Boron isotope compositions of fluids and plumes from the Kueishantao hydrothermal field off northeastern Taiwan: Implications for fluid origin and hydrothermal processes

Marine Chemistry 157 (2013) 59–66

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Boron isotope compositions of fluids and plumes from the Kueishantao hydrothermal field off northeastern Taiwan: Implications for fluid origin and hydrothermal processes Zhigang Zeng a,⁎, Xiaoyuan Wang a, Chen-Tung A. Chen b, Xuebo Yin a, Shuai Chen a, Yunqi Ma c, Yingkai Xiao c a b c

Seafloor Hydrothermal Activity Laboratory of the Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China Institute of Marine Geology and Chemistry, National Sun Yat-Sen University, Kaohsiung 804, Taiwan, ROC Qinghai Provincial Key Laboratory for Geology and Environment of Salt Lakes, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China

a r t i c l e

i n f o

Article history: Received 29 March 2013 Received in revised form 3 September 2013 Accepted 4 September 2013 Available online 9 September 2013 Keywords: Boron Hydrothermal fluid and plume Kueishantao hydrothermal field

a b s t r a c t Boron is a common element in vent fluids of seafloor hydrothermal fields, and it has been used to understand the hydrothermal flux and water–rock interaction in hydrothermal systems. We have measured the boron concentration and isotope composition of seawater, andesite, hydrothermal fluid and plume samples from the Kueishantao hydrothermal field. The δ11B value of ambient seawater near the field is 40.05 ± 0.01‰, and the boron concentration is 3.81 mg/L. Andesite rocks from the hydrothermal field have an average boron content of 15.3 ppm. The hydrothermal fluids from the yellow spring and white spring span a small range of δ11B values, from 33.27 ± 0.22 to 36.84 ± 0.11‰, and plumes from both springs also cover a small range, from 37.56 ± 0.01 to 40.37 ± 0.21‰. Hydrothermal fluids from both springs in the Kueishantao hydrothermal field have variable B enrichments relative to seawater between 7 and 21%. They have B concentrations (4.10–4.64 mg/L) that are slightly higher and δ11B values (33.27–36.84‰) that are lower than those of the hydrothermal plumes (3.94–4.17 mg/L, 37.56–40.37‰). Hydrothermal fluids and plumes display a very regular array of data points in a δ11B–B diagram, suggesting that the boron of hydrothermal fluids and plumes is mainly from seawater and that little of it is, from andesite. This implies that the interaction of subseafloor fluid and -andesite at the Kueishantao hydrothermal field is of short duration. In all the fluids, from springs to hydrothermal plumes, the pH values, B concentrations and B isotopic compositions show significant correlations with each other suggesting that the δ11B/B and pH/B ratios of hydrothermal plumes have stable values over the small distance form vent to plume (b15 m). Thus the B concentrations and B isotopic compositions of hydrothermal plumes can be used to describe the diffusive processes governing the chemical compositions of hydrothermal plumes in the seawater environment. The water/rock ratios, based on the B concentrations and δ11B values, are between 1.96 and 3.63. The hydrothermal flux of boron from the yellow spring into the oceans is between 1.17 × 105 mol/yr and 1.32 × 105 mol/yr, and from the white spring it is between 6.69 × 104 mol/yr and 7.17 × 104 mol/yr, assuming that only andesites are present in the reaction zone. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Hydrothermal circulation can cause extensive boron isotopic and chemical exchange depending on the fluid temperatures and the water/rock ratios. Boron is not believed to be solubility-controlled in hydrothermal fluids (Seyfried et al., 1984). Boron isotopes can provide a distinctive geochemical tracer. It is known that the boron isotopes in seafloor hydrothermal fluids have recorded the degree of water–rock interaction, and they are especially useful in the study of fluid origin and hydrothermal processes (Spivack and Edmond, 1987; ⁎ Corresponding author at: Seafloor Hydrothermal Activity Laboratory of the Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao 266071, China. Tel./fax: +86 532 82898525. E-mail address: [email protected] (Z. Zeng). 0304-4203/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.marchem.2013.09.001

