647 ANNALS OF GEOPHYSICS, VOL. 48, N. 4/5, August/October 2005 Key words noble gases – solubility – degassing – silicate melts 1. Introduction When investigating processes involving magmas and coexisting vapor phases, a direct observation of the system is obviously impossi- ble. The information available comes from out- gassed vapors that are released at the surface and from fluid inclusions trapped in igneous products. This implies that the observation con- ditions are different from those at the source, and that the pristine gas content may be modi- fied by chemical reactions, unless unreactive species are considered. Furthermore, degassing of the dominant volatiles (H 2 O, CO 2 , S, Cl and F) strongly modifies the physico-chemical properties of the magmatic system and plays an active role in driving the degassing process, whereas components at trace concentrations passively follow the evolution of the system. Among these latter, poorly soluble species mark out the degassing process evolution very well, as degassing involves the exsolution of the vapor phase where they are preferentially partitioned. All the highlighted features can be Noble gas solubility in silicate melts: a review of experimentation and theory, and implications regarding magma degassing processes Antonio Paonita Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Italy Abstract Noble gas solubility in silicate melts and glasses has gained a crucial role in Earth Sciences investigations and in the studies of non-crystalline materials on a micro to a macro-scale. Due to their special geochemical features, noble gases are in fact ideal tracers of magma degassing. Their inert nature also allows them to be used to probe the structure of silicate melts. Owing to the development of modern high pressure and temperature technologies, a large number of experimental investigations have been performed on this subject in recent times. This paper reviews the related literature, and tries to define our present state of knowledge, the problems encountered in the experimental procedures and the theoretical questions which remain unresolved. Throughout the manuscript I will also try to show how the thermodynamic and structural interpretations of the growing experimental dataset are greatly improving our understanding of the dissolution mechanisms, although there are still several points under discussion. Our improved capability of predicting noble gas solubilities in conditions closer to those found in magma has allowed scientists to develop quantitative models of magma degassing, which provide constraints on a number of questions of geological impact. Despite these recent improvements, noble gas solubility in more complex systems involving the main volatiles in magmas, is poorly known and a lot of work must be done. Ex- pertise from other fields would be extremely valuable to upcoming research, thus focus should be placed on the structural aspects and the practical and commercial interests of the study of noble gas solubility. Mailing address: Dr. Antonio Paonita, Istituto Nazio- nale di Geofisica e Vulcanologia, Sezione di Palermo, Via Ugo La Malfa 153, 90146 Palermo (Italy); e-mail: paoni- [email protected]
Transcript
647
ANNALS OF GEOPHYSICS, VOL. 48, N. 4/5, August/October 2005
Key words noble gases – solubility – degassing –silicate melts
1. Introduction
When investigating processes involvingmagmas and coexisting vapor phases, a directobservation of the system is obviously impossi-ble. The information available comes from out-gassed vapors that are released at the surface
and from fluid inclusions trapped in igneousproducts. This implies that the observation con-ditions are different from those at the source,and that the pristine gas content may be modi-fied by chemical reactions, unless unreactivespecies are considered. Furthermore, degassingof the dominant volatiles (H2O, CO2, S, Cl andF) strongly modifies the physico-chemicalproperties of the magmatic system and plays anactive role in driving the degassing process,whereas components at trace concentrationspassively follow the evolution of the system.Among these latter, poorly soluble speciesmark out the degassing process evolution verywell, as degassing involves the exsolution ofthe vapor phase where they are preferentiallypartitioned. All the highlighted features can be
Noble gas solubility in silicate melts: a review of experimentation
and theory, and implications regardingmagma degassing processes
Antonio PaonitaIstituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Italy
AbstractNoble gas solubility in silicate melts and glasses has gained a crucial role in Earth Sciences investigations andin the studies of non-crystalline materials on a micro to a macro-scale. Due to their special geochemical features,noble gases are in fact ideal tracers of magma degassing. Their inert nature also allows them to be used to probethe structure of silicate melts. Owing to the development of modern high pressure and temperature technologies,a large number of experimental investigations have been performed on this subject in recent times. This paperreviews the related literature, and tries to define our present state of knowledge, the problems encountered in theexperimental procedures and the theoretical questions which remain unresolved. Throughout the manuscript Iwill also try to show how the thermodynamic and structural interpretations of the growing experimental datasetare greatly improving our understanding of the dissolution mechanisms, although there are still several pointsunder discussion. Our improved capability of predicting noble gas solubilities in conditions closer to those foundin magma has allowed scientists to develop quantitative models of magma degassing, which provide constraintson a number of questions of geological impact. Despite these recent improvements, noble gas solubility in morecomplex systems involving the main volatiles in magmas, is poorly known and a lot of work must be done. Ex-pertise from other fields would be extremely valuable to upcoming research, thus focus should be placed on thestructural aspects and the practical and commercial interests of the study of noble gas solubility.
Mailing address: Dr. Antonio Paonita, Istituto Nazio-nale di Geofisica e Vulcanologia, Sezione di Palermo, ViaUgo La Malfa 153, 90146 Palermo (Italy); e-mail: [email protected]
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recognized in the geochemical group of the no-ble gases: they are all chemically inert andvolatile, and are present in trace concentrationsin magmatic systems. Moreover, their physico-chemical properties gradually vary within thegroup with the increase in their atomic mass orsize. Thus, although noble gases do not thermo-dynamically define the magmatic system, theirabundance ratios are a very powerful tool instudying degassing processes, on a micro to amacro-scale, from magmatic intrusions to man-tle outgassing and the development of atmos-phere.
For these studies, noble gas solubilities are acrucial parameter and a number of investiga-tions show a growing interest in the subject(Carroll and Draper, 1994; Carroll and Webster,1994 and reference therein; Draper and Carroll,1995; Chamorro-Perez et al., 1996, 1998; Shi-bata et al., 1996, 1998; Shackelford, 1999; Nuc-cio and Paonita, 2000; Paonita et al., 2000; Wal-ter et al., 2000; Schmidt and Keppler, 2002), al-so thanks to the development of modern experi-mental and analytical technologies. Experimen-tal devices make it possible to reproduce mag-matic temperatures from atmospheric to mantlepressures, and to measure the noble gas concen-trations down to parts per million (ppm). Never-theless, technical questions that mainly concernthe distinction between the dissolved and ex-solved fractions of gas must be solved to obtainaccurate vapor-melt partition coefficients.Moreover, available experimental data on noblegas solubility are far from exhaustive, particu-larly as regards helium. The existing studies on-ly partially cover the compositional ranges ofmagmatic systems and possible thermo-baricconditions, thus their use in modeling naturalprocesses has still to be optimized.
Due to their inert nature, noble gases devel-op van der Waals-type interactions with silicatemelts and seem to display a preferentially«physical solubility» (i.e. the noble gas atom issimilar to a fixed-volume sphere entering arigid network). Nonetheless, their dissolutionmechanism is not fully defined. Empirical orsemi-empirical (Lux, 1987; Jambon, 1987;White et al., 1989; Chennaoui-Aoudjehane andJambon, 1990; Carroll and Stolper, 1993; Shi-bata et al., 1998; Nuccio and Paonita, 2000) ap-
proaches are still the most frequently used toolsto describe the noble gas solubility and to ex-trapolate the experimental data towards com-plex magmatic systems, whereas statistical-me-chanics models (Doremus, 1966; Studt et al.,1970; Shacklford et al., 1972) work within avery narrow range of temperature, pressure (T,P) and composition. Current efforts also aim atinvestigating the structural environment of no-ble gas atoms inside the silicate network, prom-ising results coming from the use of X-ray tech-niques (Wulf et al., 1999).
