Liquid CO 2 venting on the seafloor: Yonaguni Knoll IV hydrothermal system, Okinawa Trough

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L16607, doi: /2006gl026115, 2006 Liquid CO 2 venting on the seafloor: Yonaguni Knoll IV hydrothermal system, Okinawa Trough Uta Konno, 1 Urumu Tsunogai, 1 Fumiko Nakagawa, 1 Miwako Nakaseama, 2 Jun-ichiro Ishibashi, 2 Takuro Nunoura, 3 and Ko-ichi Nakamura 4 Received 24 February 2006; revised 3 July 2006; accepted 14 July 2006; published 19 August [1] We determined the chemical and isotopic compositions of the liquid CO 2 found on Yonaguni IV knoll hydrothermal site, as well as those in hydrothermal fluid venting from the surrounding chimneys. The d 13 C of both CO 2 and CH 4 in the liquid CO 2 almost coincide with those in the hydrothermal fluid, suggesting that the liquid CO 2 must be derived from the hydrothermal fluid. While showing homogeneous d 13 C, the hydrothermal fluids exhibit wide variation in gas contents. Active phase separation must be taking place within the conduits. Besides, H 2 -depletion in the liquid CO 2 suggests formation of solid CO 2 -hydrate must also precede the venting of liquid CO 2. In conclusion, liquid CO 2 must be produced through following subseafloor processes: phase separation of hydrothermal fluid due to boiling, formation of solid CO 2 -hydrate due to cooling of vapor phase, and melting of the solid CO 2 -hydrate to liquid CO 2 due to a temperature increase within the sedimentary layer. Citation: Konno, U., U. Tsunogai, F. Nakagawa, M. Nakaseama, J. Ishibashi, T. Nunoura, and K. Nakamura (2006), Liquid CO 2 venting on the seafloor: Yonaguni Knoll IV hydrothermal system, Okinawa Trough, Geophys. Res. Lett., 33, L16607, doi: /2006gl Introduction [2] In 2000, an active hydrothermal site, venting hightemperature fluid up to 300 C, was discovered by Shinkai 6500 on the top of Yonaguni Knoll IV during YK cruise in Okinawa Trough [Matsumoto et al., 2001] (Figure 1a). Major hydrothermal vents discovered at the site have been named Lion, Tiger, Swallow and Mosquito, respectively, from north to south (Figure 1b). During the subsequent subseafloor survey using Shinkai 6500 in 2003 (YK03-05), vents of liquid CO 2 droplets were found at the site. Similar liquid CO 2 droplets had previously been found at the active hydrothermal sites at JADE hydrothermal field, Okinawa Trough, during the extensive seafloor survey using submersibles in 1989 (Figure 1a) [Sakai et al., 1990]. Based on the chemical and isotopic compositions of both the liquid CO 2 samples taken using a plexiglass 1 Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan. 2 Department of Earth and Planetary Sciences, Faculty of Science, Kyusyu University, Hakozaki, Japan. 3 Subground Animalcule Retrieval Program, Extremobiosphere Research Center, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan. 4 National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan. Copyright 2006 by the American Geophysical Union /06/2006GL cylinder [Sakai et al., 1990] and the nearby hydrothermal fluid samples taken by a non-gas tight water sampler, Sakai et al. [1990] concluded that the liquid CO 2 had the same magmatic origin as the nearby hydrothermal fluid. That is to say, the liquid CO 2 may be produced either from hydrothermal solutions through subsurface boiling of hydrothermal fluid or from volcanic rocks that retain a substantial amount of volatiles in their vesicles. In addition, the presence of solid CO 2 -hydrate has also been assumed to exist within the subseafloor sedimentary layer [Sakai et al., 1990]. Because the samples were taken using a non-gas tight sampler and pressure change during sample recovery at sea level results in vigorous gas evolution from the sampler, the estimated chemical compositions were altered from their original values, so that it was difficult to discuss the production processes quantitatively. As a result, the relation between seafloor venting liquid CO 2 and the surrounding high-temperature hydrothermal fluid was not clarified in their studies. Furthermore, no definite evidence was obtained for the presence of CO 2 -hydrate in the