Mineralogy and hydrogen isotope geochemistry of clay minerals in the Ohnuma geothermal area, Northeastern Japan

Earth and Planetary Science Letters, 47 (1980) 255-262

255

© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [21

MINERALOGY AND HYDROGEN ISOTOPE GEOCHEMISTRY OF CLAY MINERALS IN THE OHNUMA GEOTHERMAL AREA, NORTHEASTERN JAPAN KATSUMI MARUMO

1 KEINOSUKE

NAGASAWA 2 and YOSHIMASU KURODA 3

1 Department o f Earth Sciences, Nagoya University, Nagoya 464 (Japan) 2 Institute of" Geosciences, Shizuoka University, Shizuoka 422 {Japan) 3 Department of Geology, Shinshu University, Matsumoto 390 {Japan)

Received July 13, 1979 Revised version received December 5, 1979

Mineralogical and hydrogen isotopic studies have been made on clay minerals occurring in the Ohnuma geothermal area, northeastern Japan. Here, clay minerals such as smectite, kaolinite, dickite, sericite, and chlorite were formed by hydrothermal alteration of Miocene rocks. A chemical equilibrium can be assumed to be attained from the fact that the amount of expandable layer in the interstratified chlorite/smectite decreases and the polytype of sericite changes from 1M to 2M1 with increasing depth and temperature. The hydrogen isotopic composition (D/H) of the clay minerals is lighter than that of the geothermal and local meteoric waters by about 20-40%0. The hydrogen isotopic fractionation factors C~mineral.water are as follows: 0.972-0.985 for kaolinite and dickite, 0.973-0.977 for sericite, and 0.954-0.987 for chlorite. In the temperature range from 100 to 250°C, the hydrogen isotopic fractionation factors between these minerals and water are not sensitive to the temperature. C%hlorite.water depends on the kind of octahedrally coordinated cations which lie close to the hydroxyl groups; it becomes large with an increase of Mg content of chlorite.

1. Introduction Clay minerals such as kaolinite, sericite, and chlorite commonly occur in hydrothermal mineral deposits and geothermal areas. They contain hydrogen as hydroxyl groups. It is important to study the hydrogen isotopic composition (D/H) of these hydrous minerals to obtain information regarding the role o f water in the formation and subsequent history of the minerals and the rocks containing them. Savin and Epstein [1] and Lawrence and Taylor [2,3] obtained a value o f 0.97 for the hydrogen isotopic fractionation factor between kaolinite and water at room temperature from D/H data of kaolinites o f weathering origin. Wenner and Taylor [4] estimated the temperature dependence of the hydrogen isotopic fractionation factor between serpentine and water based on D/H ratios of oceanic antigorite, lizardite, and deweylite. Taylor [5] estimated the hydrogen isotopic fractionation factors between hy-

drous minerals such as kaolinite and chlorite and water in the temperature range from 25 to 300°C by using D/H data of natural and synthetic minerals cited from the literature. Suzuoki and Epstein [6] experimentally studied the hydrogen isotopic fractionation factors in hydrous mineral-water systems, and showed that they are not only a function o f the temperature but also that of the Mg, A1, and Fe contents of the octahedral sites in the hydrous minerals. Sakai and Tsutsumi [7] experimentally studied the hydrogen isotopic fractionation factor between serpentine and water at temperatures from 100 to 500°C and a water pressure o f 2 kbar. The hydrogen isotopic fractionation factors in the temperature range from 100 to 300°C are not y e t known well, because the exchange rate of hydrogen isotopes between clay minerals and water is too low. Geothermal areas may provide natural laboratories for the study o f such low-temperature reactions. The purpose o f this study is to determine hydrogen iso-

256 topic fractionation factors between clay minerals and water in the temperature range from 100 to 250°C based on isotopic data from natural systems in geothermal areas.