In this framework of growing high-qualityscientific literature, this paper describes the sol-ubility of noble gas in silicate melts, which ismainly applied to geological problems involv-ing magma degassing in its wide range of pos-sible conditions. Throughout the text, this paperwill try to draw attention to existing literatureand the main problems encountered in experi-ments and modeling. While stressing the im-portance of noble gases as tools in studyingmagma degassing, the paper shows two things:a) what type of data is needed for geological ap-plications; b) which are the most promisingguidelines of research for achieving these re-sults.
2. Experimental techniques
The investigation of noble gas solubility insilicate materials at magmatic conditions needsto experimentally reproduce high temperatureand atmospheric up to mantle pressures. Mod-ern technology has designed and optimized re-sistance furnaces and Externally Heated and In-ternally Heated Pressure Vessels (EHPV andIHPV) to perform experiments at atmosphericpressure up to 500-600 MPa at magmatic tem-peratures. In this equipment, the noble gas un-der investigation is normally used as the pres-sure medium in the high pressure line and en-ters the unsealed noble-metal capsules wherethe sample is contained. In EHPV and IHPVexperiments, noble gas can also be contained ina sealed capsule, that is loaded prior to the runby using a custom-designed loading device(Boettcher et al., 1989) or a gas-bearing mate-rial (Paonita et al., 2000). Experiments at pres-
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sure up to mantle values (several GPa) havebeen performed using piston cylinder apparatusor multi-anvil cells. In these instruments, asealed capsule (single or double) containing thesample and the noble gas is inserted, and highpressures are reached using a solid pressuremedium.
Run duration has to be long enough to attainequilibrium. Based on the diffusion rates of no-ble gases, the appropriate duration ranges fromhours to months, depending on the melt or glasscomposition and temperature (longer periodsare necessary for heavy gases in silica glasses).The use of small grain sized sample powdersyields a shorter diffusive walk of gas atoms inmelt, because the gas penetrates through thegrain interstices before melting, resulting infaster equilibration (Jambon et al., 1986; Lux,1989; Roselieb et al., 1992).
The noble gas content in quenched samplesis analyzed using different methods. The elec-tron microprobe has been frequently used for insitu analyses of Ar or heavier noble gases(White et al., 1989; Carroll and Stolper, 1991,1993; Montana et al., 1993; Draper and Carroll,1995; Chamorro-Perez et al., 1996; Schmidtand Keppler, 2002), and has obtained concen-tration profiles in quenched glasses with a spa-tial resolution of around 10 µm. X-ray inducedemission by synchrotron radiation and protonbeams have been applied to the analysis of Krin albite (Carroll et al., 1993b). Light gases(He, Ne) cannot be detected by these methods,and bulk extraction techniques have been ap-plied to extract the dissolved gases (Hayatsuand Waboso, 1985; Jambon et al., 1986; Lux,1987; Broadhurst et al., 1992; Roselieb et al.,1992; Shibata et al., 1996, 1998; Paonita et al.,2000). Solid samples are melted in a vacuumline, in order to extract the dissolved volatiles.Various analytical methods are capable ofmeasuring the amount of extracted gas, namelyGas-Chromatography (GC), Mass Spectrome-try (MS) and Knudsen cell Mass Spectrometry(KMS). The main problems consist in distin-guishing between gas fractions from surface ad-sorption or trapping in fractures, secondary flu-id inclusions and truly dissolved volatiles in thesilicate melt (see Roselieb et al., 1992). In orderto avoid the gas fraction released from incorpo-
rated bubbles, powdering of quenched glasseshas been used and grain sizes below 100 µmwere found to be necessary to remove most ofthe physically trapped gas. However, an exces-sively small grain size could cause significantgas loss by diffusion before analysis, at least asregards helium and, in part, neon, if the samplesare not analyzed within a matter of hours (Pao-nita et al., 2000). Stepwise degassing methods,coupled to KMS or quadrupole MS measure-ments, have been developed to overcome thesedifficulties, based on the fact that different gascontributions are extracted in different rangesof temperature (fig. 1) (Roselieb et al., 1992,1995; Paonita et al., 2000).
In situ extraction techniques (ultra-violetlaser beam) have been combined with massspectrometry (Kelly et al., 1994; Brooker et al.,1998; Chamorro et al., 2002), and the concen-tration of dissolved argon in glass volumes hav-ing around a 10 µm diameter size have been ob-tained. These methods are very promising asthey are able to remove the contribution fromphysically trapped gas and they can be used forlighter noble gases.
Fig. 1. Release spectrum of helium from basalticglasses heated at a rate of 10°C/min. Data fromPaonita et al. (2000). Peak 2 provides the amount ofdissolved gas, whereas peak 1 has been attributed tothe decrepitation of fluid inclusions.
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3. Noble gas solubility: experimental resultsand modeling
The main parameters affecting the solubili-ty of noble gases in silicate melts are pressure,
temperature and composition of the coexistingliquid and vapor phases. In general (see table I),the concentration of any dissolved noble gasdisplays a linear relation with the gas pressureup to several hundred MPa (figs. 2a,b and 3). At
Table I. Noble gas solubility in melt and glasses, and parameters for pressure and temperature dependence.
(a) Noble gas solubility at P° and T°; the dissolved concentrations are normalized by noble gas pressure. Uncer-tainties are normally within 10%.(b) V° and ∆H° are molar volume and enthalpy of solution of the reference state (see text).(c) Calculated from the isothermal data of 8 by fitting eq. (3.6).(d) Solubilities and related parameters are reliable for both pure Ar and Ar in He-Ar mixtures.(e) Pressure range of the experiments. (f) NS refers to the system Na2O-SiO2 (1, 2, 3 have about 35-65, 31-70, 24-77 wt% respectively). NCS is thesystem Na2O-CaO-SiO2 (1, 2, 3 have about 25-9-64, 19-19-60, 11-31-56 wt% respectively). CMS is the systemCaO-MgO-SiO2 (1, 2, 3 have about 37-8-54, 26-16-56, 14-26-59 wt% respectively). (g) Calculated from data from Frank et al. (1961), Shackelford et al. (1972), Shelby (1972a,b, 1976).(h) Values in hydrous melts at Ptot≈PH2O and He concentration in vapour > 0.1 mol%. Numbers in parenthesis are
Antonio Paonita
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A review on noble gas solubility and degassing in magmas
wt% of dissolved water. Solubilities at Ptot≈PH2O + PCO2 were obtained at PH2O very close to that given in tableand do not display appreciable differences with respect to given values, therefore they were not reported.Ref.: 1 – Kirsten (1968); 2 – Fisher (1970); 3 – Hayatsu and Waboso (1985); 4 – Jambon et al. (1986); 5 – Lux(1987); 6 – White et al. (1989); 7 – Carroll and Stolper (1991); 8 – Roselieb et al. (1992); 9 – Broadhurst et al.(1992); 10 – Carroll and Stolper (1993); 11 – Carroll et al. (1993); 12 – Montana et al. (1993); 13 – Draper andCarroll (1995); 14 – Chamorro-Perez et al. (1996); 15 – Shibata et al. (1998); 16 – Paonita et al. (2000); 17 –Schmidt and Keppler (2002).
