ELSEVIER ,KulDlNG

ISOTOPE GEOSCIENCE Chemical Geology 125 (1995) 19-28

Ion microprobe determination of water in silicate glasses: methods and applications

fitienne Deloule a,*, Olivier Paillat a, Michel Pichavant b, Bruno Scaillet b a C.R.P.G.-CNRS. BP 20, F-54501 Vandmwe-l&Nancy Cedex, France

’ C.R.S.C.M.-CNRS, IA rue de la Fernllerie, F-45071 Orl6ans Cedex 2, France

Received 18 November 1994; accepted 29 April 1995 after revision

Abstract

Ion probe measurements offer the advantage of in situ determination of water content with a few micrometres spatial resolution. After a brief review of ion microprobe procedures used for the determination of water concentration in silicates, we examine three important effects on analytical precision: ( 1) the effect of energy filtering on the background; (2) the variation of the H/S1 ratio with time; and (3) the dependence of the H/Si ratio on the composition of the matrix, which requires the use of standards close in chemical composition to the unknowns.

Finally, applications of this method to the determination of water in glasses quenched from water-saturated melts, to experi- mental products consisting of a mixture of glass and crystals, and to natural melt inclusions illustrate the advantages and the limitations of the ion microprobe.

1. Introduction

Precise knowledge of the water content of silicate glasses is important when studying both experimental and natural magmatic systems. Different techniques have been used for this purpose (Dingwell, 1986). Most of them give bulk concentrations, but ion micro- probe analysis and micro-infrared spectroscopic tech- niques allow in situ analysis for water. In this paper we discuss three improvements of the analytical procedure for ion probe analysis of Hz0 in silicate glasses and present applications of the method to both experimental and natural glasses.

Many different ion microprobe procedures have been used over the past twenty years. Hinthorne and Anderson ( 1975) first determined water in silicates by

[CA1 * Corresponding author.

0009-2541/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved ssDI0009-2541(95)00070-4

ion microprobe. Hz0 contents were determined using a working curve of H+/Si+ vs. Hz0 in wt%, based on analyses of standards with known water content. The primary beam was 17-keV mass-separated O- ions with an intensity of 10 nA. The average errors for hydroxylated mineral standards were N f 20% of the hydrogen concentration, with a background level of 100 ppm by weight. Delaney and Karsten ( 1981) obtained three distinct calibration curves of H+/O+ vs. Hz0 for basaltic, albitic and rhyolitic glasses using a 15-keV NO; primary ions beam with an intensity of 10 nA; they showed the existence of a pronounced matrix effect. The background level was 0.5 wt% HzO. Hervig and Williams ( 1988) compared working curves with and without energy filtering, and concluded that the selection of high-energy secondary ions decreased the background from -0.1 to 0.01 wt% H20. In posi- tioning the energy window of the secondary ions at 50 eV, they obtained different working curves of H+

20 E. Deloule et al. /Chemical Geology 125 (1995) 19-28

counts vs. HZ0 in wt% for rhyolitic and basaltic glasses, with slopes differing by 30%. Due to the quite large errors (estimated at + 15%)) however, they concluded that the matrix effects were quite small. Under extreme analytical conditions ( lOO-nA O- primary beam accel- erated to 14.5 kV, a spot size 100 pm in diameter, a secondary beam collected from the central area 60 pm in diameter, and a sample voltage offset of - 100 V with a 20-eV energy window), Yurimoto et al. ( 1989) obtained a background level lower than 10 ppm atomic for H in quartz.

After a detailed description of our analytical proce- dure, we will present several applications to illustrate the advantages and limitations of water analysis in experimental and natural silicate glasses by ion micro- probe.

