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ANNALS OF GEOPHYSICS, VOL. 48, N. 4/5, August/October 2005

Key words volatiles – magmatic rocks – basalticglass – degassing – Hawaiian lava – remelting – blacksand

1. Introduction

The analysis of volatiles in natural wholerocks has historically attracted little attention

from igneous petrologists, in contrast to major-,trace-elements and isotope analysis. As Carolland Holloway (1994) wrote (1.1): «the words ofBowen 1928 are still broadly applicable … tomany petrologists a volatile component is exact-ly like a Maxwell demon; it does just what onemay wish it to do».

The LOI (Loss On Ignition) is in many cur-rent publications the only information aboutvolatiles in natural samples. Special analyticalmethods as well as KFT, SIMS or FTIR havebeen developed for several volatile species.DEGAS was developed in the last 20 years fora simultaneous determination of complex mix-

Determination of volatiles in volcanic rocksand minerals with a Directly Coupled

Evolved Gas Analyzing System (DEGAS) –Part I: Interpretation of degassing profiles(DEGAS-profiles) of minerals and rocks

on the basis of melting experiments

Klaus Heide and Christian M. SchmidtInstitute of Geosciences, University of Jena, Germany

AbstractVolatile components in magma strongly influence many physical properties of melts and minerals. The temperatureresolved degassing analysis of volcanic crystalline and vitreous rocks gives detailed information about volatile com-pounds in the melt. A special high-temperature mass-spectrometry device in combination with a thermo-balance al-lows a quantitative determination of different volatile species. It enables a differentiation between the primary gascontent in the magma and the gas released from decomposition of secondary alteration products. The gas releaseprofiles give the following indications: i) during the littoral explosions of Pahoehoe lava the content of volatiles isnot changed by interaction with air or sea water; ii) the degassing profiles of vitreous black sand verify the primarycontent of volatiles in the erupted melt, only CO2 was detected; iii) the oxygen release profile gives significant in-dications for oxygen undersaturation of the erupted magma; iv) remelting of black sand in air at 1450°C for 0.45 hcauses an oxygen saturation of the basaltic melt; v) remelting of black sand in argon atmosphere confirms the oxy-gen undersaturation of the melt; vi) remelting of black sand-black shale mixtures affects a significant change in thedegassing profiles, especially in CO2-release. With the first investigations we can demonstrate that gas releasecurves of volcanic rocks are qualified for a) detection of the primary gas content of erupted magma; b) detection ofalteration processes of the igneous glass; c) detection of contamination of the magma with adjacent rocks.

Mailing address: Dr. Klaus Heide, Institute of Geo-sciences, University of Jena, Burgweg 11, D-07749 Jena,Germany; e-mail: ckh@rz.uni-jena.de

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tures of volatiles. This method has been usedwith success to solve problems of quality inglass production by uncontrolled formation ofbubbles in the melt (Heide et al., 2000). It wasdemonstrated by systematic studies of industri-al and laboratory produced specimens that aquantification of volatiles dissolved in industri-al glass as well as in the matrix of magmaticrocks is possible. This allows the discrimina-tion of residual volatiles in whole rocks.

For individual gasses like water the resultsare in agreement with other analytical tech-niques such as SIMS, FTIR, LECO or KFT(Leschik et al., 2004). Unlike these volatile-specific techniques DEGAS allows us to deter-mine the amount of all volatiles evolved fromthe glass or the minerals in a single run.

This contribution presents preliminary re-sults of degassing experiments performed onremelted basaltic glasses. The aim of the pa-per is to show the technical potential of DE-GAS in the study of magmatic degassing andto draw the attention of the geological com-munity to the possibilities of our analyticaltechnique.

It is well known that the degassing behav-iour is related to solubility properties of differ-ent volatile species, depending on factors suchtemperature, pressure and composition.

It has been demonstrated that the degassingbehavior of a silicate melt in vacuum conditionsis a complex process of evolution of differentvolatile species (Heide, 1989; Stelzner and Hei-de, 1992). A systematic study of vitreous rimsfrom specific positions in the borehole HSDP2(Hawaiian Scientific Drilling Project II) by Hei-de and Schmidt (2003) gave detailed informationabout temperature ranges of degassing and theconcentrations of volatile species. The degassingrate and the degassing mechanism (formation ofbubbles or/and diffusion from the bulk) is differ-ent for different species. A new aspect is the phe-nomenon of oxygen emplacement in the melt.Such a decrease of the partial pressure of oxygenduring the degassing experiment was frequentlyobserved by DEGAS (Heide and Schmidt, 2003).

