Infrared microspectroscopy analysis of water distribution...

E L S E V I E R Tectonophysics 245 (1995) 263-276

TECTONOPHYSICS

Infrared microspectroscopy analysis of water distribution in deformed and metamorphosed rocks

Satoru Nakashima a,., Hiromi Matayoshi b, Takako Yuko b, Katsuyoshi Michibayashi b, Toshiaki Masuda b, Noriko Kuroki a, Hiraku Yamagishi a

Yuki Ito a, Akira Nakamura c a Geological Institute, University of Tokyo, Hongo 7-3-1, Tokyo 113, Japan

b Department of Geosciences, Shizuoka University, Ohtani 836, Shizuoka 422, Japan c Department of Chemistry, College of Education, Akita University, Tegatagakuen-cho 1-1, Akita 110, Japan

Received 10 October 1993; revised version accepted 26 September 1994

Abstract

Infrared microspectroscopy has been applied to thin sections of various deformed and metamorphosed rocks in order to investigate water content of quartz in these rocks. The broad IR band absorbance around 3400 cm- I probably due to fluid-inclusion molecular water (H20) was used to calculate water contents. Deformed granitic rocks from the Yanazawa-Kamimura area near the Median Tectonic Line (MTL) showed an increase of water content in quartz from about 300 ppm to 2500 ppm toward the MTL with increasing degree of deformation. Metacherts from Sambagawa metamorphic rocks (Asemigawa route) showed a systematic decrease of water content in quartz from about 1000 ppm to 200 ppm with increasing metamorphic degree from chlorite, garnet, albi te-bioti te to oligoclase-biotite zones. An Archaean metachert from the Napier Complex (granulite facies) has only 40 ppm water. Comparing this with an Inuyama unmetamorphosed chert sample having water contents of 3500 to 7000 ppm (a starting point of metamorphism), the systematic decrease of water in quartz with increasing metamorphic grade may extend from unmetamorphosed cherts to the highest-grade metacherts. Water contents in high-pressure metamorphic rocks in Japan are mostly of the order of 500 ppm (300-700 ppm), except for those from Kurosegawa with a water content as high as 1700 ppm. These results represent an exploratory analysis of intragranular water contents in quartz in various deformed and metamorphosed rocks. Further micro FT-1R studies of the distribution of water in crustal rocks will provide a quantitative basis for examining the geochemical cycle of water in the earth's crust.

I. Introduct ion

Flu ids have a s ignif icant inf luence on the de- f o rma t ion o f rocks (Pa te r son , 1990; C a r t e r et al., 1990; see also o t h e r p a p e r s in this issue). O n e of

* Corresponding author.

the bes t -known mechan i sms by which wa te r affects d e f o r m a t i o n is hydrolyt ic weaken ing of silica te minera ls , in pa r t i cu l a r quar tz (Griggs , 1967) and olivine (Blacic, 1972). Hydro ly t i c weaken ing of hyd ro the rma l ly grown " w e t " quar tz , l ead ing to a r educ t i on in flow stress by up to 90% c o m p a r e d to dry na tu ra l quar tz , has been r e p o r t e d in various l abo ra to ry d e f o r m a t i o n expe r imen t s (e.g.,

0040-1951/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0040-1951(94)00239-8

264 s. Nakashima et a l . / Tectonophysics 245 (1995) 203-270

Griggs and Blacic, 1965; Kronenberg et al., 1986, 1990; Paterson, 1990; Kronenberg and Wolf, 1990).

Recently, Kronenberg and Wolf (1990) first applied Fourier transform infrared spectroscopy (FT-IR) to natural rock samples for the determi- nation of water content and speciation. They used a focused IR beam on a FT-IR spectrometer and analyzed thin sections of quartzite, novac- ulite, granite and mylonite samples down to a 100 × 100-#m area. Intragranular water contents found in quartz grains of these deformed rocks are high, around 2400-3900 ppm. They con- cluded that most of the intragranular water in quartz occurs as " f reezable" fluid inclusion water based on the comparison of IR spectra of O-H stretching vibrations at room and liquid-nitrogen temperatures with the peak shift due to the crys- tallization of water to ice. The same method was also used in Kronenberg et al. (1990) to analyze intragranular water contents of minerals within granitic rocks along a small ductile shear zone. They found the first direct evidence of increased water content in naturally deformed quartz, traversing a ductile shear zone, from 60 to 4000 ppm H / S i (30-1900 ppm in quartz) for aplite and 2000 to 11000 ppm H / S i (900-5100 ppm in quartz) for granodiorite. Kronenberg et al. (1990) also reported in these sheared rocks two distinct populations of fluid inclusions, one forming pla- nar arrays along healed microcracks (0.4-3 p,m in diameter) and the other fluid inclusions being fine (20-140 nm in diameter) and spherical ones lying along dislocations. These authors suggested that fluid infiltrated along open microcracks and that then pipe diffusion of water occurred along mobile dislocations toward grain interiors to form water-related defects. However, since their spatial resolution of FT- IR measurements was lim- ited to 100 #m, further systematic and higher- resolution analyses of a variety of deformed rocks, including fine-grained rocks such as ultramy- lonites, are needed to determine water distributions in natural rock systems.