2. Standards and analytical procedure

2.1. Description of the glass standards

Four synthetic albitic glasses (provided by S. New- man and E. Stolper; see Silver and Stolper, 1989), chips

Table 1 Standard glasses used in this study

of anorthitic and albitic glasses (Paillat, 1992)) a nat- ural obsidian glass from the Macusani area, SE Peru (Pichavant et al., 1987) and five synthetic haplogran- itic glasses were used as standards in this study (Table 1). The haplogranitic glasses were synthesised from gel or industrial glass (Schott Company) for the purpose of this study. The amount of water added to the starting materials was slightly greater than that required for saturation. Pressure and temperature were chosen to obtain a large range of water concentrations in the glasses. Two cycles of grinding, melting and quenching were carried out to ensure a homogeneous water distribution in the glasses. Experiments carried out in cold-seal vessels (glasses 343-l to 343-3, Table 1) were quenched at constant pressure. Glasses Schott- lop and Schott-2@@ were synthesised in an inter- nally-heated pressure vessel without pressure control during the quench. The glasses were examined after the experiment by normal petrographic methods.

The water concentrations of standards were deter- mined by manometry [see Newman et al. ( 1986) for a discussion]. The water was extracted from single millimetre-sized chips by heating up to 1300-1400°C with an induction furnace in a covered Pt crucible in a

Glass Apparatus P (kbar)

T SiOq Al@; CaO” Na20= K&Y Hz0 Total H/Sib (“C) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%)

Cl' IHPV” 0.99 1,150 68.74 20.29 0.12 11.77 0.16 0.21’ 101.29 0.02 G8’ IHPV 5.0 1,150 65.86 19.24 0.14 11.07 0.12 4.15’ 100.58 0.42 Ab12’ IHPV 2.0 1,150 64.09 18.65 0.21 10.51 0.18 6.30” 99.94 0.65 G7’ 1HPV 3.68 1,150 61.32 17.71 0.14 10.24 0.13 9.60” 99.14 1.04 Macusani’ - 72.26 15.83 0.22 4.14 3.66 0.19 96.30 0.017 Schott I “‘A. IHPV 0.52 900 76.08 13.37 0.01 4.61 5.40 1.82” 101.29 0.16 Schott 2’“.’ IHPV 0.52 900 74.92 13.28 0.01 4.62 5.36 2.96h 101.15 0.26 343-lP CSPV’ 2.0 800 73.47 12.48 0.01 3.09 6.31 5.03” 100.39 0.46 343-2r CSPV’ 3.0 800 72.86 12.28 0.00 2.90 5.88 6.84” 100.76 0.62 343-3” CSPV’ 4.0 800 72.59 12.25 0.01 2.91 5.76 8.08h 101.60 0.74

“Major oxides analysed by electron microprobe (Cameca”’ Can@bax, University of Nancy I), under the following operating conditions: 8 nA, 15 kV, spot size 12 urn, 6 s, standards albite, corundum, orthoclase. NaxO and KZO corrected by using secondary hydrous granitic glasses of known Na*O and K20 values (Pichavant, 1987). “Atomic ratio. ‘Provided by S. Newman and E. Stolper. See Silver and Stolper ( 1989) for more detailed information. dlHPV=internally-heated pressure vessel. “From S. Newman (pers. commun., 1989). See also Silver and Stolper ( 1989). ‘Macusani glass (Pichavant et al., 1987). “This study. “Micromanometric analyses (C. France-Lanord and F. Holtz). ‘CSPV = cold-seal pressure vessel.

E. Deloule et al. /Chemical Geology 125 (1995) 19-28 21

0.01

0.M) 0.0 0.1 0.2 0.3 0.4 0.5 0.6

fI/si MAN

Fig. 1. Calibration curves of (H’/Si+),,, vs. (H&i),,,. Sample compositions are given in Table 1. Lines show the linear regression of data for each secondaq ion energy: 0 eV (full squares) ; 60 eV (open squares) ; and 80 eV (diamonds).

vacuum line. The Pt crucible was pre-heated under vacuum to eliminate water from both it and the enclos- ing Pyrex@ chamber. Water released from the glasses was trapped in a liquid nitrogen cooled loop, and the non-condensable (m.ainly H2) collected as gas. After eliminating CO2 and HP by step warming, water was passed over hot U metal for conversion to Hz. Total H2, collected by a Toppler pump, was measured manomet- rically. Water concentrations in the glasses range between 0.21 and 9.60 wt% (Table 1). Numerous ion microprobe measurements have been performed on each standard, and no significant heterogeneity were observed.

