Petrography and uplift history of the Quaternary Takidani Granodiorite: could it have hosted a supercritical (HDR) geothermal reservoir?

Journal of Volcanology and Geothermal Research 120 (2003) 215^234 www.elsevier.com/locate/jvolgeores

Petrography and uplift history of the Quaternary Takidani Granodiorite: could it have hosted a supercritical (HDR) geothermal reservoir? Masatoshi Bando, Greg Bignall, Kotaro Sekine, Noriyoshi Tsuchiya
Department of Geoscience and Technology, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan Received 29 January 2002; received in revised form 19 June 2002; accepted 19 June 2002

Abstract The Quaternary Takidani Granodiorite (Japan Alps) is analogous to the type of deep-seated (3^5 km deep) intrusive-hosted fracture network system that might support (supercritical) hot dry/wet rock (HDR/HWR) energy extraction. The I-type Takidani Granodiorite comprises: porphyritic granodiorite, porphyritic granite, biotitehornblende granodiorite, hornblende-biotite granodiorite, biotite-hornblende granite and biotite granite facies; the intrusion has a reverse chemical zonation, characterized by s 70 wt% SiO2 at its inferred margin and 6 67 wt% SiO2 at the core. Fluid inclusion evidence indicates that fractured Takidani Granodiorite at one time hosted a liquiddominated, convective hydrothermal system, with 6 380‡C, low-salinity reservoir fluids at hydrostatic (mesothermal) pressure conditions. ‘Healed’ microfractures also trapped s 600‡C, hypersaline (V35 wt% NaCleq ) fluids of magmatic origin, with inferred minimum pressures of formation being V600^750 bar, which corresponds to fluid entrapment at V2.4^3.0 km depth. Al-in-hornblende geobarometry indicates that hornblende crystallization occurred at about 1.45 Ma (7.7^9.4 km depth) in the (marginal) eastern Takidani Granodiorite, but later (at V1.25 Ma) and shallower (V6.5^7.0 km) near the core of the intrusion. The average rate of uplift across the Takidani Granodiorite from the time of hornblende crystallization has been 5.1^5.9 mm/yr (although uplift was about 7.5 mm/yr prior to V1.2 Ma), which is faster than average uplift rates in the Japan Alps (V3 mm/yr during the last 2 million years). A temperature^depth^time window, when the Takidani Granodiorite had potential to host an HDR system, would have been when the internal temperature of the intrusive was cooling from 500‡C to V400‡C. Taking into account the initial (7.5 mm/yr) rate of uplift and effects of erosion, an optimal temperature^time^depth window is proposed: for 500‡C at 1.54^1.57 Ma and V5.2 E 0.9 km (drilling) depth; and 400‡C at 1.36^1.38 Ma and V3.3 E 0.8 km (drilling) depth, which is within the capabilities of modern drilling technologies, and similar to measured temperature^depth profiles in other active hydrothermal systems (e.g. at Kakkonda, Japan). F 2002 Elsevier Science B.V. All rights reserved. Keywords: Takidani Granodiorite; Al-in-hornblende geobarometry; £uid inclusion microthermometry; uplift history; Deep-Seated Geothermal Reservoir (DSGR)

* Corresponding author. E-mail address: [email protected] (N. Tsuchiya).

1. Introduction Volcano-plutonic complexes provide the heat

0377-0273 / 02 / $ ^ see front matter F 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 ( 0 2 ) 0 0 3 9 9 - 2

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source for many active geothermal ¢elds (Reyes, 1990; Browne et al., 1992; Hulen and Nelson, 1993), and have attracted attention as systems for hot dry/wet rock (HDR/HWR) energy extraction (Muraoka et al., 2000; Hashida et al., 2000). At Kakkonda (NW Japan), exploration well WD-1a encountered partly solidi¢ed Kakkonda Granite and (inferred) reservoir temperatures of s 500‡C (Ikeuchi et al., 1998; Kato et al., 1998; Muraoka et al., 1998), that prompted a 5-year project at Tohoku University entitled ‘Investigation on Design Methodology for a Supercritical Subsurface Boiler, for Next Generation Geothermal Energy Extraction’ (Nakatsuka, 1998). The Takidani Granodiorite (Japan; Fig. 1) is the ‘youngest exposed pluton on Earth’ (Harayama, 1992), and analogous to the type of deep-seated (3^5 km depth) intrusive-hosted fracture network that might support a ‘supercritical HDR/HWR energy extraction system’ (Hashida et al., 2001). Here, new geobarometry and £uid inclusion microthermometric data for the Takidani Granodiorite are used to deduce the cooling history of the intrusion, and physico-chemical conditions in its fossil magmatic^hydrothermal system. Our work contributes to understanding the uplift history of young intrusive bodies and their potential as reservoir rocks for deep-seated HDR/HWR geothermal resources, particularly at temperature^pressure conditions that are greater than the brittle^plastic transition temperature of the granitoid. Geological evidence indicates that geothermal systems, and the hydrothermal circulation which maintains them, can be active for tens of thousands of years (Bignall and Browne, 1994). Several workers have attempted to determine how long a single intrusive episode might be expected to provide su⁄cient heat to fuel an active geothermal system. Cathles (1981) argued that the time required for a near-equidimensional plutonic dyke to cool to V25% of its initial temperature is a function of the pluton’s size and the permeability of the environment into which it intruded. Subsequently, Cathles et al. (1997) suggested that high-temperature hydrothermal systems suitable for energy (electricity-generating) exploita-

tion probably have a useful lifetime of less than 1 m.y., and possibly 6 10 000 years. A large granitic intrusion may take several million years to conductively cool, but is unlikely to produce a convective hydrothermal system, since circulating hot water requires good permeability. Intrusions producing large, moderate^hightemperature ( s 200‡C) hydrothermal systems are likely to have intruded an environment that is permeable enough to be cooled by convection, with geothermal activity closely tied to speci¢c intrusives events. If granitoid rock masses ascend too slowly, or have temperatures too low to establish an arti¢cially created fracture network at supercritical conditions, then it will be necessary to reconsider the type of rock masses that are presently the target for high-temperature HDR geothermal utilization. By studying the uplift history of the Takidani Granodiorite, we aimed to deduce a time^depth^temperature window when the intrusive might have retained su⁄cient internal heat in which to sustain an arti¢cial, supercritical Deep-Seated Geothermal Reservoir (DSGR), at depths that could be reached by present drilling technologies.

2. Geological setting The Takidani Granodiorite (21 km2 ; s 1.2 km vertical exposure) comprises part of a volcanoplutonic complex that extends over the Hotaka Mountain Ridge (within the Hida Mountain Range; Fig. 1), and forms part of the Norikura Volcanic Chain. Basement rocks in the Takidani area comprise Mesozoic chert, unmetamorphosed sandstones and alternating mudstones of the Sawando Complex, and Paleogene granite, which are covered and in places intruded by Hotaka Andesite (including dacitic-andesitic welded tu¡, diorite porphyry and tu¡ breccia) and Takidani Granodiorite (Harayama, 1990). The Norikura Volcanic Chain (Fig. 1a) has been active from the late Pliocene, with monogenetic basaltic volcanism in its southern part coincident with initial uplift of the Hotaka Mountain Ridge (Ujiie et al., 1992; Nakano, 1993, 1994; Harayama, 1994). In the middle Pleistocene, vol-

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Fig. 1. Location, geology and lithofacies map of the Takidani area (Harayama, 1992, 1994). Active volcanoes of the Norikura Volcanic Chain are indicated by b in panel a, and include (from north to south): Mt. Tateyama, Mt. Kumonotaira, Mt. Yake, Mt. Norikura and Mt. Ontake. See Fig. 3 for an enlarged map of the Shiradashi-zawa/Nishiho-zawa area. Samples locations: (A) 2403, (B) 2105, (C) 2401, (D) 1702 and (E) 2005.