Spivack et al., 1987, 1993; Morris et al., 1990; Palmer and Sturchio, 1990; You et al., 1993, 1996; Butterfield and Massoth, 1994). The mobile B is a good indicator of source- rock composition and subseafloor alteration processes in hydrothermal systems (Schmidt et al., 2011), boron in hydrothermal fluids is derived from different sources, depending on the hydrothermal circulation system and local host rocks (Kasemann et al., 2004), and the boron isotope ratio of hydrothermal fluids can identify different source rocks. Boron is present in the form of two dissolved species in seawater: the trigonal, boric acid, B(OH)3, and the tetrahedral, borate anion, B(OH)− 4 , the proportions of which depend on the pH of the solution (Rollion-Bard et al., 2011). Boron isotopes are fractionated between these two species, B(OH)3 being enriched in 11B by more than 25‰ (Pagani et al., 2005; Zeebe, 2005; Klochko et al., 2006) compared to B(OH)− 4 . There are relatively large fractionations during the interaction

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of dissolved boron with solid phases, mainly because of differential uptake of the two species (Spivack and Edmond, 1987). Boron is highly incompatible during magmatic processes and is strongly depleted in mantle rocks (Salters and Stracke, 2004) relative to N-type mid-ocean ridge basalts. During high-temperature fluid–rock interaction, boron is easily mobilized and its concentration in the fluid is thought to depend primarily on the initial boron concentration in the host rock and the effective water–rock ratio (Seyfried et al., 1984). The boron isotope ratios of hydrothermal fluids can reflect variations in the boron isotope composition of the source rocks, and they do not necessarily reflect secondary processes such as phase separation or precipitation of secondary minerals (Palmer and Sturchio, 1990; Kasemann et al., 2004). Experimental

studies and comparisons of water/rock ratios derived from boron concentrations and heat-flow rates have also shown that boron is readily leached from volcanic rocks with comparatively little partitioning of boron into secondary minerals (Ellis and Mahon, 1964, 1967). These studies are largely supported by boron isotope compositions of seafloor hydrothermal fluids (Palmer and Sturchio, 1990). During lowtemperature recrystallization of smectite (~60 °C), B may be adsorbed on the clay surfaces or may substitute for Si (Williams et al., 2001). The B enrichment of the oceanic crust is the result of secondary mineral formation, mainly smectites, chlorites and serpentines, during lowtemperature alteration (Spivack and Edmond, 1987). Both adsorption on mineral surfaces and substitution for Si in silicates could be involved as B enrichment mechanisms in altered oceanic rocks (Simon et al.,

Fig. 1. (A) The map showing Taiwan and Okinawa Trough (Bathymetric map and data from http://www.geomapapp.org/index.htm). (B) Bathymetric map showing the tectonic setting and location of Kueishantao islet (from Chen et al., 2005a). (C) The location of springs in the Kueishantao hydrothermal field. Yellow star indicates yellow spring (108 °C) and white star indicates white spring (51 °C).

