquids: solution, di¡usion, bubbles, and surface e¡ects, with applications to natural samples, Geochim. Cosmochim. Acta 51 (1987) 1549^1560. [5] B.S. White, M. Brearley, A. Montana, Solubility of Argon in silicate liquids at high pressures, Am. Mineral. 74 (1989) 513^529. [6] C.L. Broadhurst, M.J. Drake, B.E. Hagee, T.J. Bernatowicz, Solubility and partitioning of Ne, Ar, Kr, and Xe in minerals and synthetic basalt melts, Geochim. Cosmochim. Acta 56 (1992) 709^723. [7] K. Roselieb, W. Rammensee, H. Buttner, M. Rosenhauer, Solubility and di¡usion of noble gases in vitreous albite, Chem. Geol. 96 (1992) 241^266. [8] M.R. Carroll, E.M. Stolper, Argon solubility and di¡usion in silica glass: implications for the solution behavior of molecular gases, Geochim. Cosmochim. Acta 55 (1991) 211^225. [9] M.R. Carroll, E.M. Stolper, Noble gas solubilities in silicate melts and glasses: new experimental results for Ar and the relationship between solubility and ionic porosity, Geochim. Cosmochim. Acta 57 (1993) 5039^5051. [10] D.S. Draper, M.R. Carroll, Argon di¡usion and solubility in silicic glasses exposed to an Ar^He mixtures, Earth. Planet. Sci. Lett. 132 (1995) 15^24. [11] T. Shibata, E. Takahashi, J. Matsuda, Noble gas solubility in binary CaO^SiO2 system, Geophys. Res. Lett. 23 (1996) 3139^3142. [12] T. Shibata, E. Takahashi, J. Matsuda, Solubility of neon, argon, kripton and xenon in binary and ternary silicate system: a new view on noble gas solubility, Geochim. Cosmochim. Acta 62 (1998) 1241^1253.
EPSL 5570 5-9-00
604
A. Paonita et al. / Earth and Planetary Science Letters 181 (2000) 595^604
[13] J.E. Shelby, Pressure dependence of helium and neon in vitreous silica, J. Appl. Phys. 47 (1976) 135^139. [14] S.L. Boettcher, Q. Guo, A. Montana, A simple device for loading gases in high-pressure experiments, Am. Mineral. 74 (1989) 1383^1384. [15] A. Jambon, J.E. Shelby, Helium di¡usion and solubility in obsidians and basaltic glasses in the range 200^300³C, Earth Planet. Sci. Lett. 51 (1980) 206^214. [16] B.O. Mysen, R.J. Arculus, D.H. Eggler, Solubility of carbon dioxide in melts of andesite, tholeiite, and olivine nephelinite composition to 30 kbar, Contrib. Mineral. Petrol. 53 (1975) 227^239. [17] E.M. Stolper, G.J. Fine, T. Johnson, S. Newman, The solubility of carbon dioxide in albitic melt, Am. Mineral. 72 (1987) 1071^1085. [18] L.A. Silver, P.D. Ihinger, E. Stolper, The in£uence of bulk composition on the speciation of water in silicate glasses, Contrib. Min. Petrol. 104 (1990) 142^162. [19] J.E. Dixon, E.M. Stolper, J.R. Holloway, An experimental study of water and carbon dioxide solubilities in midOcean Ridge basaltic liquids. Part I: calibration and solubility models, J. Petrol. 36 (1995) 1607^1631. [20] G. Moore, T. Vennemann, I.S.E. Carmichael, An empirical model for the solubility of H2 O in magmas to 3 kilobars, Am. Mineral. 83 (1998) 36^42. [21] J.G. Blank, E.M. Stolper, M.R. Carroll, Solubilities of carbon dioxide and water in rhyolitic melts at 850³C and 750 bar, Earth Planet. Sci. Lett. 119 (1993) 27^36. [22] D. Gozzi, F. Cellucci, P.L. Cignini, G. Gigli, M. Tomellini, E. Cibsani, S. Frullani, G.M. Urciuoli, X-ray, Heat Excess and 4 He in the Pd/D System, J. Electroanal. Chem. 435 (1997) 113^136, erratum, 452 (1998) 251^271. [23] P. Papale, Modeling of the solubility of a two component H2 O+CO2 £uid in silicate liquids, Am. Mineral. 84 (1999) 477^492. [24] J.R. Holloway, J.G. Blank, Experimental results applied to C^O^H in natural melts, in: volatiles in magmas, Rev. Mineral. 30 (1994) 231^279. [25] M. Nowak, H. Behrens, The speciation of water in haplogranitic glasses and melts determined by in situ nearinfrared spectroscopy, Geochim. Cosmochim. Acta 59 (1995) 3445^3450.