subsurface. Recently, similar liquid CO 2 venting has also been recognized in NW Eifuku hydrothermal site on Izu- Bonin-Mariana arc [Lupton et al., 2006]. It thus appears that liquid CO 2 venting might be usual phenomenon in some submarine arc volcanoes. [3] In this study, based on precise chemical and isotopic compositions of both liquid CO 2 and high-temperature hydrothermal fluid, we will discuss the subseafloor processes responsible for producing liquid CO 2 at the Yonaguni Knoll IV site, as well as the possibility of the occurrence of solid CO 2 -hydrate within the sediments. To this end, we used the WHATS II gas-tight sampler [Tsunogai et al., 2003; Saegusa et al., 2006] together with the manned submersible Shinkai 6500, for sampling both seafloor liquid CO 2 and high temperature hydrothermal fluids. The sampler was designed to collect either high temperature fluid or gas samples venting on the deep sea floor. Details of the sampler have been presented elsewhere [Tsunogai et al., 2003; Saegusa et al., 2006]. 2. Sampling [4] The samples of both the venting liquid CO 2 and hydrothermal fluids were collected using a gas-tight fluid sampler (WHATS II) [Tsunogai et al., 2003; Saegusa et al., 2006]. By using the sampler, we can recover both hydrothermal fluid and liquid CO 2 samples on board without changing the pressure at which they exist on the seafloor (approximately 13 MPa in this study). Thus, we were able to obtain samples without altering gas compositions through degassing. L of5

2 Figure 1. (a) Location of Yonaguni Knoll IV hydrothermal site together with that of JADE field in the south-eastern part of Japan, and (b) locations of chimneys in the Yonaguni Knoll IV hydrothermal site, together with the sampling locations of liquid CO 2 droplets. [5] After onboard recovery of the samples, the collected fluids in gas-tight bottles (150 ml) were immediately opened to a vacuum line (ca. 1,500 ml) in the onboard laboratory to recover all gaseous components in the bottles. We added reagent-grade solid amidosulfuric acid (HOSO 2 NH 2 ) and mercury chloride (HgCl 2 ) to each fluid in the vacuum line so as to decrease the ph of the fluid to promote degassing of CO 2 and to promote precipitating of HgS. After the degassing, approximately 50 cm 3 of the gas phase and all of the filtered liquid phase were subsampled for chemical and isotopic measurements at the onshore laboratory. [6] The liquid CO 2 samples were opened to a vacuum small volume in a line (ca. 15 ml) via Swagelok fitting to measure inner pressure. Then, a portion of the gases in the small volume (ca.15 ml) were expanded to a 50 cm 3 gastight bottle to recover the subsample and to determine both chemical and isotopic compositions of sampled liquid CO 2 sample. 3. Analysis [7] The volatiles extracted from hydrothermal fluid samples and subsampled from liquid CO 2 samples on board were analyzed in a shore based laboratory after the cruise. The concentration and isotopic composition of CO 2 and CH 4 were determined using a CF-IRMS system previously described [Tsunogai et al., 2000]. Briefly, the procedures are as follows (see Ijiri et al. [2003], Tsunogai et al. [2000], Ishimura et al. [2004], and Nakagawa et al. [2004] for details). To determine both the concentration and isotopic composition of CO 2, an analytical system consisting sequentially of a liquid N 2 -ethanol temperature trap and a liquid N 2 temperature trap [Ijiri et al., 2003] was used. The Table 1. Chemical (in mmol kg 1 ) and Isotopic (in % VPDB) Compositions of Sampled Fluids, as Well as Those of Pure Hydrothermal Fluids Estimated for Each Sampled Fluid ([H 2 ] c, [CH 4 ] c, [CO 2 ] c, and [Cl ] c ; See Text for Detail) Location H 2 CH 4 d 13 C CH4 CO 2 d 1 C CO2 Cl Mg 2+ [H 2 ] c [CH 4 ] c [CO 2 ] c [Cl ] c Lion Lion Tiger a Tiger a Mosquito Mosquito Tiger a Swallow Swallow a Samples are taken from different vents on the same, very large Tiger chimney. 2of5