Fig. 2a,b. Dissolved Ar as a function of Ar pressurein a) rhyolite (800-900°C) and basalt (1200°C); b)haplogranite and basalt (1500°C). Data are from Car-roll and Stolper (1993) and White et al. (1989).Curves were calculated by using eq. (3.6) with pa-rameters given in table I. The linear relation betweenconcentration and pressure, as well as the higher sol-ubilities in the acidic compositions, should be noted.
higher pressure, the solubility (hereafter de-fined as the ratio between gas concentration inmelt and gas fugacity, unless differently speci-fied) decreases, and recent experiments indicate
Fig. 3. Neon and krypton solubilities in albite glassat 1000 and 750°C, respectively. Data are taken fromRoselieb et al. (1992).
Fig. 4. Effect of very high pressure on Ar solubilityin basaltic and haplogranitic melts between 1500 and2000°C. Data from Schmidt and Keppler (2002);curves calculated by using eq. (3.6) with the param-eters given by the authors (see table I). At about 5GPa, dissolved Ar in the melt reaches a thresholdconcentration which does not increase even when Arpressure is increased by some GPa.
a
b
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Antonio Paonita
that a threshold concentration is reached, atleast for Ar and Xe, at very high pressures (4-5GPa and 5 GPa, respectively, fig. 4; Schmidtand Keppler, 2002). The dependence of solubil-ity on temperature is very modest and is oftenlower than the experimental uncertainties.
3.1. Dependence on the composition of meltand glass
The parameters which most affect the solu-bility of noble gases in silicate melts and glassesare the composition of the solvent and the atom-ic radius of the noble gas (Doremus, 1966;Shackelford et al., 1972; Shelby, 1976; Hayatsuand Waboso, 1985; Jambon et al., 1986; Lux,1987; Broadhurst et al., 1992; Roselieb et al.,1992; Carroll and Stolper, 1993; Shibata et al.,1996, 1998; Shackelford, 1999; Paonita et al.,2000; Schmidt and Keppler, 2002). In general,solubility increases with the increasing concen-tration of SiO2 in the melt while smaller noblegases display higher solubilities (fig. 5). Thesevariations suggest that noble gas atoms dissolvein holes (free spaces) of the silicate melt struc-ture, following the so-called physical mecha-nism of dissolution (Doremus, 1966; Studt et al.,
1970; Shackelford et al., 1972; Shelby, 1976).Five basic structural units form the main net-work of silicate melts and glasses, namely SiO2
(three-dimensional network), Si2O22− (sheet),
Si2O64− (chain), Si2O7
6− (dimer), SiO44− (mono-
mer) (Virgo et al., 1980). Silicon and othertetrahedrally coordinated cations (Al, Fe3+, Ti4+)act as network-formers, whereas Na+, K+, Mg2+,Ca2+, Fe2+ break the silicate polymers and thusare called network-modifiers. According to Shi-bata et al. (1998), the fully polymerized, three-dimensional SiO2 structure of silicate melts cre-ates a number of holes where gas atoms can beaccommodated, whereas the less polymerizedstructures of the other units have fewer freespaces, therefore less volume is available to storegas atoms (i.e. the holes are too small). This iswhy silica-rich compositions display the highestnoble gas solubilities. A quantitative expressionof the degree of polymerization is given by theratio NBO/T, namely the number of Non-Bridg-ing Oxygens per atom of a Tetrahedrally coordi-nated cation (Brawer and White, 1975; Mysen etal., 1985). The three-dimensional network hasbridging oxygens only (NBO/T=0, that is to saythe highest polymerization), whereas the ratiogrows when other units are present; if TO4
4−
monomers are the only units in the melt, thenNBO/T=4. Shibata et al. (1998) found a directrelationship between noble gas solubility and de-gree of polymerization in the silicate melts, ac-cording to which gas atoms dissolve preferen-tially in more polymerized, low NBO/T melts(fig. 6). They proposed the following equation tocalculate the vapor-melt equilibrium constant ofnoble gas i:
(3.1)
where ABO and ANBO are the fraction of the unitswith bridging oxygens and the units with bridg-ing and non-bridging oxygens, while Ki
BO andKi
NBO are the equilibrium constants that refer toeach group of units. The model was calibratedfor noble gases from Ne to Xe, but unfortunate-ly helium was not treated. From fig. 6 it can beseen that the relationship between solubility andthe NBO/T ratio is also quite evident when plot-ting the available experimental data for helium,
ln ln lnK A K A Ki i iBO BO NBO NBO= +
Fig. 5. Solubility (expressed under form of Henry’sconstant) of the noble gases as a function of their sizeat about 1200°C. Solubility exponentially increasesfrom light to heavy noble gases.
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A review on noble gas solubility and degassing in magmas
thereby suggesting that the solubility mecha-nism is analogous. A similar approach has al-ready been proposed by Jambon (1987) andChennaoui-Aoudjehane and Jambon (1990), al-though no clear interpretation in terms of meltstructure was given by the authors. Further-more, even though the model by Shibata et al.(1998) does work for a large range of naturalmelt compositions, significant deviations fromthe relationship solubility-degree of polymer-ization occur in melts containing large amountsof Al (or network-formers other than Si). How-ever, the measured solubilities appear to belower than those predicted from the NBO/T ra-tio (fig. 6). Although further data on noble gassolubilities in aluminosilicate melts are neces-sary to confirm this hypothesis, it is possiblethat cations (Na, K, Ca, Mg), added to balancethe surplus of negative charges due to Si versusAl substitution, occupy structural vacanciesotherwise available for noble gases.
Fig. 6. Effect of melt polymerization (expressed bythe NBO/T ratio, see text) on the solubility of somenoble gases (modified from Shibata et al., 1998). Da-ta for Ar are a compilation selected by Shibata et al.(1998), He and Ne from Jambon et al. (1986) andLux (1987). Ar data are divided into two classes:T/(T+Si) ratio higher (circles) than or lower (trian-gles) than 0.1, where T is the number of tetrahedral-ly coordinated cations other than silicon (i.e. Al, Ti).It is evident that the NBO/T ratio fails to predict thelow Ar solubilities in aluminosilicate melts.
Fig. 7a-c. Relationships between noble gas solubil-ity and a) ionic porosity of melt, b) melt density andc) melt molar volume (modified from Carroll andStolper, 1993). In all the graphs, the squares are sol-ubilities in CaO-MgO-Al2O3-SiO2 (CAMS) meltsfrom Broadhurst et al. (1992). Henry’s constant andmolar volumes were computed on eight-oxygen ba-sis melts. Linear fits of data, with the related R 2 val-ues for Ar, are also displayed. Solubilities are muchbetter correlated to ionic porosity than other parame-ters. The anomalous behavior of CAMS melts is un-clear (see text for discussion).