[26] A. Shen, H. Keppler, Infrared spectroscopy of hydrous silicate melts to 1000³C and 10 kbar: direct observation of H2 O speciation in a diamond-anvil cell, Am. Mineral. 80 (1995) 1335^1338. [27] E.B. Watson, Di¡usion in volatile-bearing magmas, in: volatiles in magmas, Rev. Mineral. 30 (1994) 371^412. [28] M.R. Carroll, J.D. Webster, Solubilities of sulfur, noble gases, nitrogen, chlorine, and £uorine in magmas, in: volatiles in magmas, Rev. Mineral. 30 (1994) 231^279. [29] P.M. Nuccio, M. Valenza, Magma degassing and geochemical detection of its ascent, in: G.B. Arehart, J.R. Hulston (Eds.), Water^Rock Interaction, Balkema (1998) 475^478. [30] C.J. Allegre, T. Staudacher, P. Sarda, Rare gas systematics: formation of the atmosphere, evolution and structure of the Earth's mantle, Earth Planet. Sci. Lett. 81 (1986) 127^150. [31] A. Jambon, Earth degassing and Large scale geochemical cycling of volatile elements, in: volatiles in magmas, Rev. Mineral. 30 (1994) 479^510. [32] B.E. Taylor, Magmatic volatiles: isotopic variations of C, H, and S, Rev. Mineral. 16 (1986) 185^225. [33] W.F. Giggenbach, Chemical composition of volcanic gases, in: R. Scarpa, R.I. Tilling (Eds.), Monitoring and Mitigation of Volcanic Hazards, Springer, Berlin (1996) 221^256. [34] G. De Astis, L. La Volpe, A. Peccerillo, L. Civetta, Evoluzione vulcanologica e magmatologica dell'isola di Vulcano, in: L. La Volpe, Dellino, P.M. Nuccio, E. Privitera, A. Sbrana (Eds.), Progetto Vulcano, Risultati dell'attivita© di ricerca 1993^1995 Felici Editore, Pisa (1997) 155^177. [35] M.S. Ghiorso, I.S.E. Carmichael, M.L. Rivers, R.O. Sack, The Gibbs Free Energy of mixing of natural silicate liquids; an expanded regular solution approximation for the calculation of magmatic intensive variables, Contrib. Mineral. Petrol. 84 (1983) 107^145. [36] A. Paonita, Magma degassing of multicomponent gas mixtures: thermodynamic modeling and applications, Ph.D. Thesis, Universita© di Palermo, Palermo (1999) 161. [37] R.C. Weast, J.A. Melvin, W.H. Beyer, CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1985.
EPSL 5570 5-9-00
Investigation of the He solubility in H2O^CO2 bearing silicate liquids at moderate pressure: a new experimental method A. Paonita a , G. Gigli b , D. Gozzi b , P.M. Nuccio a;c , R. Trigila d a
Dipartimento di Chimica e Fisica della Terra ed Applicazione alle georisorse ed ai rischi naturali (CFTA), Universita© di Palermo, Via Archira¢ 36, 90123 Palermo, Italy b Dipartimento di Chimica, Universita© La Sapienza, P.le Aldo Moro, 5, I-00185 Roma, Italy c Istituto di Geochimica dei Fluidi, INGV, Via Ugo La Malfa, 153, 90123 Palermo, Italy d Dipartimento di Scienze della Terra, Universita© La Sapienza, P.le Aldo Moro, 5, I-00185 Roma, Italy Received 2 July 1999; received in revised form 1 March 2000; accepted 4 July 2000
Abstract We have designed the first available experimental method capable to investigate the solubility of inert gases in H2 O^ CO2 bearing silicate melts in a large range of pressures. The method overcomes the difficulties imposed by the physical state of volatiles at room conditions. Experiments were done by using an internally heated pressure vessel, where sealed capsules containing the sample are introduced. The peculiarity of the method consists in the capability of loading, in accurately known proportions (even lower than ppm), volatiles in a gaseous state at room conditions. Gas is loaded as a weighed amount of a gas-bearing glass, which was previously prepared by using the same gas as a pressure medium of a superliquidus experimental run. By analyzing the gas concentration in the prepared glass, the glass itself can be used as a gas source. We applied this method in order to investigate the helium solubility in silicate melts having dissolved volatile mixtures mostly composed of H2 O and CO2 , in a range of pressure suitable to provide useful information regarding pre-eruptive magma degassing. Our results indicate that helium solubility is strongly affected by the H2 O content in silicate liquid, displaying an increase by about a factor three as a consequence of the addition of 3 wt% H2 O to the melt. Taking into account the physical mechanism of He dissolution, the sites of the silicate melt structure, where noble gases can be accommodated, drastically increase by adding modest aliquots of H2 O. The influence of dissolved CO2 on helium solubility is not certain, although few signs would suggest an opposite effect with respect to H2 O, at least in basalt melts. On the basis of the obtained results, we could reasonably expect that the helium behavior during the degassing of H2 O^CO2 bearing magmas is sensibly different with respect to a H2 O^CO2 free silicate liquid. Therefore, the effect of the major volatiles on noble gas solubility in magmas should be taken into account in the investigation of several geological processes, such as magma genesis, ascent and degassing, or mantle evolution. ß 2000 Elsevier Science B.V. All rights reserved. Keywords: helium; solubility; silicate melts; pressure; experimental studies
1. Introduction The investigation of a wide range of Earth evolution processes (i.e. mantle degassing and atmos-
pheric evolution, magma genesis and migration) must also take into account the noble gas solubility in natural magmas. In magmatic systems, noble gases are always minor or trace constituents of
0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 2 1 5 - 6
EPSL 5570 5-9-00
596
A. Paonita et al. / Earth and Planetary Science Letters 181 (2000) 595^604
Table 1 Composition of the starting materials, expressed as wt% Rhyolite Basalt
SiO2
TiO2
Al2 O3
FeO
Fe2 O3
MnO
MgO
CaO
Na2 O
K2 O
73.43 48.49
0.11 0.77
13.42 12.5
1.67 6.01
0.03 5.41
0.07 0.21
0.01 8.64
0.86 12.48
4.07 2.2
4.95 2.08
Duplicate samples (PVL10, VL168/1) from Vulcano Island were used [34].
a volatile mixture usually dominated by H2 O and/ or CO2 . Nevertheless, the existing data on noble gas solubility refer to a single species or a noble gas mixture [1^12], and their application to mixed-volatile systems is uncertain because major volatiles dissolved in melts could a¡ect noble gas solubility. In high-pressure experiments, noble gas solubility has been studied by using noble gas as a pressure medium [5,7^9,13], but this technique cannot be adopted for H2 O^CO2 ^noble gas mixtures. Moreover, the Boettcher et al. [14] method, which loads the noble gas in its gaseous state into capsules, cannot be applied for our purposes seeing that the amount of noble gas to be loaded is too light (W1034 mg) and henceforth cannot be weighed. Nevertheless, in experiments with sealed capsules containing gas mixtures, our knowledge of both the charged quantity of noble gas and its concentration in the melt, is necessary to evaluate its partial pressure in the gas phase. Moreover, helium is di¤cult to con¢ne and could partially escape from the crimped capsule prior to sealing.