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canic activity resumed after a period of dormancy and formed a series of stratovolcanoes (e.g. Mt. Ontake). During this time extensive pyroclastic £ows occurred (e.g. Okuhida Pyroclastic Flow), which accompanied continued uplift of the Hotaka Mountain Ridge, of V2000 m since the middle Pleistocene (Harayama, 1999). Ando (1986) used seismic wave analysis to show that an anisotropic mass, up to 100 km long and 50^150 km deep, occurs beneath central Japan, associated with the SW subduction of the Paci¢c plate. Low-velocity bodies have been inferred, 6 100 km below the Hida Mountain Range, in the wedge portion of the upper mantle (Hirahara et al., 1989), and are likely to have played a major role as heat sources for partial melting which produced the Takidani Granodiorite. Based on dating of eruption products and geo-

chronological history of the Hotaka Andesite, a late Pliocene age has been inferred for initial uplift of the Hotaka Mountain Ridge (Harayama, 1999). K^Ar (biotite) dating of Takidani Granodiorite has indicated ages from 1.27 to 1.07 Ma, with K^Ar (hornblende) measurements yielding K^Ar ages of 1.93 E 0.16 and 1.46 E 0.10 (Harayama, 1992) and 1.20 E 0.34 (Harayama, 1994), although concerns have been expressed regarding the validity of some K^Ar ages (Harayama, 1999). Post-emplacement uplift resulted in eastward tilting of the Takidani Granodiorite (Harayama, 1992), with an exposure age inferred to be 0.10^0.65 Ma, based on dating of Pleistocene pyroclastic £ow deposits and Quaternary Yakedake Volcanics which cover the southern part of the pluton (Harayama, 1992, 1994). Volcanic activity in the Takidani area has continued to the present, forming Mt. Yake and Mt. Norikura (Fig. 1a).

Table 1 Average modal content (vol% and S.D.) and rock texture of lithofacies in the Takidani Granodiorite Lithofacies Porphyritic granodiorite

Porphyritic granite

Biotitehornblende granodiorite

Biotite-hornblende granite

Hornblendebiotite granodiorite

Biotite granite

n (vol%)

8

7

35

6

12

16

Q Pl Or Bi Hb Mt

27.5 ( E 2.5) 46.5 ( E 4.0) 14.9 ( E 3.4) 6.8 ( E 1.5) 3.5 ( E 1.9) 0.1 ( E 0.1)

32.8 ( E 2.8) 34.9 ( E 4.6) 22.5 ( E 5.3) 6.5 ( E 1.2) 2.6 ( E 2.3) 0.4 ( E 0.1)

27.8 ( E 3.0) 43.7 ( E 3.8) 15.1 ( E 3.7) 7.2 ( E 1.2) 5.3 ( E 2.4) 0.6 ( E 0.4)

29.2 ( E 1.9) 38.8 ( E 2.9) 22.0 ( E 4.3) 5.3 ( E 1.0) 3.7 ( E 1.4) 0.8 ( E 0.4)

32.5 ( E 3.1) 41.5 ( E 3.6) 18.2 ( E 3.2) 5.9 ( E 1.8) 1.3 ( E 0.9) 0.3 ( E 0.3)

31.6 ( E 4.8) 39.7 ( E 6.8) 23.4 ( E 3.8) 4.1 ( E 1.2) 0.3 ( E 0.5) 0.5 ( E 0.4)

Texture

Porphyritic (phenocrysts predominate)

Porphyritic (groundmass predominates)

Medium-grained equigranular

Medium- to ¢ne-grained equigranular

Medium- to coarse-grained equigranular

Medium-grained equigranular

Pl phenocrysts ( 6 10 mm long) in equigranular mosaic of Q (V0.4^0.5 mm), Or, Pl, Bi and Hb

Zoned, euhedral Pl phenocrysts ( 6 8 mm). Matrix of Or, anhedral Q, minor Pl, Bi, Hb and accessory minerals

Abundant Pl, in interlocking mosaic of Q, Or, lesser Bi, Hb and opaques

An interlocking mosaic of subhedral Q, Pl and Or, plus subordinate Bi and Hb, with accessory Mt, Tt, Ap and Al

Less Pl and Hb than in other lithofacies, but with more Or. Minor Bi and Hb

Most leucocratic. Mosaic of anhedral perthitic-textured Or, subhedral Pl, anhedral Q; sparse Bi, Hb and opaques

Accessory minerals

Tt, Ap

Tt, Ap, Px, Rt

Tt, Ap

Tt, Ap, Al

Tt, Ap

Tt, Ap

Enclave

Rare

Rare

Abundant

Abundant

Intermediate

Intermediate

9.5 ( E 3.2)

13.1 ( E 3.3)

9.8 ( E 2.4)

7.5 ( E 2.3)

4.9 ( E 1.6)

Color index 10.9 ( E 2.5)

Key: Q, quartz; Pl, plagioclase; Or, orthoclase; Bi; biotite; Hb, hornblende; Mt, magnetite; Tt, titanite; Ap, apatite; Px, pyroxene; Rt, rutile; Al, allanite.

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3. Petrography

3.2. Hotaka Andesites

3.1. Takidani Granodiorite

Dacitic-andesitic welded tu¡, up to V1500 m thick (Harayama, 1992, 1994), comprises the main body of the Hotaka Andesite. The welded tu¡ is hornfelsed by Takidani Granodiorite (characterized by coarse groundmass plagioclase within 120 m of the contact). In distal parts, interstitial biotite replaces opaque minerals, whilst orthopyroxene is replaced by biotite and hornblende. Cordierite occurs as a groundmass phase. Diorite porphyry intruded the welded tu¡ as a stock, sheet or dike (Harayama, 1990). The diorite porphyry has a hypabyssal texture, containing euhedral, 6 10-mm-long, zoned plagioclase phenocrysts, in a groundmass of euhedral olivine, orthopyroxene, quartz, magnetite and rare spinel crystals.

Takidani Granodiorite samples are classi¢ed as ‘granite’ or ‘granodiorite’, using the total alkalis versus silica diagram of Cox et al. (1979; adapted by Wilson, 1989), and further subdivided into one of six lithological facies (biotite-hornblende granodiorite, hornblende-biotite granodiorite (Fig. 2a), biotite-hornblende granite, porphyritic granodiorite, porphyritic granite (Fig. 2b) and biotite granite) that form a zonation across the intrusion (Fig. 1c). The porphyritic granite is equivalent to the ‘upper lithofacies’ of Harayama (1994), whilst the biotite-hornblende granodiorite, hornblendebiotite granodiorite and biotite-hornblende granite facies correspond to a ‘lower lithofacies’. There are no obvious contacts between lithofacies, which grade from one facies into another. The common primary mineral assemblage in the medium-grained Takidani Granodiorite comprises plagioclase, quartz, K-feldspar, biotite, hornblende, magnetite and various accessory minerals (e.g. titanite, apatite, E -pyroxene, allanite and rutile) ; summarized in Table 1. Circular^ovoid ma¢c enclaves are locally abundant (Fig. 2c) and may, based on their interlocking occurrence in some streambed boulders, derive from conduit wall and/or roof rocks. The microgranular enclaves are typically a few tens of centimeters in diameter, with an indistinct ‘reaction’ halo characterized by alkali feldspar and paucity of ferromagnesian minerals in enveloping Takidani Granodiorite.

4. Rock chemistry 4.1. Major elements Takidani Granodiorite major element compositions follow the trend of Japanese granitoids (Aramaki et al., 1972; Table 2). The Takidani Granodiorite has a ‘reverse’ chemical zonation, with biotite-hornblende granodiorite having 66^ 70 wt% SiO2 , whilst rocks in marginal parts have greater silica contents (e.g. porphyritic granite near the eastern contact has 70^73 wt% SiO2 ; Fig. 3). The Na2 O content ( s 3.2 wt%), occurrence of magmatic titanite, and Al2 O3 / (CaO+Na2 O+K2 O) molar ratio (Fig. 4) indicate

Fig. 2. Photographs of (A) hornblende-biotite granodiorite (#2105); (B) porphyritic granite (#2001); (C) sub-rounded enclaves in Takidani Granodiorite (at Shiradashi-zawa).