2. Geological setting The Ohnuma geothermal area is located in the northwestern part of Hachimantai volcano, northeastern Japan (Fig. 1). Here, wells drilled for exploitation of geothermal steam have reached depths of over 1500 m, and the bottom temperature of these wells are above 220°C. The geology of this area has been described by Yora et al. (personal communications, 1973, 1977). The surface of this area is almost entirely covered with Hachimantai volcanic products of Quaternary age. The underlying Miocene formations are composed mainly of andesitic and dacitic volcanic and pyroclastic rocks accompanied by argillaceous sediments. Hydrothermal water is contained in fractures

Alteration minerals Sericite Mica / Smectite~ Smectite Chlorite Chlorite / Smectite* Kaolinite, Dickite PyrophyUite Clinoptilotite Mordenite Laumontite Wairakite Epidote Carbonate

I

I

1

I

500 I

6'0 1~o 1~o

I

Depth (meters) 1000 I

I

I

I

I

I

I

I

1500 I

I

2~o Temperature ( °C )

Fig. 2. Distribution of alteration minerals in the Ohnuma geothermal area. * Interstratified mineral. of Miocene volcanic and pyroclastic rocks. It is acid (pH 2 - 3 ) on shallower levels and gradually changes from neutral to weakly alkaline (pH 7 - 8 ) with increasing depth. The distribution of clay minerals and other alteration products is shown in Fig. 2. Sericite and chlorite are the most common clay minerals in this area. The following trend of change in clay minerals is observed with increasing depth: smectite -+ interstratified mica/smectite and chlorite/smectite -+ sericite and chlorite. Kaolinite and pyrophyllite occur locally; the former being observed only in boreholes O-5T and O-6T, and the latter O-6R.

3. Experimental

procedures

3.1. Mineral separation and identification

,,O

P

g

o~ Fig. 1. Location of the Ohnuma geothermal area. The Matsukawa and Ohtake geothermal areas are also shown.

Less-than-241m fractions were separated from about 200 core samples by the sedimentation method. Although they are usually mixtures of more than one kind of clay minerals, about 40 are monomineralic and were used for the present hydrogen isotopic study. The X-ray diffraction method was used for identification of clay minerals. Diffraction patterns were obtained for air-dried and glycolated oriented aggregates on glass slides as well as unoriented powder sam pies. Chemical composition of chlorite was estimated by measuring bo and dool with an X-ray diffractometer using silicon as an internal standard [8].

257 3.2. Isotopic analysis

~ Hygroscopic moisture and interlayer water were removed by heating the sample in vacuo at 270°C for one hour [1 ]. Then, the extraction of constitutional water from the clay minerals was performed by a pyrolytic dehydration technique using an induction furnace as described by Friedman and Smith [9] and Godfrey [10] and a dehydration vessel as described by Epstein and Taylor [ 11 ]. The evolved water vapor is usually accompanied by hydrogen gas, which was converted to water vapor by heating with Cue at 500-600°C. The water vapor thus extracted was passed over hot uranium metal at 700°C, and converted to molecular hydrogen. After measuring the volume of the hydrogen gas, its isotopic composition was determined by using a dual gas feeding mass spectrometer. The D/H ratio is presented by the following expression:

O-5T279mkaolinite

io

lb

3'0

O-5T62m

i

I

O-

i

p i

~ I

io

O,-ST1,'-3rn

i

~

5T228rn

~'o

6'o

7doo(2e Cuko)

O-5T1685m

t

O-ST240m

-5 ; d Tq~ i

,

D(%o) = (D/H)sample -- (D/H)sM°w X 1000 (D/H)sMOW where SMOW refers to Standard Mean Ocean Water. The hydrogen isotopic fractionation factor between mineral and water is expressed as: Otmineral.wate

r

=

(D/H)mineral/(D/H)water

i~0°(20

Cuko)

Fig. 3. X-ray diffraction patterns of seven kaolin mineral sam ples from borehole O-5T. Pure dickite (Matsukawa 7-6-5) from the Matsukawa geothermal area is also shown.

4. Mineralogy of clay minerals 4.1. Kaolin minerals In the Ohnuma geothermal area, kaolin minerals occur in boreholes O-5T and O-6T. These kaolin minerals are usually identified as mixtures of kaolinite and dickite based on their X-ray diffraction patterns. Fig. 3 shows a change in X-ray pattern from kaolinite-rich to dickite-rich samples in borehole O-5T. Infrared absorption spectra in Fig. 4 show a variation in the OH stretching vibration bands of kaolin minerals in the borehole O-5T, which corresponds to the change in the kind of kaolin minerals, kaolinite and dickite. 4.2. Sericite Here, sericite is defined as fine-grained dioctahedral mica with or without a small amount of

expandable layer. In the Ohnuma geothermal area, sericite occurs generally below the depth of 250 m. Fig. 5 shows X-ray diffraction patterns of sericite. The basal spacing of these sericites is a little larger than that of ideal muscovite, suggesting interstratification with expandable layers. The X-ray diffraction patterns show that sericite on shallower levels is of 1M polytype, and that on deeper levels is a mixture of 1M and 2M1. This is supported by infrared absorption spectra which were examined according to the criteria of Nishiyama et al. (personal communication, 1977). Velde [12] experimentally showed that muscovite crystallizes as 1M or 1Md polytypes at lower temperatures, and as 2M1 at higher temperatures. Therefore, the change in polytype of sericite with depth is explained by the difference in temperature of formation.