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2006). At low temperatures (lower than 150 °C), the mineral–water partition coefficients for dissolved boron are larger than 1, and 10B is incorporated preferentially in hydrous minerals such as clays and chlorites (Palmer et al., 1987; You et al., 1995). Study of the boron isotope composition of vent fluids will provide useful tools for diagnosis of geochemical processes in subseafloor hydrothermal systems. The understanding and quantification of geochemical processes in seafloor hydrothermal systems are crucial for elucidating both modern and ancient boron budgets of the ocean. However, the influence of vent fluids of hydrothermal systems affected by seawater–andesite rock interaction on global lithosphere–hydrosphere exchange budgets is still poorly known. The boron isotope composition of deep-sea hydrothermal fluids from mid-ocean ridges and back-arc basins has been described by Spivack and Edmond (1987), Campbell et al. (1988), Berndt and Seyfried (1990), Fouquet et al. (1991), James et al. (1995), and Mottl et al. (2011). However, the boron isotope composition of shallowwater hydrothermal vents still remains poorly documented, and the few boron isotope data that have been obtained for hydrothermal fluids cannot provide a good estimate of the water–rock interaction. This lack of data could lead to a bias in the quantification of the global hydrothermal flux of boron into the oceans. The major aims of the present study were to (1) examine the origin and geochemical evolution of hydrothermal fluids and their plumes, (2) investigate the interaction between the host rock and fluids as recorded by hydrothermal plumes and vent fluids, and (3) assess the hydrothermal flux of boron into the oceans. 2. Geological setting The Kueishantao hydrothermal field (121°55′E, 24°50′N, about 0.5 km2) is a shallow-water field situated southeast of Kueishantao islet (Fig. 1). The area surrounding the field is characterized by an andesite seafloor with lava and pyroclastics. The last major eruption occurred about 7000 years ago (Chen et al., 2001) off northeastern Taiwan, near the southern Okinawa Trough. There are N30 hydrothermal vents at water depths of 10–30 m in the Kueishantao hydrothermal field. The vents can be divided into “yellow spring” and “white spring” types. The temperature of the yellow-spring fluids is between 78 and 116 °C, and the temperature of the white-spring fluids is between 30 and 65 °C (Chen et al., 2005a). The yellow-spring fluids have very low pH (as low as 1.52) and wide ranges of chemical compositions. The white-spring fluids are characterized by relatively low concentrations of methane, iron, and copper (Chen et al., 2005a). The temperature variation of the vent fluids is associated with tides, and the hydrothermal fluids reach their highest temperatures about 3.5 h after each high tide (Kuo, 2001; Chen et al., 2005a,b). The hydrothermal product at the Kueishantao hydrothermal field is mostly native sulfur in the form of chimneys, mounds, and balls. A large yellow chimney, about 6 m high at a water depth of 20 m, was discovered on 12 August 2000. Study of the geochemical characteristics of the native sulfur chimneys and balls suggests that the interaction between subseafloor fluid and -andesite at the Kueishantao hydrothermal field is of short duration and that the rare earth elements and trace elements in the native sulfur chimneys and balls are mostly from the Kueishantao andesite and partly from seawater (Zeng et al., 2007, 2011).

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The fluids were also collected with two-valve polyethylene tubes to assess the reliability of the data obtained with the Pyrex bottles. Hydrothermal plume samples were collected in 1 L Nalgene polypropylene bottles at about 2 m, 5 m, and 7 m below sea level at the yellow spring vent (at a water depth of 7.2 m) and about 5 m, 10 m, 13 m and 15 m below sea level at the white spring vent (at a water depth of 15.1 m). Ambient seawater was collected at a depth of 10 m near Kueiwei to exclude the hydrothermal influence. The temperatures and fluxes of the fluids were measured in situ. The methods of fluid collection and in-situ determination of temperature and flux are given in detail by Kuo (2001). 3.2. Analytical methods In the shore laboratory, the pH of each aqueous sample was determined with a portable pH meter (JENCO 6010, resolution 0.01, automatic temperature compensation). Before determination, the pH meter was calibrated with buffer solutions of pH 4.00 and 6.86 (i.e., potassium hydrogen phthalate (0.05 mol/L, 25 °C, pH = 4.00) and mixed phosphate (0.025 mol/L, 25 °C, pH = 6.86) buffers were used for calibration). All the aqueous samples were filtered into 1 L Nalgene polypropylene bottles (previously soaked in 1:1 HNO3 for 48 h, washed to neutral pH with distilled water, then dried) on shore. The concentrations of boron in aqueous samples were determined with a precision of ±1% by ICP-AES (Thermoelectricity Company) at the Experiment and Testing Center of Marine Geology, Qingdao Institute of Marine Geology, China Geological Survey. The contents of boron in andesite samples were determined by ICP-MS (ELAN DRC II, American PE Company) (relative standard deviation b 5%) at the Institute of Oceanology, Chinese Academy of Sciences. Boron isotope compositions of hydrothermal fluid, plume and seawater samples were measured according to Xiao et al. (1988). The boron isotope compositions were analyzed by Triton thermal ionization mass spectrometry at the Salt Lake Analytical and Test Department, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences. Boron isotope ratios were determined conventionally in relation to the NIST 951 boric acid reference material. The average 11B/10B ratio in NIST 951 was 4.05359 with a relative standard deviation of 0.006%. 4. Results We first report the boron (B) concentration and isotopic compositions of fluids from the yellow spring, the white spring, and the plumes, which are formed by emissions from both springs. At both spring sites, the yellow and white hydrothermal fluids issue directly from the andesite rock, there being a complete absence of sediment cover. The sample locations are shown in Fig. 1, and the δ11B values are listed in Table 2 along with information on temperature, pH, and B concentration. The B concentration in the yellow spring varied from 4.01 mg/L at 0 mbsl (meters below sea level) to 4.52 mg/L at 7.2 mbsl (Fig. 2). The maximum concentration of 4.64 mg/L, with δ11B of 36.22 ± 0.09‰, occurred where the in-situ temperature was 108 °C. The B concentration in the white spring varied from 3.97 mg/L at 0 mbsl to 4.59 mg/L at 15.1 mbsl (Fig. 2). The maximum concentration of 4.59 mg/L, with δ11B of 33.27 ± 0.22‰, occurred where the in-situ temperature was 51 °C.

