- Open Access
Distribution of CO2 fluids in the Shimanto belt on Muroto Peninsula, SW Japan: possible injection of magmatic CO2 into the accretionary prism
© Okamoto et al.; licensee Springer. 2014
- Received: 18 November 2013
- Accepted: 14 April 2014
- Published: 15 May 2014
Carbon dioxide and methane are major components in geofluids; however, there is little evidence showing how C-H-O fluids evolve in a subduction zone. We investigated fluid inclusions in quartz veins from the Eocene-Oligocene Shimanto belt (Murotohanto subbelt) on Muroto Peninsula, SW Japan using microthermometry and laser Raman spectroscopy. Quartz veins that cut the cleavage of the host rocks in the Murotohanto subbelt contain one-phase carbonic inclusions (CH4) and two-phase aqueous inclusions (CH4 ± CO2 vapor and H2O liquid). The vapor in the two-phase inclusions is essentially CH4 in the northern part of the belt and a CO2-CH4 mixture in the southern part; values of (=CO2 / (CO2 + CH4)) vary from 0 to 0.9. Within a single CO2-bearing vein, values decrease from the vein wall ( = 0.5 to 0.9) to the vein center ( = 0), and the homogenization temperature increases from approximately 180°C to 240°C-250°C, indicating a transition of the carbonic species from CO2-CH4 to CH4 during vein formation. CO2-dominant fluids are rare in most accretionary prisms formed under low-grade metamorphic conditions, and the generation of CO2 cannot be explained by diagenesis of organic matter in sediments under the P-T conditions of formation of the CO2-bearing veins (235°C to 245°C, 165 to 200 MPa). The CO2 fluids are distributed preferentially near an out-of-sequence thrust that brings the Murotohanto subbelt into contact with the late Oligocene-early Miocene Nabae subbelt and its many volcanic and intrusive rocks. We therefore suggest that the CO2 fluids were generated in association with near-trench magmatism during the middle Miocene and that the fluids were injected and mixed with the CH4 pore fluids of the sediments in the accretionary prism.
- C-H-O fluid
- Fluid inclusion
- Shimanto belt
- Magmatic CO2
- Quartz vein
Fluids in the crust consist mainly of water, carbon dioxide, and methane, as well as electrolytes, and they may be carried down into the Earth's interior at subduction zones. The dominant volatiles in the pore fluids of sediments evolve during plate subduction, and based on thermodynamic calculations of C-H-O fluids coexisting with graphite at geologically common oxidation conditions, the dominant species changes from methane to water to carbon dioxide with increasing temperature (e.g., Holloway 1984). In accretionary prisms, the volatile species are products of microbial activity, hydrothermal water-rock interactions, or thermal decomposition of organic matter under low-grade metamorphic conditions (e.g., Tissot and Welte 1984).
The evolution of fluids during low-grade metamorphism has been studied extensively by analyzing fluid inclusions in the fissure quartz of the Central Alps, Switzerland (e.g., Mullis et al. 1994; Tarantola et al. 2007, 2009), and four fluid zones that correlate with increasing metamorphic conditions have been identified. These zones are a hydrocarbon zone formed at temperatures lower than 200°C, a methane zone formed at temperatures ranging from 200°C to 270°C, a water zone formed at temperatures in excess of 270°C, and a carbon dioxide zone at even higher temperatures. However, there is no direct evidence for a transition from CH4 to CO2 during prograde metamorphism.
Many studies have examined fluid inclusions in various ancient accretionary prisms formed under low-grade metamorphic conditions (e.g., Fisher and Brantley 1992; Moore and Vrolijk 1992; Sakaguchi 1996; Hashimoto et al. 2002, 2012; Kondo et al. 2005; Raimbourg et al. 2014), and they showed that methane is the only carbonic species in the fluids. Given this general lack of variety in the carbonic species found, it is not surprising that there is still little information on the variation in the chemistry of C-H-O fluids along subduction zones.
The Shimanto belt is one of the best-studied ancient accretionary complexes in the world (Taira et al. 1988). The metamorphic temperatures of the Shimanto belt ranged from 100°C to 300°C (e.g., Underwood et al. 1993; Sakaguchi 1996; Hashimoto et al. 2002, 2012; Kondo et al. 2005; Raimbourg et al. 2014), which overlaps the temperature range proposed for the seismogenic zone along subduction zones (100°C to 350°C; Hyndman and Wang 1993). Except at one locality on Muroto Peninsula, SW Japan, the carbonic component of the fluids in the Shimanto belt is always CH4 ( e.g., Hashimoto et al. 2002, 2012; Kondo et al. 2005; Raimbourg et al. 2014). At the locality on Muroto Peninsula, quartz veins contain a mixture of CO2 and CH4 vapor in two-phase aqueous inclusions (Lewis et al. 2000; Lewis and Byrne 2003). However, the origin and distribution of CO2 fluids in the Shimanto belt remain unclear.