a
b
c
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Antonio Paonita
Based on the mechanism of interstitial disso-lution in silicate networks, relationships have al-so been found between noble gas solubility andthe physical properties of melt that give the gaugeof the free space in the silicate network (fig. 7a-c). In particular melt density (Lux, 1987; White et al., 1989), melt molar volume (Broadhurst et al., 1992) and melt Ionic Porosity (IP, Carrolland Stolper, 1993). Carroll and Stolper (1993)clearly show that ionic porosity works better thanthe other properties (density, molar volume) inpredicting solubility, and is thus a better expres-sion of the free space in the melt structure (seefig. 7a-c). In fact, ionic porosity is defined as
(3.2)
where vca is the total volume of cations plus an-ions in one gram of melt and vL is the specificvolume of melt. The relation found by Carrolland Stolper (1993) between the solubility loga-rithm and ionic porosity is
(3.3)
where m and n are fitting parameters to be cali-brated by linear regression of the experimental-ly measured solubility of the noble gas in sev-eral melt compositions, and Henry’s constantKh is defined as
(3.4)
where fi is the noble gas fugacity in the gasphase. Carroll and Stolper (1993) estimatedthese parameters for all stable noble gases, byusing experimental data acquired at 1000-1400°C and 0.1 MPa. In spite of its general ac-complishment, ionic porosity fails to predictsolubility in CaO-MgO-Al2O3-SiO2 (CMAS)melts. Furthermore, although it estimates theoverall free volume in melt, the same volumecould be distributed either in a few large holesor in many small cavities. Following Shibata et al. (1998), the incorporation of network-modifier cations probably generates a kind offree space that is unable to accommodate noblegas atoms (i.e. too small interstitial sites).
Nearly all of the available experimental data(Carroll and Webster, 1994 and reference there-
/K xfh i i=
ln K m nIPh- = +
( / )v vIP 100 1 Lca= -
in; Draper and Carroll, 1994, 1995; Chamorro-Perez et al., 1996, 1998; Shibata et al., 1996,1998; Shackelford, 1999; Walter et al., 2000;Schmidt and Keppler, 2002) have been obtainedby equilibrating a silicate liquid with a gas phasecomposed of solely noble gases, which intro-duces a large uncertainty in the determination ofthe real solubility of noble gases in magmas. Infact, the gas phases coexisting with magmas arenormally H2O-CO2 dominated vapors; morerarely, sulfur can reach several percents, where-as halogens and N2 remain minor componentsand the noble gases occur in trace concentrations(Gerlach and Nordlie, 1975; Giggenbach, 1996).Even at some GPa of total pressure, the partialpressures of noble gases are very low (a fewbars; Carroll and Webster, 1994). In a recent ar-ticle, Paonita et al. (2000) experimentally inves-tigated the effect of H2O and CO2 on helium sol-ubility, in order to simulate data at more geolog-ically relevant conditions. They measured the
Fig. 8. Effect of dissolved water concentration onthe helium Henry’s constant, the latter expressed inMPa and calculated on 8-oxygen basis melt (modi-fied from Nuccio and Paonita, 2000). Experimentaldata (symbols) from Paonita et al. (2000) were ob-tained in the condition Ptot≈PH2O in the range 100-200 MPa, and about 0.1 mol% He in the vapor.Curves were computed by using the model by Nuc-cio and Paonita (2000), which accounts for the effectof H2O and CO2 on both noble gas solubility in meltand fugacity in vapor (see text).
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A review on noble gas solubility and degassing in magmas
isothermal He solubility in basaltic and rhyoliticmelts at pressures of 100 up to 200 MPa, in thepresence of mixed H2O-CO2 vapors (0 to 50mol% H2O) with ≈0.1 mol% He. The solubilityincreased by about a factor of three with 3 wt%H2O added to the melt, whereas a further addi-tion of water, up to 6 wt%, seemed to have verylittle effect on the solubility (fig. 8). The effect ofCO2 addition (up to 0.05 wt%) was within theexperimental uncertainties or slightly negative.Based on these data, Nuccio and Paonita (2000)found that the helium solubility is in very goodagreement with the model of Carroll et al. (1993)(eq. (3.3) with the same parameters as those fordry melts, fig. 9), provided that the ionic porosi-ty melt is computed by taking into account thepresence of dissolved H2O and CO2 (namely theH2O and CO2 partial molar volumes). It is obvi-ous that the H2O and CO2 concentrations in thecoexisting gas and liquid phase must be con-strained by the solubility of these two volatiles.Nuccio and Paonita coupled their model to thethermodynamic model by Papale (1999) for sol-ubility of H2O-CO2 mixed vapors in silicatemelts. They obtained the following equation forionic porosity:
(3.5)
where r and i are ionic radius and molar fractionof each of the I ions forming the melt; x and v° arethe molar fraction and the reference molar vol-ume of each of the n oxides in melt at P and T,and wi,CO2 are binary interaction coefficients be-tween dissolved CO2 and other oxides (wi,CO2 val-ues are taken from Papale, 1999; reference molarvolumes of oxides are from Lange and Char-michael, 1987; and ionic radii from Shannon andPrewitt, 1969). By inserting the ionic porosity
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value from eq. (3.5) into eq. (3.3), the heliumHenry’s constant can be considered valid forH2O-CO2 bearing melts. The empirical parame-ters m and n were the same as those obtained fordry melts (fig. 9). In order to calculate the heliumconcentration in the melt by eq. (3.4), the authorscomputed the noble gas fugacity by using a mod-ified Redlich-Kwong equation of state for theH2O-CO2-noble gas (De Santis et al., 1974).
With the Nuccio and Paonita (2000) model,you can theoretically calculate the solubility ofother noble gases by assuming that m and n pa-rameters in eq. (3.3) are the same as those fordry melts. Although the last assumption has on-ly been experimentally confirmed for He, dueto the lack of experimental data, the authorspredicted that the solubilities would also in-crease with water addition, and become similarwhen 6-7 wt% H2O are dissolved in melt.
The relation between He solubility and wa-ter content of the melt can be tentatively dis-
Fig. 9. Relationship between helium solubility (Kh
in MPa and on 8-oxygen basis melt) versus IonicPorosity (IP) in H2O and H2O-CO2 bearing silicatemelts (H2O and CO2 contents up to 6 and 0.05 wt%,respectively). IP was calculated by taking into ac-count the effect of the main volatiles (see text). Linewas calculated by using eq. (3.3) with m and n pa-rameters in Carroll and Stolper (1993) for dry melts,and it displays a very good agreement with the ex-perimental data (data from Paonita et al., 2000).
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Antonio Paonita
cussed from a structural point of view. Waterdissolution mainly occurs as OH− groups at lowconcentrations of dissolved H2O, whereas mo-lecular H2O becomes more and more importantwith increasing water content in basaltic to rhy-olitic melts (McMillan, 1994; Dixon et al.,1995; Nowak and Behrens, 1995, 2001; Ihingeret al., 1999; Withers et al., 1999; King and Hol-loway, 2002). Although it is generally acceptedthat water depolymerizes silica and alkali-sili-cate compositions, a similar effect is not clearin aluminosilicate melts (such as magmas). Asfirst recognized by Sykes and Kubicki (1993),water could break Si-O-Al bonds and have adepolymerizing effect on the network. On thecontrary, the Si-O-Al bond could be main-tained, but weakened by the formation of a hy-drogen bond, following which the silicate net-work would undergo merely modest deforma-tions (Kohn et al., 1989, 1998; Oglesby et al.,2001). Based on recent studies (Zeng et al.,1999, 2000; Schmidt et al., 2000, 2001), bothmechanisms could occur in the melt. In fact, thesignificant effect of OH− formation on He solu-bility matches better the hypothesis of signifi-cant structural changes rather than small defor-mations, suggesting that noble gases can pro-vide valuable insights into the structure of alu-minosilicate melts. We are unable to evaluatewhether or not Shibata et al.’s approach (1998)predicts the observed increase in the solubilityof helium, because the uncertainties regardingthe structural effect of water and its depolymer-izing role prevent us from obtaining a reliableestimation of the NBO/T ratio in water-bearingmelts. On the other hand, the higher He solubil-ity is consistent with the calculated increase inthe ionic porosity due to the addition of water.At higher concentrations of dissolved water, he-lium solubility seems to decrease with the in-crease of the molecular H2O/OH ratio, suggest-ing that the H2O molecules probably occupyfree spaces in the silicate structure.