2.2. Ion microprobe analysis

The measurements were performed on the CRPG modified Cameca@ IMS 3f ion microprobe (Deloule et al., 1991). Chips of glass were mounted in epoxy, pol- ished, gold coated, and stored in an oven at 70°C to minimise water adsorption. During analysis, a liquid

nitrogen trap was used to remove instrumental mois- ture. A negative primary oxygen beam, with 2-6-nA intensity, was focused to produce a IO-pm-diameter spot. For measurements of large samples and associated standards, the primary beam was scanned over a 25 pm-side square, with use of the dynamic transfer to refocuse the secondary beam. The positive secondary beam was centred in a 150-pm image-field aperture, imaging a 60-km-diameter area of the sample, wide enough to transmit the whole beam, but small enough to limit the contribution from the surrounding surface. The mass resolution was set at 500. ‘H+, **Si+ and 3oSi+ were measured by peak switching; possible inter- ferences or instrumental bias were monitored by check- ing the Si isotope ratio. The energy slit width was kept at 10 eV and the slit lined up for a nominal accelerating voltage of 4.5 kV; energy filtering was achieved by maintaining fixed the energy window and decreasing the accelerating voltage applied to the sample. The choice of a relatively thin energy window ( 10 eV) was made to allow a precise determination of the energy distribution of secondary ions, measured on 3oSi, and then a precise and reproducible positioning of the energy window during analysis. The electron multiplier was used in pulse-counting mode, with counting rates up to 50,000 cps on **Si. Counting times were 3 s on each peak and waiting time was 1 s. Successive mea- surement cycles were accumulated for 10-15 min on one sample position.

2.3. Energyjltering

Hervig and Williams (1988) demonstrated that the background level of H20 in haplogranitic glasses can be effectively reduced by applying an energy offset. In this study, measurements made without energy filtering (Fig. 1, E = 0 f 5 eV) gave high H/Si ratios even for low water concentrations. The data revealed a rough linear correlation between (H+/Si+),r, and (H/

Table 2 Comparison of (H/Si)siMs and (H/Si)MAN on synthetic bubbly glasses

Samples P T WWSIMS War) (“C)

Ab3a 3.5 1,400 0.112-0.120 0.91 8.0-8.3 8.1 Abla 5 1,400 0.106-0.192 1.44 6.7-12.8 12.75 Ab5a 5.5 1,400 0.102-0.112 1.71 6.4-7.1 15.14

22 E. Deloule et ul. /Chemical Geology 125 (1995) 19-28

Secondary ion Energy (ev)

Fig. 2. Hydrogen and silicon secondary ion energy distribution. AE is the energy window aperture. Primary beam 5 nA, negative oxygen.

Si>,n,9 where (H+/Si+),r,, is themeasuredsecondary ion ratio, and (H/Si),,, is the one calculated from manometric and electron probe measurements (Table 2); a (H+/Si+)sims intercept of 0.018 was found, corresponding to a background of 3.3 wt% nom- inal water (i.e. the value expected for a water-free sam- ple from the regression line). The detection limit (i.e. the minimum water concentration providing a signifi- cant increase of (H+/Si+)sims compared to the preci- sion on the determination of the background level) was calculated to be f0.4 wt% H20. Although the high background could be lowered by pumping the sample and the ion probe overnight (Deloule et al., 1991)) this technique is time consuming and was not employed in this study. Rather, we chose to measure only high- energy ions. Calibration curves were obtained for energy filtering of 60 _t 5 and 80 f 5 eV (Fig. 1) . The slope of the working curve increases with the energy of the secondary ions, due to the different energy dis- tributions of Hf and Si+ (Fig. 2). The H+ energy distribution, measured on glass (Fig. 2)) mica (Okano and Nishimira, 1984) or amphibole (Deloule et al., 1991) is characterised by a rapid decrease in the low- energy range and a very gradual one in the high-energy range, when other elements, e.g. Si have aregulardecay (Fig. 2). Background levels are much lower, corre- sponding respectively to 0.26 and 0.16 wt% nominal H,O and detection limits of 0.036 and 0.040 wt%. The minimisation of the background for the high-energy ions is due to surface or vacuum residual species that are only weakly bound and therefore are mostly ionised by low-energy processes such as surface potential ion- isation, electron bombardment or secondary collisions.