In order to investigate He solubility in H2 O^ CO2 -bearing magmas, we have developed a simple experimental technique for the preparation of sealed capsules containing rock powder as well as accurately known proportions of H2 O, CO2 and noble gas. We have used this method to study the helium solubility in basaltic and rhyolitic melts equilibrated with a £uid phase of H2 O^ CO2 ^He. 2. Experimental and analytical procedures 2.1. Preparation of a He-bearing glass A rhyolitic obsidian and trachy^basaltic scoriaceous lava, collected at Vulcano Island (Aeolian Islands, Italy), was used as starting material (Table 1). Grains 2^3 mm in size of each rock were accurately degassed for about 10 h at subsolidus temperature in a high vacuum line, while we monitored volatile loss by using a quadrupole
Table 2 Experimental conditions and results for He starting silicates
Rhyolite
Basalt
Sample
T (³C)
P (MPa)
t (h)
grain size (Wm)
Dissolved He (molar fraction)
Kh (MPa)
s1r a s1r b s1h s3 s2r s2h rip1 rip2 rip3 rip4
1090 1090 1090 1130 1180 1180 1180 1180 1180 1180
119 119 119 160 119 119 137 137 137 137
32 32 32 30 32 32 25 25 25 25
W1000 W1000 6 500 W1000 W1000 W1000 W1000 W1000 W1000 6 100
0.0195 0.0156 0.0123 0.0203 0.0089 0.0103 0.0099 0.0101 0.0114 0.0065
7123 8904 11303 9624 15440 13273 16282 15871 14151 24522
Kh = f/x, where f is the He fugacity in gas and x is the molar fraction of He in melt, calculated on 8-oxygen basis [35]. Grain size refers to grains used for the analysis. Powder loaded in IHPV had a grain size less than 10 Wm. Analyses of samples s1r and s2r were performed at 10 K/min heating rate. He content in s1r (Fig. 2) was computed both by integration of the total area in (sample s1ra), and by excluding the second peak (sample s1rb). Temperature and pressure were accurate to þ 5³C and þ 4 MPa respectively.
EPSL 5570 5-9-00
A. Paonita et al. / Earth and Planetary Science Letters 181 (2000) 595^604
mass spectrometer. About 100 mg of degassed and ¢nely powdered rock was loaded into Au75%^Pd25% alloy capsules (20 mm l, 3 mm d, 0.15 mm wall thickness) to prevent Fe2 loss. Unsealed capsules were pressurized at about 100 MPa and a superliquidus temperature in the internally heated pressure vessel (IHPV) apparatus, using He as a pressure medium in a Haskel two stage pneumatically driven gas booster. Experiments lasted 24^36 h and were terminated by an almost isobaric (2^4 MPa pressure increase) quench, with a rate of temperature decrease of 400³C/min. The He concentration of the obtained
597
He-bearing glass was subsequently determined by quadrupole mass spectrometry (QMS) (see Section 2.4). The temperature was measured with a Pt/Pt^10%Rh thermocouple in contact with the capsule, which had an accuracy of þ 5³C. The pressure was monitored by a Manganin Cell with an accuracy of þ 4 MPa. Run conditions for each sample are listed in Table 2. 2.2. Experiments with H2 O^CO2 ^He mixture Weighed amounts (on the order of 100 mg) of the degassed rock-powder were loaded into the
Table 3 Experimental conditions and results for H2 O+CO2 +He starting silicates
Rhyolite R20L R20M R20H R15L R15H R10L R10M R10H Basalt B20L B20M B20H B15M B15H B10L B10M
Total H2 O a (wt%)
CO2 liq.b H2 O CO2 gasb liq.b (wt%) (wt%) (molar fraction)
He liq.c QMS
(MPa) (h)
Total CO2 a (wt%)
(molar fraction 105 )
Kh Hee He gasd balance (molar (MPa) fraction 104 )
1140 1140 1140 1130 1130 1130 1130 1130
215 215 215 165 165 112 112 112
40 40 40 24 24 30 30 30
0.000 0.806 2.216 0.000 2.342 0.000 1.743 3.669
7.470 5.566 4.405 5.937 3.316 5.310 4.495 3.566
0.0E+00 3.0E-02 4.8E-02 0.0E+00 3.7E-02 0.0E+00 1.4E-02 2.1E-02
5.96 4.57 3.42 5.05 2.58 3.92 2.90 2.18
0.000 0.247 0.466 0.000 0.542 0.000 0.295 0.500
( þ 0.00) ( þ 0.05) ( þ 0.06) ( þ 0.00) ( þ 0.04) ( þ 0.00) ( þ 0.02) ( þ 0.02)
8.49 5.83 4.11 4.19 2.93 2.57 0.937 0.764
7.1 7.1 6.5 6.7 7.7 6.8 3.6 3.5
( þ 1.13) ( þ 1.01) ( þ 0.51) ( þ 1.46) ( þ 0.52) ( þ 0.83) ( þ 0.20) ( þ 0.12)
1788 2604 3401 2642 4327 2977 4297 5148
( þ 320) ( þ 413) ( þ 317) ( þ 616) ( þ 344) ( þ 407) ( þ 299) ( þ 232)
1160 1160 1160 1150 1150 1160 1160
215 215 215 165 165 112 112
40 40 40 24 24 30 30
0.000 0.669 2.255 0.819 1.801 0.000 0.508
6.213 4.825 3.878 4.262 2.986 4.436 3.241
0.0E+00 1.8E-02 2.9E-02 1.0E-02 1.6E-02 0.0E+00 3.8E-03
5.14 3.98 2.90 3.30 2.32 3.32 2.60
0.000 0.231 0.462 0.252 0.502 0.000 0.236
( þ 0.00) ( þ 0.04) ( þ 0.03) ( þ 0.03) ( þ 0.04) ( þ 0.00) ( þ 0.03)
3.07 2.57 1.98 0.855 1.37 3.82 2.16
5.1 ( þ 0.98) 5.2 ( þ 0.76) 5.1 ( þ 0.34) 3.0 ( þ 0.33) 6.5 ( þ 0.47) 15 ( þ 1.91) 12 ( þ 1.55)
3592 4353 5496 5754 7743 4395 6149
( þ 784) ( þ 709) ( þ 451) ( þ 715) ( þ 660) ( þ 626) ( þ 879)
T
P
(³C)
t
a