The presence of intragranular water in minerals is also important in material transport that affects wa te r - rock interactions. Diffusion processes in water-bearing rocks are much faster

than corresponding solid-state diffusions not only through grain boundary pore spaces but also through intragranular pore spaces which are in- terconnected (Nakashima, 1995). Fresh granite (lnada, Japan) with only 0.8% porosity can have effective diffusion coefficients (D r) of aqueous ions through pore water as large as 2 × 10 12 m: s ~ at 25°C, 1 atm, only 3 orders of magnitude lower than ionic diffusivities in pure water. Rock-forming minerals themselves have intragranular pore spaces as large as about 2 vol.% for feldspars and about 0.3 vol.% for quartz (Nakashima, 1995). The pore sizes in Inada granite vary from about 8 nm to about 10 #m. Large and less tortuous "micro-pore" networks in quartz provide fast diffusion pathways, while more abundant and tortuous "nano-pore" networks in feldspars give slower pathways (Nakashima, 1995). Consequently, diffusivity through intragranular pore water present in these micro- and nano-pores together with that via grain boundaries is considered to be effective in enhancing material transport and thus to assist in rock deformation. Since water-bearing pores can be well under the resolution of optical or IR microscopes, analysis of intragranular water in naturally deformed rocks by IR microspectroscopy can provide information on not only dislocation-related water but also small microscopic fluid inclusions.

The fluid flow during metamorphism and deformation has received much attention in recent years in the quantitative t reatment of mass transport CRumble I I | , 1989; Brenan, 1991). However, little is known about the behaviour of aqueous fluid (amount, fluid flow rate, excess water pressure etc.). Since water in the fractures and grain boundaries is no longer present, we are obliged to use bulk water content as hydrous minerals (Fyfe et al., 1978), mass balance equation and wate r - rock ratios based on bulk chemistry and stable isotope analyses (Ferry, 1986, 1991). The intragranular water content in quartz in metamorphic rocks as analysed by IR microspectroscopy may provide a new means of studying aqueous fluids present during metamorphism and which are now trapped in minerals.

Recent developments of IR microspectroseopy (Nakashima et al., 1989, 1992; Akiyama et al.,

S. Nakashima et al. / Tectonophysics 245 (1995) 263-276 265

1992) have enabled analyses of the forms and amounts of water in natural rock thin sections and the spatial distribution of water with spatial resolutions down to about 5 × 5 /xm. This paper aims to report exploratory results on distributions of water in various deformed and metamorphosed rocks from this IR microspectroscopy, especially for two representative series of rocks: granitic mylonites around the Median Tectonic Line (MTL), central Japan with increasing degree of deformation toward the MTL (Hara et al., 1980; Takagi, 1986; Michibayashi and Masuda, 1993; Michibayashi, 1993; Yamamoto, 1994; Ma- suda et al., 1995) and the well studied Asemigawa sequence of Sambagawa metamorphic rocks, Shikoku, Japan with increasing metamorphism (Higashino et al., 1981; Masuda, 1982; Banno and Sakai, 1989; Wallis and Banno, 1990).

JASCO Micro FT- IR , / anssen

Eyepieces Polarizer

b

2. Experimental

2.1. Fourier transform infrared microspectroscopy (Micro FT-IR)

We have already succeeded in developing Fourier transform infrared microspectroscopy (abbreviated as Micro FT-IR) for the analysis of hydrous species in rock thin sections in areas as small as 10 x 10 ~m (Nakashima et al., 1989). By using this Micro FT-IR, we have succeeded for the first time to characterize hydrated surface layers of red feldspar during hydrothermal alteration along fractures. The apparatus consists of a FT-IR spectrometer JEOL JIR-3505 and an IR microscope unit JEOL I R - M A U l l 0 (abbreviated as JEOL Micro FT-IR hereafter) and was used during a part of our analyses. The system configuration is: a globar light source, a CsI beam splitter, MCT (HgCdTe) detector and a Cassegrainian mirror objective with a magnification of 10 x (final magnification of 400 x ) . The rectangular aperture is used to mask the image to select desired areas for analysis of about 100 x 100 /~m or 50 X 50 ~m. IR absorption spectra in the 4000-700 cm -1 wavenumber range were measured by adding 100 to 500 scans for improve- ment of the signal to noise ( S / N ) ratio. This

Variable aperture

ATOS

Cassegrainian objective

Sample stage

Cassegrainian condenser

;-I Changeover mirror

Interfer- omeler

Light path of Light path of C

re fe rence spectrum sample spectrum

L. L_ / / Measured area toin ess

Fig. 1. Schematic figures of the Fourier transform infrared microspectrometer Janssen which has been commercialized by JASCO, Ltd. incorporating the authors ' suggestions (a), the IR microscope configuration (b) and the transmission measurement method on a thin section of rock (c).