Therefore those species have a narrower energy distri- bution than the ions extracted with a large energy dis- tribution from under the surface by the collision cascade process (Sigmund, 1972; Gries and Riiden- auer, 1975). Thus, energy filtering largely increases the ratio of the original strongly bound structural water vs. the contamination water. The variation of the (H+ / Si+)sims ratio as a function of the secondary ion energy implies an accurate control of the value of the selected energy band, made routinely every 4 min during mea- surements. For further measurements, we used an 80+5-eV energy band, combining a good emission yield and a low instrumental background.

2.4. In-depth pro$ling

A consistent feature of ion-probe analyses was the systematic decrease of the (H+/Si+)sims ratio during the first minutes of the analysis, followed by an approach to a steady state (Fig. 3). This behaviour was observed for all silicates independent of Hz0 content and may result from: ( 1) a decrease in surface contam- ination during initial sputtering, and/or (2) an elec- tromigration of H caused by charging (Wilson et al., 1989)) and/or (3) a preferential sputtering of H from the sample relative to Si, until a steady state is reached for both elements.

We could exclude the effect of surface contamina- tion, since the variation of the (H+/Si+),r, ratio at the beginning of a measurement is observed even when the contamination is made negligible by energy filter- ing. Due to the respective energy distributions of H and Si (Fig. 2)) electrostatic charging would increase,

0.2” + 1

Rs RMRs+Ro)

0.15 -.

9 z

p - - ” _ Gl 0.012 0x4 -------..__ __ _ ____-- _.___. 0.M). ’ : ’ ; . : . : . : ’ : ’

0 120 240 360 480 600 720 840

Time (WC.)

Fig. 3. Time dependence of the (H+ IS+),,, ratios during the first minutes of analysis. Sample compositions are given in Table 1. The dart& lines show the model evolution calculated from the regression on the data, with the values of A, Rs and RJ(Rs + Ro) for each case.

E. Deloule et al. / Chemical Geology 12.5 (1995) 19-28 23

rather than decrease, the (H+ /Si+),i, ratio, which allows us to eliminate the possibility of electromigra- tion of H+ caused by charging. Furthermore, Perny et al. ( 1992) pointed out that no migration was observa- ble on Naf for primary beam intensities lower than 20 nA.

A preferential sputtering of H relative to Si would increase the (H + /Si+ ) sims ratio at the beginning of the profile, until the surface layer becomes depleted in H. It can be explained by: ( 1) a higher sputter yield for H than for Si, which results in a depletion of H relative to Si in the subsurface layer, and in a decrease of the (H+ / Si + )sims ratio until the sputtered layer thickness equals the extraction thickness (i.e. the thickness of the layer from which ions are {extracted) ; (2) by an extraction thickness larger for H than for Si, which would amplify the (H+/Si+),i,,ratiovariation both in timeandinten- sity. We report in Fig. 4 the calculated variations of the (H+ /Si+)sims ratio R as a function of time until steady state is reached, for different H and Si extraction yields ( eY) and extraction thicknesses ( ed) . When the extrac- tion yields are different (Fig. 4a), the curves can be fitted by exponential functions of the form:

R=Rs+Roexp( -Al)

where Rs is the steaey value of R and Rs + R,, its initial value. On the other hand, when the extraction thick- nesses are different, lthe shape of the curve is inverted and can be fitted by a Gaussian function (Fig. 4b). When both extraction yields and extraction thicknesses are different, various shapes may be produced (Fig. 4~). The difference between the initial and steady-state ratios is strongly dependent on the values of both parameters e, and ed, and can change from 0% to 20% (Fig. 4)) as observed in the samples. Using the sputtering rate of our sample (10 nm min-‘), the extraction depth of hydrogen can be estimated to be in the range of 100 nm when steady state is reached in 10 min. The thickness of the mixing layer under sputtering is known to be a few tens of nm in the case of semi- conductors and metaJs (Hofmann, 1982; Amour et al., 1988)) but is unknown for silicates. Moreover, hydro- gen may be extracted by processes other than ion col- lision, such as photon absorption (Deloule et al., 199 1) , from deeper llevels than elements more strongly bound in the glass. ‘The experimental curves (Fig. 3) more closely resemble those of Fig. 4b than of Fig. 4a, suggesting that the dominant effect is a deeper extrac-

^o cr: t 0.9 8 z?

0.8

v1 0.8

2 0.7

0.6 0 2 4 6 8 10

Time (arbitrary unit)

Fig. 4. Model evolution of (HI /Si+ ).i, during the tirst stage of sample sputtering. The amounts of ion counted by time unit are the products of the amount of element extracted multiplied by their ionisation yields, which are assumed constant. The amount of ele- ment extracted is the product of the amount of element included in the extraction depth ed multiplied by the extraction yield ey. The element content in the sample decreases by a factor eY over a depth ed, for each time step, and its in depth profile will change until the sputtered thickness equals the extraction depth. R, the measured ratio of two elements with different ed and/or eY, also change during this time. The extraction depths e, and extraction yields eY are repotted on the graph. In depth variations are shown in: (a) for elements with the same extraction depth ed; (b) for two elements with the same extraction yield e,; and (c) for two elements with different ed and eY. In (a), the curves are exponential functions [R = Rs + R. exp( - hr)] fitted to the calculated points.

tion for H than Si. This could be due to electromigration of H toward the surface, or to degassing of water under vacuum.

It should be noted that for some samples the initial decrease is very small (see Fig. 7), suggesting in these cases very small difference between H and Si extraction yields or extraction depths. This could be due to vari- able mobility of H or water in synthetic or natural glasses, probably depending on their formation condi- tions. Therefore hydrogen concentrations are deter- mined by waiting until a steady state is reached,

24 E. Deloule et al. /Chemical Geology 125 (1995) 19-28

0.12

g

+

5 0.08

&

0.00 0 0.2 0.4 0.6 0.8 1 1.2

I-I&i M*N Fig. 5. Composition dependent calibration curves, with a 80f lo- eV secondary ion energy. Sample compositions are given in Table 1 or in Paillat ( 1992) for the two samples with anorthite-like compo- sition.

ignoring the initial decrease of the (H+ /Si+),ims ratio during a fixed time for both samples and standards. If an exponential time-dependent relationship is valid, all the data from one analysis may be fit by a simple curve. The values of Rol (R, + R,) obtained for each sample reflect the relative extraction yields and depths for H and Si and we use the values obtained to test the relative behaviour of H and Si.

2.5. Matrix effects

Measurements of three standard glasses of albitic, granitic and anorthitic compositions are reported in Fig. 5. The slopes of the lines clearly depend on the composition of the glass, as shown by Hervig and Wil- liams ( 1988) for rhyolitic and albitic glasses. It should be noted that the lines differ only slightly for albitic and granitic compositions, whereas the difference is large for anorthitic glass. The lines were obtained by plotting the values of (H+ /Si+),i, as a function of (H/Si)_. The variations in slope do not depend on the Si concentration of the glasses, as would be the case for a plot of (H+/Si+),ims as a function of the HZ0 concentrations. The H emissivity increases with the Si concentration, however, as shown for other elements in silicates by Shimizu ( 1986).