3.2. Pressure and temperature effects
Equilibrium partitioning of noble gases be-tween vapor and silicate melt has been de-scribed using the following thermodynamic re-
lation, which is based on the equality conditionof the chemical potential of the gas species invapor and liquid phases
(3.6)
where a and f refer to the activity of the noblegas in melts and its fugacity in vapor; P° and T°are the standard state conditions; ∆H0
P°,T is theenthalpy of dissolution of the noble gas at P°and T; V 0
P, T is the molar volume of noble gas inits reference state at P and T, and R is the gasconstant. Noble gas fugacity is normally com-puted, as a function of P and T, by using a mod-ified Redlich-Kwong equation of state (Hol-loway, 1977).
The parameters a°/f°, V° and ∆H° must becalibrated using experimental data for each no-ble gas as a function of pressure, temperatureand silicate melt composition. Use of eq. (3.6)and the consequent parameter estimation de-pends on the theoretical model adopted for thedissolution process. Based on the previous con-siderations concerning the physical dissolutionof the noble gases, one can hypothesize that no-ble gas atoms dissolve into a fixed populationof holes in the silicate melt network. One canalso assume that the available sites for a givennoble gas are identical, that their number is in-dependent of T and P, and that gas atoms do notinfluence each other reciprocally. This allowsus to define an activity-composition relation,namely a=n/(N−n), where n is the number ofdissolved atoms and N is the number of avail-able sites in the melt, referenced to a hypothet-ical reference state of pure noble gas. By fixingthe temperature and assuming that the volumeof any solubility site is unchanged after the dis-solution of a noble gas atom (namely, V°=0),eq. (3.6) and the previous definition of activitycan be combined to obtain the well-known for-malism of the Langmuir adsorption isotherm
(3.7)
where KL(T) indicates that the equilibrium con-stant depends on temperature. The above for-
( ) /( )K T n N n fL = -
( )
ln lnfa
fa
RH
T T
RTV
P P
1 1,
,
,
, ,
,
P T
P T
P T
P T T
P T
= - - +
- -
∆ P
0
0
cc
c
c
c c
c c cb l
659
A review on noble gas solubility and degassing in magmas
malism has been widely applied to describe no-ble gas solubility in melts and glasses by con-sidering the number of sites as a parameter tobe evaluated (Shelby, 1976; Roselieb et al.,1992; Carroll and Stolper, 1991; Walter et al.,2000). The very high-pressure experiments re-cently performed by Schmidt and Keppler(2002) reached a threshold Ar concentration atwhich the dissolved gas content did not in-crease with increasing Ar pressure in either sili-cic or tholeiitic melts (fig. 4). The authors sug-gested that the sites available to store the Aratoms were fully occupied, therefore the N val-ue was accordingly constrained to 13.8×1020
sites/cm3 and the obtained Langmuir constantwas able to fit the data very well. Approximat-ing the structure of the granitic liquid as atridymite-like network, the number of intersti-tial holes can also be computed. When compar-ing such a number with the N value obtained bySchmidt and Keppler, the latter represents only60% of the interstitial holes. This underlines thefact that only a fraction of the holes is availableto accommodate Ar atoms. In accordance withits much more compact structure, the estimatedN value in the tholeiitic melt was almost one or-der of magnitude lower (3.1×1020 sites/cm3).The number of holes available for the physicaldissolution of Xe was also significantly lowerthan that for Ar, which is consistent with thelarger size of the former. Although the Lang-muir isotherm was successfully applied, noblegas dissolution could violate one of its main as-sumptions, namely that gas atoms do not affectthe structure of the solvent (V°≠0). In fact, a re-cent X-ray absorption spectroscopy study in-vestigated the structural environment of dis-solved krypton in vitreous silica, revealing thatKr atoms are coordinated by oxygen atoms hav-ing a characteristic geometry (Wulf et al.,1999): thus, Kr atoms create or adjust to theirown solubility sites. However, whether or notsmaller gas atoms have a similar network-mod-ifier role must be verified.
Models involving additional parameterswhich also contain information on the structureof the silicate network have been developed fol-lowing statistical-mechanical approaches (Dore-mus, 1966; Studt et al., 1970; Shackelford et al.,1972). In addition to the number of sites, these
models include the vibrational frequency of thedissolved atom, which is treated as a harmonicoscillator in the structural cavity, and the bindingenergy to the interstitial hole. In silica glass, thesetwo parameters depend upon the atomic size ofthe noble gas (lower frequency and more nega-tive binding energy for larger noble gases; Shel-by, 1976; Shackelford and Brown, 1980; Nakaya-ma and Shackelford, 1990). A sharp increase inthe binding energy and decrease in the vibrationalfrequency were obtained along the join Na2O-SiO2 when moving from the silica-rich to thesodium-rich edge of the miscibility gap (Shelby,1973). This is in agreement with both the strongerstructural constraints and the necessity to spendenergy in creating adequate holes in a less poly-merized solvent (see also below).
A different theoretical approach in applyingeq. (3.6) considers that the units of the silicatemelt and noble gas atoms mix ideally. In thiscase, the activity of the dissolved gas is equal toits molar fraction when a Henrian referencestate of pure noble gas is assumed (a hypothet-ical material composed by noble gas arrangedlike a silicate melt). The two parameters V° and∆H° define the dependence of the solubility onP and T, respectively (see table I). Consideringthem as constants in the fitting procedure, themodel is generally able to successfully repro-duce the experimental data in a large range of Pand T conditions (Lux, 1987; White et al.,1989; Carroll and Stolper, 1991, 1993; Draperand Carroll, 1995; Shibata et al., 1998; Schmidtand Keppler, 2002). The calculated molar vol-umes for heavy noble gases (Ar, Xe) in meltsare comparable to their respective co-volumesin MRK equations (the parameter named b),which reflects the atomic volume (White et al.,1989; Schmidt and Keppler, 2002). Slightlylower values than the co-volumes were deter-mined in glasses (Carroll and Stolper, 1991,1993; Draper and Carroll, 1995), suggestingthat the noble gases fit into pre-existing holesand only a small amount of new space must becreated in the melt. Similar results were foundfor helium in glasses, which displays V° valueslower than its co-volumes (Carroll and Stolper,1991). The small V° values of the noble gasesaccount for their Henrian behavior at low pres-sure (at least up to some hundreds of MPa), due
660
Antonio Paonita
to the negligible weight of the V°(P−P°)/RTterm in eq. (3.6). The Ar molar volumes com-puted for different melts (tholeiite to graniticmelts; White et al. 1989, Schmidt and Keppler2002) display a negligible dependence on theliquid composition, suggesting that a similarenvironment surrounds the dissolved atom inspite of the notable structural differences of thesilicate structure.