3 Figure 2. CO 2 content in each end-member hydrothermal fluid plotted as a function of Cl- content. The dotted line indicates the least square fit line to the whole data set. CO 2 in each sample was separated from H 2 O, N 2, and other noncondensable gases using this system. The eluted CO 2 was carried continuously into a Finnigan MAT 252 isotoperatio mass spectrometer for determining both quantity and carbon isotopic ratio. To determine both concentration and isotopic composition of CH 4, an analytical system consisting sequentially of a CO 2 -trapping port with Ascarite II, and a liquid O 2 temperature trap (Porapak-Q) [Tsunogai et al., 2000; Nakagawa et al., 2004] was used. With this system, the CH 4 in each sample is separated from most of the H 2 O, CO 2,N 2, and other noncondensable gas in the sample. Then they are concentrated at the head of a PoraPLOT-Q analytical capillary column at liquid O 2 temperature and by final separation under the column oven temperature. The eluted CH 4 is separated and quantitatively converted to CO 2 by passage through a 960 C combustion furnace (CuO/Pt catalyst), this is carried continuously into a Finnigan MAT 252 isotope-ratio mass spectrometer for determining both carbon content and carbon isotopic ratio. The concentration of H 2 was measured by the analytical system consisting sequentially of H 2 O and CO 2 traps (liquid O 2 ), and a Molecular Sieve 5A column gas chromatograph with a TRD (Trace Reduction gas Detector) [Seiler et al., 1980]. Firstly, H 2 is concentrated at the head of the Molecular Sieve column together with N 2,O 2 and CO at liquid N 2 temperature. Then they are separated by the Molecular Sieve column at room temperature. Finally they are introduced into the TRD to measure the concentration of H 2. The Mg 2+ and Cl concentrations in the hydrothermal fluid were measured with an ion chromatograph. 4. Results and Discussion 4.1. Hydrothermal Fluid [8] Chemical and isotopic compositions of sampled fluids are presented in Table 1. Recovered hydrothermal fluid samples typically become diluted with seawater to some extent during sampling on the seafloor. In order to clarify subseafloor processes using the geochemical characteristics of the fluid, we correct for the seawater contribution, using the Mg 2+ concentration in sampled fluids ([Mg 2+ ] = mmol/kg) and assuming [Mg 2+ ] = 0 in the pure hydrothermal fluids [Von Damm et al., 1985] ([H 2 ] c, [CH 4 ] c, and [CO 2 ] c in Table 1). Because the maximum observed temperature of the Yonaguni Knoll IV hydrothermal fluid exceeds 325 C, Mg 2+ must precipitate at deep subsurface. The observed minimum Mg 2+ concentration close to 0 ([Mg 2+ ] = 0.8 mmol/kg) also support this hypothesis. If the geochemical differences among the vents were slight except for the contribution of seawater during sampling, the corrected endmember composition ([H 2 ] c, [CH 4 ] c, and [CO 2 ] c ) of each hydrothermal fluid would be identical. While the samples taken at same vent with the exception of the Tiger chimney exhibited almost similar endmember concentration (815-1 and 815-2, and 821-1, and 821-3), there was wide variation among the vents ([CO 2 ] c : mmol/kg, [CH 4 ] c : mmol/kg, [H 2 ] c : mmol/kg), suggesting that some significant fractionation processes are occurring within the system, including the fluid conduits in the Tiger chimney Phase Separation [9] The correlation between the end-member concentrations of both CO 2 and Cl are shown in Figure 2. They exhibit reverse correlation in the plot. Phase separation beneath the hydrothermal site is one of the most probable processes producing Cl heterogeneity of fluids at such a site. In addition, the observed maximum temperature (325 C) corresponds almost exactly to the boiling temperature of seawater at this pressure (13MPa) [Bischoff and Rosenbauer, 1985, 1988], which also supports phase separation. [10] The unequivocal identification of phase separated effluents from a seafloor venting system was reported by Massoth et al. [1989]. They found a variety of Cl concentrations in hydrothermal fluid, ranging from 35% to 115% of that of seawater, within a 60-m diameter of the ASHES vent field in the Juan de Fuca Ridge. Butterfield et al. [1990] studied the fluid chemistry in detail and pointed out its unique characteristics, namely: (1) a wide range of Cl and