In the present study, we investigated the C-H-O compositions and homogenization temperatures of fluid inclusions in the mineral veins of the Eocene-Oligocene Shimanto belt on Muroto Peninsula. We revealed the spatial distribution of CO2 fluids on both the regional and single-vein scale, and we discuss the possibility of CO2 injection related to middle Miocene magmatism.
Geological background and occurrences of veins
Ditullio and Byrne (1990) described the polyphase deformation of the Paleogene-Neogene Shimanto belt in the following order: (1) synsedimentary deformation, (2) penetrative cleavage formation due to solution mass transfer, and (3) syn- to post-cleavage shearing and deformation. Vitrinite reflectance and illite crystallinity show that the peak temperature of the Murotohanto subbelt increased from north (approximately 215°C to 230°C) to south (270°C to 290°C) (DiTullio et al. 1993; Underwood et al. 1993). Across the Shiina-Narashi fault to the south, the peak temperature dropped from 270°C-290°C to 135°C-205°C. The Nabae subbelt is made up of several mélanges with pillow lavas, volcanic breccias, and intrusive gabbros and dolerites. Based on structural, chronological, and geochemical studies (e.g., Hibbard and Karig 1990; Mizoguchi et al. 2009), these igneous rocks are interpreted to have formed in association with near-trench magmatism during the middle Miocene (approximately 15 Ma). Offshore, the active equivalent of the Shimanto belt is referred to as the Nankai wedge (Figure 1a).
We also analyzed a quartz vein hosted by pillow lava at locality E4 (Figures 1b and 2b). This sample was taken from a large floating block, interpreted to have been derived from Shiina volcanic rocks (e.g., Hibbard and Karig 1990; Mizoguchi et al. 2009). The Shiina volcanic rocks have been interpreted as intrusions into the Hioki mélange in the Miocene Nabae subbelt (Mizoguchi et al. 2009).
Occurrences and compositions of fluid inclusions
We prepared doubly polished thin sections of the veins, cut perpendicular to the vein walls. The compositions of the fluid inclusions were measured by laser Raman microprobe spectroscopy (Kaiser Optical system, HoloLab 5000, Ann Arbor, MI, USA). The size of the laser spot on the sample surface was about 2.0 μm. The veins are composed of quartz, and some also contain minor calcite. They include many primary, pseudo-secondary, and secondary inclusions, and most of these inclusions are smaller than 10 μm. For this study, we focused on the primary and pseudo-secondary inclusions. Representative Raman spectra of the fluid inclusions are shown in Additional file 1: Figure S1.
Except for sample E4 (hosted by pillow lava), the quartz veins contain two types of inclusions: one-phase carbonic inclusions and two-phase aqueous inclusions (Figure 2c). The one-phase carbonic inclusions have a smoky appearance (Figure 2c) and show a Raman band for CH4 at 2,918 cm-1. The two-phase aqueous inclusions are composed of liquid and vapor (Figure 2c), and they show a broad band around 3,200 to 3,600 cm-1, indicating liquid H2O. Vapors in the two-phase inclusions show either a sharp band for CH4 or bands for both CH4 (at 2,918 cm-1) and CO2 (at 1,287 and 1,389 cm-1). The secondary inclusions contain only CH4 vapor; CO2 is not present.
Quartz veins in sample E4 (Shiina volcanics) also contain two-phase aqueous inclusions, but they do not contain one-phase inclusions. The vapor in the two-phase inclusions is entirely CO2.
Variations of and Th within the Murotohanto subbelt
The molar proportions of CH4 and CO2, = CO2 / (CO2 + CH4), in the vapor of the two-phase inclusions were determined from the intensity (peak area) ratios of the CO2 and CH4 Raman spectra following the method of Burruss (2003). The error of the estimates is within approximately 0.1. The intensity data of the CO2 and CH4 bands and calculated are listed in Additional file 1: Table S1.
The homogenization temperatures (Tc) of the one-phase carbonic inclusions (CH4) were measured by cooling experiments with samples E1 and W4 (Figure 4f). Methane bubbles were observed below –90°C, and solid carbon dioxide was not observed. Tc was determined as the temperature at which the methane bubbles disappeared during re-heating of the inclusions.
