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Geochimica et Cosmochimica Acta 72 (2008) 117–125

Hydration of rhyolitic glass during weatheringas characterized by IR microspectroscopy

Tadashi Yokoyama a,*, Satoshi Okumura b,1, Satoru Nakashima a,2

a Department of Earth and Space Science, Graduate School of Science, Osaka University,

Machikaneyama 1-1, Toyonaka, Osaka 560-0043, Japanb Institute of Mineralogy, Petrology and Economic Geology, Graduate School of Science, Tohoku University, Sendai 980-8571, Japan

Received 14 November 2006; accepted in revised form 19 October 2007; available online 1 November 2007

Abstract

The mechanism and rate of hydration of rhyolitic glass during weathering were studied. Doubly polished thin sections oftwo rhyolites with different duration of weathering (Ohsawa lava: 26,000 yr, Awanomikoto lava: 52,000 yr) were prepared.Optical microscope observation showed that altered layers had developed along the glass surfaces. IR spectral line profileanalysis was conducted on the glass sections from the surface to the interior for a length of 250 lm and the contents of molec-ular H2O (H2Om), OH species (OH) and total water (H2Ot) were determined. The diffusion profile of H2Om in Ohsawa lavaextends beyond the layer observed by optical microscope. The content of H2Om in the hydrated region is much higher thanthat of OH species. Thus, the reaction from H2Om to OH appears to be little and H2Om is the dominant water species movinginto the glass during weathering. Based on the concentration profiles, the diffusion coefficients of H2Om ðDH2OmÞ and H2Ot

ðDH2OtÞ were determined to be 2.8 · 10�10 and 3.4 · 10�10 lm2 s�1 for Ohsawa lava, and 5.2 · 10�11 and 4.1 · 10�11 lm2 s�1

for Awanomikoto lava, respectively. The obtained DH2Om during weathering are more than 2–3 orders of magnitude largerthan the diffusion coefficient at �20 �C that is extrapolated from the diffusivity data for >400 �C. This might suggest thatthe mechanism of water transport is different at weathering conditions and >400 �C.� 2007 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

It is widely known that volcanic glasses hydrate duringweathering. The hydrated glass region has a higher refrac-tive index than the unhydrated region and a boundarycan be recognized at the rim of the glass under an opticalmicroscope. The hydrated glass often shows optical anisot-ropy under crossed polars, which is thought to be due tostrain birefringence (Ross and Smith, 1955).

The water in glass can be classified into several species;molecular H2O, OH species (Si–OH, Al–OH, etc.), protons

0016-7037/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.gca.2007.10.018

* Corresponding author. Fax: +81 6 6850 5480.E-mail addresses: tadashi@ess.sci.osaka-u.ac.jp (T. Yokoyama),

sokumura@mail.tains.tohoku.ac.jp (S. Okumura), satoru@ess.sci.osaka-u.ac.jp (S. Nakashima).

1 Fax: +81 22 795 7764.2 Fax: +81 6 6850 5480.

(H+) and hydronium ions (H3O+). In this paper the molec-ular H2O, OH species and total water are referred to asH2Om, OH and H2Ot, respectively. The H2Om and OHare the stable species in rhyolitic melts/glasses (e.g., Zhanget al., 1991; Behrens and Nowak, 1997; Okumura andNakashima, 2005). The mechanism of water transport inrhyolitic glasses/melts at >400 �C, i.e., above glass transi-tion temperature, has been extensively studied (for details,see Behrens and Nowak, 1997; Zhang, 1999). At these tem-peratures, water moves into glass predominantly as H2Om

and OH are formed due to the rapid reaction betweenH2Om and silicate structure. The reaction can be expressedas follows (e.g., Stolper, 1982; Zhang, 1999):

H2Om þX–O–X$ X–OHþHO–X; ð1Þ

where X can be Si, Al, Na, etc. (not H). For weatheringconditions, the characterization of hydrated glass hasbeen conducted by various surface analytical techniques,

Table 1Mineral composition and chemical composition of unaltered glassdetermined by electron microprobe

Ohsawa lava(26,000 yr)

Awanomikoto lava(52,000 yr)

Mineral compositiona

Glass 89.3 86.3Plagioclase 6.5 10.1Quartz 3.8 2.7Biotite 0.3 0.7Magnetite + ilenite 0.1 0.2