Uncertainties in the total loaded H2 O are within 2^3 wt% and they derive from a weighing error of about 0.02 mg. Since CO2 is loaded as Ag2 CO3 , the same weighting error gives a negligible e¡ect on the total loaded CO2 . b H2 O and CO2 dissolved in liquid (H2 O liq. and CO2 liq.) and CO2 in gas phase (CO2 gas) were recalculated by the already tested model of Papale [23], on the basis of their loaded amounts. Showed uncertainties in CO2 gas derive from propagation of a 5% error in dissolved H2 O (the average error in dissolved H2 O, obtained by testing the Papale model for our melt compositions [36]). The given error in total H2 O propagates sharply lower uncertainties in CO2 gas. A 10% error in dissolved CO2 will have a negligible e¡ect on the value of CO2 gas. c Molar fraction of He in glass (He liq.) after quenching, measured by QMS. Uncertainties are about 10%. d Molar fraction of He in gas phase (He gas), derived by mass balance between melt and gas phase. Showed uncertainties (12% average value) derive from the propagation, in mass balance, of the error given for the dissolved H2 O; instead, the error in dissolved CO2 had a negligible e¡ect. Uncertainties deriving from errors in the measurement of the dissolved He in the sample or in the He-bearing glass used for loading, are lower or similar. e PHe = Kh HeUxHe where PHe is the partial pressure of He in gas mixture and xHe is its molar fraction in melt, calculated on 8-oxygen basis of non-volatile species [35]. On the basis of very low He concentration in gas phase, we assume an ideal behavior of He in the H2 O^CO2 mixture; therefore Kh was computed by using He partial pressure in vapor. Showed uncertainties (12% average value) derive from the error in He gas.
EPSL 5570 5-9-00
598
A. Paonita et al. / Earth and Planetary Science Letters 181 (2000) 595^604
Au75%^Pd25% capsules and, subsequently, known quantities of H2 O, as deionized liquid, and CO2 , as silver carbonate, were added. A weighed amount of the previously prepared Hebearing glass (as grains of W1 mm size) was also loaded. The capsules were welded shut in order to con¢ne the gas mixture and they were pressurized by argon in the previously described IHPV apparatus; the run duration was 24^40 h at 1130^ 1160³C and a rapid quench was performed (see above). To check gas loss during experiments, capsules were weighed before and after each run by a high precision Sartorius microbalance (0.001 mg precision). Detailed run conditions are given in Table 3. 2.3. Attainment of equilibrium Several workers [3,4,7] put into evidence the importance of grain and sample size on the equilibration time. Small grain size implies short di¡usive walks of the gas atom within the melt, because gas always ¢lls grain interstices before melting. In this work, we have estimated that 24 h were more than enough for the attainment of equilibrium. This estimation was made possible by using the di¡usion coe¤cient (D) for He in basaltic [4] or in rhyolitic melts (extrapolated from [15]), as well as a critical evaluation of the run's duration and equilibrium time reported in literature at several temperatures and melt compositions ([4,5,7,9], for noble gases and [16^21] for H2 O and CO2 ). 2.4. He analysis QMS was employed for the He analysis of the run products coming from both He and H2 O^ CO2 ^He experiments. The apparatus [22] consists of a furnace including a graphite crucible, a tantalum resistance and a Pt^Rh thermocouple, connected to a high vacuum extraction line and a quadrupole mass spectrometer. Quenched glasses were crushed and bubble-free grains (size W1 mm) were selected by microscopic inspection. Prepared samples were inserted (from 5 to 40 mg for He and H2 O^CO2 ^He bearing glasses respectively) into a manipulator connected to the pre-
Fig. 1. He molar fractions analyzed in the same sample. Cases 1^3 concern analyses of bubble-free grains of W1 mm size. Data agree within the given error bar, with dispersion from the mean value (dashed line) within 10%. In case 4, a bubble-free grain was powdered to about 10 Wm, causing signi¢cant He loss.
evacuated line, where they remained at least 1 h in order to remove adsorbed gas. Two types of procedures were adopted: (a) the sample was let to drop from the manipulator while the crucible was already at the temperature necessary for complete silicate melting; or (b) the drop of the sample was done into a crucible which had a temperature of about 400³C and the He release was accomplished by increasing the temperature at 10³C/min. In both cases, the 4 He signal was continuously acquired in the multiple ion detection (MID) mode, obtaining a intensity vs. time curve, and the signal was recorded until the blank value was again reached. The integral of the curve was converted into He atoms by calibration with known air volumes. The error of analysis was within þ 10% (see Fig. 1). The two analytical procedures, for a bubble-free sample, are in accordance within the error bar (Table 2: samples s2r and s2h). According to Roselieb et al. [7], the He signal obtained for a bubble-containing sample by procedure b, showed a peak at high temperature (2 in Fig. 2), which was attributed to the decrepitude of the gas bubbles. 2.5. Gas inclusions in glasses The run products, microscopically inspected,
EPSL 5570 5-9-00
A. Paonita et al. / Earth and Planetary Science Letters 181 (2000) 595^604
599
pared to the total amount of dissolved helium (W1038 mol). Consequently, we generally used large grains for the He analyses, which were performed within a few hours after opening the capsules too. Anyway, future experiments will minimize problems due to bubble formation by ¢rst equilibrating melts at low He pressure and then raising that pressure to its ¢nal value, so as to cause the bubble collapse. 2.6. Helium loss during the procedure Fig. 2. QMS spectra of the released He from a basaltic glass (sample s1r). Heating rate 10 K/min. According to [7], we attribute the peak (b) to decrepitude of gas bubbles, which become mobile at temperature above glass transition, while peak (a) represents the dissolved gas. He content was computed by integration of the displayed area.