Variations of and Th within a single vein
Across the vein, the values of in the two-phase aqueous inclusions (line A to B in Figure 5a) show symmetrical patterns with respect to the central plane of the vein (Figure 5c). Near the vein wall (less than approximately 1.0 mm from the vein wall), where growth zoning of quartz is clear (Figure 5b), the vapor in the fluid inclusions is dominated by CO2 with values ranging from 0.5 to 0.9 (Figure 5c). The value decreases toward the center of the vein, and the vapor is entirely CH4 in the central part of the vein (3 to 6 mm from the vein walls) (Figure 5c). The one-phase carbonic inclusions, which are made up of CH4 regardless of position within the vein, are distributed uniformly across the vein (Figure 5c).
The homogenization temperature of the two-phase inclusions, Th, changes across the vein: the Th value is 180°C to 200°C near the vein walls but increases to 240°C to 250°C at the vein center (Figure 5c). In summary, decreased and homogenization temperature, Th, increased during the growth of the vein.
P-T conditions of vein formation
The pressure and temperature conditions of formation of the post-cleavage veins were estimated on the basis of fluid inclusion analyses (Figures 3, 4 and 5). When one-phase carbonic inclusions of CH4 coexist with the two-phase aqueous inclusions with pure CH4 vapor ( = 0), the homogenization temperature (Th) of the two-phase aqueous inclusions directly indicates the fluid trapping temperature (Vrolijk et al. 1988), because the value of Th is consistent with the temperature of methane saturation in water. Given the trapping temperatures of the two-phase aqueous inclusions, pressures were determined by calculating isochores for the one-phase carbonic inclusions from cooling experiments (Vrolijk et al. 1988). In the cases where > 0, the Th value indicates the minimum possible trapping temperature. Although great care is needed to identify cogenetic trapping of fluid inclusions (Raimbourg et al. 2014), we believe that the aqueous and carbonic fluid inclusions coexisted with each other in the analyzed veins because (1) both inclusions always exist in all the analyzed veins (except for sample E4), (2) they have close spatial associations in single crystals (Figure 2c), and (3) the aqueous inclusions contain CH4 ± CO2 vapor.
It is important to consider the origin of the CO2 fluids within the Murotohanto subbelt because CH4 is the dominant carbonic component of fluids under low-grade metamorphic conditions in other parts of the Shimanto belt (e.g., Sakaguchi 1996; Hashimoto et al.2002, 2012; Kondo et al. 2005), in ancient accretionary prism complexes (e.g., Fisher and Brantley 1992; Moore and Vrolijk 1992), and in active accretionary prisms (e.g., Tsunogai et al. 1998; Mau et al. 2006).
(Tarantola et al. 2007, 2009) showed that the thermal conversion of CH4 into CO2 and H2O occurred at 270°C ± 5°C in Alpine fissure quartz. Lewis and Byrne (2003) suggested that the CO2 fluids in the Murotohanto subbelt were produced by thermal conversion of CH4, as in Alpine fissure quartz, based on the facts that temperature increases from north to south in the Murotohanto subbelt (Underwood et al. 1993) and that pure CH4 occurs in the north, but CH4-CO2 mixtures occur in the south. However, we reject this scenario because (1) the fluid in the post-cleavage veins of the Murotohanto subbelt was initially dominated by CO2 and then changed to CH4 (Figure 5), (2) the temperature of the vein formation (200°C to 250°C; Figure 6a) is lower than the temperature of the thermal conversion proposed by Tarantola et al. (2007, 2009) (270°C ± 5°C), and (3) only CH4 fluids were found in even the higher-grade parts of the Shimanto belt (>300°C at the Nobeoka thrust; Kondo et al. 2005). Figure 6b shows the thermodynamic calculations obtained by using the Perple_X computer program package (Connolly 1995) for the mole fractions of C-H-O species coexisting with graphite under the quartz-fayalite-magnetite (QFM) buffer along the geothermal gradient of vein formation (35°C/km, lithostatic). This diagram shows that the dominant species changes with increasing temperature from CH4 (T < 170°C) to H2O (170°C < T < 450°C) to CO2 (T > 450°C) and that is very low (<0.1) at the temperatures of vein formation (200°C to 250°C; Figure 6a) under both lithostatic and hydrostatic conditions. Therefore, diagenesis of organic matter in the sediments is unlikely to explain the origin of the CO2.