covariation of most major elements concentrations, (2) an inverse correlation of CO 2 content and Cl. They demonstrated that the entire range of fluid compositions is best explained by subcritical phase separation during the rise of fluids through the crust, followed by partial segregation of the two phases. Subsequent geochemical studies of various seafloor hydrothermal systems verified that phase separation can be a ubiquitous process in seafloor hydrothermal fluids [Butterfield et al., 1990; Massoth et al., 1989; Charlou et al., 1996; Ishibashi et al., 1994]. [11] The vent field in Yonaguni Knoll IV shows a similar trend in the concentrations of CO 2 and Cl. Therefore, we conclude that active phase separation is going on within the seafloor conduit of this hydrothermal circulation system. Table 2. Determined Chemical and Isotopic Compositions of Seafloor Venting Liquid CO 2 Sampling Location H 2,% CH 4,% d 13 C, %VPDB CO 2,% d 13 C, %VPDB south of Tiger north east of Swallow < of5

4 Figure 3. The relation between H 2 /CO 2 ratio and CH 4 / CO 2 ratio for samples of liquid CO 2 together with those for each end-member Liquid CO 2 Bubbles [12] The stable carbon isotopic compositions (d 13 C) of CO 2 and CH 4 dissolved in liquid CO 2 samples exhibit the values 7.3 ± 0.4 (1s) % VPDB and 26.3 ± 1.0 (1s) % VPDB, respectively (Table 2), both of which are almost identical to those of hydrothermal fluid, 7.6 ± 0.4 % VPDB and 26.4 ± 1.6 % VPDB, respectively. Therefore, we conclude that the liquid CO 2 must be derived from the hydrothermal fluid. Abundant CO 2, however, must accumulate from the hydrothermal fluid below the seafloor to produce the liquid CO 2 droplets. In order to clarify the subseafloor processes producing the liquid CO 2 from hydrothermal activity, we compared its chemistry with that of the hydrothermal fluid. Figure 3 shows the relation between the CH 4 /CO 2 and H 2 /CO 2 ratio. [13] The liquid CO 2 samples are characterized by a higher CH 4 /CO 2 ratio than the hydrothermal fluid. The CH 4 /CO 2 ratio can be explained by fractionation during phase separation because the Henry s Law constant of CH 4 is substantially lower than that of CO 2 under hydrothermal conditions [Wilhelm et al., 1977], so that CH 4 tends to concentrate more in the vapor phase than in the brine phase relative to CO 2. The liquid CO 2 showing CH 4 enrichment must be derived from the vapor phase of the hydrothermal fluid. If liquid CO 2 samples were determined only through the Henry s Law constant, however, their H 2 /CO 2 ratios would, like those of CH 4, be higher than those in the hydrothermal fluid. The liquid CO 2 samples, however, tend to show lower H 2 /CO 2 ratios than those in the hydrothermal fluid (Figure 3). This discrepancy can be explained by an additional subseafloor fractionation process such as producing solid CO 2 -hydrate. The H 2 molecule is too small to be taken into the crystal of CO 2 -hydrate [Sloan, 1998]; H 2 molecules must be excluded from the CO 2 -hydrateand preferentially reside in the vapor phase. The H 2 -depletion in the liquid CO 2, therefore, suggests that formation of CO 2 - hydrate must be also going on prior to the venting of liquid CO 2 on the seafloor. [14] To produce CO 2 -hydrate at pressure of 13 MPa, however, both extraordinary CO 2 -enrichment relative to H 2 O (at least 1000 mmol/kg) and a temperature 3.5 C (the temperature of deep sea water around this site) are needed [Diamond and Akinfiev, 2003]. The observed maximum CO 2 concentration in hydrothermal fluid is 350 mmol/kg and the temperature of hydrothermal fluid is C. Hence, it is unlikely that CO 2 -hydrate is produced directly from hydrothermal fluid actually venting on the seafloor. The vapor phase segregated entirely from the high-temperature hydrothermal fluid, however, must be highly enriched in CO 2 relative to H 2 O, making it a probable source for the CO 2 -hydrate. The CO 2 concentration of the pure vapor phase hydrothermal fluid (Cl =0) can be roughly estimated to be 700±330 mmol/kg using the observed linear relation in Cl -CO 2 plot (Figure 2), which is almost sufficient to produce CO 2 -hydrate. If further Figure 4. Schematic cross section of hydrothermal systems at Yonaguni Knoll IV hydrothermal site. The dotted line represent the depth where boiling begins. 4of5