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Table 2 Representative whole rock major (wt%) and trace element (ppm) composition of the Takidani Granodiorite

Sample No: SiO2 TiO2 Al2 O3 Fe2 O3 MnO MgO CaO Na2 O K2 O P2 O5 Total Ba Co Cr Cu Nb Ni Rb Sr V Y Zn Zr

a

Porphyritic granodiorite 2004 69.34 0.60 13.88 4.44 0.09 1.60 3.40 3.17 2.91 0.14 99.56 565 15 33 0 11 11 110 286 65 20 56 181

Porphyritic granite 2003

Biotite-hornblende granodiorite 2704

Biotite-hornblende granite 2505E

72.95 0.37 13.98 2.44 0.05 0.79 1.93 3.09 4.65 0.06 100.33

66.73 0.65 16.02 4.56 0.08 1.88 4.45 3.58 2.79 0.14 100.87

67.79 0.58 15.36 4.14 0.08 1.71 3.87 3.41 3.36 0.11 100.40

573 5 33 6 12 18 161 251 46 25 33 151

539 13 39 12 9 17 98 358 84 17 53 172

543 10 49 12 10 25 128 272 69 25 51 157

Hornblende-biotite granodiorite 2105 70.59 0.33 14.90 2.96 0.07 0.74 2.66 4.00 3.31 0.06 99.61 683 7 22 3 10 12 113 255 26 23 54 161

Biotitegranite 2403 72.50 0.26 13.86 2.80 0.06 0.46 1.57 3.80 4.21 0.04 99.54 930 5 72 3 13 35 130 167 12 34 56 182

Samples locations are shown in Fig. 1 or 3. a Fe2 O3 as total Fe.

that the Takidani Granodiorite is an ‘I-type granite’ (Chappell and White, 1974). 4.2. Trace elements The Rb content of the Takidani Granodiorite follows a similar pattern to SiO2 , with 82 to 127 ppm in biotite-hornblende granodiorite near the core of the granitoid, and s 200 ppm Rb in porphyritic granite and biotite-hornblende granite in marginal parts (Fig. 3). Porphyritic granodiorite is enriched in Ba and Sr, and depleted in Rb, which may have been produced by fractional crystallization due to plagioclase accumulation (e.g. Langmuir, 1989). Co, Sr, V and Zn abundances in Takidani Granodiorite decrease with increasing SiO2 . Incompatible trace element patterns indicate that Takidani Granodiorite and Hotaka Andesites may have the same magmatic source. The

Takidani Granodiorite is inferred to have formed by partial melting (Bando et al., submitted), with ma¢c enclaves comprising a possible restite component, as in many other I-type granites (Chappell et al., 1987). Hotaka Andesite is unlikely source material for the ma¢c enclaves, due to major petrological and geochemical di¡erences between themselves and the host rock. Chemical variation between the Takidani Granodiorite and their ma¢c enclaves indicates that magma mixing is also unlikely to be the major reason for lithofacies variation. There is no evidence for recrystallization to explain compositional zoning in the granitoid.

5. Aluminum-in-hornblende geobarometry Numerous studies have demonstrated that amphibole composition in (granitic) magmas varies

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221

Fig. 3. Spatial distribution of wt% SiO2 (and ppm Rb; in parentheses) in Takidani Granodiorite in the Shiradashi-zawa area. Samples listed in Tables 2^4, used for Al-in-hornblende geobarometry, £uid inclusion analysis or mentioned in the text are: (F) 2505E, (G) 2506, (H) 2507, (I) 1406, (J) 1401, (K) 2703, (L) 2704, (M) 2701, (N) 2003 and (O) 2004.

with bulk composition and oxygen fugacity, as well as temperature/pressure conditions (Wones, 1981; Wones and Gilbert, 1982). In this study, electron probe microanalysis (EPMA) was undertaken to determine the major element chemistry of hornblende and plagioclase in Takidani Granodiorite, for application in the Al-in-hornblende geobarometer of Anderson and Smith (1995), and hornblende-plagioclase geothermometer of Blundy and Holland (1990). Mineral analyses were made using the wavelength-dispersive electron microprobe (JXA-8800M, JEOL) at the National Institute of Polar Research (Japan), with 15 kV, 8 nA and beam size 2 Wm. Quantitative data processing was based on the ZAF correction algorithm. By estimating the depth of hornblende crystallization, constraints could be placed on the uplift history of the pluton, in order to establish a time^ depth^temperature pro¢le for the Hotaka^Taki-

Fig. 4. Major element Harker variation diagram of wt% SiO2 versus [Al2 O3 /(CaO+Na2 O+K2 O)molar], showing that the Takidani Granodiorite may be classi¢ed as an ‘I-type granite’, after Chappell and White (1974). Key: a : porphyritic granodiorite; b : porphyritic granite; +: biotite-hornblende granodiorite; E : biotite-hornblende granite; O: hornblende-biotite granodiorite; U: biotite granite.

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222 Table 3 Representative EPMA of hornblende phenocrysts from the Takidani Granodiorite, with averaged EPMA from two or more crystals used for Al-in-hornblende geobarometry (after Anderson and Smith, 1995) Porphyritic granodiorite

Sample No.

2703

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SiO2 TiO2 Al2 O3 FeOð2þÞ MnO MgO CaO Na2 O K2 O Total

46.74 1.66 7.15 14.47 0.35 13.62 11.05 1.57 0.62 97.21

Biotitehornblende granodiorite 1702 46.63 1.67 7.32 13.85 0.26 13.60 11.52 1.51 0.67 97.02

Biotitehornblende granodiorite 2005ð1Þ 47.23 1.32 6.55 15.64 0.55 12.30 10.62 1.56 0.54 96.32

Biotitehornblende granodiorite 2005ð2Þ 46.37 1.51 7.10 15.50 0.40 12.75 10.95 1.62 0.62 96.83

Biotitehornblende granodiorite 2401 47.65 1.38 6.28 16.10 0.64 12.72 11.29 1.15 0.48 97.69

Biotitehornblende granite 2505E 48.55 1.02 5.49 15.50 0.61 13.44 10.97 1.37 0.38 97.32

Hornblendebiotite granodiorite 2105 45.09 0.88 6.77 21.65 1.24 8.42 10.75 1.36 0.63 96.80

Hornblendebiotite granodiorite 2003 46.72 1.68 6.93 14.73 0.30 13.06 11.24 1.59 0.62 96.89

Si AlIV AlVI Ti Fe3þ Mg Mn Fe2þ Ca NaM4 NaA K

6.820 1.180 0.050 0.182 0.751 2.961 0.043 1.013 1.728 0.272 0.173 0.115

6.843 1.157 0.109 0.184 0.503 2.975 0.033 1.196 1.812 0.188 0.240 0.125

6.992 1.108 0.135 0.147 0.660 2.713 0.069 1.276 1.685 0.315 0.131 0.103

6.836 1.164 0.071 0.168 0.717 2.802 0.049 1.193 1.730 0.270 0.193 0.118

6.955 1.045 0.036 0.036 0.761 2.768 0.080 1.205 1.766 0.234 0.091 0.089

7.068 0.932 0.011 0.112 0.818 2.916 0.075 1.069 1.711 0.289 0.098 0.070

6.859 1.141 0.074 0.101 0.835 1.909 0.160 1.921 1.753 0.247 0.155 0.123

6.886 1.114 0.090 0.186 0.503 2.975 0.033 1.196 1.812 0.188 0.240 0.125

Fe/(Fe+Mg) Fe3þ /(Fe2þ +Fe3þ )

0.373 0.419

0.364 0.294

0.416 0.341

0.405 0.375

0.415 0.389

0.393 0.434

0.591 0.303

0.364 0.294

Si (average/sample) Al (total; average)

6.757 1.241

6.843 1.259

6.994 1.076

6.937 1.103

6.924 1.066

6.997 0.975

6.825 1.227

6.886 1.204

T (‡C) P (kbar)

720 2.37

721 2.48

680 2.07

689 2.13

706 1.80

673 1.65

Total Fe content is de¢ned as FeO. NaM4 and NaA refers to Na content (apfu) in sites M4 and A, respectively. Average Si and Al (total) abundances (apfu) were used for Hb^Pl geothermometry^barometry. See text for further details.

