water distribution in low-grade siliceous metamorphic rocks by micro-ftir and its relation to grain...

Water distribution in low-grade siliceous metamorphic rocks by

micro-FTIR and its relation to grain size: a case from the

Kanto Mountain region, Japan

Yuki Ito *, Satoru Nakashima1

Geological Institute, The University of Tokyo, Hongo 7-3-1, Tokyo 113, Japan

Received 27 February 2001; accepted 30 January 2002

Abstract

Infrared microspectroscopy reveals that liquid-like water is always present in polycrystalline quartz grains from low-grade

metamorphic cherts and shale in the Chichibu Group and the Mikabu Greenrock Complex of the Kanto Mountains area. The

water distribution in these rocks is heterogeneous and is related to rock textures. In the microcrystalline quartz, O–H species

due possibly to Si–OH are generally present. The liquid-like water is also present at grain boundaries and/or in sub-micron fluid

inclusions in the microcrystalline parts. On the other hand, in quartz grains larger than 20 Am in diameter within veins, only the

liquid-like water is present, possibly as fluid inclusions trapped during quartz vein formation. In the microcrystalline cherty

matrix of the metamorphic rocks, a general decrease in liquid-like water content is observed with increasing metamorphic grade,

associated with an increase in grain size. Two representative grain shapes, a cube and a regular tetradecahedron (having 14

planes), are used to estimate the surface area between grains per unit volume with the grain size D (Am): 3/D for cubic and 2.37/

D for tetradecahedron. Grain boundary volumes were then calculated assuming grain boundary widths from 0.5 to 20 nm and

normalized by unit volume of the rock. The measured IR data on low-grade metamorphic cherty rocks fall closely on these

curves with grain boundary width of around 10 nm for the both models, assuming saturation of H2O at grain boundary. The

observed decrease in water content with increase in grain size can be rigorously explained by the decrease in grain boundary

volume per unit volume. These results suggest that liquid-like water occurs mainly between grain boundaries in the

microcrystalline quartz of low-grade metamorphic rocks. Although this grain boundary water model is the first approximation

and requires further details of quantitative grain boundary textures, the present model can provide a new approach to understand

water distribution in polycrystalline rocks.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Water; Infrared spectroscopy; Microcrystalline quartz; Grain size; Grain boundary; Low-grade metamorphic siliceous rock

1. Introduction

Water in the crust plays an important role in the

global cycle of water. Water saturates the pore space

of many sedimentary rocks and many lines of evi-

dence indicate that bulk water contents decrease in

metamorphic rocks with increasing metamorphic

0009-2541/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0009 -2541 (02 )00022 -0

* Corresponding author. Current address: Geosphere Environ-

mental Science Department, Central Research Institute of Electric

Power Industry (CRIEPI), 1646, Abiko, Chiba 270-1194, Japan.

Tel.: +81-4-7182-1181; fax: +81-4-7183-8700.

E-mail address: [email protected] (Y. Ito).1 Current address: Interactive Research Center of Science,

Graduate School of Science and Engineering, Tokyo Institute of

Technology, 2-12-1,Ookayama,Meguro-ku,Tokyo152-8551, Japan.

www.elsevier.com/locate/chemgeo

Chemical Geology 189 (2002) 1–18

grade (Fyfe et al., 1978). Stoichiometric dehydration

of rocks during metamorphism by sequential trans-

formations from one hydrous mineral assemblage to

another anhydrous one gives rise to an important

water flux in the crust. In deeper parts, water in the

earth’s mantle has been supposed to be entrapped

mainly in pyroxenes and olivines, nominally anhy-

drous minerals, and to provide a possible mechanism

to recycle water from the earth’s surface into the deep

mantle (Bell and Rossman, 1992).

Water plays crucial roles in deformation and meta-

morphism of rocks. Water distributions in deformed

rocks have been discussed by Kronenberg and Wolf

(1990), Kronenberg et al. (1990), Paterson (1990)

and Nakashima et al. (1995) in terms of the effects of

water on the rock deformation. However, the mech-

anism of hydrolytic weakening of quartz remains

unknown (Kronenberg, 1994). On the other hand,

during metamorphism, water mediates migration of

materials in rocks and H2O chemical potential deter-

mines the equilibria of metamorphic reactions involv-

ing dehydration (Thompson, 1955). In addition, water

between grain boundaries promotes diffusion of

materials (Paterson, 1990); thus, increases transforma-

tion rate of rocks. Water is also essential in metamor-

phic reactions including recrystallization and grain

growth.

Despite the crucial role of water during metamor-

phism, the amounts of metamorphic fluids are not

known quantitatively and the speciation of water is

not known (Ferry, 1994). Water is known to have

profound effects on nominally anhydrous minerals

such as quartz; yet water contents in quartz aggregates

of siliceous metamorphic rocks have not been inves-

tigated in detail. Nakashima et al. (1995) reported that

7000–3500 ppm of water is present in unmetamor-

phosed chert in Inuyama, Aichi, Japan, while only

1000–200 ppm of water was found in medium-grade

siliceous metamorphic rocks in Asemigawa, Ehime,

Japan. These results suggest that water contents in

polycrystalline quartz drop upon metamorphism.

Water might be released from hydrous polycrystalline

quartz aggregates into more anhydrous ones during

increasing metamorphic degree, possibly related to

grain growth of quartz grains. Based on this hypoth-

esis, water contents in low-grade metamorphic rocks

might be intermediate around 3500–1000 ppm and

can be related to grain size of quartz.

During metamorphism, recrystallization and the

grain growth of quartz grains are known to occur.

Grain size can be one of the major quantitative pa-

rameters that represent relative metamorphic grade, as

grain growth is strongly influenced by high temper-

ature. The larger the grain size is, the lower the ratio

of grain boundary area to the unit volume of the rock.

If water is present mainly at grain boundaries, less wa-

ter content is expected for the polycrystalline quartz

aggregates with larger grain sizes. Based on this hy-

pothesis, some relation between the grain size and

water content might be observed in siliceous metamor-

phic rocks.

In addition, the speciation of water may vary with

grade in metamorphic rocks. Water may occur in a

variety of forms, including: (1) liquid water in pores

and grain boundaries, (2) fluid inclusion water in

quartz, (3) surface OH species on grain surfaces and

(4) OH species bound to crystalline interior structures

including structural defects (Kronenberg, 1994). The

distribution of each different species of water in

metamorphic rocks should provide critical informa-

tion to understand the role of water in metamorphic

processes.