Total 100.0 100.0

Chemical composition of glass (wt%)b

SiO2 77.5 77.9Al2O3 12.2 12.1TiO2

c 0.09 0.09Fe2O3 + FeO 0.57 0.53CaO 0.51 0.54MnOc 0.07 0.07K2O 3.81 3.67P2O5 0.00 0.00Na2O 3.71 3.78MgOc 0.08 0.08

Total 98.5 98.7

a

118 T. Yokoyama et al. / Geochimica et Cosmochimica Acta 72 (2008) 117–125

including electron microprobe (Jezek and Noble, 1978), nu-clear reaction (Lee et al., 1974; Laursen and Lanford, 1978),sputter-induced optical emission (Tsong et al., 1978), andSIMS analyses (Anovitz et al., 1999, 2004; Riciputi et al.,2002). Several candidates for the dominant water speciesin glass have been proposed, for example, H3O+ (Laursenand Lanford, 1978), H2Om (Jezek and Noble, 1978) andhydrogen (Riciputi et al., 2002). However, the precise spe-cies are still unclear and the mechanism of water transportduring glass weathering remains a topic of debate.

Infrared (IR) spectroscopy has been widely used todetermine the concentrations of water species in naturaland synthetic glasses/melts (e.g., Stolper, 1982; Newmanet al., 1986; Zhang et al., 1991; Nowak and Behrens,1995; Okumura et al., 2003). In this study we analyze waterdistribution profiles in glass sections of weathered rhyolitesby using IR spectroscopy to investigate water transportduring weathering. The concentration distribution ofH2Om, OH and H2Ot are determined from IR spectral lineprofile. Based on the analytical results, the dominant waterspecies in glass are discussed and the rates of water trans-port during rhyolite weathering are determined. The resultsare also compared with the behavior of water in glass at>400 �C which, in contrast to at low temperatures, has beenwell characterized experimentally.

Data from Taniguchi et al. (1990).b Data from Yokoyama and Banfield (2002).c Below detection limit.

Fig. 1. Scanning electron microscope image of the rhyoliteweathered for 26,000 years. Weathering products (arrow) arerecognized on the glass surface.

2. SAMPLES AND METHODS

Two rhyolites from Kozushima, Japan, different in erup-tion age and duration of weathering were used in the pres-ent study: Ohsawa lava and Awanomikoto lava. The agesof the lavas were determined to be 26,000 yr (+5000 to�4000 yr) for Ohsawa lava and 52,000 yr (+4000 to�3000 yr) for Awanomikoto lava, respectively (Yokoyamaand Banfield, 2002). Kozushima is a volcanic island in thePacific Ocean, located �180 km southeast from Tokyo,composed of at least 16 rhyolitic monogenetic volcanoes(Isshiki, 1982). To estimate the eruption ages of those vol-canoes, Taniguchi (1980) measured the thicknesses of thehydrated glass layers under optical microscope. The tworhyolites chosen in the present study have relatively sharppeak in the histogram of hydrated layer thickness. For Ohs-awa lava, 85% of hydrated layer has the thickness of 12.6–16.8 lm. For Awanomikoto lava, 58% of hydrate layer is12.6–18.9 lm, 20% is 8.4–12.6 lm and 14% is 18.9–23.1 lm. The hydrated layer thicknesses of the other lavasyounger than Ohsawa lava show sharper peak in the histo-gram but the thicknesses (<�6 lm) are not thick enoughfor IR measurement due to too young ages (<3400 yr)(Taniguchi, 1980). Table 1 shows the modal compositionand chemical composition of unaltered glass determinedby electron microprobe. Since the two rhyolites have similarcomposition and commonly porous (porosity 30–37%),similar mechanism of glass hydration can be expected.Kozushima has high rainfall through the year (total2872 mm yr�1), ensuring constant water–rock interaction.The two rhyolites are fairly homogeneously weathered atleast in the outcrop scale due to high porosity and abundantcooling joints (Oguchi et al., 1999). The mean temperatureof water–rock reaction during weathering of the rhyolites

has been estimated to be 15–21 �C (Yokoyama and Ban-field, 2002).

For these rhyolites, detailed evaluations on the rates ofglass dissolution and weathering products formation havebeen conducted in the author’s previous researches (Yokoy-ama and Banfield, 2002; Yokoyama and Nakashima, 2005).Fig. 1 is a scanning electron microscope (SEM; S4500, Hit-achi) image of Ohsawa lava (26,000 yr). Weathering prod-ucts consisting mainly of halloysite and amorphousaluminosilicate are recognized on the surface of the glass.