were found to be completely glassy. The He-bearing glasses showed dispersed gas bubbles (W100 Wm): following Roselieb et al. [7], it can be interpreted that these bubbles are not produced by exsolution during quenching, but rather represent primary bubbles originated in grain boundaries and cavities during melting of the powdered starting material. Such bubbles posed a severe problem in the evaluation of the actual dissolved gas. The powdering of glass below 100 Wm, which allows to entirely eliminate the gas inclusions [7], causes relevant He loss (see Section 2.6). In order to overcome this problem, we selected grains of He-bearing glass having a size of W1000 Wm. As concluded from optical inspection, grains of this size were generally bubble-free or contained bubbles 6 10 Wm. On the contrary, H2 O^CO2 ^He-bearing glass exhibits large semi-spherical bubbles in contact with the wall of the capsule. This suggests that the coalescence of bubbles has occurred because of the low viscosity of the liquid which is linked to the presence of dissolved water. Similarly to the experimental results of Blank et al. [21], we found rare entrapped vesicles smaller than 10 Wm, and therefore we neglected them as their helium contribution was insigni¢cant (W10313 mol) com-
Helium loss may potentially occur during several stages of the experimental procedure. Bubblefree grains of the He-bearing glass were analyzed both in their entirety and after their powdering, so as to reveal a signi¢cant loss of dissolved He during powdering (Fig. 1). The He loss which occurred from the grains during our procedure, could be roughly calculated by considering the He di¡usion coe¤cient (extrapolated from [15]) as well as the spherical shape of the grains. Seeing that we used grains approximately 1 mm in size, which were prepared 1 h or less before the analysis, we were able to compute that the helium loss from the He-bearing grains was within 5%. In preparing the capsules containing H2 O^ CO2 ^He, we made W1 mm grains of He-bearing glass just before their loading, selecting the bubble-free ones. The entire procedure for each sample lasted at most 2 h, implying that the computed He loss before the capsule sealing was estimated at less than 10% by using the previously mentioned calculation. Considering the larger size of grains of the H2 O^CO2 ^He containing glass (see Section 2.5), we were able to perform the He analyses within a few hours after the capsules were opened. 2.7. Determination of the He solubility in H2 O^CO2 bearing melts The aliquots of H2 O and CO2 which were loaded into capsules (gTH2 O e gTCO2 ), were converted in weight percentages by using the relations:
EPSL 5570 5-9-00
600
A. Paonita et al. / Earth and Planetary Science Letters 181 (2000) 595^604
wt%H2 O T
gTH2 O
wt%TCO2
gTH2 O 100; pgTCO2 ps
gTCO2 100 gTH2 O gTCO2 ps
1
where ps is the weight of loaded rock. A corresponding wt% of dissolved volatiles was obtained through calculation using a proper thermodynamic model of solubility of H2 O^CO2 mixtures in silicate melts [23] at experimental pressure and temperature. The result obtained by subtracting the dissolved volatile from the loaded one furnished us the amount of H2 O and CO2 existing in the gas phase. Consequently, the CO2 molar fraction in vapor was computed by the relation : yCO2
gCO2 =MWCO2 gCO2 =MWCO2 gH2 O =MW H2 O
2
where g and MW are the weight of volatile in gas phase and its molecular weight, respectively. The weight and helium concentration of Hebearing glass permitted us to know the total number of He moles existing in a capsule. Moreover, the number of He mol dissolved in melt (mHe ) was subsequently obtained by analyzing the dissolved He (xHe ) in the H2 O^CO2 bearing melt: mHe mH2 O mCO2 MxHe
The amount of helium loaded was decided in order to obtain its molar fraction of about 0.001^ 0.0005 in the gas phase; this implied the use of 1^ 4 mg of the He-bearing glass. So, the H2 O^CO2 silicate system was not signi¢cantly perturbed by the presence of a small amount of He, and we were able to study the He partitioning under physico-chemical conditions very similar to those of the majority of natural magmatic systems. We also evaluated the possible amount of helium due to the presence of air in the sealed capsule, so concluding that it could be neglected (atmospheric He was at least 104 times lower than the quantity loaded by He bearing glass). 3. Results and discussion In the absence of other volatiles, He solubility sharply increases from basaltic to rhyolitic melts (Fig. 3), in agreement with the solubility model of Carroll and Stolper [9] which is based on the concept of ionic porosity. A sharp increase of the He solubility, with respect to anhydrous melts, is caused by the presence of dissolved H2 O (Figs. 4 and 5). Nevertheless, it seems that a further increase of dissolved H2 O from 3 up to 6 wt%
3
where mH2 O and mCO2 are the amount (mol) of the two volatiles in the melt, while M is the total amount loaded (mol) of silicate rock. The result obtained by subtracting the dissolved from the loaded amount furnished us the He (mol) existing in gas phase at equilibrium ; therefore He molar fraction (yHe ) in vapor was computed using the equation : Y He
mvHe mvCO2
mvH2 O
where mv represents the amount in vapor phase (mol). Then, the value of the Henry constant was obtained by knowing both yHe and xHe , (see Table 3).