In this paper, we analyze the distribution of differ-

ent species of water in quartz aggregates from low-

grade metamorphic siliceous cherts in Chichibu,

Kanto Mountains, Japan, by means of infrared micro-

spectroscopy (micro-FTIR). We examine (1) the oc-

currence of water in low-grade metamorphic rocks

with intermediate water contents of 3500–1000 ppm,

lower than unmetamorphosed silica and higher than

medium to high degree of metamorphic rocks; (2) the

relation of water content to grain size of quartz

aggregates; and (3) the distribution of different water

species in siliceous metamorphic rocks.

2. Geologic setting

The rock samples for water measurement were taken

from the easternmost exposure of the Chichibu Group

in the Kanto Mountains (Mts.) region, 140 km north-

west of Tokyo, Japan (Fig. 1a). In the Kanto Mts., the

Mikabu Greenrock Complex and the Chichibu Group

show a zonal structure from north to south with as-

cending stratigraphic order (Fig. 1b). The Kanto Mts.

are considered to belong to the high-pressure type

Y. Ito, S. Nakashima / Chemical Geology 189 (2002) 1–182

metamorphic rocks according to detailed studies of

mineral assemblages (Hirajima, 1983, 1985; Shimizu,

1988).

2.1. The Chichibu Group

The Chichibu Group is composed of trench-filling

sediments that contain olistoliths of Carboniferous to

Triassic age and was deposited from late Jurassic

onwards (Tagiri et al., 1992). The main rock types

are phyllitic mudstone and sandstone containing olis-

toliths of chert, volcaniclastics, basic lava and lime-

stone (Tagiri et al., 1992). It has been suggested that

the metamorphic zoning is concordant with the strati-

graphic succession, and each metamorphic zone

roughly corresponds to each stratigraphic unit (Tor-

Fig. 1. (a) Geological map of the Chichibu Group in the Kanto Mountains region, 140 km northwest of Tokyo, Japan (after Hirokawa, 1982). (b)

Geological map of the Mikabu Green Rock Complex and the Chichibu Group in the Kanto Mountains region compiled from Toriumi (1975),

Shimizu (1988) and Hirajima and Banno (1989). Sampling localities are indicated.

Y. Ito, S. Nakashima / Chemical Geology 189 (2002) 1–18 3

iumi, 1975; Hirajima, 1985). The Chichibu group

experienced prehnite zone and pumpellyite–actinolite

facies metamorphism. The pressure and temperature

conditions (P–T conditions) of this group are esti-

mated to be 250–300 jC and 4–6 kbar (Banno and

Sakai, 1989).

2.1.1. The Mamba Formation (MA)

This formation is dominantly composed of green-

stones and contains a small amount of chert (Shimizu,

1988). The southern part of the formation is consid-

ered to have experienced prehnite zone metamor-

phism, while the northern part reached low-grade

pumpellyite–actinolite facies conditions (samples

were not collected in this study for the pumpellyite–

actinolite facies) (Hirajima, 1985).

2.1.2. The Kashiwagi Formation (KW)

This formation consists of red-bedded chert,

muddy chert, acidic tuff, and alternating layer of

acidic tuff and mudstone (Shimizu, 1988). This for-

mation is assigned to the medium-grade zone of the

pumpellyite–actinolite facies (Hirajima, 1985).

2.2. The Mikabu Greenrock Complex (MI)

The Mikabu Greenrock Complex in this area is

composed mainly of basic tuff, tuff breccia, lava

(partly pillow lava) and dolerite, and these are found

intercalated with mudstone and sandstone. In the

uppermost part, red shale with Jurassic radiolaria

overlay conformably basic pyroclastic rocks (Shi-

mizu, 1988). This complex is considered to have

reached high-grade conditions of the pumpellyite–

actinolite facies (Hirajima, 1985).

3. Sample description

Six samples for this study were collected from

each siliceous part of low-grade metamorphic cherts

from outcrops of the Chichibu Group and the Mi-

kabu Greenrock Complex in the Kanto Mts. (two

samples from MA, three samples from KW and one

sample from MI). Sampling localities are shown in

Fig. 1b.

The Mamba Formation experienced the lowest

metamorphic grade in this area, from prehnite zone to

low-grade part of pumpellyite–actinolite facies (Hir-

ajima, 1985). Grayish chert (MA6) and red chert

(MA3) from the Mamba Formation were sampled in

this study. Purplish shale/chert (KW7), pinkish chert

(KWT) and pink–white chert containing many small

red particles in the pinkish part (KW9B) from the

Kashiwagi Formation (medium-grade part of pum-

pellyite–actinolite facies), and red shale (WMI) from

the Mikabu Greenrock Complex (high grade part of

pumpellyite–actinolite facies) were taken for the anal-

ysis.

In all cases, the metamorphic grades were eval-

uated for rocks containing metamorphic minerals such

as prehnite, pumpellyite, actinolite and chlorite (Tor-

iumi, 1975; Hirajima, 1983, 1985; Shimizu, 1988;

Banno and Sakai, 1989). The siliceous rocks of this

study were sampled from the same formations and are

supposed to have experienced the same degrees of

metamorphism.

Average quartz grain sizes in the cherty matrix

were determined by optical microscopy, with aid of

image processing and particle analysis software. How-

ever, they were difficult to measure precisely owing to

their small sizes and their elongate shapes in some

samples. The grain sizes were therefore determined as

diameters of circles of equal area to that of grains

imaged in thin section. Since grain size values were

thus estimations, only the average grain size of each

sample is reported here. Average grain sizes in cherty

matrix are, from smaller to larger ones:

MA3 ð2:45 AmÞ < MA6 ð3:54 AmÞ

< WMI ð6:12 AmÞ < KW7 ð8:66 AmÞ

< KWT ð16:2 AmÞ < KW9B ð17:5 AmÞ:

With only one exception (WMI), samples exhibit a

systematic trend of larger grain sizes with increasing

metamorphic grade.