266 S. Nakashima et al. / Tectonophysks 245 (199.5) 263-276

equipment uses a housing purged with dried air to avoid moisture fluctuations during the measurement, which is essential for accurate analysis of small amounts of water in minerals. This system requires purging for 5 to 10 rain to replace the humid air initially within the sample compartment with dried air and the moisture fluctuation cannot always be eliminated successfully.

A second instrument used in this study was a Bomem DA3.02 FT- IR spectrometer (similar to that used by Kronenberg and Wolf, 1990) equipped with a small IR microscope (Spec- t raTech 's SpectraScope). A globar lamp, KBr beam splitter and MCT detector were used to obtain IR absorption spectra in the 5000-800 cm ~ wavenumber region. The rectangular aperture is used to mask the image to select desired areas for analysis of about 67 × 67 ~m. 256 scans were generally added to obtain spectra with a good S / N ratio. Since the normal configuration of the IR microscope unit attached to the FT-1R spectrometer prevents measurement under vacuum, we constructed a special acrylic box to cover the IR microscope down to the spectrometer ' s sample compar tment in order to measure moisture-free spectra. Calibration data were taken using this vacuum system with the Bomem Micro FT-IR, and some of the natural samples were analyzed using both the Bomem and J E O L Micro FT- IR instruments to test the reproducibility of the results.

In addition to the above instruments, we have recently developed a new Micro FT- IR (JASCO Janssen) exclusively for high-sensitivity microscopic measurements (Fig. la). The IR light emit- ted by the light source passes through an interfer- ometer with a Ge-coated KBr beam splitter to form an interferogram. This light is focused on a sample by means of concave mirror systems (Cas- segrainian mirrors) and then passes through a rectangular aperture to reach an MCT (HgCdTe) detector (Fig. lb). The resulting interferogram, which contains a particular signature of IR absorption by the sample, is converted via an in- verse Fourier transform to an intensity vs. energy (usually expressed in units of wavenumber cm-~) diagram in the 5000 to 700 cm-1 region. The IR microscope unit (Fig. lb) can use two objective

Cassegrainian mirrors with magnifications of 16 or 38 for both infrared and visible light, which can be selected by sliding a mirror holder. A binocular lens with a magnification of 10 gives visual images of the sample at magnifications of 160 or 380, respectively. A visible-light source can be used to observe the magnified images of the sample by both transmitted and reflected light. A visible-light polarizer and an analyzer can be used to observe the sample under polarizing light. The rectangular aperture is used to mask the image with an area of about 200 × 200 ~ m to about 3 × 3 ~m. This new apparatus has several advan- tages over the other instruments. Micro-sampling becomes easy by means of the Aperture Through Optical System (ATOS) permitting the visual ob- servation of not only the apertured area but also the surrounding area (Fig. lb). This ATOS system facilitates the selection of the desired area fl)r analysis. The optics of this instrument is very compact and mostly kept in a constant humidity inside a sealed compartment . This enables high- sensitivity signals to be obtained with reduced contributions from water vapour. The spatial resolution of this instrument is 3 × 3/xm. This apparatus has been proven useful for the non-destruc- tive characterization of natural organic mi- crophases such as pollen and kerogen in sedimen- tary rocks (Akiyama et al., 1992) and in chondrites and cosmic dust particles (Nakashima et al., 1992). Further measurements are being conducted on trace amounts of water in mantle minerals (olivines), natural and synthetic silicate glasses and also on amorphous iron hydroxides associated with granite weathering. This new instrument was also used for the analysis of water in Inuyama cherts and in quartz of granitic mylonites.

2.2. Sample preparation and measurement

Thin sections of rock samples were prepared with thicknesses of about 100 to 500/xm according to water contents. The thickness was generally measured with a micrometre. Cutting and polishing procedures may include the use of water. However, a comparison of the procedures with those conducted only with oils revealed no


introduction of external water to the thin sections. The thin sections were removed from glass plates and adhesives were dissolved with organic solvents such as acetone. Both surfaces of the thin sections were generally polished to several tens of microns of roughness, since much finer polishing up to micron levels sometimes resulted in interference fringes due to the parallelism of both surfaces compared with the wavelength of IR light (about 3 g m for O-H absorptions). Rough surfaces of samples give rise to scattering of IR light causing the increase of spectral baselines. This scattering effect can be eliminated by sub- tracting appropriate baselines from absorption bands.