Hydration of rhyolitic glass 119

The thickness of the weathering products layer is �0.2 lm.Larger amount of aluminosilicate clays are present in Awa-nomikoto lava (52,000 yr). Hydration of glass progressesconcurrently with these glass dissolution and clayformation.

In order to evaluate the distribution of water species inglass, doubly polished thin sections of the rhyolites, havingthickness of 25.8 lm for Ohsawa lava and 16.2 lm forAwanomikoto lava were prepared. Before the polishing,pores in the rhyolites were embedded with a low viscosityepoxy resin (E205, Konishi) to avoid breaking of rhyoliteduring the polishing. The thicknesses were measured witha laser scanning confocal microscope (VK-8500 + VK-8510, KEYENCE) using a glass refractive index of 1.47.The refractive index of glass was measured by the immer-sion liquid method and the identical values of 1.47 were ob-tained for the two lavas. Fig. 2a and c are opticalmicroscope photographs of the thin sections of two rhyo-lites under plain polarized light and Fig. 2b and d are thoseunder crossed polars. A dark boundary can be recognizedat the rim of the glass along pores and cracks and the hy-drated region shows birefringence under crossed polars.The distances between glass surface and dark boundaryare �12 lm for Ohsawa lava (26,000 yr) and �14 lm forAwanomikoto lava (52,000 yr). Awanomikoto lava con-tains more microphenocrysts than Ohsawa lava. The loca-tions for the IR analysis were chosen according to the

Fig. 2. Optical microscope images of glass sections for Ohsawa lava (2Abbreviations are G, glass; R, resin and Bt, biotite. (a and c) Photosboundaries are recognized at the rim of the glass along pores (resin) and(b and d) A photo taken under crossed polars. Note the birefringence in

following basis: (1) hydrated layer is distinctly visible byoptical microscope under plain polarized light and crossedpolars, (2) hydrated layers that develop from other side ofglass–pore interface are not overlapped (thus, width of glassis as large as possible), and (3) enough width of void (resin)is present at outside of the glass.

IR spectral line profile analyses were conducted on theglass sections for a length of 250 lm by 5 lm step usingan aperture size of 15 lm · 50 lm. An FT-IR microspec-trometer (FTIR620 + IRT30, Jasco) with a wavenumberresolution of 4 cm�1 was used in the analyses. The analyzedareas are shown in Fig. 2a–d. In the IR microspectrometer,IR light passes through the sample with the entry angles of17–35�. Although IR signals are predominantly from theaperture area, some amounts of signals are from peripheryof the aperture because of the oblique incident of IR light.Based on the thicknesses of thin section and the glassrefractive index of 1.47, the maximum expansion of IR lightfrom the aperture is estimated to be 10.9 lm for Ohsawalava and 6.9 lm for Awanomikoto lava.

Typical IR spectra of resin, hydrated glass and unhy-drated glass obtained from Ohsawa lava are shown inFig. 3a. A large IR absorption band around 3000–3800 cm�1 corresponds to the overlap of OH-stretchingvibrations under different chemical states including isolatedor hydrogen bonded X–OH (X = H, Si, Al, etc.) (e.g., Kro-nenberg, 1994). The absorption peak for OH-stretching

6,000 yr) (a and b) and Awanomikoto lava (52,000 yr) (c and d).taken under plain polarized light. As indicated by arrows, dark

cracks. Rectangles are the areas analyzed by IR microspectroscopy.hydrated glass region (arrows).

Fig. 3. (a) IR spectra of resin, hydrated glass and unhydrated glassobtained for the thin section of Ohsawa lava (26,000 yr). Theabsorption band at �1630 cm�1 is due to the bending mode ofmolecular H2O. The large absorption band around 3000–3800 cm�1 corresponds to the overlap of OH-stretching vibrationsunder several different chemical states. The number in lm at theleftmost of spectrum corresponds to the position in Fig. 4a, b andc. (b) IR spectra for hydrated glass of Ohsawa lava, hydrated glassof Awanomikoto lava (52,000 yr), and the obsidian experimentallyhydrated at about 1000 �C and 100 MPa for 240 h (sample SAI1 inOkumura and Nakashima, 2005).