Pinkish and red chert (Inu3) from Inuyama area,

Aichi-Gifu prefecture, exhibits little evidence of meta-

morphism and water contents of these were studied for

comparison with those of metamorphosed silica sam-

ples. Agate (AG) from Chayagawa, Oshamambe, Hok-

kaido, containing more water (about 0.8 wt.% from

thermogravimetry) than the cherts was also examined

by micro-FTIR. These agate grains have elongated

thread-like shapes and their average size is about 3


Am in length and 0.3 Am in width (i.e., 0.95 Am for the

diameter of equivalent circular area).

4. Experimental methods

4.1. IR measurement

Rock samples were cut and ground to thin sections

of about 50–200-Am thick and polished on both

sides. The thin sections were removed from the glass

slides by immersing them in acetone to dissolve a

cyanoacrylate instant adhesive (TOAGOSEI, ARON

ALPHA). The thin sections were put on a sample

stage of an infrared microspectrometer and were

apertured to about 50� 50 Am2 area. Small samples

were put on the sample stage upon a stainless plate

with a hole.

Water in quartz grains of the rock samples was

measured with a microscopic FTIR spectrometer

(Jasco, MFT-2000) (Nakashima et al., 1995). All

spectra were obtained by collecting 100 scans with a

spectral range from 4000 to 700 cm� 1 and at a 4-

cm � 1 resolution. The precision of peak positions is

within 2 cm� 1 for the measurement with this wave-

number resolution.

Five to ten representative positions in each thin

section were selected for the infrared measurements

by observing magnified images (160� ) of the sample

texture with a Cassegrainian mirror objective of 16�and an ocular of 10� . Mafic and hydrous minerals,

colored substances and clear discontinuities were

avoided, excluding them from the apertured area.

A reference transmission spectrum (T0) was meas-

ured for the apertured area without a sample in place

(only the air), and a sample transmission spectrum (T)

was measured on the desired position of the sample. A

final absorption spectrum was obtained by taking

absorbance A ( =� log10T/T0) as a function of the

wavenumber (cm � 1) (Fig. 2). Linear baselines were

drawn from 3700 to 2500 cm � 1 and from 2000 to

1450 cm � 1, in order to exclude dispersion effects. All

spectra were processed with this baseline correction.

A typical IR spectrum of this study is shown in Fig.

2, in this case of sample (KWT). A broad absorption

band due to O–H stretching vibration is observed

around 3400 cm � 1. The spectral features of this OH

band around 3400 cm � 1 differed from one sample to

another. Seven peaks were observed from 2000 to

1400 cm � 1 (1990, 1870, 1790, 1680, 1610, 1524,

1490 cm� 1) due to overtone and combination modes

of Si–O vibrations. In the region of lower wave-

number than 1400 cm� 1, signals were mostly satu-

rated due to strong fundamental Si–O vibrations. The

seven peaks in the 2000–1400 cm � 1 region are

characteristic peaks of quartz or agate. The occurrence

of these seven peaks at the same wavenumbers with

the same proportion of the peak heights proves the

existence of quartz for the measured area. In this

study, spectra that did not exhibit these seven peaks

were excluded from study. The specimen thickness

can be estimated from the peak heights at 2000–1400

cm� 1, together with a measurement by micrometer.

According to the Lambert–Beer’s law, absorbance

A is proportional to the water concentration in a

sample C (mol l� 1) and the sample thickness d (cm);

A ¼ eCd ð1Þ

where e (l mol� 1 cm� 1) is the molar absorption

coefficient. The 3400 cm� 1 band is considered to be

due mainly to the ‘‘liquid-like’’ molecular water

(H2O) in quartz (Aines and Rossman, 1984; Rossman,

1988; Kronenberg and Wolf, 1990; Kronenberg et al.,

1990; Kronenberg, 1994). The ‘‘liquid-like’’ water

contents in quartz were determined based on the

Fig. 2. Representative IR spectrum of chert from the Kashiwagi

Formation (KWT). A broad band around 4000–2500 cm� 1 is due

to OH vibration. Seven peaks in the region of 2000–1400 cm � 1

(1990, 1870, 1790, 1680, 1610, 1524, 1490 cm � 1) are due to

combinations and overtones of Si–O vibration, characteristic of

quartz and agate. At lower wavenumbers, the signal is saturated, due

to strong fundamental Si–O vibrations.


absorbance A at 3400 cm � 1 (Fig. 2). The molar

absorption coefficient e for this band was assumed

to be 81 (l mol� 1 cm � 1), that determined for liquid

water (Thompson, 1965). This value has been pre-

viously used in the determination of liquid-like water

content in quartz (Kronenberg, 1994; Nakashima et

al., 1995). In fact, for the agate sample, which proved

to consist of pure fine-grained quartz aggregates by

XRD analysis, the water content measured by ther-

mogravimetry (TG) was in agreement with the

‘‘liquid-like’’ water content based on 3400 cm � 1

absorbance (Yamagishi et al., 1997). Sample thick-

nesses d is calculated from the peak height of the 1790

cm� 1 Si–O band, again making use of Lambert–

Beer’s Law. The thickness was also measured by

micrometer directly on each sample and it is generally

close to that determined by the IR spectra of agate.

However, the thickness by micrometer measurements

was not adopted in this study, because this thickness

contains large errors (of F 10 Am) and does not

always represent the true thickness of the area meas-

ured.

The relationship between thickness d and peak

height of the 1790 cm � 1 band was determined using

an agate standard consisting of microcrystalline quartz

with a random crystallographic orientation. The grains

of the chert samples employed in this study are small

( < 20 Am) in microcrystalline part and such sizes of

grains are predominant within the chert. The IR

measurements were conducted over many grains ran-

domly oriented, so agate standard was considered to

be appropriate for the thickness determination of the

chert samples. Agate thin sections of about 60-, 110-,

140-, 170-Am thick (averaged values of micrometer

measurements on five representative positions within

a thin section) were prepared and five points were

measured for infrared spectra in one thin section (Fig.

3a). After applying the baseline correction, the peak

height (absorbance) at 1790 cm � 1 was calculated.

The absorbances were plotted against thickness meas-

ured by the micrometer (Fig. 3b). The absorbance at

1790 cm� 1 of agate was employed in this study

because of its highest correlation coefficient (R =

0.963) with thickness, compared with similar analyses

of the other Si–O bands in the 2000–1400 cm � 1

range (Fig. 3b). Two strong peaks at 1990 and 1610

cm � 1 cannot be used due to their saturation for

thicker samples. In addition, it should be noted that

Kats (1962) showed that the difference of the peak

height at 1790 cm� 1 is medium when polarized light

was introduced for parallel to the c-axis and perpen-

dicular to the c-axis.