258

-1

0-5 T143m

0 5TISllSm

pl i

P

i

~

i

T

i O-5T228m

- T 334m or i

334m or EG

i

i

0-5T 240m

O-2T 392m or i

O-5T279m

l'

|/

O-2T 624.7m o r

O-5T374m

~

~

R

i

IA /

Fukenoyu

/•

~3623,CM-t I 3695CM-

370~C ~

i

840 m or i

Matsukawa 7-6-5 °"'"

~/ O

-

6

~

~ J , q

j

~

~

1458.

or

3628CM "=

365J I~ Fig. 4. Infrared absorption spectra o f the OH stretching vibration region of seven kaolin mineral samples from borehole O-5T. Pure kaolinite (Fukenoyu, Akita Prefecture) and pure dickite (Matsukawa 7-6-5, the Matsukawa geothermal area) are also shown.

10

20

O-6R 1458m

30

40

50

60

70°(2eCu ka I

Fig. 6. X-ray diffraction patterns o f five oriented and one unoriented sample o f chlorite, or = oriented sample; or EG = oriented sample after ethylene glycol treatment; pl = plagioclase; q = quartz.

tO

20

~

30

40

50

60

i

70*(200Jko)

O-4T 3~K],9m

,

~i p, , ,

i

O-4T 695m

i r i i ,

zs'

i f i

3b ....

3~*(zo c,,<) ....

z'~ . . . .

~0 . . . .

3~*(zo c,,~)

Fig. 5. X-ray diffraction patterns o f four sericite. Sericite on shallower level (O-4T 380.9 m) is o f 1M polytype, whereas those on deeper levels (O-4T 695 m and O-6R 800 m) are of b o t h 1M and 2M1.

259 fl

5. Isotopic results and discussion

~f

//

1.5 //

/

/

/

/

/ i

/

/

• •

e°

<

• •

..'."" •

t':

• •

\

c1.0

\

% \

/ / / / / •,

0.5

///

•

X X \\

\

I 0.1

I

/I

I

0.2

I

I

0.3 0.4 0.5 Total Fe/(Fe -Mg)

I

0.6

Fig. 7. Chemical composition of chlorites. Broken line indicates the field of IIb chlorite [ 13].

The 6D values and the hydrogen isotopic fractionation factors between clay minerals and water (O~mineral.water) are shown in Tables 1 and 2. The temperature of formation of these clay minerals is assumed to be equal to the temperature in the boreholes at present which was measured with thermocouple or, when actual measurement has not been done, was estimated based on the extrapolation of the actually measured temperature and on the boiling point of water at that depth (Yora et al., personal commuydcation, 1977). The hydrogen isotopic composition of water in equilibrium with these minerals is assumed to be -70%0, the same as that of the present-day hydrothermal water (Sakai and Matsubaya, personal communication), which, in turn, is similar to that of local meteoric water in this area. In Fig. 8, the temperature dependence of these O~minerai_wate r values is illustrated.

4.3. Chlorite

Temperature

800 500 T

Chlorite, like sericite, occurs generally below the depth of 250 m. Chlorite on shallower levels contains a small amount of expandable layer, judging from the diffraction pattern of oriented aggregate which show stronger 1st order basal reflection than that on deeper ones, and from a shift of basal reflections by ethylene glycol treatment (Fig. 6). The X-ray diffraction patterns of unoriented samples show that chlorites in this area are of lib polytype irrespective of depth and temperature. Chemical composition of the chlorites was estimated by measuring b0 and dl0o (Fig. 7). Most of the chlorites fall in the compositional field of lib polytype of Bailey and Brown [13].

300

T T

1

200 [

F

75

50

25

•

]

I

I

20 lOJ~O~

Ok

'°7 -,ol/ -.oL !

-so,-

/

...."<<"'<'~/

\~,

o

lc

<

• oo+ ,,

', \

+ ",~:

-

.