The amount of material needed is in the or-der of 10 to 100 mg, i.e. about the size of syn-thetic products obtained in experimental petrol-ogy (Geiger et al., 2001; Leschik et al., 2004).

Fig. 1. Black sand beach, Kaimu, Big Island Hawaii, U.S.A.

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Determination of volatiles in volcanic rocks – Part I: DEGAS-profiles

At present we are far from having a completeunderstanding of the degassing curves, but exper-iments have shown that the degassing profiles aresensitive regarding the following parameters: i)the melting temperature; ii) the starting materialsfor the melt, meaning the «raw materials» for theglass forming melt e.g. silicates, carbonates, sul-fates, hydroxides, hydrates etc.; iii) the «finingagents» in the melt, meaning polyvalent ionswhich form oxygen during a redox reaction. In

case of natural systems the main process is the re-duction of iron oxide

(1.1)

This knowledge coming from industrial use al-lows us to ask the questions:

– Which results can be obtained from thedegassing behavior of a natural volcanic glass-es or rocks?

– Which effects result from the smelting ofsedimentary rocks from the lithosphere in theextrusive volcanic melt?

2. Experimental procedures

2.1. Samples and treatment

The volcanic glasses studied here were sam-pled in 1999 from the black sand beaches ofKalapana (Kaimu), Big Island, Hawaii. Theglassy volcanic debris was formed during thelittoral explosions of Pahoehoe lava flows atKaimu in 1990 and 1991 (fig. 1). As shown infig. 2 the single black particles consist of abrownish fresh glass with euhedrial crystals ofolivine and magnetite. These vitreous frag-ments are similar to glassy rims on the basalticlava (Heide and Schmidt, 2003).

.Fe O Fe O O221III II

2 3 2" +

Fig. 2. Optical micrograph of vitreous particlesfrom the black sand. Ol – olivine phenocrysts; M –magnetite; Vi – vitreous matrix.

Table I. Chemical composition of Hawaiian basaltic lava.

Tholeiitic Hawaiian basalt Hawaian shield Kaimu black Remelted black sand(average, Macdonald et al., 1983) basalt (Bevens and sand (XRF) (Kaimu), vitreous

Wright, 1992) matrix (EDX)

SiO2 49.4 48 51.0 47.6TiO2 2.5 3 2.5 2.2Al2O3 13.9 14 13.0 11.2Fe2O3 3.0 3 12.8total -FeO 8.5 8 - 12.7total

MnO 0.2 0.1 0.2 0.5MgO 8.4 7 8.7 15.9CaO 10.3 9 11.0 8.6Na2O 2.1 3 2.2 1.7K2O 0.4 1 0.4 0.4P2O5 0.3 0.5 0.2 -

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Klaus Heide and Christian M. Schmidt

The chemical composition of glassy vol-canic debris is characterized by tholeiitic lavatype (table I).

Particles between 2 and 6 mm size wereseparated from the bulk for remelting experi-ments. A black melt with a volume of 10 cm3

was obtained by heating in an alumina crucibleat 1400°C. The melting process was carried outfor 0.45 h in an oxidizing (air) or inert (argon)atmosphere. As shown in fig. 3 a characteristicphase separation occurs in the melt. Olivinecrystals from the starting material were concen-trated on the bottom of the crucible. The bulkmaterial is a brownish glass (table I) with inclu-sions of olivine and pyroxene crystals and mag-netite dendrites. Olivine and pyroxene crystalsshow the characteristic spinnifex structure.

To demonstrate the influence of a remeltingof sedimentary rocks in the basaltic melt 10wt% of a powdered black shale was added tothe starting mixture.

2.2. Experimental techniques

Evolved gas analysis was carried out usinga special high-temperature mass-spectrometer(Heide et al., 2000). Measurements were per-formed under a vacuum of 10-3 to 10-4 Pa using

a constant heating rate of 10 K/min from RoomTemperature (RT) to 1450°C. The ion currentfor different mass/charge ratios (m/z=1 … 200)was recorded in a Multiple Ion Detection(MID) mode as a function of time by a Quadru-pole Mass-Spectrometer (QMA-125 BalzersAG). Temperature was controlled by a type Sthermocouple at the bottom of the alumina cru-cible. The degassing process occurs under high-ly non-equilibrium conditions. Volatile speciesare rapidly removed from the sample and hencethe probability for reverse reactions betweenthe volatiles and the melt can be avoided. Fur-thermore, interactions between the evolved gasspecies are minimized and a determination ofthe primary evolved species is possible.