The molar concentration of ‘‘liquid-like’’ water in

quartz C (mol l� 1) calculated by the Lambert–Beer’s

law (Eq. (1)) can then be converted to weight ppm of

water (w), by using the value of 2.65 (g cm � 3 = kg

l � 1) for the density of quartz, and the value of 18 (g

mol � 1) for the molecular weight of water. The final

practical equation to determine them is:

w ðppmÞ ¼ C � 18=2650 ð2Þ

Fig. 3. (a) Representative IR spectra of agate with various

thicknesses; 60, 110, 140, 170 Am. Absorbances due to Si–O at

1990, 1870, 1790, 1680, 1610, 1524, 1490 cm � 1 increase with

increasing sample thickness. (b) Calibration lines of Si –O

absorption against thickness of agate thin sections. We used the

regression curve of the 1790 cm� 1, as it shows the best correlation

coefficient (R).


Water volume ratio F against the volume of rocks

(quartz) and molar ppm R (H/106Si) are also calcu-

lated to compare previous studies by using SiO2 = 60

(g mol � 1):

F ¼ C � 18� 10�3 ð3Þ

R ¼ C � 2� 106=2650=60 ð4Þ

Major errors of the calculation of water (weight/

volume/moles) arise from errors of the sample thick-

ness d. The thickness determined by the IR spectra of

agate is close to that determined by micrometer

generally, but thicknesses of some samples deter-

mined by IR spectra show smaller values with an

error of 20 Am. However, since some samples showed

relatively large errors of 20–60 Am, water contents

were calculated by the both methods (1790 cm� 1

peak height and micrometer) and compared. The

variation of water content for the two thicknesses is

within the dispersion range of water content in one

thin section. As for average water contents, a max-

imum error is estimated to be 460 weight ppm for

MA3. Therefore, this error bar was added in some of

the figures (Fig. 8b,c). The errors of other water

contents by the two different methods for thickness

fall within F 160 weight ppm, which are smaller than

the size of the filled circle symbols in Fig. 8b,c.

4.2. Line IR analysis and 2D IR mapping

A line profile analysis was conducted with the mi-

cro-FTIR (Jasco, MFT-2000) equipped with an X–Y

mapping stage with a stepping precision of 1 Am. The

line analysis was conducted across a quartz vein in the

chert sample MA6 in order to investigate spectral dif-

ferences associated with textural variation. The aper-

ture of about 50� 50 Am2 area was shifted every 50

Am to obtain 16 measurements across a quartz vein of

about 400 Am width (see Fig. 6a).

A 2D IR imaging was conducted on a chert sample

of KWT, using an infrared spectrometer equipped

with a Focal Plane Array system (Bio-Rad, FTS-

6000). The spectral range for this analysis was chosen

over 4000 to 1400 cm� 1 and the area was mapped at

64� 64 pixels (one pixel corresponds to 3.3 Am). The

region selected for mapping includes two quartz veins

of about 50–80 Am width (see Fig. 7a). Gray scale

contoured images were made for the absorbances at

400 and 3605 cm � 1, taking the ratio of their absor-

bances to represent quantitative measures of the dis-

tribution of different OH species.

5. Results

5.1. Liquid-like water contents in siliceous rocks

All of the IR spectra obtained for siliceous rocks in

this study have a broad band around 3400 cm � 1. This

broad band is due to O–H stretching vibrations of

various lengths of hydrogen bonds and is assigned

mainly to liquid H2O (Aines and Rossman, 1984).

Therefore, we can assume that the liquid-like water is

present in all the studied samples.

Water contents calculated from the absorbances at

3400 cm � 1 are plotted in Fig. 4. Data originating

from one thin section (one locality) represent one

column of the plot. The formation/zones are plotted

in the reported order of metamorphic grade. Different

samples within one formation/zone are plotted from

left to right with decreasing water content. For com-

Fig. 4. Liquid-like water contents (wt ppm) based on 3400 cm � 1

absorbance for cherts, shale and agate. Data originating from one

thin section represent one column of the plot. Different samples

from one formation/zone are plotted from left to right with

decreasing absorbance and with increasing metamorphic grade.

The samples shown are agate (AG), unmetamorphosed chert (Inu3),

and cherts and shale from Chichibu Group (MA, KW, MI).


parison, data of the agate (AG) and the unmetamor-

phosed chert (Inu3) are also plotted in Fig. 4.

Water contents tend to decrease with increasing

metamorphic grade (Fig. 4). The water contents are

9600–8700 weight ppm (64,000–58,000 Si/106 H;

water volume ratio F = 0.025–0.023) for the agate

(AG) and 6200–1900 ppm (41,000–12,000 Si/106

H; F = 0.017–0.005) for the unmetamorphosed chert

(Inu3). They are 5200–1700 ppm (35,000–11,000 Si/

106 H; F = 0.014–0.005) for the MA6 (prehnite zone),

3300–1400 ppm (22,000–9500 Si/106 H; F = 0.009–

0.004) for the MA3 (prehnite zone), 3400–940 ppm

(22,000–6300 Si/106 H; F = 0.009–0.003) for the

KW7 (medium-grade of pumpellyite–actinolite fa-

cies), 2200–450 ppm (15,000–3000 Si/106 H; F =

0.006–0.001) for the KWT (medium-grade of pum-

pellyite–actinolite facies), 1370–290 ppm (9000–

2000 Si/106 H; F = 0.004–0.001) for the KW9b

(medium-grade of pumpellyite–actinolite facies), and

820–410 ppm (5000–3000 Si/106 H; F = 0.002–

0.001) for theWMI (high grade of pumpellyite–actino-

lite facies). Nakashima et al. (1995) reported that water

contents were 7000–3500 ppm for the same chert

(Inuyama) and 1000–200 ppm for metacherts from

Sambagawa metamorphic rocks that experienced

higher grade metamorphism (Asemigawa route). The

values for the metamorphic rocks obtained in this study

are intermediate (5200–290 ppm) to these earlier re-

sults. Therefore, the results in this study are in agree-

ment with the water contents for lower and higher

degrees of metamorphic rocks, indicating a general

trend of the water content decrease with increasing

metamorphic grade.