Among three different measurement methods ( t r a n s m i s s i o n , t r a n s m i s s i o n - r e f l e c t i o n , reflection), the transmission measurement has been found to be the most appropriate for the quanti tat ive analysis of water in minerals (Nakashima et al., 1989). This is because the reflection signals can include larger effects of adsorbed water vapour on the sample surface and a quantitative analysis requires very careful treatment using the Kramers -Kron ig transform (Mc- Millan and Hofmeister , 1988). For the measurement of transmission spectra of a sample, the sample was simply placed over a hole in a metal plate but leaving a part of the hole open (Fig. lc). The desired area was selected using the rectangular aperture of about 100 × 100 to 10 x 10 /xm under visible light and then the optical path was changed to IR light. 100 to 500 scans of the interferogram were measured for the sample spectrum. The same procedure was then applied to the sample-free area keeping the same aperture to obtain the reference spectrum. The ratio of these two spectra gives an IR transmission spectrum of the desired position of the sample.

Quartz and feldspar grains in deformed and metamorphosed rock thin sections were measured by Micro FT-IR. The water content of feldspars can be difficult to determine from the spectra because of the presence of fine-grained alteration products such as sericite. Quartz is the most resistant to secondary processes such as alteration and weathering and it can record the primary information of rock deformation and

Absorbace (Arb. Unit)

Bomem DA3 Micro FT-IR 67 x 67 pm area

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I ~ ' X ~ Ag ate t ~ L / 2 . . . . .mt j-v -

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Fig. 2. Typical IR transmission spectra of Brazilian agate (a) and synthetic fused quartz (b).

metamorphism. As a result, we report here only results on water in quartz. Thin sections were generally selected from quartzose parts of rock samples, and intragranular positions in quartz grains in thin sections were principally selected for analysis. Positions including grain boundaries, clearly visible fluid inclusions and micaceous layers were generally avoided in the Micro FT- IR analysis.

2.3. Calibration and accuracy

In order to quantify the water content of quartz from IR absorption bands, calibrations of IR absorption peak heights of O-H stretching vibrations were conducted by using Brazilian agate (fine-grained hydrated silica) and synthetic fused quartz glass. Thin sections of agate with thicknesses of 43, 82, 98 and 108 /~m and sections of fused quartz with thicknesses of 54, 106, 441, 790 and 954 /xm were prepared. Typical Bomem Mi- cro FT- IR spectra of these thin sections for 67 × 67/xm areas under vacuum are presented in Fig. 2. Agate has a broad IR absorption band at about 3400 c m - l and a smaller one at 3600 c m - 1, while fused quartz has a sharp peak at about 3600 cm -~. The 3400 cm 1 band is considered to be due mainly to asymmetric (3445 c m - l ) and sym- metric (3220 cm ~) O-H stretching vibrations of "liquid-like" molecular water (H20) (Aines and Rossman, 1984; Rossman, 1988; Kronenberg and Wolf, 1990), while the 3600 cm -1 band can be

268 S. Nakashirna et al. / Tectonophysics 245 (1995) 263-276

attributed to X-"O-H" species (Si-O-H in the present case) (Graetsch et al., 1985).

IR absorption intensities were determined, after the baseline (appropriate lines as shown in Fig. 2) subtraction, by the peak heights at 3600 and 3400 cm ~. IR peak heights in absorbance units for agate at 3400 cm ] and 3600 cm-J and for fused quartz at 3600 cm-~ were determined and plotted against the thickness of the thin sections (Fig. 3). The data show a fairly good linearity, indicating the possibility of a quantitative analysis of water in silica. For agate, the analysis errors for both bands are as high as about 50% for thin sections of 43 /xm, but decrease to less than 10% for 108-/xm-thick samples. Fused quartz data have smaller errors than agate because of the sharp peak. Our materials were structurally close to agate (cryptocrystalline hydrated silica). In fact, the IR spectra for most of the samples studied had IR bands mainly at 3400 cm ~ sometimes with a smaller one at 3600 cm-1. Consequently, we will use the agate data for our quantitative treatment, and generally a sample thickness of about 200 # m was used for IR measurements.