For degassing experiments we used careful-ly separated single particles with a total weightof about 20 mg.

In the following the gas release profiles ofhydrogen, hydrocarbons (methane), water, car-bon monoxide, carbon dioxide, oxygen, hydro-gen chloride and sulfur dioxide are discussed.

3. Results

The degassing profiles of original black sandfrom Kaimu (I), remelting products in oxidizing(II) and argon atmosphere (III), and melting prod-ucts from black sand mixed with 10 wt% blackshale in oxidizing (IV) and argon atmosphere (V)are summarized in figs. 4 to 9.

H2 (m/z2=H2+=molecule ion of hydrogen) –

Samples (I) and (III) show a hydrogen-releaseabove 1150°C with a broad maximum at1310°C (fig. 4). Sample (II) shows an increaseof hydrogen evolution above 1200°C up to1450°C without a maximum. Samples (IV) and(V) are similar to (I) and (III).

Hydrocarbons (fragment ions from CH4+=

=16 like CH3+=m/z=15) – No significant re-

lease of hydrocarbons, here characterized byfragment ion CH3

+ (m/z 15) was detectable inany samples (fig. 5).

H2O (m/z=18=H2O+= molecule ion of wa-ter) – Only very small quantities of water evolved

Fig. 3. Optical micrograph of remelting productfrom black sand. Melting condition: 1400°C, 0.45 hin air; Ol – residual olivine phenocrysts; SOL– skele-tal olivine

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Determination of volatiles in volcanic rocks – Part I: DEGAS-profiles

from the original black sand sample (fig. 6). Thesmall spikes at approximately 900°C and 1110°Cresult from bursting of bubbles. The decrease ofpartial pressure in the degassing profiles fromblack sand and black shale mixtures is due tochemical reactions in the melt.

CO, CO2 and N2 (m/z=28: molecule ions ofCO and N2 as well as fragment ion of CO2 m/z==44−16=28) – For determination of CO-de-gassing a more complex analysis of degassingprofiles is necessary (Schmidt and Heide, 2001,fig. 7). The intensity of molecule ion CO+ (m/z28) can be superposed by the molecule ion ofnitrogen N2

+ (m/z 28) and by the fragment ionCO+ from the CO2 molecule. By comparison of

the degassing profiles of m/z=14 (fragment ionN+ or CO++, fig. 8) with the runs of m/z 28 andm/z 44 (molecule ion of CO2, fig. 9) a separa-tion of N2, CO and CO2 is possible.

Most of the carbon dioxide evolved from theblack sand by bubbling from 1050 up to 1130°C.The degassing behavior is completely changed byremelting in air. Carbon dioxide evolved mainlyby a diffusion process with a maximum diffusionrate at 1240°C. Remelting in inert atmosphereshows a degassing by diffusion and by bubbling.Addition of black shale to the black sand showssimilar degassing profiles as the samples from in-ert remelting. Only in these mixtures are traces ofnitrogen observed in the melt, which escaped bybubbling at approximately 1200°C (see fig. 8).

Fig. 4. Hydrogen (m/z=2) release profiles from black sand and remelting products.

Fig. 5. Degassing profile of hydrocarbon fragment of methane CH3+-(m/z=15) from black sand and remelting

products.

4 5

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Klaus Heide and Christian M. Schmidt

O2 (m/z=32=molecule ion of 16O2+ and 32S+)

– Especially interesting are the oxygen releasecurves (m/z 32). As shown in fig. 10 during de-gassing of black sand the oxygen partial pres-sure decreases above 1100°C up to 1300°C.

After remelting in air still above 1200°C,oxygen releases from the melt by diffusion witha maximum at 1210 °C.

Gas release profiles of non-oxidizing melt-ing products are similar to degassing of the nat-ural sample.

Mixtures with black shales molten in oxi-dizing as well as non-oxidizing atmospheresshow similar trends.

The discrimination between oxygen andsulfur is possible using specific fragment ionsas well as m/z=64 for SO2

+ or m/z=48 for SO+

and by correlation of intensities of molecule

ions with fragment ions (more details see inHeide, 1982).