However, a wide range of water contents is ob-

served in the same metamorphic grade and even within

the same thin section (Fig. 4). Differences in water

contents may be due to heterogeneity of the rock

texture within a thin section. The distribution of water

was examined with respect to rock texture to test this

hypothesis.

5.2. Water species and rock texture

The rock samples used in this study are texturally

heterogeneous. Textures can be divided into two

groups; a cherty matrix composed of microcrystalline

quartz and larger quartz grains (over 20 Am in

diameter), which include grains within quartz veins.

In the microcrystalline quartz, IR spectra of water are

characterized by the presence of a sharp peak or a

shoulder around 3600–3500 cm� 1 superimposed on

the broad 3400 cm� 1 band (Fig. 5a). For larger quartz

grains (including grains within quartz veins), IR spec-

tra of water are characterized by the presence of only

the broad band at 3400 cm� 1 (Fig. 5d).

In the microcrystalline part of the whole samples, a

sharp peak or a shoulder around 3600–3500 cm� 1,

due probably to OH, is generally present beside the

broad 3400 cm � 1 band. The positions of the sharp

peak are at 3600 cm � 1 for samples of MA6 and

KWT, and around 3600–3580 cm � 1 for samples of

KW7 (Fig. 5b). As for samples of MA3 and WMI,

either a peak around 3600–3580 or 3560–3550

cm � 1 is observed (Fig. 5c). In samples of KW9B,

either a peak or shoulder at 3600 cm � 1, or a shoulder

around 3540–3530 cm � 1 is observed. Besides those

peaks, a shoulder at 3380 cm� 1 is observed in all

samples of MA3 (Fig. 5c).

Most of these peaks occur at wavenumbers greater

than 3400 cm� 1, and they are considered to represent

O–H species other than molecular ‘‘liquid-like’’ water.

The sharp peak at 3600 cm � 1 is not well known, but it

resembles 3600 cm� 1 bands due to Si–OH (Graetsch,

1994). The peak at 3380 cm � 1 is often observed in

quartz and assigned to OH associated with Al3 +

substituted for Si (Kronenberg, 1994; Suzuki and

Nakashima, 1999). The peaks around 3550 and 3530

cm � 1 are not well known in quartz (Kronenberg,

1994); these may be due to hydrous minerals such as

nontronite and glauconite (Russell, 1987) at grain

boundaries.

In the larger quartz grains (over 20 Am in diame-

ter), the broad 3400 cm� 1 band due to ‘‘liquid-like’’

water is the major OH species. (Fig. 5d). In almost all

samples of KW9B and KWT, only the broad band

around 3400 cm � 1 is observed and no other peaks

have been found. Only one sample of KWT shows

peaks at 3600 and 3570 cm � 1. In the sample of MA6,

some spectra show a shoulder or peak at 3380 cm� 1

(Fig. 5e) and a shoulder at 3550 cm� 1, superposed on

the broad band around 3400 cm � 1. In the sample of

KW7, a peak is observed at 3550 cm � 1 or a shoulder

at 3620 cm� 1. In the sample of WMI, some spectra

have a peak at 3590 cm � 1 in addition to the broad

band around 3400 cm� 1. However, the probability of

these peaks due to OH other than liquid water pre-


Fig. 5. Representative IR spectra of the siliceous parts in the low-grade Chichibu cherts. (a) Spectrum with characteristic peak at 3600 cm � 1

superimposed on the broad band around 3400 cm� 1 from sample of microcrystalline quartz of chert of the Kashiwagi Formation (KW7). (b)

Spectrum with peak at 3585 cm� 1 superimposed on the broad band around 3400 cm� 1 from chert sample of the Kashiwagi Formation (KW7).

(c) Spectrum with peaks at 3550 and 3800 cm � 1 superimposed on the broad band around 3400 cm� 1 from chert sample of the Mamba

Formation (MA3). (d) Spectrum characteristic of the large crystalline part ( > 20 Am) of chert with only a broad band around 3400 cm � 1,

measured for chert from the Kashiwagi Formation (KWT). (e) Spectrum with a peak at 3380 cm� 1 superimposed on the broad band around

3400 cm� 1 from chert of the Mamba Formation (MA6).


sent in these coarse-grained regions is much lower

than in the microcrystalline parts.

5.3. Line IR analysis

In order to examine the transition in IR spectra

between coexisting large quartz grains (a quartz vein)

and microcrystalline quartz, a line profile analysis was

carried out by traversing a quartz vein in microcrystal-

line quartz matrix in a chert from the prehnite zone

(MA6, 80–90-Am-thick sample). IR spectra were

obtained in areas of 50� 50 Am2 with a 50-Am step

as indicated in Fig. 6a. The resulting IR spectra of

water for this line profile analysis are shown in Fig.

6b. Although the broad band around 3400 cm� 1 is

observed in all of the spectra, significant differences

in spectra are noted that correspond to changes in rock

texture.

In the coarse-grained quartz vein, spectra are domi-

nated by the broad band around 3400 cm� 1 due to

liquid water with some presence of a peak at 3380

cm � 1. In contrast, the microcrystalline quartz exhibits

a sharp peak at 3600 cm� 1 in addition to the broad

band around 3400 cm � 1. At the boundary of the

microcrystalline quartz and larger quartz grains (posi-

tions 6 and 14 in Fig. 6a and b), a weak shoulder at 3600

cm � 1 is observed superposed on the 3400 cm� 1 band.

In addition to these spectral differences, the inten-

sities of the OH bands vary. Intensities of the 3400 and

3600 cm � 1 bands are plotted against the position (Fig.

6c). Within the vein, the 3600 cm � 1 band is smaller

than that in the microcrystalline matrix. In contrast, the

Fig. 6. (a) Photomicrograph of chert from the Mamba Formation (MA6) under a polarizing optical microscope (crossed nicols). The 16 squares

indicate positions of an IR line profile analysis with measurement areas of 50 Am2. The scale bar is 50 Am. (b) IR spectra for positions 1 to 16

shown in (a). Spectra are offset vertically for clarity. (c) Absorbances at 3400 and 3600 cm� 1 are plotted against spatial positions 1 to 16.


peak intensity of the 3400 cm� 1 band in the vein is

larger than that in the microcrystalline matrix. These

results show that the distribution of OH species differ

significantly in the microcrystalline quartz and in the

larger quartz grains.