+

As shown in Fig. 2, a systematic change is observed in the kind of clay minerals and zeolites. Detailed mineralogical studies described above revealed that there are systematic changes in the polytypes of sericite and in the amount of expandable layer in chlorite. This fact suggests that the alteration minerals approach equilibrium with the geothermal water in this area.

........ I

'-..

+ + . c~L°~r~ o

_60i -70--

-I .......... S

"'--. s~e~vr~J,;

?0 (~eltesch~t)

106/T 2

4.4. Genesis of clay minerals

( °C )

100 •

.........

~,ut W ~ n e

r ancl T i y e r )

(°K-2)

O Kaolinite, Ok:kite ( Ohnuma ) • Kaolinite, Oickite ( Matsukawa ) • Dickite ( Ohtake ) • $ericite ( Ohnuma ) + Chlorite ( O h n u m a ) - --- 5akai and Tsutsumi ( 1978 ) - 5uzuokl and Epstein ( t 976 ) ....... Empirical curve Taylor ( 1974 ) IA 5avin and Epstein ( cited in 5heppard et al from zones ~ deep weathering

J I

B Sheppard et a[ ( 1969 ) Pot, Yettowst'one.

4

1969 )

Data on natural kaolinit e~

Coexisting kaolinite and water from Fountain Paint

C 5heppard and Taylor ( cited in 5heppard et al., 1969 ) calibcat~ns of kaolinite-H20

Preliminary laboratory

Fig. 8. Temperature dependence of fractionation factors between clay minerals and water.

TABLE 1 Hydrogen isotopic analyses of kaolin minerals and sericite Sample No.

Mineralogy

Sample location

Estimated temperature (°C)

6D (%~)

a *

1000 In a

50 80 90 100 120 140 150 160

-95.9 -91.2 -92.2 -89.6 -89.6 -86.2 -84.2 -84.9

0.972 0.977 0.976 0.979 0.979 0.983 0.985 0.984

-28.4 -23.1 -24.2 -21.2 -21.2 -17.6 -15.4 -16.2

180 200 180

-87.8 -85.8 -90.1

0.981 0.986 0.978

-19.3 -14.1 -21.9

100

-76.5

0.979

-21.2

130 200 130 250

-91.7 -94.8 -94.2 -94.8

0.977 0.973 0.974 0.973

-23.6 -27.0 -26.3 -27.0

Kaolin minerals 1 2 3 4

K+D K K K+ D

5

K + D

6 7

K K

8

K + D

9 10 11

K+D K+ D D

12

D

Ohnuma O-5T 62 m O-6T 165 m O-5T 143 m O-5T 168 m O-5T 228 m O-5T 240 m O-5T 279 m O-5T 374 m Matsukawa No. 8 400 m No. 8 500 m AR-1 305 m Ohtake 2T 290 m

Sericite 13 14 15 16

1M 1M + 2M 1 1M + 2M1 1M + 2M 1

Ohnuma O-4T 380.9 m O-4T 695 m HM-2 492 m O-6R 800 m

K = kaolinite; D = dickite. 1M and 2M 1 represent the polytypes of sericite. * The hydrogen isotopic composition of present-day hydrothermal water is -70%~ in the Matsukawa geothermal area and -56%c in the Ohtake geothermal area [ 14]. TABLE 2 Hydrogen isotopic analyses of chlorite from the Ohnuma geothermal area Sample No.

Fe/(Fe + Mg)

Sample location

Estimated temperature (°C)

6D (%c)

c~ *

1000 In a

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

0.28 0.23 0,36 0.33 0.35 0.43 0.43 0.43 0.44 0.5 1 0.43 0.36 0.39 0.36 0.44 0.36 0.35 0.5 2 0.48 0.34 0.36

O-2T 352 m O-2T 392 m O-2T 507 m O-2T 624.7 m O-4T 587.1 m O-4T 599.4 m O-6T 356 m O-6T 471 m O-6T 492 m O-6T 711 m O-5T 436 m O-3Ra 1236.5 m O-3Ra 1429.5 m O-5T 756.5 m O-5R 1019 m O-5R 1373 m O-5R 1485 m O-6R 1234 m O-6R 1390 m O-6R 1458 m O-6R 1585 m

130 150 160 180 170 170 220 230 230 240 200 250 250 250 250 250 250 250 250 250 250