The slopes of the linearity in Fig. 3 correspond to the molar absorption coefficient • (L mol- mm -~) of corresponding O-H absorptions in Lamber t -Beer ' s law:

A = • × d × c (1)

where A is absorbance, d is the thickness of the material (mm) and c the concentration of water (mol 1 ~). For "liquid-like" molecular water with a broad band at 3400 c m - ] a molar absorption coefficient (•) of liquid water of 8.1 L mol - I mm-~ (Thompson, 1965) is generally used for a quantitative calculation of the water content of minerals (Aines and Rossman, 1984; Rossman, 1988; Kronenberg and Wolf, 1990). If we use this value for the 3400 cm i band of agate, we find the water content c of agate:

0.5 c - - 1 . 5 m o l H 2 0 per litre

8.1 × 0.04

Using the density of quartz (2.65 g cm-3), this corresponds to about 1.0 wt.% (10,000 ppm) of water, which is consistent with the water content

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= 840 × absorbance at 3400 cm l /d (mm) (2)

Strictly speaking, the calibration standards used here (agate and fused quartz) are not "good" analogues for our deformed and metamorphosed quartz grains, because of the differences in the crystal structures, speciation of water and the levels of water contents. This applies not to the absolute quantity standards of water but to the evaluation of magnitudes of errors associated with the procedures of sample preparation and measurement. Much more realistic calibrations are now being carried out using several quartz grains together with attempts to determine absolute water content by TG-DTA and other methods.

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3. Deformed and metamorphosed rock samples

3.1. Deformed Ryoke granitic rocks near the MTL, Japan

Ryoke granitic rocks near the Median Tec- tonic Line (MTL) in central Japan were sampled successively from the MTL towards the north (Ryoke belts) (Fig. 4). These granitic rocks are classified as undeformed granites, deformed granites to granitic mylonites along a traverse from nor th to sou th a p p r o a c h i n g the M T L (Michibayashi and Masuda, 1993; Michibayashi, 1993; Masuda et al., 1993) (Fig. 5). A microstructural study in this area showed that the degree of plastic deformation increases toward the margin of the granitic body (and the MTL) based on the grain-size distributions of dynamically recrystallized quartz aggregates combined with deformation textures in feldspar (Michibayashi and Ma- suda, 1993; Michibayashi, 1993).

Quartz grains in deformed granites were mainly measured for water contents and their relation with increasing deformation towards the MTL. Porphyroclastic mylonite was also measured by

Fig. 4. Locations of quartz schist samples from high-pressure metamorphic belts in Japan: 1 = Kamuikotan, 2 = Yamagami, 4 = Sambagawa, 5 = Kurosegawa, 6 = Y a e y a m a , and 3 = Ryoke deformed granitic rocks.

Micro FT-IR. Thin sections of about 200 /xm thick were prepared.

3.2. Sambagawa metamorphic rocks (Asemigawa route), Japan

The Sambagawa metamorphic belts in Japan are known to be the widest high-pressure regional belts formed during the Jurassic to Late Cretaceous (Fig. 4). Because of its northward-increasing metamorphic grade from chlorite, garnet to biotite zones, the Asemigawa route is the best studied by many investigators for its pelitic, psam- mitic and quartz schists (Higashino et al., 1981; Masuda, 1982; Banno and Sakai, 1989; Wallis and Banno, 1990) (Fig. 6). Quartz schist samples were taken successively from different metamorphic zones. Recrystallized quartz grains become larger in the higher-metamorphic-grade terrains. Thin sections of about 200 p~m thick were prepared for IR measurement.

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Fig. 6. Locations of metamorphic rock samples from the Asemigawa route, Sambagawa belt, Shikoku, Japan. The metamorphic grades are after Higashino et al. (1981).

taken from the Kurosegawa belt, south of Yachiyo-shi, Kumamoto. This rock is considered to have been metamorphosed ~ 400 Ma ago to glaucophane facies. A quartz schist from the Yaeyama belt, Ishigaki-jima, of a glaucophane facies (200 Ma) was also measured for water content.

3.4. Precambrian metamorphic rocks

Archaean metachert samples from the Napier Complex, Enderby Land, Antarctica were selected for water content analysis as an example of a dry end-member rock. This rock is considered to have been metamorphosed ~ 2.5 Gy ago at about 1000°C and 10 kbar (granulite facies) and contains almost no hydrous minerals (Motoyoshi and Hensen, 1989). Quartz grains are very large, up to 8 mm. Thin sections of about 450/zm thick were used for Micro FT-IR analysis.

Archaean Isua rocks, Greenland, were analyzed to examine very old water within inclusions. This metamorphic conglomerate is known to be very old, around 3.8 Gy (Compston et al., 1986; Nutman and Collerson, 1991), and quartz grains were analyzed by Micro FT-IR.