Fig. 3. Dissolved He vs. He fugacity for rhyolitic and basaltic melt. Molar fraction of He in melt is computed on 8-oxygen basis (see Table 3), while He fugacity is obtained by using a Redlich^Kwong equation of state with (a) and (b) parameters calculated by the He critical constants given in Weast et al. [37]. Lines represent solubility predicted by the ionic porosity model [9].
EPSL 5570 5-9-00
A. Paonita et al. / Earth and Planetary Science Letters 181 (2000) 595^604
(PH2 O from 100^200 MPa) causes a decline in the initial rate of increment of the He solubility, both in rhyolitic and basaltic melts. Taking into account the physical mechanism of He dissolution ([23] and reference therein), the sites where noble gases can be accommodated drastically increase by adding modest quantities of H2 O to a melt, reaching a maximum value almost unmodi¢ed by the further addition of H2 O. This result could depend on the solubility mechanism of H2 O above 2^3 wt% in melt: in such conditions, dissolved molecular H2 O becomes signi¢cant and it occupies the free spaces in the silicate structure ([19,24] and their reference, [25,26]). Moreover, when CO2 is present in the gas phase (Figs. 4 and 5), the He solubility appears to decline in accordance with the decrease of dissolved H2 O from 6 to 2^3 wt% due to the CO2 partitioning into vapor at constant Ptot . Nevertheless, a lower He solubility is exhibited by an H2 O^ CO2 -bearing basalt with respect to an H2 O-bearing basalt having a similar content of dissolved H2 O (see Fig. 5). This ¢nding suggest that the dissolved CO2 would have an opposite e¡ect with respect to H2 O on He solubility, although the very low CO2 concentration in the melt likely makes its ultimate e¡ect di¤cult to detect. How-
Fig. 4. He solubility (as Kh , see Table 3) vs. dissolved water in a rhyolitic liquid which contains H2 O or H2 O and CO2 . Composition of gas phase is approximately indicated as CO2 molar fraction (XCO2 g). Samples with the same composition of gas phase, have been performed at di¡erent total pressures (see Table 3).
601
Fig. 5. He solubility vs. dissolved water in a basaltic liquid which contains H2 O or H2 O and CO2 . Samples (a) and (b) have the same aliquot of dissolved H2 O, but (a) is CO2 -free and (b) contains 100 ppm of dissolved CO2 . Therefore, they would suggest the He solubility somewhat decreases by adding CO2 to basalt melt.
ever, only further He solubility data, concerning the CO2 ^He-silicate system, could con¢rm this conclusion. Analogous to the described H2 O in£uence on the helium solubility, we argue that this volatile very probably a¡ects the solubility of all the noble gases. The design of our experimental method obviously makes it possible to investigate solubility of any inert gases in H2 O^CO2 bearing melts. Moreover, given the lower di¡usivity of heavier noble gases with respect to helium [4,8,27], the noble gas bearing glass could be powdered before analysis or loading, in order to avoid the microscopic check of grains for bubble absence. Our data show the helium solubility in H2 O rich melts can also be one order of magnitude higher than that of CO2 [23] in the same melt (i.e. a 5 wt% H2 O basalt with respect to a dry basalt). By using the fractionation equations of Jambon et al. [3], we can roughly evaluate how such di¡erences of solubility modify the He behavior throughout magma degassing (Fig. 6). Although this is only a preliminary computation, nevertheless predictions at low melt vesicularity should result nearly indicative. In fact, more accurate results should be achieved taking into account the variation of the noble gas solubility
EPSL 5570 5-9-00
602
A. Paonita et al. / Earth and Planetary Science Letters 181 (2000) 595^604
Fig. 6. Predicted variations of the CO2 /He ratio in basalt as a function of magma vesicularity. Calculations for dry melt were performed by using equations of Jambon et al. [3] for closed degassing system. Because the small CO2 e¡ect on He solubility, the same curve can be practically assumed for a CO2 -rich melt. Curve for 5 wt% H2 O bearing magma has been calculated by using, in the same equation, the CO2 solubility for the H2 O^CO2 bearing basalt [23] and the He solubility from this work. The initial CO2 /He ratio was within the range for MORB basalts.