727 2.27

716 2.29

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Lithofacies

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dani volcano-plutonic system. We mainly analyzed samples from the Taki-dani and Shiradashi-zawa areas (Fig. 1), because they provide a convenient transverse (from margin to core to margin) across the Takidani Granodiorite. 5.1. Methodology The Al-in-hornblende geobarometer is based on a linear correlation between crystallization pressure and total Al content in hornblende (Eq. 1 ; Anderson and Smith (1995) ; after Johnson and Rutherford, 1989; Schmidt, 1992): Pð0:6 kbarÞ ¼ 4:76 Alt 33:013f½T3675 =85g Uf0:530 Alt þ 0:005294½T3675 g

ð1Þ

where P is pressure in kbar, Alt is total Al content (atoms per formula unit, apfu) in hornblende and T is temperature in ‡C. Blundy and Holland (1990) also suggested that hornblende composition depends on pressure and temperature, and proposed the following hornblende-plagioclase geothermometer for silica-saturated rocks, which is based on the AlIV content of amphibole co-existing with plagioclase :

223

T ¼ ð0:677P348:98 þ Y Þ= ð30:042930:008314 lnKÞ

ð2Þ

where T is temperature in K, P is pressure in kbar, Y = 0 for XAb s 0.5; or 38.06+25.5 (13XAb )2 for XAb 6 0.5, K = XAb (Si34)/(83Si), XAb is albite molar content of plagioclase, Si is silica content in hornblende. Hornblende equilibrium pressure and temperature estimates were obtained by iteration of Eqs. 1 and 2 (after Anderson and Smith, 1995). The Takidani Granodiorite satis¢es all compositional constraints to be considered a trivariant reaction system (with respect to pressure, temperature and oxygen fugacity; Schmidt, 1992). Oxygen fugacity has a marked e¡ect on the mineral system, so only hornblendes with Fe/(Fe+Mg) 6 0.65 (Anderson and Smith, 1995), SiA7.5 and Caw1.6 (apfu) were used for geobarometry (Hammarstrom and Zen, 1986). 5.2. Mineral chemistry 5.2.1. Hornblende Representative major element EPMA of unaltered magmatic hornblende from Takidani Grano-

Fig. 5. Hornblende classi¢cation (symbols are the same as in Fig. 4). Si and Mg/(Mg+Fe) compositions were determined from averaged major element EPMA, calculated to an apfu of 23 oxygen, and normalized to Total cations3(Ca+Na+K) = 13, after Leake (1978), see Table 3.

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224 Table 4 Representative EPMA of rim and core plagioclase crystals from the Takidani Granodiorite, in direct contact with hornblende phenocrysts used for Al-in-hornblende geobarometry Porphyritic granodiorite

Porphyritic granodiorite 2703 (rim)

Biotitehornblende granodiorite 1702 (core)

Biotitehornblende granodiorite 1702 (rim)

Biotitehornblende granodiorite 2005 (1) (rim)

Biotitehornblende granite 2401 (rim)

Hornblendebiotite granodiorite 2505E (rim)

Hornblendebiotite granodiorite 2105 (rim)

Sample No.

2703 (core)

SiO2 TiO2 Al2 O3 FeO MnO MgO CaO Na2 O K2 O Total

57.22 0.03 26.50 0.18 0.00 0.06 8.41 6.46 0.34 99.21

63.88 0.01 22.47 0.28 0.00 0.04 2.96 9.15 0.68 99.48

57.09 0.02 26.58 0.21 0.01 0.02 9.04 6.17 0.47 99.60

61.60 0.02 23.85 0.18 0.02 0.01 5.64 8.04 0.34 99.70

64.31 0.04 22.56 0.10 0.01 0.12 4.34 8.19 0.34 100.00

61.95 0.00 23.48 0.20 0.03 0.00 5.14 8.31 0.45 99.56

65.57 0.08 21.64 0.07 0.00 0.11 3.39 8.81 0.33 99.99

61.66 0.01 23.73 0.08 0.03 0.04 5.28 7.94 0.45 99.22

Si Al Na Ca K Ti Fe Mn Mg Total

2.585 1.412 0.565 0.408 0.020 0.001 0.007 0.004 0.004 5.005

2.835 1.175 0.787 0.141 0.039 0.000 0.010 0.002 0.002 4.992

2.575 1.413 0.540 0.437 0.027 0.001 0.008 0.001 0.001 5.002

2.743 1.252 0.694 0.269 0.019 0.001 0.007 0.000 0.000 4.986

2.832 1.171 0.700 0.205 0.019 0.001 0.004 0.008 0.008 4.948

2.762 1.234 0.718 0.246 0.026 0.000 0.007 0.000 0.000 4.992

2.880 1.121 0.750 0.159 0.019 0.002 0.003 0.007 0.007 4.948

2.754 1.250 0.688 0.252 0.026 0.000 0.003 0.003 0.003 4.979

XAlbite ðmolar%Þ XAnorthite ðmolar%Þ XOrthoclase ðmolar%Þ XAlbite ðsampleÞ Average Range

56.75 41.26 1.99

81.42 14.57 4.01

53.71 43.58 2.71

70.67 27.37 1.95

75.81 22.15 2.04

72.59 24.80 2.61

80.79 17.19 2.02

71.22 26.14 2.64

58.18 51^64

77.68 74^82

53.61 51^56

69.99 66^74

76.77 74^83

72.59 71^74

81.42 76^84

71.22 68^77

Fe content reported as total FeO. The average albite content (molar%) for analyzed crystals (two or more for each sample), was used for Hb^Pl geothermometry^ barometry. Sample locations are given in Figs. 1 and 3.

M. Bando et al. / Journal of Volcanology and Geothermal Research 120 (2003) 215^234

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Lithofacies

M. Bando et al. / Journal of Volcanology and Geothermal Research 120 (2003) 215^234

diorite are presented in Table 3. Major element EPMA compositions were calculated to an apfu of 23 oxygen, and normalized to Total cations 3(Ca+Na+K) = 13, with Fe3þ /Fe2þ ratios calculated by charge balance. For all amphiboles, (Ca+Na)M4 s 1.34, NaM4 6 0.67 and Ti 6 0.5, so they belong to the calcic amphibole group of Leake (1978; Fig. 5). Most are magnesio-hornblendes, with (Na+K)A 6 0.5 and 6.50 6 Si 6 7.25, with [Mg/(Mg+Fe2 )] s 0.50 (Leake, 1978). Fe/(Fe+Mg) values range from 0.354 to 0.420, which is similar to hornblendes in other plutonic complexes (Hollister et al., 1987; Hammarstrom and Zen, 1986; Anderson and Smith, 1995). None of the analyzed hornblendes are zoned, although only EPMA from amphibole rims were used for hornblende-plagioclase geothermometry. Hornblende-biotite granite (e.g. sample #2105) contains ferro-hornblende, with average Fe/(Fe+Mg) values of 0.589. The EPMA of rim hornblende, for individual crystals, have a mean CaðapfuÞ composition in the M4 site that ranges from 1.687 (#2005) to 1.812 (#1702). Similarly, NaðapfuÞ in the M4 site varies from 0.188 to 0.313, and in the A site from 0.066 (#2701) to 0.240 (#1702). SiðapfuÞ in the tetrahedral (T) site varies from 6.751 (#2704) to 7.171 (#2701), whilst total AlðapfuÞ varies from 0.933 (#2701) to 1.277 (#2704) (from 0.829 to 1.246 in the AlIV (T) site). High Fe3þ /(Fe3þ +Fe2þ ) values, up to 0.506 (for #2704), point to a moderately high oxidation state of the magma.

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5.2.2. Plagioclase Representative major element (core and rim) EPMA of unaltered plagioclases, in contact with calcic amphiboles from Takidani Granodiorite samples, and calculated to an apfu of 8 oxygen, are listed in Table 4. The majority of the sub- to euhedral plagioclase crystals have an andesine composition (XAN in the plagioclase core varies from 0.29 to 0.53 ; average = 0.40). The rims of plagioclase are lower in XAN than in the core, with a composition from oligoclase to andesine (XAN ranges from 0.16 to 0.28; average = 0.23). 5.3. Geobarometry and depth estimation Based on Al-in-hornblende geobarometry, we infer that the Takidani Granodiorite crystallized at lithostatic pressures of 6 2.48 ( E 0.6) kbar, i.e. somewhat shallower than plutons of the Sierra Nevada (Hollister et al., 1987; Hammarstrom and Zen, 1986). A pressure of 2.07^2.48 kbar is indicated for samples from the Taki-dani area (#1702, 2005, 2105; 1850 mRSL) and 2.07^ 2.37 kbar from the Shiradashi-zawa^Gankirido (#2703, 2704; V1950 mRSL), compared to 1.65 ^1.80 kbar for samples further west (e.g. #2505E, 2506 and 2401; at V1600 mRSL); Table 5. Assuming a crustal density of 2.66 g/cm3 we may estimate the depth of the solidi¢cation of the granitic magma. The temperature of hornblende crystallization, by the hornblende-plagioclase geothermometer

Table 5 Summary of £uid inclusion microthermometric (‡C) and salinity data (wt% NaCleq ), for the Takidani Granodiorite Type

Number of inclusions

Homogenization temperature (‡C) Range (and average)

Salinity (wt% NaCleq ) Range (and average)

Location

1. Low-salinity, liquid-rich £uid inclusions from a mesothermal setting

306

154^385 (276)

0.4^18.8 (5.7)

Taki-dani, Chibi-dani, Nishiho-zawa, Shiradashi-zawa, Kamoshika-zawa

2. Moderate- to high-salinity liquidand vapor-rich (boiled) £uid inclusions, from mesothermal setting

84 {5}

157^393 (285) {203^325 (257)}

18.0^48.9 (34.0) Taki-dani, Chibi-dani, Nishiho-zawa, {60.3^73.9 (68.8)} Shiradashi-zawa, Kamoshika-zawa Yonagi-dani

482^ s 600 ( s 570)

33.0^41.2 (36.3)

3. High-salinity, polyphase, (halite11 bearing) magmatic £uid inclusions from a magmatic-hydrothermal setting

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Chibi-dani, Shiradashi-zawa, Yonagi-dani

226

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Fig. 6. Results of Al-in-hornblende geobarometry (after Blundy and Holland, 1990; Anderson and Smith, 1995). Values: (top row) pressure (kbar), (second row) calculated depth (km), (third row) uplift rate (mm/yr), based on K^Ar (hornblende) ages of 1.46 Ma (western) and 1.93 Ma (eastern) parts of the Takidani Granodiorite, and (bottom row) sample number.