120 T. Yokoyama et al. / Geochimica et Cosmochimica Acta 72 (2008) 117–125

vibrations generally shifts to lower wavenumbers as thelength of hydrogen bonding (O–H� � �H) decreases and asthe strength of hydrogen bond increases (Nakamotoet al., 1955). An IR absorption band around 1630 cm�1 cor-responds to the bending mode of molecular H2O. The con-tents of H2Ot, H2Om and OH are described as wt% of H2Oin the current study, and hereafter they are denoted asCH2Ot ;CH2Om and COH, respectively. Because two OH isneeded to make one H2O, COH = 0.5 wt% means that 1 g

of a rock sample contains 1 · 0.5/100/18.02 · 2 mol ofOH (for details, see Zhang, 1999). The CH2Om and CH2Ot

were determined as follows. Firstly peak heights at 1630and 3580 cm�1 were determined after baseline correctionwith straight lines (Fig. 3a). Since these peak heights in-clude the fraction of the IR absorbance of resin, to removethe contribution of resin the peak height at 2854 cm�1 dueto resin was measured. By using the peak height ratio of1630 cm�1/2854 cm�1 and 3580 cm�1/2854 cm�1 in thespectrum of resin, the contribution of resin at 1630 and3580 cm�1 was evaluated for each spectrum. The removalof the effect of resin changes CH2Ot by 60.11 wt% andCH2Om by 60.03 wt% near the resin–glass interface andthe contribution of resin becomes negligible at >15 lm inte-rior the glass. The CH2Om and CH2Ot are determined by thefollowing Lambert–Beer’s law:

C ¼ 18:02Abs

qd1

e; ð2Þ

where C is the content of either H2Om or H2Ot (wt%), 18.02is the molecular weight of H2O (g mol�1), Abs is the absor-bance (unitless, corrected for resin), q is the density (g L�1),d is the thickness of thin section (cm), and e is the molarabsorptivity of either H2Om or H2Ot (L mol�1 cm�1). A va-lue of 2350 g L�1 was used as a typical density of rhyoliticglass. The values of molar absorptivity for CH2Om in rhyo-litic glasses was reported as 55 ± 2 L mol�1 cm�1 at1630 cm�1 band (Newman et al., 1986) and we used this va-lue. As to the molar absorptivity for CH2Ot , Stolper (1982)listed the values for rhyolitic glasses ranging from 52 ± 4to 78 ± 14 L mol�1 cm�1 at �3550 cm�1 band and reportedthat for many samples the value was within67 ± 7 L mol�1 cm�1, with pointing out the uncertainty ofthe lower or higher values due to technical problems. Inconsideration of the uncertainty of molar absorptivity forCH2Ot , we used 60 and 74 L mol�1 cm�1 for determinationof CH2Ot . After CH2Om and CH2Ot are determined, COH is cal-culated by subtracting CH2Om from CH2Ot .

3. RESULTS

Fig. 4a and e show the optical microscope images of theanalyzed glass sections of the two rhyolites. The aperturearea (solid line box) and the maximum expansion of IRlight (dotted line) are depicted in the figures. Fig. 4b, c, fand g show the concentration profiles of H2Ot, H2Om andOH, where CH2Ot was determined by using either the molarabsorptivity of 60 L mol�1 cm�1 (Fig. 4b and f) or74 L mol�1 cm�1 (Fig. 4c and g). Each data point is theaveraged value of three measurements for an identical posi-tion and errors correspond to the maximum and minimumconcentrations among the three measurements. CH2Ot andCH2Om gradually increase to reach maximum values asapproaching from resin to glass surface. These gradualchanges of concentration for distances of �35 lm in Ohsa-wa lava and �20 lm in Awanomikoto lava can be largelyattributed to the artifact at resin–glass interface due tothe aperture size plus the effect of the oblique incidence ofIR light. At the interior of the glass, CH2Om continuously de-creases from the glass surface toward interior to a depth of

Fig. 4. (a) Optical microscope image of the section of rhyolitic glass in Ohsawa lava (26,000 yr) analyzed by IR microspectroscopy. Solid linebox denotes the aperture size of 15 lm · 50 lm and dotted line box represents the maximum expansion of IR light from the aperture. (b and c)Concentration profiles of total water, molecular H2O and OH species by using molar absorptivity values of 60 and 74 L mol�1 cm�1,respectively (d) Numerical fitting results for the determinations of DH2Ot and DH2Om based on the (b) profile. (e–h) The same data sets forAwanomikoto lava (52,000 yr).