4. Results and discussion

3.3. High-pressure metamorphic rocks in Japan

Several high-pressure metamorphic rocks were taken from the Kamuiko tan , Yamagami , Kurosegawa and Yaeyama areas in Japan (Fig. 4) in order to compare their water contents with stress values estimated from analyses of microboudinage (Masuda et al., 1990). Quartz schist was taken from a serpentinite zone at the Numapoporo River to the north of the Ka- muikotan belts. This rock is considered to have been metamorphosed ~ 130 Ma ago at about 8 kbar (glaucophane facies) and includes lawsonite and jadeite. Quartz schist from the Yamagami belt was taken from a location 60 km south of Sendai, northeast Japan. This rock is considered to have been metamorphosed ~ 350 Ma ago to epidote-amphiboli te facies. Quartz schist was also

4.1. Water in deformed granitic rocks near the Median Tectonic Line, Japan

Typical Micro FT-IR spectra for the OH region are presented in Fig. 7. Quartz grains of deformed granitic rocks exhibit an OH absorption band at 3400 cm ~, probably due to fluid inclusion molecular water (Kronenberg and Wolf, 1990), but some also show smaller bands at 3600 cm -~. The water contents calculated using Eq. (2) based on the 3400-cm i band are plotted against the horizontal distance from the Median Tectonic Line (MTL) for the Yanazawa- Kamimura area (Fig. 8; see Fig. 5 for sample locations). The data clearly indicate an increase in water content of quartz from about 300 ppm to 2500 ppm within the deformed granitic rocks towards the MTL. The water content appears to

272 S. Nakashima et al. / Tectonophysics 245 (1995) 263-276

Absorbance (Arb. Unit)

YG 5

- - i / / --YG2 ~ J ~ ~ Y G 2 - 2

~ l - " - - - ' ~ I ~ " ~ ~ ' - YG 11 ~ ~ Y G 3

~ ~ Y G 8

4000 3400 30100 Wavenumber (cm "1 )

Fig. 7. Typical IR transmission spectra by JEOL Micro FT-IR of water of a 50×50-/*m area in quartz in granitic mylonite from the Yanazawa route (6 sampling locations in Fig. 5b). A broad band around 3400 cm = is considered to be due to fluid inclusion water, and absorbance values at 3400 cm I are

used to calculate the water content of quartz.

Water in quartz of deformed granites from Yanazawa and Kamimura near MTL

3000.

.= 20o0 •

E D. D.

0 looo - r

0 Kamimura (Bomern} A Yanazawa (Jeo

A

A o 0 A~ 0 • = , J • i • J • i , i •

200 400 600 800 1000 1200 1400

D i s t a n c e f r o m M T L ( m )

Fig. 8. Water content of quartz of deformed granitic rocks of the Kamimura route and the Yanazawa route as a function of the distance from the MTL (see Fig. 5). The data were obtained using a JEOL Micro FT-IR for the Yanazawa route and a Bomem Micro FT-IR for the Kamimura route. One sample from the Yanazawa route showing an anomalously high water content of ca. 2000 ppm includes lots of "Augen"

K-feldspar porphyroclasts.

rise remarkably at ca. 700 m from the MTL (Fig. 8).

A microstructural study in this area showed that the degree of plastic deformation increases toward the margin of the granitic body (and the MTL) based on the grain-size distributions of dynamically recrystallized quartz aggregates combined with deformation textures in feldspar (Michibayashi and Masuda, 1993; Michibayashi, 1993). Michibayashi (1993) reported a drastic reduction, at ca. 700 m from the MTL, of mean grain size of recrystallized quartz from 400-100 # m to less than 50 / ,m (average "stable" grain size of about 37 #m) for several traverses toward the MTL. This grain-size reduction appears to be correlated with the drastic jump of the water content of quartz at the same position (Fig. 8). A mechanistic explanation for these results needs further detailed studies on water contents and rock textures together with analyses of bu[k rock chemistry and stable isotopes. However, the systematic increase of water in quartz with increasing rock deformation is consistent with the evidence for hydrolytic weakening of quartz reported by Kronenberg et al. (1990) in a much smaller granitic ductile shear zone.

In addition, we have currently analyzed quartz "ribbons" from some granitic mylonites for the mapping of water contents with the newly developed 1R microspectrometer JASCO Janssen. The preliminary result shows that a higher water content is clearly detected at highly sheared thinner parts of these ribbons (about 900 ppm), in con- trast to the central parts with less strain (about 200 ppm) (H. Yamagishi, pers. commun., 1994). Since the data in Fig. 8 are interpreted to be some kind of "average" water content of quartz grains as a first approximation, the study on microscopic heterogeneity of water contents will give us further details on the rock deformation and its relation with microscopic textures.

4.2. Water in the Sambagawa metamorphic rocks (Asemigawa route)

Typical Micro FT-IR spectra for the OH region of quartz grains in Sambagawa quartz schists are presented in Fig. 9. Quartz of these rocks

s. Nakashima et al. / Tectonophysics 245 (1995) 263-276 273

have an O H absorpt ion band at 3400 cm l, probably due to fluid inclusion molecular water (Kronenbe rg and Wolf, 1990), but some also show smaller bands at 3600 c m - ] . The water contents calculated with Eq. (2) based on only the 3400 cm t band , are plot ted against the increasing me tamorph ic grade from chlorite, garnet , a l b i t e - bioti te to o l igoc lase-b io t i te zones (see Figs. 6, 9 and 10a). Data for the same samples collected by two different methods, Micro F T - I R J E O L and Bomem, gave satisfactorily consis tent results. This indicates that the reproducibi l i ty is good and that water conten ts in me tamorph ic quar tz grains are homogeneous except for the data inc luding micaceous layers. The data clearly indicate a decrease in water con ten t of quartz from about 1000 ppm to 200 ppm with increasing metamorph ism. It is well known that the increasing me tamorph i sm at the deepe r parts of the ear th ' s crust is character- ized by the dehydra t ion of hydrous minera l s (Fyfe et aI., 1978). However, these results are the first direct evidence of a systematic decrease of water