3. Applications

3.1. Synthetic bubbly albitic glasses

The first application concerns the analysis of water- rich albitic glasses produced in an experimental study

of water solubility in albitic melts at high pressures and temperatures (Paillat et al., 1992). Two samples are of particular interest for the present study. These samples are quenched albitic melts saturated in water at 5 and 5.5 kbar and 1400°C. At low pressure ( <4 kbar) an isobaric quench of water saturated albitic melts gives a clear glass; water does not exsolve during the quench [see Paillat et al. ( 1992), for more details]. The two samples we discuss here correspond to P-T conditions where the quench begins to affect drastically the pres- ervation of water content during the liquid-to-glass transition. These two samples have bubbles attributed to water exsolution from the melt upon quenching.

Ion probe analysis of the glasses gave lower water concentrations than manometry (Fig. 6a; Table 2) ; for these samples, the (H+/Si+),ims ratio decreases quickly with time and a steady state is not reached even

0.20 - _

g 0.15 -

5 ‘;- 0.10 - X

0.05 -

ai

0 0 0.4 0.8 1.2 1.6

HiSi MAN

-1

0.20 14

V..”

0 200 400 600 800 1cKlO 1200

Distance from horder (pm)

Fig. 6. (H+/Si+),, measured on bubbly glasses. a. Measured values vs. calibration curves. Compositions given in Table 1 for the standards and in Paillat et al. (1992) for Abla and Ab5a (runs IHPV 6 and -7) b. Concentration profiles along two samples with different water contents. Sample Abla profile done from one border to the opposite one, sample Ab3a (run HIPV 3, in Paillat et al., 1992) done from the border to the center.

E. Deloule et al. / Chemical Geology 125 (1995) lS28

Table 3 Measured (SIMS) and calculated H,O contents of water-saturated experimental glasses from phase equilibrium studies

25

Sample P T WWSIMS Rd(Rs+Rd H,O (wt%)

(kbar) (“0 measured calculated’

Gangotri granite:

GB 15 4 149 0.157 0.15 9.5 8.8 GB 19 4 720 0.142 0.19 8.7 8.9 GB 35 4 682 0.145 0.14 9.9 9.1

Manoslu granite:

DK 15 4 749 0.153 0.16 8.9 8.9 DK 19 4 720 0.117 0.14 7.6 8.8 DK 34 4 682 0.096 0.20 7.1 9.1 DK44 4 663 0.1136 0.12 7.0 9.1

Topaz rhyoliteb:

l -84-2a 2 900 3.9 3.2 cs 85-10 1 191 3.1 4.1 cs-85-49 0.5 800 - 3.2 2.9

“Calculated after Bumham ( 1975). bFrom Webster et al. ( 1987).

after 15 min. Moreover, the water concentrations meas- ured by ion probe across a whole slug of a bubbly sample are higher in ,the rim than in the core (Fig. 6b). For comparison a traverse across a slug of a bubble- free glass quenched from lower pressure shows a fairly homogeneous water distribution. The inhomogeneity in Hz0 content in Abla (Fig. 6b) is expected because of the presence of I$0 bubbles that locally decrease the Hz0 in the adjacent glass. The higher water content observed at the margin suggests that the surface of the

Table 4 Analytical results for the glass inclusion from Bishop Tuff and stan- dard glasses

Bishop Tuff( H/Si)sIMs R,J (R, + R,) HZOSIMS H,OFTIR (wt%) (wt%)

Rim 0.0686 0.152 5.7 Core 0.0887 0.204 8

5.7

Standard (H/Si)slMs R,/(R,+R,) (HISi),,

Macusani 0.0032 0.254 0.017 Schott I”’ 0.0196 0.032 0.16 Schott Ym 0.03 15 0.026 0.264 343.1 0.0704 0.013 0.457 343.3 0.0984 0.087 0.743

sample has been quenched faster than its core, which is consistent with the expected process of the outwards heat loss of the sample during the quench. However, no calculation has been performed to evaluate the dif- ference of cooling rate between the core and the surface of the sample.