Hydration of rhyolitic glass 121

>50 lm for Ohsawa lava and >30 lm for Awanomikotolava. Thus, the hydrated region in Ohsawa lava extends be-yond the dark boundary in glass observed under opticalmicroscope, even if the effect of artifact is considered. Thediffusion profile of water in Ohsawa lava is thicker thanAwanomikoto lava contrary to the thickness observed byoptical microscope and duration of weathering. COH in

unweathered glass region corresponds to the amount inher-ently contained in rhyolitic magma. In both rhyolites COH

somewhat increases near the glass surface ife3580 = 60 L mol�1 cm�1 is used whereas COH decreasesnear the glass surface if e3580 = 74 L mol�1 cm�1 is used.Since COH should be P0 wt%, e3580 near the glass surfaceneeds to be lower than 74 L mol�1 cm�1 at least for Awa-

122 T. Yokoyama et al. / Geochimica et Cosmochimica Acta 72 (2008) 117–125

nomikoto lava. Although true values of CH2Ot and COH ap-pear to present between the two profiles, we do not haveother constraints on increase/decrease of COH and this ex-tent of error needs to be recognized.

Fig. 4d and h show the results of fitting for the concen-tration profiles of H2Om and H2Ot determined by e3580 =60 L mol�1 cm�1. The concentration profiles were fitted tothe following equation (cf., Lasaga, 1998):

C ¼ ðCS � C0Þerfcx

2ffiffiffiffiffi

Dtp þ C0; ð3Þ

where CS denotes CH2Ot or CH2Om at the glass surface, C0 isthe CH2Ot or CH2Om in unweathered glass, D is the diffusioncoefficient, t is the duration of weathering (age of eachlava), and x is the distance from glass–pore interface.Itwas assumed that the initial concentration was C = C0 forx > 0 and the boundary concentration at x = 0 was main-tained at CS for all the time of weathering. The diffusioncoefficient of H2Ot in obsidian has been reported as concen-tration-dependent, based on the analysis of hydrated obsid-ian using SIMS (Anovitz et al., 1999). For rhyolitic melts/glasses at 400–550 �C, the diffusion coefficient of H2Om

was found to be constant while that of H2Ot is concentra-tion-dependent due to the combination of the constantH2Om diffusivity and the interconversion reaction betweenH2Om and OH (Reaction 1) (e.g., Zhang et al., 1991). Theanalysis of diffusion coefficient is further complicated byvarious factors such as dissolution of glass, leached layerformation, reaction from H2Om to OH, ion exchange, andstress and volume changes induced by diffusion (Caseyand Bunker, 1990; Anovitz et al., 1999). IR microspectros-copy has an advantage for obtaining information on thechemical state of water, but its spatial resolution is limitedto about 10–20 lm. Thus, detailed curvature of water distri-bution profile, which may have information for the factorsaffecting diffusion coefficient, is not sufficiently obtained forthe rhyolites studied here. Therefore, as a first approxima-tion we assumed constant DH2Om and DH2Ot in the fitting,i.e.diffusion coefficient is not concentration-dependent andis constant during the duration of weathering. The fittingcalculates the values of CS, C0 and D. The obtainedDH2Om and DH2Ot were 2.8 · 10�10 (+1.3 · 10�10 to �0.8 ·10�10) lm2 s�1 and 3.4 · 10�10 (+1.4 · 10�10 to �1.1 ·10�10) lm2 s�1 for Ohsawa lava (26,000 yr), and5.2 · 10�11 (+0.6 · 10�11 to �1.0 · 10�11) lm2 s�1 and4.1 · 10�11 (+0.3 · 10�11 to �0.6 · 10�11) lm2 s�1 forAwanomikoto lava (52,000 yr), respectively. The errors cor-respond to the values derived from each triplicate IR mea-surement. Note that DH2Om and DH2Ot are not affected byusing either e3580 = 60 or 74 L mol�1 cm�1. These valuesare generally consistent with the results by Anovitz et al.(2004) that reported DH2Ot during weathering of Pachucaobsidian at Chalco sites as 10�12 to 10�9 lm2 s�1 basedon the analysis using SIMS.

Fig. 5. Evaluation of the effect of glass dissolution on the diffusioncoefficient in Awanomikoto lava. The dissolved thickness during52,000 yr of weathering is assumed as 3 lm. Solid trianglescorrespond to the measured raw data of H2Ot content in glass.Open triangles are the contents parallel-shifted to 3 lm from rawdata, as indicated in arrows for several data points. Solid curvesand diffusion coefficients show the results of fitting.