3. Sampling and methods 3.1. Sample collection Hydrothermal fluids and plumes were sampled by divers from two locations in the Kueishantao hydrothermal field on 31 May, 2011 (Table 1, Fig. 1). Hydrothermal fluids were collected in 4 L Pyrex bottles at and within the vent, two bottles for each vent.

Table 1 Location, fluid temperature and fluid flux of yellow spring and white spring in the Kueishantao hydrothermal field. Spring type

Latitude

Longitude

Depth

Fluid temperature

Fluid flux (m3/h)

Yellow spring White spring

24.8349°N 24.83412°N

121.96194°E 121.96196°E

7.2 m 15.1 m

108 °C 51 °C

35.1 19.3

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Table 2 pH, boron concentrations and δ11B values of hydrothermal fluids and plumes in the Kueishantao hydrothermal field. Sample

Depth (m)

pH

B (mg/L)

δ11B (‰)

Error (±)

Ambient seawater Yellow spring plume, 0 m Yellow spring plume, −2 m Yellow spring plume, −5 m Yellow spring fluid, out, bottle Yellow spring fluid, in, bottle Yellow spring fluid, out, tube Yellow spring fluid, in, tube White spring plume, 0 m White spring plume, −2 m White spring plume, −5 m White spring plume, −10 m White spring fluid, out, bottle White spring fluid, in, bottle White spring fluid, out, tube White spring fluid, in, tube

10 0 2 5 7.2 N7.2 7.2 N7.2 0 2 5 10 15.1 15.1 15.1 15.1

8.02 6.15 6.12 5.60 2.81 2.29 – – 6.14 6.12 5.91 5.51 5.11 4.67 5.10 4.67

3.81 4.01 4.08 4.14 4.52 4.64 4.10 4.27 3.97 3.94 3.96 4.17 4.28 4.59 4.40 4.45

40.05 39.91 40.37 39.55 36.84 36.33 – – 39.97 40.05 39.83 37.56 36.37 33.27 36.52 36.73

0.01 0.11 0.21 0.02 0.11 0.09 – – 0.14 0.15 0.18 0.01 0.12 0.22 0.19 0.24

Sample

Range of B (ppm)

Average of B (ppm)

Kueishantao andesite

8.35–23.2

15.3 (n = 29)

“–” no detect.