A b s o r b a n c e ( A r b . U n i t )

" L i g u i d ' l ! k e

i J l J l , , , i I

4 0 0 0 3 5 0 0 3 0 0 0

W a v e n u m b e r ( c m "1 )

Fig. 9. Typical IR transmission spectra of water for several points of a 50x50-~tm area of quartz from Sambagawa metamorphic rocks (locality 13 in Fig. 6). A broad band around 3400 cm 1 is considered to be due to fluid inclusion water, and absorbance values of 3400 cm- 1 are used to calculate the water content of the quartz. A smaller band at 3600 cm 1 may be due to OH species• Spectrum 7 includes a micaceous layer giving a very high absorption at 3600 cm 1.

1400

1200

1000

. - 800

g ~ 6(?0

O 400

200

)~ Micro FT-IR JEOL JIR3505

Bomem DA3 O Average

a

0 x

x

x

x O x x x

x x

x t 0

Water contents of quartz i n x Sambagawa metamorphic

rocks at Asemigawa route

Locali ty ° ~ 5 o l i g o c l a s e albite garnet chlorite z o n e _o,o1,o i .o,o1,o I I

1 8 0 0 '

1 6 0 0

1 4 0 0 '

1 2 0 0 ' .E

1 0 0 0 E 800

O 6 0 0 '

b

O O

O O

4 0 0 0

2 0 0 " 0

0 0 , • , ' , ' J ' , ' , • , ' ,

I s u a N a p i e r M o t o Y a m K a m V a e A s e r n i K u r o

L o c a l i t y

Fig. 10. Water content of quartz from Sambagawa metamorphic rocks (see Fig. 6) with increasing metamorphic grade from chlorite, garnet, albite-biotite to oligoclase biotite zones (a) and from Japanese high-pressure metamorphic rocks and Precambrian metamorphic rocks (b). Kuro = Kurosegawa; Asemi = average value of the chlorite zone at Asemigawa, Sambagawa belt; Yae = Yaeyama; Kam = Kamuikotan; Yam = Yamagami; Moto = quartz schist from the Motomiyayama area, Ryoke belt; Napier = Precambrian Napier dry rock (granulite facies), Antarctica; lsua = quartz in conglomerate from the Precambrian ]sua supracrust, Greenland.

in quartz with increasing metamorphism. Since the rmodynamics of water solubility in quartz is not yet well unders tood (Paterson, 1986, 1990), fur ther invest igat ions on the mechanisms of water decrease with increasing me tamorph i sm are nec- essary.

In order to compare the water conten ts of quartz of these Sambagawa metamorph ic rocks and of u n m e t a m o r p h o s e d rocks, a chert sample has been selected from the Inuyama area, Gifu,

274 S. Nakashima et al./ Tectonophysics 245 (1995) 263-276

central Japan. The chert is Triassic in age with abundant radiolarian microfossils (Isozaki ct al., 1990; Matsuda and Isozaki, 1991). Several points of a 100 x 100-p.m area on the chert's thin sections of about 120 /.tin thick were analyzed by a JASCO Janssen. The IR spectra consist almost always of two major OH bands: one at 3400 cm and a smaller one at 3600 cm -~. The water content calculated with Eq. (2) based on the 3400-cm ~ band varied from 3500 ppm to 7000 ppm. This range of water content in chert can be considered as a starting point of the decrease of water content in metacherts with decreasing metamorphic grade. Between these cherty rocks (7000-3500 ppm) and the Asemigawa metacherts (1000-200 ppm), an intermediate water content is expected for the low-grade Sambagawa metamorphic rocks. In fact, our preliminary analyses on low-grade metacherts from the Chichibu area, central Japan indicate a water content of quartz of about 4000-1000 ppm.

4.3. Various m e t a m o r p h i c rocks

Water contents in various metamorphic rocks from high-pressure Japanese terrains, and old metamorphic rocks from Greenland (Isua) and Antarctica (Napier) are given in Fig. 10b. A high-grade (granulite facies) metachert of the Napier Complex has only a 3400 e m i band and has a very low water content in the order of 40 ppm, which is the lowest value among the samples studied. This is consistent again with the above decrease of water in quartz with increasing metamorphism which continues up to the granulite facies. It is interesting that lsua Archaean quartz grains in conglomerate exhibit a different weak IR band, at around 3200 cm i. We do not yet have an explanation for the chemical state of the water here. If we apply the same quantitative calculation as above for this 3200 cm i band of Isua, the water content is about 200 ppm.