The veins and host rocks in the Shimanto belt commonly contain calcite (e.g., Okamoto and Sekine 2011). Decarbonation reactions in carbonate-bearing metasediments could occur at temperatures less than approximately 350°C (e.g., Massone, 2010). Therefore, it is possible that CO2 fluids were generated by decarbonation reactions in the deeper part of the subduction zone and then transported to shallower levels. Although this scenario cannot be ruled out, it is difficult to explain by decarbonation reactions why CO2-bearing fluids are distributed only in the south of the Murotohanto subbelt. In addition, the unmixing of CH4-CO2-H2O fluids from deeper regions cannot explain the variation of in the analyzed vein (Figure 5) because separation of CH4-rich fluid by unmixing would cause an increase of in the aqueous fluids, which contradicts the decrease during the growth of the analyzed vein (Figure 5c).
An alternative source of CO2 fluids is magmatism in the vicinity of the accretionary prism. Discharge of CO2 from hydrothermal vents associated with submarine volcanoes has been reported in various places, including the Okinawa trough (Konno et al. 2006), the Izu-Bonin-Mariana and Tonga-Kermadec arcs (Lupton et al. 2008), and the Mid-Atlantic Ridge in Iceland (Botz et al. 1999). Discharges of CO2 can take the form of either liquid droplets or gas bubbles, depending on the pressure-temperature conditions of the site on the seafloor, and the vent fluids commonly have values exceeding 0.99. It is well known that basic and acidic igneous bodies are distributed widely within the Shimanto belt, and these igneous rocks have been dated as middle Miocene (approximately 15 Ma; e.g., Underwood et al. 1993). On Muroto Peninsula, such igneous rocks (pillow lava, volcanic breccia, dolerite, and gabbro) are not found in the Eocene-Oligocene Murotohanto subbelt but occur only within the late Oligocene to early Miocene Nabae subbelt. Based on plate reconstructions (Underwood et al. 1993), it is considered that such magmatic activities were related to the subduction of a young oceanic plate (Shikoku basin back-arc spreading ridge) in the middle Miocene.
The veins analyzed in this study cut the cleavage of the host rocks, which suggests that they formed later than the peak temperature conditions; however, the absolute timing of cleavage and vein formation within the Murotohanto subbelt remains unknown. Nevertheless, the rocks in the Murotohanto subbelt underwent a progressive deformation during and after the development of the out-of-sequence thrust (i.e., the Shinya-Narashi fault) that bounds the Murotohanto and Nabae subbelts (Ditullio et al. 1993; Underwood et al. 1993). In addition, pure CO2 vapor is found in the inclusions only in the Shiina volcanic rocks, close to the Shinya-Narashi fault (Figure 3).
These facts support the hypothesis of CO2 injection during the middle Miocene according to the following scenario. In the Murotohanto subbelt, few veins formed during the Eocene and Oligocene, because coherent units are widely distributed. In this area, the formation of post-cleavage veins was probably triggered by the near-trench magmatism during the middle Miocene. At that time, magmatic CO2 fluids diffused along the out-of-sequence thrust and propagating fractures and mixed with CH4 pore fluids to form CO2-CH4 fluids with high values of (Figures 3 and 5). The presence of CO2 fluids at one locality in the north (W1) suggests an inhomogeneous nature of the fluid flow within the Murotohanto subbelt (Figure 3). Because the input of external CO2 was a short-term event, the dominant carbonic component in the fluid changed progressively from CO2 to CH4, as was recorded within the single vein (Figure 5).
It remains to be determined whether the thermal structure of the Murotohanto subbelt is that of an Eocene-Oligocene accretionary prism or whether it is the result of overprinting in the middle Miocene (Underwood et al. 1993). In our study, we were able to show that the CO2 fluids are distributed in the region up to 10 to 15 km from the Shinya-Narashi fault (W3, W4, W5, and E3; Figure 3). However, the trapping temperature of the fluid inclusions in the southern vein (235°C to 245°C for locality W4; Figure 6a) is lower than the maximum vitrinite reflectance temperature at the same locality (270°C to 290°C, Figure 6a; Underwood et al. 1993), implying negligible temperature overprinting due to fluid flows related to Miocene magmatism. Nevertheless, such an injection of magmatic fluids into an accretionary prism could occur during the subduction of a young plate, which may produce unique geochemical environments along the subduction zone.
We thank A. Yamaguchi for providing information in the field about Muroto Peninsula and basaltic rocks. We also thank N. Hirano and N. Watanabe for the discussions. This study was supported by a Grant-in-Aid for Scientific Research on Innovative Areas (nos. 22109501, 24109701) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan and by a Grant-in-Aid for Challenging Exploratory Research from the Japan Society for the Promotion of Science (no. 23654180) awarded to A. Okamoto.
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