-82.3 -85.0 -92.8 -95.9 -102.7 -108.4 -100.5 -106.8 -102.8 -107.3 -105.7 -104.3 -103.2 -104.4 -99.0 -102.3 -100.6 -112.5 -113.4 -104.1 -105.7

0.987 0.983 0.975 0.972 0.965 0.959 0.967 0.960 0.965 0.960 0.967 0.963 0.964 0.963 0.969 0.965 0.967 0.954 0.953 0.963 0.962

-13.3 -16.6 -24.8 -28.2 -35.8 -42.2 -33.4 -40.3 -36.0 -40.9 -39.1 -37.5 -36.4 -37.7 -31.7 -35.3 -33.5 -46.7 -47.8 -37.4 -39.1

261 I

5.1. Kaolin minerals

Fig. 8 shows that the O~kaolin_water value does not change so much with temperature. It changes from 0.972 to 0.985 with a temperature increase from 50 to 150°C, and are consistent with the room temperature values obtained by Savin and Epstein [1] and Lawrence and Taylor [2,3]. In Fig. 8, O%aolin_water values of kaolin minerals (kaolinite and dickite) from the Matsukawa geothermal area (Iwate Prefecture, northeastern Japan) and Ohtake geothermal area (Oita Prefecture, Kyushu) are also plotted. The result t h a t O~kaolin_water does not depend much on the temperature is in accordance with the suggestion by Taylor [5]. 5.2. Sericite

It seems from Fig. 8 that Otsericite_water also has a nearly constant value throughout the temperature range from 130 to 250°C. It is noteworthy that this O~sericite.wate r value is nearly equal to Otkaolin_water, because the octahedrally coordinated sites close to the hydroxyl groups are occupied almost exclusively by A1 in both kaolin minerals and sericite. 5.3. Chlorite

As shown in Table 2, the hydrogen isotopic composition of chlorite changes from -82.3 to -113.4~'~ in the temperature range of 130-250°C. This isotopic variation is much larger than those of kaolin minerals and sericite. The ~chlorite-water value does not depend on the temperature explicitly. As shown in Fig. 9, however, there is a correlation between the O~chlorite.water value and the Fe/(Fe + Mg) ratio of chlorite; the former decreases with an increase of the latter. Suzuoki and Epstein [6] experimentally showed that hydrogen isotopic fractionation factors between hydrous minerals such as hornblende, biotite, and muscovite, and water depend not only upon the temperature but also upon the chemical composition, especially the Fe content of the octahedral sites. Kuroda et al. [16] showed that 8D values of natural chlorites depend on the Fe content of the octahedral sites. Sakai and Tsutsumi [7] experimentally showed that the hydrogen isotopic fractionation factor

g _30L o

-

i

I -40~[

-50~-60 ~

I

I

I

0.1

0.2

0.3

E

04

I

t

I

I

0.5

0.6

0.7

0.8

Fel(Fe.Mg)

Fig. 9. Relationship between achlorite.wate r and Fe/ (Fe + Mg) of chlorites.

between serpentine and water becomes more positive with decreasing temperature in the range from 100 to 480°C, with a cross-over point at about 210°C. But no such tendency could be found for either chlorite or kaolin minerals in the same temperature range (Fig. 8). In Fig. 10, the change of 8 D value of successive water samples collected by stepwise heating of chlorite is shown. Chlorite occurring on shallower levels contains expandable layers. In the case of these chlorites, 8D values of water dehydrated in the low-temperature range from 25 to 270°C are higher than those of the constitutional water. This heaw water

('1.) -40 - 50

- 60

- 70

~D - 80

O-2T

392rn

i,__j - 90

i i

-100

O-6R 1458m

-110 -120

i

100

i

i

300

i

=

500

i

i

700

,

i

900

i

i

1100

,

i

1300 (°C)

Temperature

Fig. 10. The change of 6D values of successive water samples collected by stepwise heating of two chlorites. Chlorite from borehole O-2T 392 m contains expandable layers.

262

12"1.

6.1%

O-2T 352m

1.7"/* ~ G5%

O~'l, -2T 392m

I

-

sity, Dr. K. Sumi of the Geological Survey of Japan, and the staff members o f the Mitsubishi Metals Co., Ltd. and the Japan Metals Chemicals Co., Ltd.

65% 1

References

•

~.%__

-2T 62&Tm

j

t5%

0~. ShL~lka~i m i n e

.....

u~o. . . .