HCl (m/z=36=H35Cl+ molecule ion) – Thereis no indication of HCl-release in the case of thestarting material from the beach (fig. 11). Thesmall degassing peak from the in air remeltedsample can be explained by a contamination ofthe sample from the air during the remeltingprocess (2 h) in the laboratory furnace or by aninhomogeneity in the starting material. Fromthe igneous sample HCl does not escape in therange of detection limit (1×10-12 ion current isapproximately equivalent to 100 ppb using 50mg of starting material).

SO2 (m/z=64=molecule ion of SO2+ or frag-

ment ion of SO3+=m/z 80−16) – No indication

Fig. 6. Water release (m/z=18) from black sand and remelting products.

Fig. 7. Change of partial pressure of the mass number m/z=28.

6 7

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Determination of volatiles in volcanic rocks – Part I: DEGAS-profiles

of SO2-release is observed in the original sam-ple from the beach. Very small quantities of sul-fur oxides (detected as SO+ (m/z=48)) evolvedfrom the remelted black sand samples only bybubbling (fig. 12). The small spikes in the tem-perature range between 1100°C and 1200°C re-sult from bursting of bubbles.

To check the influence of olivine phenocrystson the gas release, olivine crystals were separat-ed from the glassy matrix.

As shown in fig. 13 the described techniqueallows a simultaneous characterization of es-caped gases from the matrix glass as well asfrom the crystal inclusions. The olivine phe-nocrystals show a spiky evolution of CO2 in therange at 1000°C, which can be explained by thedecripitation of fluid inclusions in the xeno-crysts.

4. Discussion

4.1. Gas release from the black sand samples

The degassing profiles from the littoral vit-reous lava fragments give significant indica-tions that the content of volatiles of magma wasnot changed by an interaction with air or seawater. From the magma CO2 only evolved bybubbling above 1000°C (fig. 9).

As shown in fig. 13 small quantities of CO2

evolved from olivine as spikes above 1000°C inthe same temperature range as from the matrixglass. The formation of fluid inclusions in theolivine crystals and the solution of CO2 in theglass forming melt take place in the same tem-perature – pressure range. This is an importantindication that the high temperature degassing

Fig. 8. Change of the partial pressure of the mass number m/z=14.

Fig. 9. CO2 release of black sand and remelting products.

8 9

726

Klaus Heide and Christian M. Schmidt

gives valuable information about the interac-tion of volatiles with the melt during the ig-neous process.

Furthermore the main CO2 volume evolvedfrom the vitreous matrix in the temperaturerange between 500 and 600°C. This is an un-equivocal indication of a carbonate decomposi-tion. A definite discrimination between carbondioxide solved in the melt and from secondarycarbonates is possible.

The non-detectable interaction of eruptedlava with air or sea water was confirmed by theoxygen degassing profile. This agrees with theobservation by Burkhard (2001) that no oxida-tion of Kilauea basalt glass is detectable. Fur-thermore Dixon and Clague (2001) describe arelatively dry plume component in the basalticglasses from Loihi Seamount.

In addition, HCl release profiles give no in-dication for a contamination by sea water.

The SO2 release is an indication of a subse-quent devolatilization of the magma during theeruption process.

Subaerially erupted Hawaiian magmas arecharacterized by sulfur contents of 100 ppm ata δ34S<0. This may indicate loss of total sulfurand δ34S compared to submarine Hawaiianmagmas (700 ppm, δ34S>0), due to syn-erup-tive degassing. This fractionation pattern alsoindicates that the degassing phase must be oxi-dized sulfur. SO2 commonly is the dominantphase at melt temperatures, although aside fromSO2 and SO3, reduced species may occur (Ger-lach and Graeber, 1985).

During the melting of the black sand parti-cles under high vacuum conditions, the affinity

10 11

Fig. 10. Change of the partial pressure of mass number m/z=32 (O2+).

Fig. 11. Change of the partial pressure of mass number m/z=35 (35Cl+).

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Determination of volatiles in volcanic rocks – Part I: DEGAS-profiles

4.2. Gas release from fresh erupted Etna lava

As fig. 14 demonstrates the fresh eruptedsubaeric lava from Etna (eruption at 31.12.2002south flank) shows a different oxygen releaseprofile. After a minor decrease of the oxygenpartial pressure above 1100°C the oxygen is re-leased from the melt by bubbling. Further sys-tematic studies are necessary to confirm thisdifference between the Island-Arc and theOcean Island basalts. The strong oxygen affini-ty of the melt of both lavas also causes oxygenincorporation by remelting in air under normalpressure. From the maximum in the gas releasecurve (fig. 10) a significant change occurring inthe oxidation state of the remelted samples canbe concluded. The results are confirmed by theremelting experiments under inert atmosphere.