The heat of the solution (∆H°), in bothglasses and melts, is generally small (see tableI), and experimental data show that temperaturedependence is close to that of the experimentaluncertainties. Negative ∆H° values, namely de-creasing solubility at increasing temperature,have been estimated for silicate glasses (Shack-elford et al., 1972; Shelby, 1972a,b; Carroll andStolper, 1991; Draper and Carroll, 1995) withcompositions from albitic and rhyolitic up tofused silica (fig. 10). Increasing Na2O and K2Oconcentrations to above 20 mol% seem to invertthis solubility-temperature relation (Shelby,1973, 1974). On the other hand, a positive en-thalpy of solution and a positive direct tempera-ture dependence of solubility have normallybeen measured in basaltic melts (Lux, 1987;Jambon et al., 1986; Hayatsu and Waboso, 1985;
see fig. 11), as well in Na2O-SiO2, Na2O-CaO-SiO2, Na2O-MgO-SiO2 melts (White et al., 1989;Shibata et al., 1988). All the described effectsare normally more pronounced for heavier no-ble gases. The dissolution of a noble gas atomin the silicate melt can be usefully divided intotwo steps, each having its own enthalpy varia-tion: i) the creation of a hole to accommodate agas atom in the melt structure, and ii) the trans-fer of the atom into the hole. The former stepneeds energy, namely its enthalpy variation ispositive; instead, moving an atom from a gasinto a more condensed phase involves the evo-lution of heat and a negative ∆H° sign. Thewhole enthalpy of solution is the sum of the twoterms and its sign will depend on which contri-bution is dominant. Glasses and silica-richmelts have open structures, so we can expectthat only a small contribution of energy is need-ed to create cavities inside. Moving atoms intothe melt will be the energetically dominantprocess and, in view of the negative ∆H° signof such a process, we can explain the negativedependence of solubility on temperature. Oppo-site considerations can be made to explain thedirect solubility-temperature relationship ofless polymerized, alkali-rich melts, where a fewpre-existing large holes are available and ener-gy has to be spent to create or enlarge cavities.
Fig. 10. Temperature effect on noble gas solubilityin a basanite melt between 1200 and 1500°C at 0.1MPa. Data are from Lux (1987) and uncertainties arein the order of the symbol size. Curves were comput-ed by using eq. (3.6) with parameters in table I. Sol-ubility slightly increases with the increase of temper-ature.
Fig. 11. Dissolved Ar in a rhyolite glass at 120-130MPa as a function of temperature (data from Carrolland Stolper, 1993). Curves were computed by usingeq. (3.6) with parameters in table I. Solubility dis-plays a slightly negative relation with the increase oftemperature.
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A review on noble gas solubility and degassing in magmas
3.3. Size distribution of solubility sites
Several researchers (Shackelford and Masa-ryk, 1978; Shackelford and Brown, 1980; Shack-elford, 1982; Nakayama and Shackelford, 1990;Carroll and Stolper, 1993) used the different sol-ubilities among the noble gases as a gas-probe toinvestigate the distribution of hole size in vitreoussilica and natural silicate melts. The logarithm ofnoble gas solubility displays a linear relation withthe atomic radius (fig. 5), meaning that hole sizesresult as having a log-normal distribution. Themode shifts towards higher hole size with in-creasing ionic porosity, thus large holes becomemore common than small ones and the solubilityof heavier noble gases increases more quicklywith respect to that of light gases. In accordancewith the above conclusions, a numerical simula-tion of the interstitial structure of silica glass con-firmed the log-normal distribution (Chan and El-liot, 1991).
Nuccio and Paonita (2000) assumed that them and n parameters in eq. (3.3) are composi-tion-independent for all noble gases, in accor-dance with what was experimentally verifiedfor helium. In these conditions, their extendedionic porosity model for helium solubility inH2O-CO2 bearing melts is valid for other noblegases. They calculated that a low percent of wa-ter in a melt causes an exponential increase ofthe number of large holes, as a consequence ofpotential structural modifications of the melt.These conclusions need to be proved using sol-ubility data for noble gases having a largeatomic radius.
4. Implications in modeling magmadegassing
Several elements in magmas are able to formchemical species which may be exsolved in va-por phases (H, C, O, S mainly, halogens, nitro-gen, noble gases etc.; Gerlach and Nordilie,1975; Giggenbach, 1996). These elements canreact with the liquid forming chemical bonds(i.e. OH groups), or they exist under the form ofvolatile atomic or molecular species dissolved inthe silicate melt (i.e. molecular H2O, noble gas-es). Any dissolved volatile species (true or po-
tential) is characterized by its own vapor pres-sure, which depends directly upon its concentra-tion in the melt. Magma reaches supersaturationwhen the sum of the vapor pressures of the indi-vidual volatiles becomes higher than the totalpressure of the system, as a consequence of as-cent and depressurization of the melt (Carrolland Webster, 1994). In such conditions, the mag-ma is potentially able to exsolve a vapor phase.As H2O and CO2 are the most abundant volatilesin natural magmatic systems, the assessment ofmagma saturation has been modeled as a gasmixture of these two compounds (Papale, 1999),although sulfur could have significant effectswhen its concentrations reach several weightpercent in the system (Moretti et al., 2003).
When the vapor phase exsolves, minorvolatiles are partitioned between the silicate liq-uid and the coexisting vapor, depending upontheir solubilities. In fact, early vapor is stronglyenriched in highly volatile species, which areconsequently depleted in the melt (Giggenbach,1996). The more soluble species are only ableto exsolve into the gas phase during the latestages of degassing. As a consequence, less sol-
Fig. 12. Fraction of noble gases remaining in atholeiitic melt with respect to the amount of exsolvedvapor during a closed system degassing. Vapor/meltratio is the volume of gas phase exsolved per volumeunit of melt. Adopted equations for calculations werethose by Giggenbach (1996), as well as the H2O andCO2 solubilities. CO2 solubility causes CO2 to degasbefore light noble gases, whereas H2O is much moresoluble and is exsolved during late degassing.
662
Antonio Paonita
uble, heavy noble gases (Ar, Kr, Xe) are de-gassed earlier, whereas He and Ne degassingoccurs later (fig. 12).
This solubility-controlled fractionation ofthe noble gases is much more significant if in-finitesimal aliquots of volatiles are exsolved inequilibrium from the liquid phase and immedi-ately removed, thereby creating open systemdegassing conditions. However, this behavior isalso evident during closed system degassingprocesses, during which the exsolved vaporstays in contact with the melt throughout thedegassing process (Carroll and Webster, 1994).Noble gas fractionation in open system de-gassing can be modeled by applying the typicalRayleigh distillation process. By consideringtwo noble gases i and j, the extent of fractiona-tion between two gases is related to the residualfraction of one of the two volatiles
(4.1)
where C is the volatile concentration in melt(m); subscript «0» refers to initial conditions(undegassed parental melt) and K is the Henry’sconstant. Calculations by eq. (4.1) have been ap-plied to ratios between noble gases as a functionof the total content of one noble gas in basalts(Taylor, 1986; Sarda and Graham, 1990; Carrolland Webster, 1994; Marty, 1995; Giggenbach,1996; Burnard, 1999; Moreira and Sarda, 2000).By writing two Rayleigh equations for a ternarysystem, such as He-Ar-CO2, and coupling them,it can be shown that a plot of ln(Ar/CO2) versusln(Ar/He) yields a straight line with a slope de-pendent upon the solubility ratios among thegases and an intercept dependent upon theAr/CO2 and Ar/He ratios in the pristine unde-gassed melt (Burnard, 1999, 2001).