(Blundy and Holland, 1990), ranges from 656 to 728‡C, with mean values of V715‡C (at 7.8^9.4 km depth) in the Taki-dani part of the Takidani Granodiorite ; V700‡C hornblende crystallization (8.5^9.1 km depth) in samples from the Shiradashi-zawa to Gankirido area ; and V705‡C (6.3^ 6.8 km depth) for the western Takidani Granodiorite (Fig. 6). The apparent di¡erence in lithostatic pressures for Takidani Granodiorite from the Taki-dani/ Shiradashi-zawa (average = 2.22 kbar) and Migimata-dani/western (average = 1.72 kbar) regions (V0.5 kbar; or V1900 m, assuming 2.66 g/cm3 crustal density), re£ect two major factors : (a) an inference that hornblende crystallization pressures

for the Taki-dani/Shiradashi-zawa area corresponds to early-stage emplacement, when the pluton was 8^9.4 km deep, whereas pressure^depth estimates for the Migimata-dani/western region re£ect a later, shallower (V6.3^6.8 km) depth of hornblende crystallization; and (b) variation in uplift rates across the Takidani Granodiorite, following initial emplacement and subsequent unroo¢ng; and other e¡ects, including: (1) di¡erences in overburden of Hotaka Andesite in western and eastern parts of the granitoid ; (2) postemplacement eastward tilting of the pluton; (3) possible NNE^SSW faulting (concealed faults may contribute to the apparent displacement between the Migimata-dani and Taki-dani/Shiradashi-zawa

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sides of the Takidani Granodiorite) ; and (4) analytical uncertainty due to plagioclase zoning.

6. Fluid inclusion microthermometry A £uid inclusion study was undertaken to provide insights into £uid^rock interaction processes within the fossil Takidani magmatic^hydrothermal system. This study expands on the work of Sekine et al. (2001), who used £uid inclusion data to evaluate the type, distribution and formation history of microcracks in the Takidani Granodiorite, whilst Kano and Tsuchiya (2002) used that information to infer fracture generation processes in the intrusion. We use the £uid inclusion data to constrain temperature and pressure conditions during uplift of the Takidani Granodiorite, to deduce the potential of the granitoid to support an arti¢cial, deep-seated geothermal resource. 6.1. Methodology Doubly polished, 100-Wm-thick plates were pre-

227

pared for 30 Takidani Granodiorite samples, and analyzed using a Linkam heating^freezing stage, with Olympus 40U long-focus lens. Fluid inclusion homogenization and ice melting temperatures were determined to infer chemical (i.e. salinity) and physical conditions at the time of £uid entrapment. 6.2. Results Thermometric and salinity data for £uid inclusions from the Takidani Granodiorite are summarized in Table 5. The £uid inclusions are secondary, occur along healed microcracks in quartz and include: (1) liquid-rich, (2) vapor-rich, and (3) polyphase types. Measured homogenization (Th ) temperatures range from V200 to s 600‡C, and £uid salinity varies from 1.2 to 73 wt% NaCleq (Fig. 7). Daughter minerals in the polyphase inclusions comprise mainly halite, although some contain unknown (ilmenite ?) crystals. Most Takidani Granodiorite samples contain liquid-type £uid inclusions typical of mesothermal conditions (mostly 250^350‡C, 6 12 wt% NaCleq at 1^2 km depth), with entrapment of hydrother-

Fig. 7. Homogenization temperature^salinity variations for £uid inclusions from Takidani Granodiorite. Key: b 1401; R 2003; E 2004; +2505E; U 2507; O 2701 and a 2704. See Figs. 1 and 3 for sample locations. Microphotographs: (a) liquid-, (b) ‘boiled’ vapour- and (c) polyphase, ‘magmatic’-type £uid inclusions.

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mal £uids that had locally boiled (to produce vapor-type inclusions) and/or mixed with near-surface waters. At Chibi-dani, Shiradashi-zawa and Yanagi-dani, ‘sinuous-healed’ microcracks occur that contain high-Th ( s 600‡C), polyphase £uid inclusions. These hypersaline ( s 30^40 wt% NaCleq ), halite-bearing inclusions have a magmatic origin, containing £uid (brine and vapor) exsolved from the crystallizing intrusion at a time when the host rock was non-brittle (i.e. at temperatures above its brittle^plastic transition). 6.3. Interpretation Fluid inclusion evidence reveals the presence of a fossil, liquid-dominated, convective hydrothermal system associated with the Hotaka-Takidani volcano-plutonic system. Meteoric-derived £uids circulated in brittle Takidani Granodiorite at hydrostatic pressure, where the temperature was 6 380‡C, with localized boiling of the meteoricderived £uids producing moderately saline brines. A hydrostatic pressure regime is inferred for the low^moderate-Th /low-salinity and moderate^ high-salinity (boiled) £uid inclusions. Boiling curve calculations follow Haas (1971), with entrapment pressures (and hence depth) estimated for averaged Th values indicating that these mesothermal-type £uids were trapped at depths ranging from 500 to 2000 m below the paleo-ground surface. Magmatic £uid (brine and vapor) exsolved from the crystallizing Takidani magma was trapped in short-lived, ‘sinuous’ microcracks, when the Takidani Granodiorite was quasi-brittle and devoid of open fractures. We suggest a lithostatic pressure regime prevailed at this time, except (possibly) during periods of high strain (Fournier, 1999). The pressure of formation of the magmatic £uid inclusions is inferred, based on the assumption of immiscibility, by assuming they were trapped in the presence of a vapor, using the NaCl^H2 O system of Bodnar et al. (1985) as a model (which is valid, since the inclusions contain halite daughter minerals ; Bodnar and Viryk, 1994). The NaCl-saturated, magmatic (liquid-rich, polyphase) inclusions were found in fracture planes with rare vapor-rich inclusions (e.g. at Shiradashi-zawa),

which provide convincing evidence of their coexistence at the time of trapping. Entrapment conditions for the magmatic-type £uid inclusions were deduced from corresponding vapor pressure curves (to give an inferred pressure of formation) and homogenization temperatures (approximating the true trapping temperature), to infer the depth at which the £uid inclusion formed (cf. Hedenquist et al., 1998 ; after Bodnar et al., 1985). Using this approach, magmatic-derived £uid inclusions hosted by Takidani Granodiorite are inferred to have a minimum pressure of formation of V600^ 750 bar (and depth of s 2.4^3.0 km, assuming 2.66 g/cm3 crustal density).

7. Discussion Volcano-plutonic complexes in island-arc and continental margin settings are potential heat sources for geothermal resources. For example, Reyes (1990) showed that young intrusive complexes (e.g. late Miocene^late Pleistocene plutonic bodies and/or dikes) underlie many Philippine geothermal systems; a 1.0-Ma (Quaternary) felsite intrusive has been identi¢ed in deep parts of the Geysers geothermal system (USA; Hulen and Nelson, 1993); and diorite has been encountered at depth in the Ngatamariki geothermal ¢eld (New Zealand; Browne et al., 1992). In northeast Japan, several studies have been carried out to deduce the role of the Kakkonda granite as a heat source for the Kakkonda geothermal system (Ikeuchi et al., 1998; Kato et al., 1998; Muraoka et al., 1998), where exploitation/production drilling of geothermal boreholes intersected high-temperature ( s 500‡C), solidi¢ed or partly solidi¢ed plutonic bodies. Petrological studies of young plutonic rocks, and their associated volcanics, contribute to our understanding of their potential as heat sources for deep-seated geothermal resources. It may take several million years for a plutonic body to be exposed at the ground surface, and those that crop out rarely have ages younger than late Pliocene. To be exposed, a Quaternary granitoid must, by inference, have had an exceedingly rapid cooling history and unroo¢ng.