We have shown above that the use of energy filtering allows to exclude water absorbed on the surface from the analysis. Similarly, water weakly bound to the structure of the glass might be partially not detected by ion probe using energy filtering, whereas it is measured by manometry. Moreover, the ion microprobe analyses do not include water present in the bubbles, which will be at least partially pumped away before to be analysed. Thus, the higher concentrations obtained by manome- try are related to the presence of water in bubbles or sub-micrometric cavities. The strong decrease of the (H+/Si+),i, ratio with time is another argument supporting the process of a faster extraction of H (see Section 2.4). The faster H extraction relative to the surface sputtering may arise from a diffusion process not related to the sputtering itself. This suggests that the water is weakly bound to the glass structure in order to be extracted so fast. Water is present in aluminosil- icate glasses as both molecular and hydroxyl species

E. Deloule et al. /Chemical Geology I25 (1995) 19-28

w 0.02 r

I / Schottl

/

/ /

core / I 343-3 /

Macusani 0.00 I I I, II.1 I I

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 H/Si calculated

Fig. 7. ( H+ /Si + ) f, in a glass inclusion (@led squares) hosted by quartz from the Bishop Tuff. The &shed line is the calibration curve. The size of the symbols decreases with increasing time (total meas- urement duration is 10 min) . Standards (open squares) are described in Table 1.

(Stolper, 1982). Dingwell and Webb (1990) have shown that the ratio of molecular water over hydroxyl groups increases with decreasing temperature. Thus, quenching will tend to form molecular water at the expense of hydroxyl groups. We can expect this molec- ular water to be more loosely bound than the hydroxyl groups.

In conclusion, ion-probe analysis of bubbly glasses allows to test the sample homogeneity, and also to explain the differences between results obtained using the various analytical methods. Results with manome- try and ion microprobe on samples Ab la and AbSa can be reconciled if the bubbles observed were formed dur- ing the quench.

3.2. Phase equilibria studies

The results of ion microprobe analyses of water in experimentally produced peraluminous glasses are listed in Table 3. Starting materials were natural rocks in both cases, the Gangotri and Manaslu granites (Him-

alaya; experiments by Scaillet et al., 1995) and the topaz rhyolite of the Spor Mountain (Utah, U.S.A.; experiments by Webster et al., 1987). The Gangotri and Manaslu samples were analysed using the instru- ment and procedure detailed in this paper (see also Scaillet et al., 1995) whereas the Spor Mountain values were obtained with the ion microprobe instrument at the University of Arizona (Websteret al., 1987). These types of experiments were, in both studies, aimed at the determination of the phase relations as a function of the Hz0 content of the melt. Hence, determination of the melt HZ0 contents was essential. Compared to the albitic glasses previously discussed, the difference here resides in that the analysed samples are partially crys- talline. Table 3 compares analytical results for H20- saturated conditions to HZ0 concentrations expected from the equimolal water solubility model of Burnham (1975). The agreement is good in most cases, with differences of < 1 wt% H20, except for samples DK34 (682’C) and DK44 (663°C) where the differences reach values higher than 2 wt% H20. In these two cases, we do not have a satisfactory explanation for the dis- crepancies. If real, the 682” and 663°C values would imply a strong positive temperature dependence of the HZ0 solubility (in the order of +0.5 wt% HZ0 per 50°C)) inconsistent with all the available information of H,O solubility in aluminosilicate systems (Burn- ham, 1979; McMillan and Holloway, 1987; Paillat et al., 1992), but presumably consistent with Holtz et al., 1992. Both samples contain crystalline phases but in low amounts so that this should not have greatly affected the measured HZ0 contents. It is possible that the results for the DK sample (which has a potassic composition) are somewhat biased because the cali- bration curve used only albitic compositions (Scaillet et al., 1995; see Fig. 5). Nevertheless apart from these samples, these data show that the ion microprobe ena- bles determination of HZ0 contents over a large range of concentrations and for chemically complex bulk compositions, even for crystal-rich glasses.