caused by the degassing of H2 O from the magma. During a gas fractionation from an H2 O-rich magma, He will be scarcely released from the melt until the less soluble CO2 is almost entirely degassed. Therefore, a higher decrease of the CO2 /He ratio during the degassing can be computed for a H2 O rich melt compared to a dry melt (Fig. 6). On the other hand, the He solubility of a CO2 rich dry melt is probably similar to one found in the system He-silicate melt, which implies that the helium solubility is 2^3 times higher than that of CO2 . Hence, CO2 -rich dry magmas exhibit a less variable CO2 /He ratio (Fig. 6), which decreases at more advanced degassing stages, when dissolved CO2 ¢nally approaches very low contents and then He can be largely degassed from the silicate melt. Therefore, during protracted degassing, CO2 -rich anhydrous melts retain the dissolved helium more e¤ciently than H2 O-rich magmas. The evidence resulting from the discussion above, is that more helium generally remains in a natural H2 O-bearing magma when it degasses
its vapor phase. Therefore, the residual fraction of helium in natural melts, which is computed by the magma vesicularity [28], could be strongly underestimated using the solubility data of He-melt systems. Finally, the He solubility in a silicate melt is also used when estimating the ascent of magma towards the surface and in evaluating volcanic hazard [29]; then, the increase of He solubility, linked to the dissolved water, a¡ects such estimations too. In particular, the computed value of magma ascent would become up to four times higher when considering the e¡ect of H2 O. In the view of the latter, an underestimation of volcanic hazard could derive when using the He solubility of the He-melt system. 4. Conclusions We propose the ¢rst available experimental method to investigate the solubility of any inert gas in H2 O^CO2 bearing magma. This method allows to experimentally reproduce chemical conditions very similar to natural ones, where inert gas is only a tracer constituent of a H2 O^CO2 dominated vapor phase. We applied this technique in order to estimate the helium solubility in a rhyolite and basalt which contained H2 O or H2 O and CO2 . As a result, a positive dependence of the helium solubility in regard to the dissolved H2 O has been demonstrated by our experiments. In particular, the He solubility appears to increase by about a factor three when adding 3 wt% of H2 O to a melt. Evidently, a modest addition of H2 O to silicate liquid modi¢es the melt network so as to create a great number of new sites, into which noble gas atoms can enter. The breaking of Si^O^Si bonds and melt depolymerization by water addition [24^26] could qualitatively explain the above e¡ect, because it would render the silicate melt closer to a sphere packing than to a structure of silica-polymer chains. Surely, more extensive information concerning the structural e¡ect of water dissolution in a silicate liquid will be needed in order to better understand this behavior. Finally, the in£uence of CO2 on helium solubility has not been clearly recognized, how-
EPSL 5570 5-9-00
A. Paonita et al. / Earth and Planetary Science Letters 181 (2000) 595^604
ever, suspicion exist that the e¡ect of CO2 can be opposite with respect to H2 O in basaltic melts. We also argued on the expected modi¢cations of helium behavior during the fractionations of a vapor phase from H2 O^CO2 bearing melts with respect to noble gas-silicate melt systems. Our preliminary computations put into evidence that the e¡ect of H2 O on helium solubility signi¢cantly in£uences the behavior of this noble gas during magma degassing. Similar results should be expected for other noble gases. The processes of gas fractionation in magmatic systems which contain H2 O and/or CO2 , concern the majority of large scale geological processes, such as geochemical cycling, mantle evolution, Earth degassing and formation of the atmosphere [7]. For these last ones, noble gases and their solubility in silicate melt provide precious insights [28,30,31]. In particular, helium is the unique tracer of the exhalation of primordial volatiles from the mantle, while noble gases generally provide secular information on their Earth reservoirs. Geological environments of magma genesis and evolution are often constrained on the basis of the initial CO2 /He ratio of the melt, which can be computed by both vesicularity and the residual CO2 /He ratio of the melt itself. The same ratio or ratios among noble gases are often used to provide the degassing extent and modalities of vesiculated MORB glasses. Moreover, noble gas fractionation has acquired great relevance in the study of pre-eruptive degassing of shallow magmatic bodies for the estimation of volcanic hazard [29]. As noble gas solubility in melts has been largely used in the study of all these processes [28^33], the geochemical impact of our results on the previous arguments becomes particularly relevant. Consequently, we hope this research may provide the tool in acquiring further experimental data on noble gas solubility in H2 O^CO2 bearing silicate melts, in order to describe magmatic systems with parameters more adequate to true natural conditions. Acknowledgements This work is part of the Ph.D. thesis of Dr.
603
A.P., supported by the European Social Fund. We appreciated useful comments and suggestions from Prof. Mike Carroll and another anonymous reviewer. We also are grateful to Eng. L. Puxeddu for his assistance during high pressure-high temperature experiments. Dr. A.P. would also like to thank Dr. Tatiana Terranova for the linguistic revisions done to this article.[FA] References [1] J.F. Shackelford, P.L. Studt, R.M. Fulrath, Solubility of gases in glass II. He, Ne, and H2 in fused silica, J. Appl. Phys. 43 (1972) 1619^1626. [2] A. Hayatsu, C.E. Waboso, The solubility of rare gases in silicate melts and implications for K^Ar dating, in: F.A. Podosek (guest Ed.), Terrestrial Noble Gases, Chem. Geol. 52 (special issue) (1985) 97^102. [3] A. Jambon, H.W. Weber, O. Braun, Solubility of He, Ne, Ar, Kr, Xe in a basalt melt in the range of 1250^1600³C: Geochemical implications, Geochim. Cosmochim. Acta 50 (1986) 401^408. [4] G. Lux, The behavior of noble gases in silicate liquids: solution, di¡usion, bubbles, and surface e¡ects, with applications to natural samples, Geochim. Cosmochim. Acta 51 (1987) 1549^1560. [5] B.S. White, M. Brearley, A. Montana, Solubility of Argon in silicate liquids at high pressures, Am. Mineral. 74 (1989) 513^529. [6] C.L. Broadhurst, M.J. Drake, B.E. Hagee, T.J. Bernatowicz, Solubility and partitioning of Ne, Ar, Kr, and Xe in minerals and synthetic basalt melts, Geochim. Cosmochim. Acta 56 (1992) 709^723. [7] K. Roselieb, W. Rammensee, H. Buttner, M. Rosenhauer, Solubility and di¡usion of noble gases in vitreous albite, Chem. Geol. 96 (1992) 241^266. [8] M.R. Carroll, E.M. Stolper, Argon solubility and di¡usion in silica glass: implications for the solution behavior of molecular gases, Geochim. Cosmochim. Acta 55 (1991) 211^225. [9] M.R. Carroll, E.M. Stolper, Noble gas solubilities in silicate melts and glasses: new experimental results for Ar and the relationship between solubility and ionic porosity, Geochim. Cosmochim. Acta 57 (1993) 5039^5051. [10] D.S. Draper, M.R. Carroll, Argon di¡usion and solubility in silicic glasses exposed to an Ar^He mixtures, Earth. Planet. Sci. Lett. 132 (1995) 15^24. [11] T. Shibata, E. Takahashi, J. Matsuda, Noble gas solubility in binary CaO^SiO2 system, Geophys. Res. Lett. 23 (1996) 3139^3142. [12] T. Shibata, E. Takahashi, J. Matsuda, Solubility of neon, argon, kripton and xenon in binary and ternary silicate system: a new view on noble gas solubility, Geochim. Cosmochim. Acta 62 (1998) 1241^1253.