4. DISCUSSION

We first evaluate the effect of glass dissolution coinci-dently occurred with the hydration of glass. Analysis usingSEM-EDS revealed that the ratio of Si/Al for the glass was

5 and that for the weathering products was 1–2. This indi-cates that substantial amount of dissolved Si was flushedout and Al was concentrated in the weathering products.Assuming that the dissolution of glass had been stoichiom-etric and that all dissolved Al had precipitated as weather-ing products (Si/Al = 1), from the thickness of theweathering products layer (�0.2 lm, Fig. 1) the thicknessof dissolved glass is estimated as 0.2 · 5/1 = 1 lm for Ohs-awa lava (26,000 yr). About threefold amount of weather-ing products is contained in Awanomikoto lava(52,000 yr) (Yokoyama and Banfield, 2002) and the thick-ness of dissolved glass can be roughly estimated as�3 lm. If we try to rebuild the profile of CH2Ot for the casethat dissolution does not occur during 52,000 yr, two possi-bilities are envisioned (Fig. 5)—the one is that CH2Ot at thecurrent positions can be simply parallel-shifted to the dis-solved layer; the other is that the current profile remains in-tact and is extrapolated to the initial surface. The actualprofile may lie between the two curves in Fig. 5, but thechange in diffusion coefficient is around 4 · 10�10 to5 · 10�10 lm2 s�1. This extent of variation is little com-pared with the difference of diffusion coefficient betweenOhsawa lava and Awanomikoto lava.

The results in Fig. 4 show that a hydration front can ex-tend beyond the dark boundary observed under opticalmicroscope especially in Ohsawa lava. Based on the obser-vation of such boundary in glass, Friedman and Smith(1960) first introduced a method for dating of obsidianusing the thickness of hydrated layer. The thickness of hy-drated layer x was originally correlated with time t and rateconstant k as x ¼

ffiffiffiffi

ktp

(Friedman and Smith, 1960). How-ever, it has revealed from the elemental concentration pro-file analyses of weathered obsidians that the boundaryrecognized under optical microscope has little relation tohydrated–unhydrated boundary and that a simple square-root-of-time model is insufficient for the purpose of dating(e.g., Anovitz et al., 1999, 2004). Our result also supports

Hydration of rhyolitic glass 123

the inconsistency of the hydration front and suchboundary.

The interconversion reaction between H2Om and OH(Reaction 1) has been extensively studied for rhyoliticmelts/glasses at >400 �C. The relative concentrations ofwater species under equilibrium state are dependent on tem-perature and CH2Ot : COH increases with temperature at a gi-ven CH2Ot ; and COH is higher than CH2Om at low CH2Ot , butCH2Om becomes a dominant species at higher CH2Ot due tosaturation of COH (Stolper, 1982; Nowak and Behrens,1995, 2001; Ihinger et al., 1999). For example, based onthe equilibrium constant reported in Nowak and Behrens(2001), COH is higher than CH2Om for whole range ofCH2Ot (0–6 wt%) at 800 �C, while at 500 �C the fraction ofOH decreases and CH2Om becomes higher than COH forCH2Ot ¼ 3:2� 6 wt%. Given that the model in Nowak andBehrens (2001) can be extrapolated to <400 �C, the fractionof OH is expected to be very low for all CH2Ot at ambienttemperatures. Although there is uncertainty of COH inweathered glass (Fig. 4b, c, f and g) due to the uncertaintyof molar absorptivity for H2Ot, significant addition of OHto the inherent OH is unlikely. Thus, our result for weath-ered glass is consistent with the expectation from the modelfor high temperatures. The low COH and high CH2Om nearthe glass surface also suggests that the dominant water spe-cies diffusing into the glass is H2Om.