5.4. 2D IR imaging

In order to examine the distribution of different wa-

ter species further, IR imaging was conducted over an

area containing both quartz veins and the microcrystal-

Fig. 7. (a) Photographic image of chert sample with two quartz veins crossed each other from the Kashiwagi Formation (KWT) under an optical

microscope with crossed nicols. This region was analyzed by IR mapping using absorbances at (b) the 3400 cm � 1 and, (c) 3605 cm� 1, and the

ratio of absorbances using (d) peak ratio 3605/3400 cm� 1.


line quartz matrix. Results of 2D IR imaging for two

quartz veins crossed each other in a cherty matrix of

KWTare shown in Fig. 7b, c and d. The 2D images are

presented as the intensity of the 3400 cm� 1 band (Fig.

7b), the intensity of the 3605 cm � 1 band (Fig. 7c), and

the ratio of peak intensities (3605/3400 cm� 1) (Fig.

7d).

The absorption intensity at 3400 cm� 1 is generally

higher for larger quartz grains within veins. However,

the smaller grains of the veins exhibit 3400 cm� 1

band intensities similar to those of the microcrystal-

line cherty matrix. On the other hand, the band

intensity at 3605 cm� 1 is high in the microcrystalline

matrix and low in the quartz veins. The peak ratio

image of 3605/3400 cm� 1 shows a clear contrast in

water species of the quartz veins and the microcrystal-

line quartz. The band at 3400 cm� 1 due to liquid-like

water is predominant in the quartz veins. On the other

Fig. 8. (a) Average values of liquid-like water contents within one sample for cherts and agate plotted against the mean grain size determined by

optical microscopy of the same samples. (b and c) The relation between water content (in weight ppm and mol/l) and grain size (Am). Filled

circles represent the liquid-like water contents of the microcrystalline part of the rock samples studied. Other symbols represent the calculated

water contents, assuming all water is at grain boundaries, that grain boundaries are saturated by water over grain boundary widths of 5, 10 and

20 nm. (b) Calculation for cubic (hexahedron) model grains with side dimension D giving a grain surface area per unit volume of 3/D. (c)

Calculation for regular tetradecahedron (having 14 planes) model grains giving a grain surface area per unit volume of 2.37/D.


hand, the microcrystalline quartz exhibits two signifi-

cant bands, one at 3400 cm � 1 due to liquid-like water

and another at 3605 cm� 1, most probably due to Si–

OH.

5.5. Liquid-like water contents in microcrystalline

quartz and grain size

The above line profile and the 2D imaging anal-

yses indicate that different water species are distrib-

uted in different types of quartz grains in met-

amorphic rocks. In the following, only the water

contents in microcrystalline matrix part in metacherts

will be discussed in relation to their texture and meta-

morphism, as the microcrystalline part is predom-

inant within the chert samples employed in this study.

The water contents of larger grains including quartz

veins will be discussed separately from the water

contents of other grains that experienced metamor-

phism, since the vein formation processes are com-

plex.

The water content data of the only microcrystal-

line quartz range from 10,000 to 300 weight ppm

(including agate), which is almost the same as the

data range including larger quartz grains. The meas-

ured microcrystalline part of 50� 50 Am2 is com-

posed of many small quartz grains and contains many

grain boundaries. If liquid-like water is mainly

present between grain boundaries, the water content

might be influenced by grain size. As the grain size

becomes larger, the specific area of grain boundaries

(normalizing by volume) becomes smaller. Re-exam-

ining the data from this perspective, a relationship is

found between liquid-like water content and grain

size. Average grain sizes determined under the optical

microscope and SEM generally increase with increas-

ing metamorphic grade, except for the Mikabu green

rock (WMI) (see Sample description). The average

grain sizes of microcrystalline quartz plotted against

the averaged liquid-like water contents in the micro-

crystalline part are shown in Fig. 8a for seven

samples including agate based on the absorbances

at 3400 cm� 1. The liquid-like water content tends to

decrease with increasing grain size in the cherty

matrix. This result suggests a possible causal relation-

ship between the decrease of liquid-like water in

polycrystalline quartz aggregates with increasing

grain size.

6. Discussions

6.1. Water speciation in low-grade metamorphic chert

The metamorphic siliceous rock samples in this

study are heterogeneous and generally divided into

two groups: the larger quartz grains over 20 Amin diameter, mostly distributed in quartz veins and

the cherty matrix composed of microcrystalline

quartz.

In the larger quartz grains (over 20 Am in diame-

ter), only the broad 3400 cm � 1 band is observed. The

broad band is attributed to liquid-like water, as the

spectrum resembles that of liquid water. The grain size

(>20 Am) is not small compared to the apertured size

(50� 50 Am2) and grain boundaries are not abundant

in the volume of material measured. Fraction of water

in grain boundary might be small in the larger quartz

grains. Most liquid water observed in this study by IR

measurement is therefore considered to be in fluid

inclusions, possibly trapped in quartz grains during

the quartz vein formation process. However, the water

contents of the larger grains (veins) will not be dis-

cussed hereafter, because they are not predominant

within the rocks and they might have been affected by

later events after metamorphism.

In the microcrystalline quartz, IR spectra of water

are characterized by the presence of a sharp peak or a

shoulder around 3600–3500 cm � 1 superimposed on

the broad 3400 cm� 1 band. Although minor bands at

3620 and 3550 cm � 1 are present in some samples

and can be due to fine hydrous minerals among quartz

grains, the main features are the 3400 and 3600 cm � 1

bands. The broad 3400 cm � 1 band is due to liquid-

like water. The 3600 cm� 1 band may be due to Si–

OH (Graetsch, 1994). The presence of the broad 3400

cm� 1 band in the cherty matrix suggests that liquid-

like water is present along grain boundaries and pores

or in fluid inclusions in the microcrystalline part.