EPSL 5570 5-9-00
604
A. Paonita et al. / Earth and Planetary Science Letters 181 (2000) 595^604
[13] J.E. Shelby, Pressure dependence of helium and neon in vitreous silica, J. Appl. Phys. 47 (1976) 135^139. [14] S.L. Boettcher, Q. Guo, A. Montana, A simple device for loading gases in high-pressure experiments, Am. Mineral. 74 (1989) 1383^1384. [15] A. Jambon, J.E. Shelby, Helium di¡usion and solubility in obsidians and basaltic glasses in the range 200^300³C, Earth Planet. Sci. Lett. 51 (1980) 206^214. [16] B.O. Mysen, R.J. Arculus, D.H. Eggler, Solubility of carbon dioxide in melts of andesite, tholeiite, and olivine nephelinite composition to 30 kbar, Contrib. Mineral. Petrol. 53 (1975) 227^239. [17] E.M. Stolper, G.J. Fine, T. Johnson, S. Newman, The solubility of carbon dioxide in albitic melt, Am. Mineral. 72 (1987) 1071^1085. [18] L.A. Silver, P.D. Ihinger, E. Stolper, The in£uence of bulk composition on the speciation of water in silicate glasses, Contrib. Min. Petrol. 104 (1990) 142^162. [19] J.E. Dixon, E.M. Stolper, J.R. Holloway, An experimental study of water and carbon dioxide solubilities in midOcean Ridge basaltic liquids. Part I: calibration and solubility models, J. Petrol. 36 (1995) 1607^1631. [20] G. Moore, T. Vennemann, I.S.E. Carmichael, An empirical model for the solubility of H2 O in magmas to 3 kilobars, Am. Mineral. 83 (1998) 36^42. [21] J.G. Blank, E.M. Stolper, M.R. Carroll, Solubilities of carbon dioxide and water in rhyolitic melts at 850³C and 750 bar, Earth Planet. Sci. Lett. 119 (1993) 27^36. [22] D. Gozzi, F. Cellucci, P.L. Cignini, G. Gigli, M. Tomellini, E. Cibsani, S. Frullani, G.M. Urciuoli, X-ray, Heat Excess and 4 He in the Pd/D System, J. Electroanal. Chem. 435 (1997) 113^136, erratum, 452 (1998) 251^271. [23] P. Papale, Modeling of the solubility of a two component H2 O+CO2 £uid in silicate liquids, Am. Mineral. 84 (1999) 477^492. [24] J.R. Holloway, J.G. Blank, Experimental results applied to C^O^H in natural melts, in: volatiles in magmas, Rev. Mineral. 30 (1994) 231^279. [25] M. Nowak, H. Behrens, The speciation of water in haplogranitic glasses and melts determined by in situ nearinfrared spectroscopy, Geochim. Cosmochim. Acta 59 (1995) 3445^3450.
[26] A. Shen, H. Keppler, Infrared spectroscopy of hydrous silicate melts to 1000³C and 10 kbar: direct observation of H2 O speciation in a diamond-anvil cell, Am. Mineral. 80 (1995) 1335^1338. [27] E.B. Watson, Di¡usion in volatile-bearing magmas, in: volatiles in magmas, Rev. Mineral. 30 (1994) 371^412. [28] M.R. Carroll, J.D. Webster, Solubilities of sulfur, noble gases, nitrogen, chlorine, and £uorine in magmas, in: volatiles in magmas, Rev. Mineral. 30 (1994) 231^279. [29] P.M. Nuccio, M. Valenza, Magma degassing and geochemical detection of its ascent, in: G.B. Arehart, J.R. Hulston (Eds.), Water^Rock Interaction, Balkema (1998) 475^478. [30] C.J. Allegre, T. Staudacher, P. Sarda, Rare gas systematics: formation of the atmosphere, evolution and structure of the Earth's mantle, Earth Planet. Sci. Lett. 81 (1986) 127^150. [31] A. Jambon, Earth degassing and Large scale geochemical cycling of volatile elements, in: volatiles in magmas, Rev. Mineral. 30 (1994) 479^510. [32] B.E. Taylor, Magmatic volatiles: isotopic variations of C, H, and S, Rev. Mineral. 16 (1986) 185^225. [33] W.F. Giggenbach, Chemical composition of volcanic gases, in: R. Scarpa, R.I. Tilling (Eds.), Monitoring and Mitigation of Volcanic Hazards, Springer, Berlin (1996) 221^256. [34] G. De Astis, L. La Volpe, A. Peccerillo, L. Civetta, Evoluzione vulcanologica e magmatologica dell'isola di Vulcano, in: L. La Volpe, Dellino, P.M. Nuccio, E. Privitera, A. Sbrana (Eds.), Progetto Vulcano, Risultati dell'attivita© di ricerca 1993^1995 Felici Editore, Pisa (1997) 155^177. [35] M.S. Ghiorso, I.S.E. Carmichael, M.L. Rivers, R.O. Sack, The Gibbs Free Energy of mixing of natural silicate liquids; an expanded regular solution approximation for the calculation of magmatic intensive variables, Contrib. Mineral. Petrol. 84 (1983) 107^145. [36] A. Paonita, Magma degassing of multicomponent gas mixtures: thermodynamic modeling and applications, Ph.D. Thesis, Universita© di Palermo, Palermo (1999) 161. [37] R.C. Weast, J.A. Melvin, W.H. Beyer, CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1985.
EPSL 5570 5-9-00