In a closed system, vesicles of an exsolvedvapor remain in the melt, so the total amount ofgas does not vary. Models have been developedthat take into account the mass balance amongthe total amount of noble gas, its dissolved con-centration and the volume of exsolved vaporper mass unit of melt (Jambon et al., 1986; Sar-da and Moriera, 2002). By coupling Henry’sLaw to the mass balance, the fractionation be-
CC
CC
CC
,j
i
j
i i
i
K
K
00
1j
i
=-
c c
d
m m
n
tween noble gases can be related to the vesicu-larity of the melt (the volume of vapor phaseexsolved per volume unit of melt)
(4.2)
where T0 is equal to 273 K; Te is the tempera-ture of the gas-melt equilibrium; ρ is the densi-ty of the silicate melt (assumed constant), andV* is the melt vesicularity.
The above models have been frequently ap-plied to study degassing on spatial scales fromlocal to planetary. For example, investigationswere performed on the gas phase trapped in vesi-cles and dissolved in glasses recovered fromMiddle-Oceanic Ridge Basalts (MORB) andOceanic Island Basalts (OIB) (Jambon et al.,1986; Taylor, 1986; Sarda and Graham, 1990;Carroll and Webster, 1994; Giggenbach, 1996;Burnard, 1999, 2001; Honda and Patterson,1999; Moreira and Sarda, 2000; Sarda and Mori-era, 2002). Furthermore, constraints were ob-tained on the outgassing history of the mantleand its primordial composition. The higherHe/Ar ratios in MORB glasses with respect totheir production/accumulation rate in the mantle(due to K and U decay), have normally been ex-plained by degassing processes (Jambon et al.,1985), because Ar is preferentially degassed inrespect of He. On this basis, Moriera and Sarda(2000) focused on the different degassingmodalities that occur from MORB and frommantle plumes, the former degassing as a closedsystem, and the latter as an open-systemRayleigh-type process. However, some ques-tions have been raised about this «degassing»hypothesis, namely that higher He/Ar ratios oc-cur at higher He contents (Honda and Patterson,1999) and in basalts erupted at greater depths(Fisher, 1997). Solubility controlled degassingdoes not explain these constraints, whereasmechanisms of helium diffusion from mantleminerals or crystals in basalt melts have been hy-pothesized (Matsuda and Marty, 1995; Fisher,1997; Honda and Patterson, 1999). In fact, Mor-eira and Sarda (2000) and Sarda and Moreira(2002) have suggested that the geochemical pat-tern of noble gas abundances in MORB is con-sistent with equilibrium vesiculation followed by
/( )/( )
CC
CC
KK
V T T KV T T K
,
,
*
*
j
i
j
i
i
j
e
e
i
j
0
0
0
0# #=
++
tt
663
A review on noble gas solubility and degassing in magmas
various episodes of bubble loss, in accordancewith the early model by Sarda and Graham(1990). However, Sarda and Moreira (2002)have also suggested that helium solubility shouldbe two times higher than that experimentallymeasured at 0.1 MPa, and that Ar and Ne solu-bilities should be lower by factors of 3.5 and 7,respectively. This opposite variation in solubili-ties makes their hypothesis about a pressure ef-fect on solubility rather unlikely. Burnard (1999)moves criticisms on the experimental values ofsolubilities too, providing evidence that the no-ble gas/CO2 solubility ratio has to be higher thanexperimentally measured. Nevertheless, a newview on noble gas data from MORB takes intoaccount the size of the vesicles, as small bubblesshould represent the previous extent of the de-gassing with respect to large vesicles (Burnard,2001). Sequential crushing of basalts allows forthe analysis of gas from progressively smallerbubbles, which can be used to determine the tra-jectory of the degassing. Preliminary data seemto reconcile with predicted solubility-controlledfractionation studies (Burnard, 2001).
The foregoing discussion stresses how noblegas abundances from natural samples are usedas a reference point and as a check of experi-mental results on noble gas solubilities. It alsohighlights the wide use of the noble gas solubil-ity data in Earth Sciences problems and thestrong inferences they provide. Considering thefact that the experimental conditions do notmatch the geological ones, degassing modelsthat use such data must be applied very careful-ly. Equations (4.1) and (4.2) describe how noblegases are partitioned in a vapor while they areprogressively exsolved, regardless of: i) whattype of volatiles dominate the vapor and if theybehave non-ideally in such a gas mixture, ii)changes in concentration and, above all, in solu-bility due to the exsolution of the main volatiles.Application to MORB degassing could disre-gard the second conditions, at least for a first ap-proximation, because the amounts of H2O andCO2 in these melts are lower than 1 wt% at leastfor P<1 GPa. However, MORB degassingshould take into account the noble gas fugacityin CO2-dominated vapor. Bottinga and Javoy(1990) discussed the exsolution of a non-idealH2O-CO2-He-Ar mixture from bas-altic melts,
although the H2O and CO2 effects on noble gassolubility were not considered. Nuccio andPaonita (2001) started from the already dis-cussed model for noble gas solubility in H2O-CO2 bearing melts (Nuccio and Paonita, 2000;see Section 3.1). The authors obtained an equa-tion that allowed them to calculate the concen-tration of noble gas in an exsolved H2O-CO2
mixed vapor as a function of the dissolved con-centration of H2O, CO2 and noble gas in meltand their total amounts in the vapor+melt sys-tem. Thus, the authors numerically simulatedthe degassing of multicomponent H2O-CO2-no-
Fig. 13. He/Ar fractionation during closed systemdegassing of basaltic melt at 1200°C. Initial He/Arratio was 3. Thick grey lines were computed by us-ing the model of Jambon et al. (1986) (eq. (4.2) inthe text), assuming noble gas solubilities were not af-fected by the presence of the main volatiles (K=con-stant) and ideal gas behavior. Thin black lines werecomputed on the basis of the model by Nuccio andPaonita (2001), which accounts for these effects. Inthis model, the initial conditions were 85 mol% CO2
in the H2O-CO2 mixed vapor (noble gases are com-puted as minor species) at 700 MPa pressure. Blackthin lines for melt and vesicles approach each otherat low pressure, due to the effect of water that makesthe solubilities of noble gases more similar. Dashedcurves were computed for an open system (grey lineby Jambon et al., 1986 model black line by Nuccioand Paonita, 2001).
664
Antonio Paonita
ble gas mixtures from depressurizing magmas.As a result, remarkable differences were high-lighted between the comprehensive approachand simpler models (figs. 13-15). In spite of theconstant ratio between He/Ar in melt and vesi-cles computed by the simpler model (namely,(He/Ar)gas/ (He/Ar)melt = KHe/KAr), curves fromthe more complex model come closer at lowpressure. In fact, taking into account the fact thathigh pressure causes very different solubilityamong noble gases and that dissolved water hasthe opposite effect, a few wt% of dissolved wa-ter at low pressure are able to significantly re-duce the solubility difference between He andAr (fig. 13 and see Section 3.1). As a conse-quence, noble gases may fractionate less thanpredicted by simpler calculations, depending onthe composition of the magma and on the mainvolatiles it contains. The application of the mod-el to very different contexts, from silicic to
basaltic volcanoes (Nuccio and Paonita 2001;Caracausi et al., 2003), shows how the inert gascontent of outgassed magmatic volatiles mayproduce information and constraints on deepmagmatic conditions: magma pressure anddepth over time, degassing and volatile content,melt composition and temperature. The currentresearch aims at developing approaches to de-scribe saturation surfaces in more complex sys-tems, including sulfur and halogens. On this ba-sis, the solubility of noble gases in such systemsprogressively more similar to true geologicalconditions, will have to be theoretically and ex-perimentally investigated, allowing for the de-velopment of more refined degassing models.