However, the contribution of fluid inclusions to the

total water content is considered to be small. The size

of the fluid inclusions in the microcrystalline matrix

( < 20 Am) is considered to be sub-micron. Water

content in this kind of fluid inclusions can be roughly

estimated from nm-scale fluid inclusion density in

quartz from granodiorite near a shear zone by TEM

observation (Kronenberg et al., 1990). The water

content in quartz by these nm-scale fluid inclusions


was estimated to about 155 weight ppm assuming that

fluid inclusion density is 15� 108 mm� 3 having a

spherical shape with a diameter of 80 nm, which is an

average value of four samples studied by Kronenberg

et al. (1990). This water content is not underestimated,

because their samples might have abundant cracks

which often accompany fluid inclusions due to an

intense deformation. The obtained water contents in

nm-scale fluid inclusions (155 ppm) is less than the

observed values of 5000 to 300 ppm for the rocks

studied from IR measurement. If we assume the

presence of these nm-fluid inclusion in all the rocks,

the total water contents minus 155 ppm would give

the grain boundary and pore water. This correction

would give just a small lower shift in water contents

and would not alter the general decreasing trend with

grain size (Fig. 8a). Therefore, water in microcrystal-

line aggregates is thought to be distributed mainly in

grain boundaries and pores, especially for low-grade

fine-grained cherty rocks, with a small contribution

from nm-fluid inclusions.

6.2. Water contents in microcrystalline part of the met-

amorphic rocks

The water contents in microcrystalline part exclud-

ing the larger quartz part (veins) calculated from the

absorbance at 3400 cm� 1 also range from 5000 to

300 ppm for low-grade metamorphic cherts. These

contents are lower than those of the unmetamor-

phosed Inuyama chert (6000–2000 ppm). In contrast,

the contents are higher than those of the medium-

grade Sambagawa metachert (chlorite zone–oligo-

clase biotite zone, 1000–200 ppm) (Nakashima et

al., 1995). Thus, these water contents in microcrystal-

line part of the low-grade metamorphic cherts have

intermediate values between those of unmetamor-

phosed chert and medium-grade metachert. Therefore,

water seems to be lost during increasing metamorphic

degree. Water, which we conclude to be held mainly

along grain boundaries and pores from the above

results, appears to have been released during meta-

morphism. This loss corresponds to changes in rock

texture with metamorphism, specifically the increase

in grain size.

We also calculated the water volume ratio F of the

rocks from the results of the IR peaks. The calculated

value of the agate is 0.025–0.023 (2.5–2.3 vol.%) and

the values for cherts decrease with increasing meta-

morphic grade. The value of the highest metamorphic

grade chert in this study (WMI) is 0.002–0.001 (0.2–

0.1 vol.%). These values should be compared with

effective porosities determined by conventional meth-

ods such the water-saturation method and mercury

intrusion porosimetry. Although effective porosity of

the rock studied was not measured, the porosity and the

pore size distribution for similar kind of siliceous sed-

imentary rocks in other previous studies were com-

pared (Nishiyama et al., 1990). A siliceous sedimentary

rock having lower diagenetic degree has a porosity of

30.8% determined by a water-saturation method and a

pore size distribution with a medium value of 162 nm

determined by a mercury intrusion method. After

undergoing diagenesis, the porosity for cherts

decreases to 4.3% and the medium value of the pore

size also decreases to less than 10 nm. The porosity of a

metamorphic rock (green schist) is as low as 0.46%

(Nishiyama et al., 1990). These reported porosity

values (4.3% to 0.46%) are close to the water volume

fraction estimated here from the IR data (2.5% to

0.1%).

6.3. Water thickness estimation at grain boundaries

In the low-grade siliceous metamorphic rocks stud-

ied, the averaged liquid-like water content in one thin

section (locality) is smaller for the larger grain size

material (Fig. 8a). In order to determine the amount of

water corresponding to grain boundary surface area,

two representative grain shapes were assumed, a cube

(hexahedron) and a regular tetradecahedron. Surface

area of a cubic grain (hexahedron) is 6D2 by using the

length of a side D. The surface area between grains

contacting each other, normalizing by one grain, is half

of the total surface area: 3D2. Since the volume of the

grain is D3, the surface area between grains (grain

surface area) per unit volume is 3/D. Similar calcula-

tion for a regular tetradecahedron (having 14 planes)

gives a grain surface area per unit volume of 2.37/D

(3.35D2/1.414D3). The grain surface area for the for-

mer is the largest surface to volume ratio of any simple

close-fitting shape and that for the latter is the smallest

(Rhines and Dehoff, 1968).

We will now calculate the amount of water pre-

sent between grain boundaries by using the 3/D and

the 2.37/D relations and by assuming that a mean di-


ameter of the grains can be taken as D (Am). To obtain

a water concentration per unit grain boundary volume,

we assume a grain boundary widthW (nm) of 0.5 to 20

nm following estimates of Farver and Yund (1992) and

Nakashima (1995). The grain boundary volume per

unit volume V of sample can be expressed as:

V ¼ ð3� 10�3ÞW=D ð5Þ

or

V ¼ ð2:37� 10�3ÞW=D ð6Þ

The grain boundary volume per unit volume V can

be converted to a molar concentration of water per

unit volume of quartz M (mol l � 1) by using the

density of water (1 g cm� 3 = 10� 12 g Am � 3) and

the molecular weights of water (18 g mol� 1):

M ¼ W=6D ð7Þ

or

M ¼ 2:37W=18D ð8Þ

By using the density of quartz, this volume can be

converted to a weight ppm of water per unit weight of

quartz x (wt ppm):

x ¼ M � ð18=2650Þ � 106 ¼ 1132� ðW=DÞ ð9Þ

or

x ¼ M � ð18=2650Þ � 106 ¼ 894� ðW=DÞ ð10Þ

These two relations are shown graphically in Fig.

8b and c for five representative W values of 0.5, 1, 5,

10 and 20 nm. Although 1–10 nm is generally

accepted value (Farver and Yund, 1992), 0.5 and 20

nm are also included in calculation in order to check

possible ranges of grain boundary width.

The measured IR data on low-grade metamorphic

cherty rocks fall close to these calculated curves. The

grain boundary width value ranges from 5 to 10 nm

for the cubic model and from 5 to 20 nm for the

tetradecahedron model; a width of 10 nm would be

compatible with both models (Fig. 8b and c). The

decrease of water volume between grain boundaries

can therefore be explained by the increase in grain

size. The results of the calculations also suggest that

the liquid-like water is present mainly between grain

boundaries in the low-grade metamorphic chert.

The magnitude of the grain boundary width esti-

mated from the water content by micro-FTIR is close

to the reported grain boundary width of polycrystal-

line quartz aggregate of about 1–10 nm (Farver and

Yund, 1992). The water film between grain bounda-

ries of 10-nm thick corresponds to about 36 H2O

molecules if we use the hydrogen bond distance for

liquid water of 0.276 nm (Stumm and Morgan, 1996).