"mo~:

ZT'I,

soo

~

-

looo*c

Fig. 11. DTA and TG curves of chlorites in borehole O-2T. Mg-chlorite from the Shakanai mine, Akita Prefecture is also shown.

probably came from the hygroscopic moisture and/or the interlayer water. On the other hand, chlorites which occur at deeper levels and contain less expandable layers could not evolve enough water to measure in this temperature range. The result of differential thermal analysis confirms this (Fig. 11). Suzuoki and Epstein [6] made a hydrogen isotopic study of Mgchlorite by stepwise heating. They showed that water of the brucite-like sheets which are dehydrated at lower temperatures has higher 8D values. Because chlorites in our samples contain Fe as well as Mg in octahedral sheets, water of the brucite-like sheets could not clearly be separated from that of the silicate layers. Difference in dehydration between these chlorites is shown on the differential thermal analysis curves in Fig. 11.

Acknowledgements We would like to thank Prof. K. Ishioka and Dr. K. Suwa of Nagoya University for their encouragement. We are indebted to Prof. N. Nakai and Mr. M. Ito of Nagoya University, Prof. S. Matsuo of Tokyo Institute o f Technology, and Prof. Y. Mizutani of Toyama University for their comments and help in the experiments. Many of the samples studied in this paper were kindly provided by Prof. S. Honda of Akita University, Dr. M. Hayashi of Kyushu Univer-

1 S.M. Savin and S. Epstein, The oxygen and hydrogen isotope geochemistry of clay minerals, Geochim. Cosmoclaim. Acta 34 (1970) 25. 2 L.R. Lawrence and H.P. Taylor, Jr., Deuterium and oxygen-18 correlation: clay minerals and hydroxides in Quaternary soils compared to meteoric waters, Geochim. Cosmochim. Acta 35 (1971) 993. 3 L.R. Lawrence and H.P. Taylor, Jr., Hydrogen and oxygen isotope systematics in weathering profiles, Geochim. Cosmochim. Acta 36 (1972) 1377. 4 D.B. Wenner and H.P. Taylor, Jr., Oxygen and hydrogen isotope studies of the serpentinization of ultramafic rocks in oceanic environments and continental ophiolite complexes, Am. J. Sci. 273 (1973) 207. 5 H.P. Taylor, Jr., The application of oxygen and hydrogen isotope studies to problems of hydrothermal alteration and ore deposition, Econ. Geol. 69 (1974) 843. 6 T. Suzuoki and S. Epstein, Hydrogen isotope fractionation between OH-bearing minerals and water, Geochim. Cosmochim. Acta 40 (1976) 1229. 7 H. Sakai and M. Tsutsumi, D/H fractionation factors between serpentine and water at 100° to 500°C and 2000 bar water pressure, and the D/H ratios of natural serpentines, Earth Planet. Sci. Lett. 40 (1978) 231. 8 H. Shirozu, X-ray powder patterns and cell dimensions of some chlorites in Japan, with a note on their interference colors, Mineral. J. (Jpn.) 2 (1958) 209. 9 I. Friedman and R.L. Smith, The deuterium content of water in some volcanic glasses, Geochim. Cosmochim. Acta 15 (1958) 218. 10 J.D. Godfrey, The deuterium content of hydrous minerals from the east-central Sierra Nevada and Yosemite National Park, Geochim. Cosmochim. Acta 26 (1962) 1215. 11 S. Epstein and H.P. Taylor, Jr., The concentration and isotopic composition of hydrogen, carbon and silicon in ApoUo 11 lunar rocks and minerals, Proc. Apollo 11 Lunar Sci. Conf. 12 (1970) 1085. 12 B. Velde, Experimental determination of muscovite polymorph stabilities, Am. Mineral 50 (1965) 436. 13 S.W. Bailey and B.E. Brown, Chlorite polytypism, I. Regular and semi-random one-layer structures, Am. Mineral. 47 (1962) 819. 14 Y. Mizutani, Isotopic composition and underground temperature of the Otake geothermal water, Japan, Geochem. J. 6 (1972) 67. 15 S.M.F. Sheppard, R.L. Nielsen and H.P. Taylor, Jr., Oxygen and hydrogen isotope ratios of clay minerals from porphyry copper deposits, Econ. Geol. 64 (1969) 755. 16 Y. Kuroda, T. Suzuoki, S. Matsuo and H. Shirozu, A preliminary study of D/H ratios of chlorites, Contrib. Mineral. Petrol. 57 (1976) 223.