5 cooling below 3.5 C did occur to the pure vapor phase hydrothermal fluid, CO 2 -hydrate could be crystallized from the vapor phase [Takenouchi and Kennedy, 1965]. Because the vapor phase has three-times lower heat capacity than the liquid phase [Chase, 1998], this cooling process is a probable process in the sedimentary layer. [15] When solid CO 2 -hydrate is crystallized from the vapor phase, chemical components other than CO 2 must be fractionated between the hydrate crystal and the vapor phase. While CH 4 is easily taken into hydrate crystal, H 2 must remain in the vapor phase [Sloan, 1998] and mix again into the liquid hydrothermal fluid (Figure 4). Therefore, the residual hydrothermal fluid that had precipitated CO 2 -hydrate must exhibit both CO 2 and CH 4 -depletion and H 2 - enrichment. [16] The vent fluid at the Mosquito chimney exhibits H 2 - enriched chemical composition, which indicates fluid that produced CO 2 -hydrate. The Mosquito chimney must be the residual fluid that produced CO 2 -hydrate. That is to say, production of CO 2 -hydrate is still active within the sedimentary layer at the Yonaguni Knoll IV hydrothermal site. On the other hand, observations on the natural production of liquid CO 2 droplets on the seafloor have shown that the CO 2 -hydrate also decomposes simultaneously, probably because of change in hydrothermal activity and the elevation of the local temperature within the site. 5. Conclusion [17] The observed liquid CO 2 on the seafloor seems to be formed through the following subseafloor processes: 1) phase separation of high-temperature hydrothermal fluid while ascending through crust, 2) segregation of the vapor phase from the brine phase, 3) crystallization of CO 2 - hydrate from the vapor phase due to cooling below 3.5 C within a closed system, and 4) melting of CO 2 -hydrate to produce liquid CO 2. A schematic cross section of the hydrothermal system at Yonaguni Knoll IV is illustrated in Figure 4. As shown in the figure, accumulation of a vapor phase in pocket-like structures within fluid conduits is one of the probable processes leading to crystallization of CO 2 - hydrate within the sedimentary layer. [18] Acknowledgments. We would like to thank the officers, crew, and scientist group, as well as the Shinkai 6500 operating team of JAMSTEC aboard R/V Yokosuka during the YK04-05 cruises for their support during the field operations. We especially gratefully acknowledge the assistance of S. Okubo (Hokkaido Univ.), and S. Saegusa (Hokkaido Univ.) in collecting the samples and data. This research was supported by the following grants: 21st century Center of Excellence (COE) Program on Neo-Science of Natural History at Hokkaido University financed by MEXT, MEXT Special Coordination Fund Archaean Park project, NEDO proposal based research and development project (02A53002d) and JSPS Japan-Russia Research Cooperative Program. References Bischoff, J. L., and R. J. Rosenbauer (1985), An empirical equation of state for hydrothermal seawater (3.2 percent NaCl), Am. J. Sci., 285, Bischoff, J. L., and R. J. Rosenbauer (1988), Liquid-vapor relations in the critical region of the system NaCl-H 2 O from 380 to 415 C: A refined determination of the critical point and two-phase boundary of seawater, Geochim. Cosmochim. Acta, 52, Butterfield, D. A., G. J. Massoth, R. E. McDuff, J. E. Lupton, and M. D. 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Ishibashi and M. Nakaseama, Department of Earth and Planetary Sciences, Faculty of Science, Kyusyu University, Hakozaki, Fukuoka , Japan. U. Konno, F. Nakagawa, and U. Tsunogai, Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N10 W8, Sapporo , Japan. (utaro@ep.sci.hokudai.ac.jp) K. Nakamura, National Institute of Advanced Industrial Science and Technology, Central 7, Tsukuba, Ibaraki , Japan. T. Nunoura, Subground Animalcule Retrieval Program, Extremobiosphere Research Center, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, Japan. 5of5