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7.1. Takidani Granodiorite K^Ar (hornblende) data indicates an age of 1.93 E 0.16 Ma for Takidani Granodiorite from the Taki-dani/Shiradashi-zawa area (Harayama, 1992, 1994), and 1.46 E 0.10 Ma for western parts of the intrusion. The K^Ar ages were determined using conventional mass spectrometric techniques for Ar and £ame photometry for K, as described by Harayama (1992). More recently, the K^Ar (hornblende) data were reassessed, in light of a 1.76 E 0.17 Ma age for the Nyukawa Pyroclastic Flow Deposit (Harayama, 1999), which comprises roof rocks for the Takidani Granodiorite. Harayama (1999) suggested that the maximum age of emplacement for the granitoid must be 1.76

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Ma, and that older K^Ar (hornblende) ages may be invalid. Subsequently, Harayama (2000) proposed a K^Ar (hornblende) age of 1.18 Ma for the western Takidani Granodiorite, with a 1.30^ 1.36 Ma age for the eastern side; however, his inference is inconsistent with the age of ¢nal emplacement and eruption of overlying welded tu¡ (V2.4 Ma; Harayama, 1992; after Saito et al., 1984; Yamada et al., 1985) of associated Hotaka Andesites. Uplift rates across the Takidani Granodiorite are 5.1^5.9 mm/yr (Fig. 6), based on K^Ar (hornblende) age determinations of Harayama (1992, 1994) and Al-in-hornblende geobarometry, which indicate hornblende crystallization occurred at 7.7^9.5 km depth in the eastern Takidani Grano-

Fig. 8. Uplift history and thermal evolution of the Takidani Granodiorite, since solidi¢cation (V1.9^2.2 Ma, before present). The time that a potential HDR energy extraction system might have been established in the Takidani Granodiorite, at 500^ 400‡C reservoir temperature, occurred from V1.54^1.57 Ma (5.2 E 0.9 km depth of intrusion) to 1.36^1.38 Ma before present (3.3 E 0.8 km depth). Analytical procedures are summarized by Harayama (1992). 87 Sr/86 Sr ratios and isotope dilution measurements were made using a VG Micromass 54E mass spectrometer at the Geological Survey of Japan. The whole rock biotite isochron age was calculated using the least squares method of York (1969). Fission track age of the zircon was calculated with a j value = 387.0; with a zircon ¢ssion track age of 0.80 E 0.02 Ma (Harayama, 1990).

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diorite, and V6.5^7.0 km depth in western areas. Biotite crystallization occurred at 1.21^1.24 Ma. (consistent with Rb^Sr biotite data), albeit at an unknown depth (Harayama, 1992, 1994). Based on the assumptions of Harayama (1999, 2000) the uplift of the intrusive would have been unrealistically rapid ( s 25 mm/yr, considering new geobarometry data, and likely depths for entrapment of the magmatic-derived £uid inclusions in non-brittle Takidani Granodiorite). Instead, uplift rates inferred for the Takidani Granodiorite are realistic (considering that the Hida Mountain Range has been uplifting about 3 mm/yr since the middle Pleistocene; Harayama et al., 1991) and similar to other plutonic bodies (e.g. Nangan Parbat Massif, Pakistan; Craw et al., 1994). Textural di¡erences provide evidence of variable cooling rates within the Takidani Granodiorite. A higher rate of cooling is inferred for ¢ne^ medium-grained (e.g. biotite-hornblende granite) lithofacies, than for medium-grained or porphyritic facies. Hornblende-plagioclase geothermometry indicates hornblende crystallization temperatures of 707^728‡C (average V718‡C) towards the core of the intrusion (e.g. at Shiradashizawa), with lithofacies in marginal areas (e.g. #2505E in the west, and #2005 near the eastern contact) having crystallization temperatures of V680‡C. The uplift history and thermal evolution of the Takidani Granodiorite (Fig. 8) combines zircon ¢ssion track/K^Ar ages (Harayama, 1990, 1994), thermoluminescence data (Tsuchiya and Fujino, 2000 ; with thermoluminescence closure at 0.07^ 0.18 Ma. matching inferred exposure at 0.1^0.65 Ma; Harayama, 1994), £uid inclusion data (after Sekine et al., 2001) and Al-in-hornblende geobarometry. Taking into account the results of our £uid inclusion study, and using all geochronological data of Harayama (1992, 1994), the eastern part of the Takidani Granodiorite was uplifted V5.5 km from 1.93 Ma, and V1.20 Ma, with the rate of uplift during this period of initial uplift and cooling being up to 7.5 mm/yr, with the rate of cooling being V550‡C/Myr (somewhat faster towards the margin of the granitoid), which is consistent with other late Pliocene plutons (Hess et al., 1993).

7.2. Fossil Takidani hydrothermal system There is presently no active geothermal system hosted by the Takidani Granodiorite, although hot springs do discharge from volcanic rocks at the western margin of the granitoid (e.g. Shin-Hotaka Hot Spring at Migimata-dani). Fluid inclusion evidence, however, points to a liquid-dominated, convective hydrothermal system in the past being hosted by fractured Takidani Granodiorite, with 6 380‡C, low-salinity reservoir conditions at hydrostatic (mesothermal) pressure. Microthermometric and salinity data for these hydrostatic £uid inclusions are useful for deducing the approximate pressure of £uid entrapment (using boiling point temperature versus pressure/depth pro¢les, for £uids of di¡ering wt% NaCleq ) in post-solidi¢cation stages of the Takidani Granodiorite, when the host rock (at depths of £uid trapping) behaved in a brittle manner. The £uid inclusions record thermal conditions in the hydrothermal system, but little information about the cooling and emplacement history of the intrusive, since the timing of £uid entrapment is unknown. Nevertheless, a high geothermal gradient is indicated, with 280 to V310‡C at V1 km depth, which is similar to subsurface conditions encountered in the Kakkonda geothermal system (V310‡C/km ; Doi et al., 1998). 7.3. A Takidani HDR hydrothermal system? Volcano-plutonic complexes have the potential to satisfy criteria necessary for development of long-lived, arti¢cial DSGR, but can such systems sustain high temperatures, at a level in the crust that can be readily accessed and utilized by present drilling technologies ? To establish an HDR energy extraction system at elevated temperature^pressure conditions, it is essential to maintain an interconnected network of fractures (Tsuchiya et al., 2001), with £uid^rock interactions su⁄ciently weak to preserve the heat exchange surface (Bignall et al., 2001). Some researchers suggest DSGR’s cannot exist above the plastic temperature of the reservoir rocks ( s 400 to V500‡C, depending on rock type), since connecting fractures are absent in

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the deformable rock and convective £uid transport is not possible. Although there is no evidence of veins containing a high-temperature ( s 400‡C) hydrothermal mineral assemblage, £uid inclusion evidence shows that fractures did form at a time when the Takidani Granodiorite was not completely solidi¢ed. We suggest that the s 600‡C, 30^40 wt% NaCleq £uid inclusions observed in several Takidani Granodiorite samples trapped exsolved magmatic £uids, from (‘episodic’ ?) injections of magmatic material into deep parts of the crystallizing granitoid (at s 2.4^3.0 km depth), into a quasi-brittle rock mass (i.e. at s 380^ 400‡C). A widespread natural fracture system is likely to have been initiated in Takidani Granodiorite from 1.08 to 1.36 Ma, when the granitoid had cooled to its brittle^ductile transition temperature. By this time, conditions would have been conducive for hydrothermal £uid circulation at hydrostatic pressures, with cooling preceded by convection and localized boiling, and entrapment of mesothermal-type ( 6 380‡C) liquid and vapordominated £uid inclusions. The ‘temperature^depth^time window’, when subsurface conditions were conducive for development of an HDR energy extraction system, would have been when the internal temperature of the Takidani Granodiorite was cooling from 500‡C to 400‡C (Fig. 8). At this stage, the granitoid was uplifted from 6.5^9.4 km (say, about 7.6 km) to 3.4^7.8 km (average V5.3 km), based on an average uplift rates, or closer to 6.0^4.5 km ( E 0.8 km) depths taking into account a faster (7.5 mm/yr) rate of uplift during initial stages. Assuming the present rate of erosion in the Takidani area of V2 mm/yr prevailed over the last million years, then about 0.7^0.8 km of overlying material would have been eroded in the 0.36^0.39 Myr that would have taken to cool to 500‡C, and 1.1^ 1.2 km of erosion by the time the pluton had cooled to 400‡C. For drilling purposes, the ‘500^ 400‡C temperature window’ occurred at 1.54^1.57 Ma to 1.36^1.38 Ma, which corresponds to 5.2 ( E 0.9) km and 3.3 ( E 0.8) km, which is well within the present capabilities of modern drilling technologies, and similar to measured temperature^ depth pro¢les within other drilled geothermal systems (e.g. at Kakkonda).