3.3. Natural melt inclusions

A spherical (diameter of w 200 pm) rhyolitic glass inclusion in a quartz phenocryst from the Bishop Tuff (California, U.S.A.), provided by A.T. Anderson and C. Skirius, with a total water content of 5.7 wt% (4.3 wt% molecular water and 1.4 wt% hydroxyl from infra-

E. Deloule et al. /Chemical Geology 12.5 (1995) 19-28 21

red (IR) spectroscopicdata; A.T. Anderson,pers. com- vacuum in the sample chamber can be lowered by a mun., 1992)) gave results shown in Table 4 and Fig. 7. factor of 10-100 by changing the sample introduction Rim analysis of the inclusion gave a HZ0 content of 5 system and filtering the primary beam, which should wt%, while its core has 7.5 wt% H20. Similar inclu- minimise the sample surface contamination. Another sions yielded water IR data ranging between 5.1 and option to minimise this contamination effect is to use 6.8 wt% HZ0 (Anderson et al., 1989). We interpret the a high-intensity primary beam focused on a larger area difference in water content obtained by SIMS and IR than the analysed one (Yurimoto et al., 1989), with techniques as resulting from the sample heterogeneity losing the high spatial resolution by sputtering a large rather than from a systematic bias between techniques. area.

The variations of (H+ /Si’ ) Sims with time are similar to the synthetic albitic samples of 3.1, but with an inversed spatial distribution of water (the core contains more water than th,e rim). The strong and steady decrease of (H+ /Si’ ) sims with time (Fig. 7) suggests that water in the glass inclusion is present either as sub- microscopic bubbles or loosely bound molecular spe- cies. Our ion probe data directly point out the existence of concentration gradients within melt inclusions, com- patible with diffusive loss of water species through the host quartz (Qin et al., 1992). Thus, in using vitreous inclusions as record of the bulk melt composition, one needs to consider both their possible heterogeneity and late diffusive re-equilibration through the host crystal.

The systematic decrease of the (H+ / Si + ) sims ratio during analysis is usually ignored and the water content calculated when it reaches a plateau. Nevertheless, this plateau is not always observed which makes difficult to choose the correct (H+/Si+),i, ratio. It is worth- while that a systematic study of the time dependence of the (H+ /Si+)sims ratio can lead to an improvement of the precision of the analysis as well as of the com- parison between several sets of analyses.

4. Discussion and conclusions

Finally, analyses of water in glasses quenched from melts at high pressure and temperature seem to behave identically: a strong decrease of the ( Ht /Si+ ) sims with time is observed and no steady state is attained even for long counting times. This is consistent with the presence of water is these glasses dominantly as loosely bound molecular species or as sub-microscopic bub- bles.

The main achievement of this study is to provide an investigation of the methodological aspects of the ion microprobe analysis of hydrogen so as to infer the water content of silicates glasses and eventually that of the silicate melts at high pressures and temperatures.

One must first calibrate the ion probe analysis with standards close to the samples in order to minimise the matrix effects. The silica content seems to be an impor- tant factor controlling this matrix effect, as observed for analysis of other elements in silicates by Shimizu ( 1986).

Among the numerous applications that can be found for the ion probe method, the most important one in determining HZ0 contents come from the high spatial resolution of the ion probe. All the applications we presented here use this spatial resolution to check sam- ple homogeneity, to analyse heterogeneous samples made of glass and crystals and to analyse small natural melts inclusions in crystals.

To lower the background (which is crucial for mea- suring water concentrations of < 1 wt%), a high vac- uum and an overnight pumping (eventually the baking of the ion probe with the sample inside) is the straight- forward solution but is time consuming. Using strong energy filtering (80 f 5 eV) is also efficient and faster. These two improvements might be used together to achieve high sensitivity for very low water concentra- tion analysis (down to hundreds of ppm) . There could also exist instrumental improvements. For instance, the

Acknowledgements

We thank F. Holtz, S. Newman and E. Stolper for providing us some of the standards, C. France-Lanord for the micromanometric analyses and A. Anderson for the Bishop Tuff sample. M. Chaussidon and I. Hutch- eon are acknowledged for helpful discussions. Careful reviews by R.W. Hinton and an unknown reviewer led to improvements of this paper. This is CRPG-CNRS contribution No. 1116.

28 E. Deloule et al. /Chemical Geology 125 (I 995) 19-28

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