Fig. 6 is an Arrhenius plot of DH2Ot and DH2Om in rhyo-litic glasses/melts. The diffusion coefficients at 403–603 �Care the values reported in Zhang and Behrens (2000), whichwere determined under the experimental condition ofCH2Ot ¼ 0:19–1:68 wt% and water vapor pressure = 0.1 M-Pa. When the values at 403–603 �C are simply extrapolatedto 20 �C, both DH2Ot and DH2Om determined from the rhyo-lite weathering profiles are larger than the extrapolated val-

Fig. 6. Arrhenius plot on diffusion coefficients of H2Ot and H2Om

in rhyolitic melts/glasses. All the data at >400 �C are experimen-tally determined values reported in Zhang and Behrens (2000). Thevalues for glass weathering are larger than the values at �20 �Cthat are extrapolated from the data at >400 �C.

ues. Especially, more than 2–3 orders of magnitude ofdiscrepancies are present for DH2Om . It has been also re-ported from the experimental study of silica glass hydrationthat the diffusion coefficients of water around 250–400 �Care larger than the values extrapolated from the higher tem-peratures (Tomozawa and Davis, 1999). The mechanism ofH2Om diffusion in rhyolitic glasses/melts at >400 �C is be-lieved to be the jump of H2Om from one cavity to anotherinvolving rapid Reaction 1 (e.g., Zhang et al., 1991; Behrensand Nowak, 1997), but the result in Fig. 6 may suggest thatthere is some difference in the mechanism of H2Om

diffusion between ambient weathering condition and at>400 �C.

For the case of alkali ions in silicate melts/glasses, it hasbeen reported that alkali ions move through the channelsembedded in silicate structure and their diffusivities increasewith their concentrations (e.g., Chakraborty, 1995; Hor-bach et al., 2002; Meyer et al., 2004). It is noteworthy thatthere is a small band around 3250 cm�1 in the IR spectra ofhydrated glass (Fig. 3b), which is more conspicuous inAwanomikoto lava (52,000 yr) than Ohsawa lava(26,000 yr). For Awanomikoto lava a band around3430 cm�1 is also recognizable. The bands around 3250and 3430 cm�1 are indistinctive for rhyolitic glasses/meltsexperimentally hydrated at >400 �C. An example of theIR spectrum of obsidian experimentally hydrated at about1000 �C and 100 MPa for 240 h is shown in Fig. 3b (sampleSAI1 in Okumura and Nakashima, 2005). Although therehas been a report that the band at �3225 cm�1 correspondsto the overtone of H2Om absorption at 1611 cm�1 (Davisand Tomozawa, 1996), overtone peak is usually signifi-cantly weak and this effect unlikely to account for the IRsignal intensities observed in the weathered rhyolites. It isknown that liquid water shows a broad absorption bandaround 3400 cm�1 and ice has a band at �3250 cm�1 (Ainesand Rossman, 1984). Water molecules in liquid water andice are generally hydrogen bonded to be linked one another(e.g., Jeffrey, 1997). The bands around 3250 and 3430 cm�1

in the weathered glass can be interpreted as the presence ofwater which is structurally similar to ice and liquid water,conceivably related to clustering of water. The presence ofwater cluster in glass has been also suggested in Schallerand Sebald (1995) and Zavel’sky et al. (1998). The condi-tion of water in glass (e.g., extent of hydrogen bond,whether channel is formed or not, size of cluster) and its ef-fect on water diffusivity are uncertain and will need to beelucidated in further work. However, the difference of spec-tra between >400 �C and weathering condition indicates thedifferent chemical states of water, which might be related tothe fast diffusion of water during weathering.

5. CONCLUSIONS

IR spectral line profile analysis was conducted on theglass sections of rhyolites with different duration of weath-ering (26,000 and 52,000 yr) and the contents of total water(H2Ot), molecular water (H2Om) and OH species weredetermined. The content of H2Om in the hydrated regionis higher than that of OH species. Thus, reactionfrom H2Om to OH is little and water migrates into glass

124 T. Yokoyama et al. / Geochimica et Cosmochimica Acta 72 (2008) 117–125

predominantly as H2Om. The diffusion coefficients of H2Om

during weathering are more than 2–3 orders of magnitudelarger than the values at �20 �C that is extrapolated fromthe reported diffusivity data experimentally determined at>400 �C. The IR spectrum of weathered glass shows a bandaround 3250 cm�1 due possibly to water molecular clusterthat is indistinct for the glass experimentally hydrated at>400 �C, which might be related to the fast diffusion ofH2Om during weathering.

ACKNOWLEDGMENTS

The manuscript was greatly improved by the thoughtful com-ments of J.M. Castro, B. Grambow, two anonymous reviewers,and editor W.H. Casey. The electron microscopy was performed inthe Electron Microbeam Analysis Facility for Mineralogy at theDepartment of Earth and Planetary Science, University of Tokyo.

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Associate editor: William H. Casey