231

8. Conclusions To establish an HDR system at supercritical conditions it is essential to maintain an interconnected network of microfractures. Many workers consider DSGR’s cannot exist above the plastic temperature of the reservoir rock, because connecting fractures are unlikely to be present in the deformable rock and convective £uid transport would not be possible. Our study focused on the ascent and cooling history of the Takidani Granodiorite, since it is analogous to intrusivehosted fracture networks that might support ‘supercritical HDR energy utilization’. Fluid inclusion microthermometry provides evidence of a fossil, convective hydrothermal system in brittle Takidani Granodiorite, at mesothermal conditions (250^380‡C, 6 12 wt% NaCleq at 1^2 km depth). However, ‘sinuous-healed’ microcracks also occur, which contain high-Th ( s 600‡C), hypersaline ( s 30^40 wt% NaCleq ), halite-bearing £uid inclusions, that provide convincing evidence of £uid (brine and vapor) exsolved from the crystallizing magma moving though non-brittle Takidani Granodiorite. We identi¢ed a ‘time^temperature^depth window’, when the Takidani Granodiorite had the potential for development of an HDR energy extraction system, which is within present capabilities of modern drilling technologies, i.e. when internal temperatures were 500^400‡C (i.e. when the rock mass was beginning to behave in a quasi-brittle manner). Using K^Ar (hornblende/biotite) and Rb^Sr (biotite) age data, Al-in-hornblende geobarometry and £uid inclusion thermometry, we infer uplift rates for the Takidani Granodiorite to be 5.1^ 5.9 mm/yr (from the time of hornblende crystallization, V1.5^1.9 Ma), although initial uplift rates were V7.5 mm/yr; with V810‡C/Myr cooling in western, and V550‡C/Myr in eastern parts of the pluton. Our time^temperature^depth constraints show that the Takidani Granodiorite had potential to host an arti¢cial, supercritical DSGR at 5.2 E 0.9 km (V500‡C; V1.54^1.57 Ma) to 3.3 E 0.8 km depths (400‡C; V1.36^1.38 Ma). The emplacement and rapid uplift of the Takidani Granodiorite demonstrate how large, fractured

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magmatic bodies in the shallow crust provide potential heat sources and reservoir rocks in which to establish a deep-seated, arti¢cial geothermal energy extraction system.

Acknowledgements The authors acknowledge laboratory assistance provided by Dr. Y. Motoyoshi and Miss K. Seno (National Institute of Polar Research) and Mr. I. Hino (Tohoku University). We also thank Prof. K. Nakatsuka and Prof. T. Hashida (Tohoku University), and Dr. T. Sawada (Hotaka Sedimentation Observatory) for their support and ¢eld assistance, respectively. Financial support was provided by the Japan Society for the Promotion of Science (JSPS-RFTF 97 P00901). Comments from Prof. B. Marsh (Editor, JVGR) and two anonymous reviewers are greatly appreciated.

References Anderson, J.L., Smith, D.R., 1995. The e¡ects of temperature and fO2 on the Al-in-hornblende barometer. Am. Mineral. 80, 549^559. Ando, M., 1986. Mantle diapirs observed in the seismic window. Bull. Volcanol. Soc. Japan second series 31, 45^53 (in Japanese with English abstract). Aramaki, S., Hirayama, K., Nozawa, T., 1972. Chemical composition of Japanese granites, part 2. Variation trends and average composition of 1200 analyses. J. Geol. Soc. Japan 78, 39^49. Bando, M., Bignall, G., Sekine, K., Tsuchiya, N., submitted. Compositional variation, and lithofacies characterisation in the chemically zoned Takidani Granodiorite, Central Honshu, Japan. Island Arc. Bignall, G., Browne, P.R.L., 1994. Surface hydrothermal alteration and evolution of the Te Kopia Geothermal System, Taupo Volcanic Zone, New Zealand. Geothermics 23, 645^ 658. Bignall, G., Hirano, N., Yamasaki, N., Tsuchiya, N., Nakatsuka, K., 2001. Experimental approach to understanding water-rock interactions in enhanced geothermal systems. Proc. 23rd New Zealand Geothermal Workshop, Auckland, November 7^9, pp. 15^20. Blundy, J.D., Holland, T.J.B., 1990. Calcic amphibole equilibria and a new amphibole-plagioclase geothermometer. Contrib. Mineral. Petrol. 104, 208^224. Bodnar, R.J., Viryk, M.O., 1994. Interpretation of microthermometric data for H2 O-NaCl £uid inclusions. In: De Vivo,

B., Frezzotti, M.L. (Eds.), Fluid Inclusions in Minerals: Methods and Applications. Short course of the working group (IMA), pp. 117^130. Bodnar, R.J., Burnham, C.W., Sterner, S.M., 1985. Synthetic £uid inclusions in natural quartz. III. Determination of phase equilibrium properties in the system H2 O-NaCl to 1000‡C and 1500 bars. Geochim. Cosmochim. Acta 49, 1861^1873. Browne, P.R.L., Graham, I.J., Parker, R.J., Wood, C.P., 1992. Subsurface andesite lavas and plutonic rocks in the Rotokawa and Ngatamariki geothermal systems, Taupo Volcanic Zone, New Zealand. J. Volcanol. Geotherm. Res. 51, 199^ 215. Cathles, L.M., 1981. Fluid £ow and genesis of hydrothermal ore deposits. Econ. Geol. 75, 442^457. Cathles, L.M., Erendi, A.H.J., Barrie, T., 1997. How long can a hydrothermal system be sustained by a single intrusive event? Econ. Geol. 92, 766^771. Chappell, B.W., White, A.J.R., 1974. Two contrasting granite types. Paci¢c Geol. 89, 173^174. Chappell, B.W., White, A.J.R., Wyborn, D., 1987. The Importance of residual source material (restite) in granitic petrogenesis. J. Petrol. 28, 1111^1138. Cox, K.G., Bell, J.D., Pankhurst, R.J., 1979. The Interpretation of Igneous Rocks. George, Allen and Unwin, London, 450 pp. Craw, D., Koons, P.O., Winslow, D., Chamberlain, C.P., Zeitler, P., 1994. Boiling £uids in a region of rapid uplift, Nanga Parbat Massif, Pakistan. Earth Planet. Sci. Lett. 128, 169^ 182. Doi, N., Kato, O., Ikeuchi, K., Komatsu, R., Miyazaki, S., Akaku, K., Uchida, T., 1998. Genesis of the plutonic-hydrothermal system around Quaternary granite in the Kakkonda geothermal system, Japan. Geothermics 27, 663^690. Fournier, R.O., 1999. Hydrothermal processes related to movement of £uid from plastic into brittle rock in the magmatic-epithermal environment. Econ. Geol. 94, 1193^1212. Haas, J.L., Jr., 1971. The e¡ect of salinity on the maximum thermal gradient of a hydrothermal system at hydrostatic pressure. Econ. Geol. 66, 940^946. Hammarstrom, J.M., Zen, E-an, 1986. Aluminum in hornblende: An empirical igneous geobarometer. Am. Mineral. 71, 1297^1313. Harayama, S., 1990. Geology of the Kamikochi district. Geological Survey of Japan, Scale 1:50,000 (in Japanese with English abstract). Harayama, S., Takeuchi, M., Nakano, S., Sato, T., Takizawa, F., 1991. Geology of the Yarigatake district. Geological Survey of Japan Report (in Japanese with English abstract). Harayama, S., 1992. Youngest exposed granitoid pluton on Earth: cooling and rapid uplift of the Pliocene-Quaternary Takidani Granodiorite in the Japan Alps, central Japan. Geology 20, 657^660. Harayama, S., 1994. Cooling history of the youngest exposed pluton in the world - The Plio-Pleistocene Takidani granodiorite (Japan Alps, central Japan). Mem. Geol. Soc. Japan 43, 87^97 (in Japanese with English abstract).