From our studies it is clear that the basalticlava of Hawaii volcanism is strongly under-sat-urated with oxygen.

Fig. 13. CO2 release curves from black sand sam-ples. A – vitreous matrix with olivine crystals; B –vitreous matrix without olivine crystals; C – olivinecrystals, separated from the vitreous matrix.

Fig. 12. Change of the partial pressure of massnumber m/z=48 (SO+).

of the melt to the oxygen concentration in thevacuum furnace is very strong. Oxygen wasdissolved in the melt and by that the partialpressure of O2 in the furnace decreased. Thusthe oxygen from the furnace devises was solvedin the melt

(4.1)

From these preliminary results we suggest thatthe oxygen fugacity in the vacuum of the ig-neous glass should be lower than Fayalite Mag-netite Quartz (FMQ)-buffer (see Gerlach, 1993).Conclusions for the oxygen fugacity of theirmantle source require more detailed investiga-tions with materials from experimental petrolog-ical studies (Leschik et al., 2004).

.FeO O Fe O6 22 2 3in the melt from the sorroundings in the melt"+

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Klaus Heide and Christian M. Schmidt

Crust contaminations by pelagic sedimentscan be excluded from our first results. For a fi-nal interpretation further systematic study isnecessary.

With these first investigations we candemonstrate that gas release curves of volcanicrocks are qualified to i) characterize primarycomplex gas content of erupted magma in arange of several order of magnitude (between0.1 ppm and several %); ii) characterize the for-mation temperature of fluid inclusions inxenocrysts and to determine simultaneous thecomplex composition; iii) detect and character-ize alteration processes of the igneous glass andthe secondary phases; iv) detect and character-ize contamination of the magma with adjacentrocks; v) of particular interest is the detection ofhydrogen as degassing product in a temperaturerange above 1000°C with a maximum at1307°C. Compared to MORB-lava degassing(Heide et al., 2000) we do not have indications

for an inorganic formation of hydrocarbons(fig. 5).

From different studies we know that thepressure, temperature and compositionally-re-lated dependence of volatile solubility is farfrom being simple (Behrens et al., 2001).

The solubility of H2O, S, Cl, and F increaseswith increasing SiO2-content, these volatiles arethus enriched in residual melts (Carroll and Hol-loway, 1994; Dixon et al., 1995). While the solu-bility increases with increasing pressure, it de-creases with increasing temperature (Williamsand McBirney, 1979), whereby the effect of pres-sure is minor (Behrens and Jantos, 2001).

Surprisingly, oxygen frequently remainsneglected in geochemical studies, although it isknown from industrial glass melting processesthat oxygen plays a crucial role during de-gassing of silicate melts (Jebsen-Marwedel andBrückner, 1980). Chemical equilibriums existbetween volatiles and ions with polyvalenciesin the melt. Different volatile molecules, likeCO, CH4 or H2, are thus formed depending onoxygen fugacity, temperature and pressure andmay be resorbed again in the melt under chang-ing conditions.

Thus it may be seen that the change in the re-dox equilibrium with temperature is an impor-tant kinetic factor during the degassing processof the melt. Reactions with polyvalent ions, es-pecially Fe2+, yield oxygen bubbles in the hightemperature range. The following reaction couldoccur between iron-rich olivine (fayalite) and theoxygen in the magma during heating under vac-uum conditions in an open system, that meansunder extreme non-equilibrium conditions

(4.2)

The oxygen bubbles act like a «fining» agent ofa melt for all volatiles; thereby, oxygen effec-tively promotes the degassing of magma.

Acknowledgements

The authors wish to thank R. Moretti and B.Scaillet for helpful discussions and review of

.

Fe O SiO Fe SiO

O

2 3 3

2 ( )

magnetite in the melt fayalite

gas bubbles

3 4 2 2 4&+ +

+

Fig. 14. Comparison of the oxygen release fromOcean Island basalt (Hawaii) and Island Arc basalt(Etna - 2002).

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Determination of volatiles in volcanic rocks – Part I: DEGAS-profiles

this report. S. Lenk assisted in obtaining the de-gassing profiles presented in this report. D.Schoeps and G. Voelksch are thanked for theirassistance with XRF and EDX analysis.

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