The water content of high-pressure metamorphic rocks in Japan are mostly on the order of 500 ppm (300-700 ppm), except for those from Kurosegawa which have a much higher content (1700 ppm)(Fig. 10b). These rocks were classified according to stresses derived from analyses of

boudinage (Masuda et al., 1990). However, no systematic relation was observed between this stress indicator and water content. Further studies on these rocks are needed to correlate water content with various other parameters such as dislocation density and recrystallized grain size.

5. Conclusions

Infrared microspectroscopy has been applied to various deformed and metamorphosed rocks in order to investigate water contents in these rocks. Micro FT-IR spectra for the OH region of these rocks consist mainly of a broad band around 3400 cm -t, probably duc to fluid inclusion molecular water (H20), sometimes with smaller bands at 3600 cm ~ due possibly to SiOH. The water contents in quartz were calculated by using a molar absorption coefficient value (e) of liquid water (8.1 1 mol I mm l) and an IR band ab- sorbancc at 3400 cm ~ with the following conve- nient equation:

H , O content in quartz (ppm)

840 x absorbance at 3400 cm I /d (mm)

Deformed granitic rocks from the Yanazawa area near the Median Tectonic Line (MTL) show an increase in water content in quartz grains from about 300 ppm to 2500 ppm toward the MTL with increasing degree of plastic deformation, being consistent with the decrease of the mean grain size of the quartz.

A series of metacherts from the Sambagawa metamorphic rocks (Asemigawa route) show a systematic decrease in water content of quartz grains from about 1000 ppm to 200 ppm with increasing metamorphic grade from chlorite, garnet, albite-biotite to oligoclase-biotite zones. A high-grade metachert of the Napier Complex (granulite facies) has only a small 3400 c m band and shows a very low water content on the order of 40 ppm; this is the lowest value among the samples studied and is consistent with the observed decrease of water in quartz with increasing metamorphic grade. A comparison with Inuyama unmetamorphosed chert samples having water contcnts of 350(I to 7000 ppm (which can


be regarded as a s tar t ing point of me tamorph i sm) fur ther i l lustrates the systematic decrease of water in quar tz with increas ing metamorph ism.

Wate r conten ts in high-pressure me tamorph ic rocks in Japan are mostly on the order of 500 ppm (300-700 ppm), except for those from Kurosegawa having much higher conten ts (as high as 1700 ppm).

The first analysis of old water as inclusions in quar tz of the Isua A r c h a e a n conglomera te ex- hibits a weak IR band, a round 3200 cm ~, that differs from the band detec ted for water in the o ther samples analyzed.

This exploratory survey of in t r ag ranu la r water conten ts in quar tz of various deformed and metamorphosed rocks provides direct evidence of systematic increases of water in quar tz with increasing deformat ion , and systematic decreases of water with increasing metamorph i sm. The presen t data represen t "average" water con ten t s of quartz grains, and the microscopic he terogenei ty of water conten ts may provide fur ther details regard ing de format ion and microscopic textures.

The d is t r ibut ion of water in crustal rocks de- t e rmined by Micro FT- IR , combined with ther- modynamic and kinetic in format ion on water sol- ubili t ies in quartz, can thus be used to investigate in detail processes involved in the geochemical cycle of water in the Ear th ' s crust.

Acknowledgements

Some of the results were ob ta ined at the Envi- r o n m e n t a l G e o c h e m i s t r y Labora to ry , J a p a n Atomic Energy Research Ins t i tu te ( J A E R I ) by using J E O L F T - I R plus IR microscope instruments . J A E R I ' s f inancial and logistical suppor t is acknowledged. Seki T e c h n o t r o n Corp. is t hanked for the collaborat ive deve lopment of a vacuum chamber for the Spec t raTech ' s SpectraScope attached to Bomem D.A.3 F T - I R at Aki ta Univer- sity. J A S C O Corpora t ion ltd. is also thanked for the collaborat ive deve lopment of the IR mi- c rospec t rometer and for the use of the J A S C O Janssen for our analyses at the Universi ty of Tokyo. The authors are grateful to C.J. Spiers, T. Takeshi ta , T. Shimamoto, I. Shimizu, R. Tada, K.

Kanagawa, M. Tor iumi and H. Taba ta for their interest and valuable discussions. A.K. Kronen- berg is specially thanked for his p ioneer ing F T - I R works on na tura l deformed rocks which gave us the motivat ion for this study. The manuscr ip t benef i t t ed from constructive reviews by A ndr e a s K r o n e n b e r g and Toru Takeshi ta .

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