Finally, vapor-melt equilibrium for noblegases has been assumed in all the discussedmodels. Experimental observations and theoret-ical modeling have been performed to investi-gate the growth of steam bubbles in silicate liq-uids (Bottinga and Javoy, 1990; Proussevichand Sahagian, 1996, 1998; Gardner et al., 1999,2000). Bubbles seem to maintain chemicalequilibrium with the surrounding melt even inthose silicic magmas whose ascent rates are
Fig. 14. He/Ar fractionation during closed systemdegassing of rhyolitic melt at 1000°C. Curves as infig. 13. In the calculation using the model by Nuccioand Paonita (2001), initial conditions were 40 mol%CO2 in H2O-CO2 mixed vapor (noble gases are com-puted as minor species) at 500 MPa pressure. Thehigher amount of dissolved water (6 wt%) in respectof the basaltic melt amplifies the discussed water ef-fect, causing very low He/Ar fractionation.
Fig. 15. He/Ne fractionation during closed systemdegassing of basaltic melt at 1200°C. Curves andconditions as in fig. 13. The behavior is qualitativelysimilar to that discussed in the case of argon, al-though the fractionation is more modest.
665
A review on noble gas solubility and degassing in magmas
three orders of magnitude higher than theircommon pre-eruptive values (∼0.01 m/s),whereas non-equilibrium exsolution could oc-cur during explosive rising. Nevertheless, thereis a total lack both of experimentation and mod-els for the investigation of possible disequilib-rium fractionation among noble gases. In addi-tion, noble gas diffusion in melts containingH2O and/or CO2, which is crucial to describesuch processes, is also poorly known.
5. Upcoming research
Although our knowledge of noble gas solu-bility in magmas has significantly improved overthe last twenty years, a lot of work has yet to beaccomplished from both experimental and theo-retical points of view. Magma is a multicompo-nent system carrying several volatile compo-nents in its gas phase; nevertheless our studies onthe solubility of noble gases in such complexvolatile-bearing systems are still in their earlystages. Few experimental data are available re-garding helium solubility in silicate melts coex-isting with H2O-CO2 mixed vapors. Althoughthese studies are not comprehensive enough tofully describe and understand the effect of themain volatile components on noble gas solubili-ty, they do highlight that it cannot be neglected.The dissolution of noble gases is strictly relatedto the availability of solubility sites in the silicatenetwork and, due to the significant structuralchanges in melts caused by the main volatiles,H2O and CO2 may well have a very significantcontrol over noble gas solubility.
On this basis, sulfur and chlorine (perhapsfluorine as well) may exhibit similar effects,however we lack critical information to evalu-ate them. As far as the gas phase is concerned,there are equations of state that can calculatethe activity-composition relations for Ar in H-C-S-O mixed vapor in a wide range of pressureand temperature conditions (Belenosko et al.,1992). More recently, equations have becomeavailable for noble gases in H-C-S-O-halogens(Churakov and Gottschalk, 2003). Neverthe-less, the activities of noble gas in S-Cl bearingsilicate melts are totally unknown, and there isa great need for experimentation in this field of
research. In addition, the inadequate knowledgeof the structural effect of sulfur and chlorinedoes not allow us to evaluate their influence onthe distribution of the hole-size in melts and, asa consequence, on noble gas solubility. Regard-less of the analytical method, the experimentaldetermination of noble gas solubility in com-plex systems requires the charging of knownamounts of volatiles inside capsules, in order toobtain vapor phases having similar composi-tions to those of magmatic ones. In view of this,the method of loading noble gases by gas-bear-ing glasses (see Section 2) should be broadlyapplicable to the investigation of the solubilityof all noble gases (even He and Ne) in melts upto very high pressures. Although the mass bal-ance between the total and dissolved noble gasallows us to calculate the concentration of no-ble gas in vapor and then the partition coeffi-cient, the ability to measure the content of no-ble gas in both the vapor and liquid phase be-comes ever more important. In this respect,coupling in situ extraction techniques and massspectrometry seems the most powerful tool, es-pecially in investigating the solubility of lightnoble gases.
In addition, X-ray absorption techniqueshave started to provide insights into how noblegas atoms are accommodated in the silicate meltstructure and the short-range order. Althoughthese data do not produce a direct measurementof solubility, they are still able to constrain theparameters used in the statistical-mechanics sol-ubility models. Such theoretical approaches givedetailed information and explanations of themechanism of noble gas dissolution in melt andthus they need to be further improved. Addition-al information could also come, in the future,from computer simulations of noble gas-bearingmelts by using Molecular Dynamics (MD) meth-ods, although a lot of work must be done to de-fine pair potentials between the noble gas and themain particles of the melt by fitting experimen-tal data or by performing ab-initio calculations.For this work, the expertise of mineralogists andmaterials scientists, interested in the consider-able inferences of noble gas solubility on thestructure of silicate glass and melt, could be veryhelpful. This co-operation should aim at devel-oping several aspects having technological and
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Antonio Paonita
commercial interests and involving the interac-tion of gas atoms with rigid glass materials. Forinstance, noble gases (He, Ar) are normally usedin the growth of silicon-made materials for use insemiconductor devices, or with the aim of pre-venting gas inclusion formation during the man-ufacture of glass and ceramics. Ceramics are al-so porous enough to allow for noble gas perme-ation, allowing Magnetic Resonance Imaging(MRI) using laser-polarized noble-gas-testing ofthe structural integrity of the material. Large dif-ferences in the solubility and diffusion of thegases can be used to separate the components ofgas mixtures.
Finally, an interesting field to be developedin the future involves the possible effects of dis-equilibrium during noble gas degassing frommagmas. In their recent paper, Caracausi et al.(2003) show that the helium isotopic composi-tion monitored in volcanic emissions at Mt. Etnadisplays variations over time, which cannot bereferred to shallow secondary processes. Kineticfractionation of helium isotopes during diffusionin growing bubbles has been suggested to ex-plain this type of signal (Nuccio and Valenza,1998; Caracausi et al., 2003). In fact, light 3Heatoms could diffuse at higher rates in growingbubbles with respect to heavier 4He atoms. In thesame way, the ratios between noble gases both involcanic gases and fluid inclusions could be af-fected by a similar disequilibrium process, espe-cially when taking into consideration that theyhave similar dissolution mechanisms and thatthere is a big difference between their atomicmasses. Experiments on this topic are not simpleand theoretical calculations, to be based on noblegas diffusion rates, require adequate knowledgeof the effects of the main volatiles on noble gasdiffusivity in silicate melts. A lot of effort stillhas to be made in this direction, which howeverseems very promising in its application to mag-matic systems.
Acknowledgements
The author wishes to thank P.M. Nuccio foruseful comments and suggestions. The reviewby D. Baker and C. Romano greatly improvedthe manuscript.
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