However, the hypothetical thin film water on silica

surface (2–3 nm, < 10 H2O molecules) has been con-

sidered often to have much more constrained structure

than the free water (Kronenberg, 1994). They may

have a different structure from normal liquid water

and may exhibit unusual physical and chemical prop-

erties (Kronenberg, 1994). Hiraga et al. (1999) dem-

onstrated that the real grain boundary width between

grains is under 0.5 nm by TEM observation. In this

study, however, films of greater thickness (10 nm) are

estimated to be present among grain boundaries in

these metamorphic rocks. Nevertheless, this thickness

agrees well with the results of mercury intrusion po-

rosimetry for the sedimentary siliceous rocks with a

peak pore radius of about 10 nm (Nishiyama et al.,

1990). Therefore, water determined by IR measure-

ment might suggest liquid water is distributed in

discrete pores along grain boundaries and the obtained

thickness (10 nm) corresponds to an average thick-

ness. Water might not always be present between the

entire grain boundaries in the real rock systems. The

grain boundary may be composed of portions of

wetted and non-wetted areas (Farver and Yund,

1992; Nakashima, 1995). In fact, the intergranular

pore sizes observed by transmission electron micro-

scopy (TEM) on similar metamorphic rocks have a

value range from 0.5 to about 30 nm having a

‘‘neckless structures’’ (Hiraga et al., 2001). If only

half of the grain boundary is filled with fluids, the

apparent average grain boundary thickness might be

20 nm. These different degrees of saturation will

cause data scattering in Fig. 8b and c on different

grain boundary width curves but will not change the

general water content-grain size relation. The esti-

mated grain boundary width here (10 nm) can only be

an apparent average value of water contents at grain

boundaries and is considered to be a kind of saturated


end-member. It should be noted that the shapes of

natural grains are not as simple as cubic and tetrade-

cahedron shapes, showing elongated or thread-like

shapes.

Water content in agate, which is not a metamorphic

rock, agrees well with the water and grain size relation

above. Therefore, the water content of grain bounda-

ries in microcrystalline quartz seems to vary primarily

with grain size and average grain boundary width (i.e.,

grain boundary volumes), rather than with some

change in surface speciation at increasing metamor-

phic grade. Thus, the relation between water content

and grain size might apply generally for many other

polycrystalline rocks, and may be a major controlling

factor of water contents of polycrystalline rocks in

natural systems. However, we only proposed here the

first simple approximation model for water distribu-

tion in a polycrystalline natural rock. Actual distribu-

tion of water is complex and difficult to simplify, as

water is present in pores, between grain boundaries,

grain boundary corners, fluid inclusions and so on.

The averaged water thickness we have calculated

from the thin film water model can vary with a

volume fraction of water in grain boundaries and with

saturation degree of fluids in void spaces. Detailed

study of distribution of water with pore structure data

will be conducted in the future.

This new grain boundary model which we have

presented for understanding distribution of water in

rocks can be applied to other fine polycrystalline rocks

containing no hydrous minerals, such as rocks com-

posed of olivine, pyroxene, calcite and so on.

7. Conclusions

Infrared microspectroscopy has revealed that liq-

uid-like water is pervasive in polycrystalline quartz

from low-grade metamorphic cherts and shale in the

Chichibu Group and the Mikabu Greenrock Complex

in the Kanto Mountains area. A general decrease in

the liquid-like water content with increasing meta-

morphic degree is observed. However, heterogeneity

in water content even in one thin section is significant.

The rock samples used in this study were texturally

heterogeneous and could generally be divided into

two groups; the cherty matrix composed of micro-

crystalline quartz and the larger quartz grains over 20

Am in diameter of quartz veins. In the microcrystalline

quartz, the 3600 cm� 1 band due probably to Si–OH

is generally present in addition to a broad 3400 cm� 1

band due to liquid-like water. In contrast, larger quartz

grains over 20 Am in diameter exhibit only the band at

3400 cm � 1 due to liquid water. This water may be

due to fluid inclusions trapped during quartz vein

formation.

In the microcrystalline cherty matrix of the meta-

morphic rocks studied, the liquid-like water content

decreases and grain size increases with increasing

metamorphic degree. Water seems to be lost during

increasing metamorphic degree in relation to the

grain growth process. Water contents at grain boun-

daries calculated for two representative grain shapes,

a cube (hexahedron) and a regular tetradecahedron,

match observed water contents and explain the

relation to grain size D. The grain boundary volume

per unit volume can be expressed by 3W/D for cube

and 2.37W/D for tetradecahedron, where W is the

grain boundary width. The measured IR data on low-

grade metamorphic cherty rocks fall close to these

curves with W=f 10 nm. These comparisons sug-

gest that liquid-like water is present between grain

boundaries in the microcrystalline quartz and thus

bulk water contents are controlled by grain size and

grain boundary width. Although the present simpli-

fied grain boundary model is the first approximation

of the complex grain boundary and pores structures,

the general trend of water decrease upon grain

growth seem to explain well the natural data sets.

Therefore, liquid water in polycrystalline fine aggre-

gates is considered to be mainly located at grain

boundaries.

Acknowledgements

Jasco is thanked for the use of the infrared

microspectrometer for our analyses at the University of

Tokyo. H. Shiratori of JASCO and M. Sunose of Seki

Technotron are also thanked for helping with spectro-

scopic data processing. Mr. Nakano, Mr. Shimada and

Mr. Kimura of Nippon Bio-Rad Laboratories are

appreciated for their technical support of 2D IR im-

aging spectroscopy. H. Yamagishi is thanked for IR

calibration against thickness for agate. We are grateful

to Y. Narita, T. Irino, N. Saito and S. Ikeda for their


technical help and useful advice. We would like to

thank N. Kuroki for providing a sample from Inuyama.

S. Suzuki kindly informed us of the way to calculate

grain sizes of polycrystalline aggregates. We are also

grateful to M. Toriumi, R. Tada, I. Shimizu, Y. Wata-

nabe for their valuable discussions. C. Spiers of Utrecht

University is acknowledged for the critical reading of

the manuscript. We gratefully acknowledge A. Kro-

nenberg and an anonymous reviewer for their con-

structive comments. We are also grateful to E.H.

Oelkers for his effective editorial handling. [EO]

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water distribution in low-grade siliceous metamorphic rocks by micro-ftir and its relation to grain...

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