VOLGEO 2531 8-1-03

M. Bando et al. / Journal of Volcanology and Geothermal Research 120 (2003) 215^234 Harayama, S., 1999. Multistage upheaval and tectonics in the Hida Mountain Range. Gekkan Chikyu/Monthly Earth 21, 603^607 (in Japanese) (translated into English by authors). Harayama, S., 2000. Thermal history of a pluton and its emplacement model, Gekkan Chikyu/Monthly Earth 30, 35^38 (in Japanese) (translated into English by authors). Hashida, T., Hayashi, K., Niitsuma, H., Matsuki, K., Tsuchiya, N., Nakatsuka, K., 2000. Investigation of heat extraction from supercritical geothermal reservoirs. In: Iglesias, E., Blackwell, D., Hunt, T., Lund, J., Tamanyu, S., Kimbara, K. (Eds.), Proc. World Geothermal Conference, WGC-2000 Kyushu-Tohoku, Japan, May 28^June 10, pp. 3725^3730, CD-ROM. Hashida, T., Bignall, G., Tsuchiya, N., Takahashi, T., Tanifuji, K., 2001. Fracture generation and water-rock interaction processes in supercritical deep-seated geothermal reservoirs. Geotherm. Res. Council Trans. 25, 225^229. Hedenquist, J.W., Arribas, A.Jr., Reynolds, T.J., 1998. Evolution of an intrusion-centered hydrothermal system: Far Southeast-Lepanto porphyry and epithermal Cu-Au deposits, Philippines. Econ. Geol. Bull. Soc. Econ. Geol. 93, 373^ 404. Hess, J.C., Lippolt, H.J., Gurbanov, A.G., Michalski, I., 1993. The cooling history of the late Pliocene Eldzhurtinskiy granite (Caucasus, Russia) and the Thermochronological potential of grain-size/age relationships. Earth and Planetary Science Letters 117, 393^406. Hirahara, K., Ikami, A., Ishida, M., Mikumo, T., 1989. Threedimensional P-wave velocity structure beneath Central Japan: low-velocity bodies in the wedge portion of the upper mantle above high velocity subducting plates. Tectonophysics 163, 63^73. Hollister, L.S., Grissom, G.C., Peters, E.K., Stowell, H.H., Sisson, V.B., 1987. Con¢rmation of the empirical correlation of Al in hornblende with pressure of calc-alkaline plutons. Am. Mineral. 72, 231^239. Hulen, J.B., Nelson, D.L., 1993. Interim report on geology of the Geysers felsite, Northwestern California. Geotherm. Res. Council Trans. 17, 249^258. Ikeuchi, K., Doi, N., Sakagawa, Y., Kamenosono, H., Uchida, T., 1998. High-temperature measurements in well WD-1a and the thermal structure of the Kakkonda geothermal system, Japan. Geothermics 27, 591^607. Johnson, M.C., Rutherford, M.J., 1989. Experimental calibration of the aluminum-in-hornblende geobarometer with application to Long Valley caldera (California) volcanic rocks. Geology 17, 837^841. Kano, S., Tsuchiya, N., 2002. Parallelepiped cooling joint and anisotropy of P-wave velocity in the Takidani granitoid, Japan Alps. J. Volcanol. Geotherm. Res. 2410, 1^13. Kato, O., Doi, N., Sakagawa, Y., Uchida, T., 1998. Fracture systematics in and around well WD-1, Kakkonda geothermal ¢eld, Japan. Geothermics 27, 609^629. Langmuir, C.H., 1989. Geochemical consequences of in situ crystallization. Nature 340, 199^205. Leake, B.E., 1978. Nomenclature of amphiboles. Mineral. Mag. 42, 533^563.

233

Muraoka, H., Uchida, T., Sasada, M., Yagi, M., Akaku, K., Sasaki, M., Yasukawa, K., Miyazaki, S., Doi, N., Saito, S., Sato, K., Tanaka, S., 1998. Deep geothermal resources survey program: igneous, metamorphic and hydrothermal processes in a well encountering 500‡C at 3729 m depth, Kakkonda, Japan. Geothermics 27, 507^534. Muraoka, H., Yasukawa, K., Kimbara, K., 2000. Current state of development of deep geothermal resources in the World and implications to the future. In: Iglesias, E., Blackwell, D., Hunt, T., Lund, J., Tamanyu, S., Kimbara, K. (Eds.), Proc. World Geothermal Conference, WGC-2000 Kyushu-Tohoku, Japan, May 28^June 10, pp. 1479^1484, CD-ROM. Nakano, S., 1993. Ueno basaltic rocks I: Heterogeneous magmas at two monogenetic volcanoes. J. Petrol. Mineral. Econ. Geol. 88, 272^288 (in Japanese with English abstract). Nakano, S., 1994. Ueno basaltic rocks II: Chemical variation in the Kiso province, to the south of the Ontake volcano. J. Petrol. Mineral. Econ. Geol. 89, 115^130 (in Japanese with English abstract). Nakatsuka, K., 1998. New project for design methodology of supercritical subsurface boiler for next generation geothermal energy extraction. Proc. 4th Int. HDR Forum, Strasbourg, France, September 28^30. Reyes, A.G., 1990. Petrology of Philippine geothermal systems and the application of alteration mineralogy to their assessment. J. Volcanol. Geotherm. Res. 43, 279^309. Saito, N., Shiono, T., Mitani, Y., 1984. On the Takara volcanic rocks and the Quaternary deposits to the East of Takayama City, central Japan. J. Geol. Soc. Japan 90, 371^382 (in Japanese with English abstract). Schmidt, M.W., 1992. Amphibole composition in tonalite as a function of pressure: an experimental calibration of the Al-in hornblende barometer. Contrib. Mineral. Petrol. 110, 304^310. Sekine, K., Bignall, G., Tsuchiya, N., Nakatsuka, K., 2001. Evidence for £uid £ow in non-brittle Takidani Granite implications for utilization of DSGR’s. Geotherm. Res. Council Trans. 25, 243^248. Tsuchiya, N., Fujino, K. (2000) Evaluation of cooling history of the Quaternary Takidani pluton using thermoluminescence technique. In: Iglesias, E., Blackwell, D., Hunt, T., Lund, J., Tamanyu, S., Kimbara, K. (Eds.), Proc. World Geothermal Conference, WGC-2000 KyushuTohoku, Japan, May 28^June 10, pp. 3939^3944, CDROM. Tsuchiya, N., Hirano, N., Bignall, G., Nakatsuka, K., 2001. Supercritical water-rock interaction for development of deep-seated HDR reservoirs. In: Cidu, R. (Ed.), Proc. 10th International Symposium on Water-Rock Interaction, WRI-10, Villasimius, Italy, 10^15 July 2000, pp. 209^ 212. Ujiie, O., Iizuka, Y., Nakano, S., 1992. K-Ar ages of the Ueno basaltic rocks. J. Petrol. Mineral. Econ. Geol. 87, 102^106 (in Japanese with English abstract). Wilson, M., 1989. Igneous Petrogenesis. Unwin Hyman, London.

VOLGEO 2531 8-1-03

234

M. Bando et al. / Journal of Volcanology and Geothermal Research 120 (2003) 215^234

Wones, D.R., 1981. Ma¢c silicates as indicators of intensive variables in granitic magmas. Mining Geol. 31, 191^212. Wones, D.R., Gilbert, M.C. (1982) Amphiboles in the igneous environment. In: Veblen, D.R., Ribbe, P.H. (Eds.), Amphiboles: Petrology and Experimental Phase Relations. Reviews in Mineralogy, pp. 355^390. Mineralogical Society of America, Washington, DC.

Yamada, N., Kato, H., Ono, K., Iwata, O., 1985. K-Ar ages of some Pliocene to Pleistocene felsic volcanic rocks around the Japanese Northern Alps, central Honshu. Geol. Surv. Japan Bull. 36, 539^549 (in Japanese with English abstract). York, D., 1969. Least squares ¢tting a straight line with correlated errors. Earth Planet. Sci. Lett. 5, 320^324.

VOLGEO 2531 8-1-03