The white spring hydrothermal plumes display a slightly larger range of boron concentration (3.94 mg/L to 4.17 mg/L) than the yellow spring hydrothermal plumes (4.01 mg/L to 4.14 mg/L) (Fig. 3), and the B concentrations of both hydrothermal plumes are slightly higher than that of ambient seawater away from the Kueishantao hydrothermal field (3.81 mg/L) (Fig. 2). The B concentration range (from 4.10 mg/L to 4.64 mg/L) of the yellow spring hydrothermal fluids covers the range (from 4.28 mg/L to 4.59 mg/L) of white spring hydrothermal fluids (Fig. 3), and the B concentrations of the hydrothermal fluids from both springs are significantly higher than that of ambient seawater. It is clear that the B concentrations of the hydrothermal fluids from both springs are significantly higher than those of the respective hydrothermal plumes (Fig. 3). These boron levels are very similar to those of the hydrothermal fluids in the North Cleft segment of the Juan de Fuca Ridge between 44°54′ and 45°00′N latitude (Butterfield and Massoth, 1994) and those of the Plume vent at South Cleft (Von Damm, 1990) (Fig. 3). The δ11B values in the yellow spring range from 36.22 ± 0.09‰ in fluid to 40.37 ± 0.21‰ in plume, and the values in the white spring range from 33.27 ± 0.22‰ in fluid to 40.05 ± 0.15‰ in plume (Table 2). The δ11B values of the two hydrothermal plumes are similar (Table 2), and they are slightly lower than the 40.05 ± 0.01‰ value of ambient seawater (Fig. 2). The δ11B values of the yellow spring hydrothermal fluids are in a narrow range,

Fig. 3. (A) Boron concentrations and (B) δ11B values of hydrothermal fluids and plumes from seafloor hydrothermal systems.Y—yellow spring; W—white spring; KST—Kueishantao; EPR—East Pacific Rise; MAR—Mid-Atlantic Ridge; JdFR—Juan de Fuca Ridge; MOR—Mid-Ocean Ridge; 1 line indicates increasing δ11B values of hydrothermal fluids with decreasing spreading rate. Hydrothermal fluid data are from Campbell et al. (1988), Berndt and Seyfried (1990), Fouquet et al. (1991), You et al. (1994), James et al. (1995), Seyfried et al. (2003), Schmidt et al. (2007), and Schmidt et al. (2011).

from 36.22 ± 0.09‰ to 36.84 ± 0.11‰, whereas those of the white spring hydrothermal fluids have a larger range, from 33.27 ± 0.22‰ to 36.73 ± 0.24‰ (Fig. 3). The δ11B values of the hydrothermal fluids of both springs are significantly lower than that of ambient seawater, and

Fig. 2. Variation of pH (A), boron concentration (B), and δ11B value (C) from hydrothermal fluid to plume in the white spring and the yellow spring.

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the δ11B values of the hydrothermal fluids of both springs are significantly lower than those of either of the hydrothermal plumes (Fig. 3). The boron concentrations and δ11B values of the hydrothermal fluids and plumes in the Kueishantao hydrothermal field have a smaller range than those (B concentration range is from 5.49 mg/L to 11.99 mg/L, δ11B range is from 17.4‰ to 30.2‰) in vent fluids from the Eastern Lau Spreading Center (Mottl et al., 2011). The pH of the hydrothermal fluids ranges from 2.29 to 5.11, and that of the plumes ranges from 5.51 to 6.15. The pH values of the hydrothermal plumes and fluids are significantly lower than the 8.02 value of ambient seawater, and from the hydrothermal fluids at the seafloor to the hydrothermal plumes at sea level, the pH value has a tendency to increase (Fig. 2). The boron concentrations of the hydrothermal plumes and fluids show a strong negative correlation with pH (R2 = 0.95–0.99, p b 0.01) and δ11B (R2 = 0.91–0.97, p b 0.01) (Fig. 4). From vent fluids to hydrothermal plumes, the pH values, B concentrations and B isotopic compositions are significantly correlated, and boron concentrations tend to decrease with increasing pH (Fig. 4). The correlations suggest that the δ11B/B or pH/B ratios of hydrothermal plumes have a stable value within a small range (the distance from vent to plume is less than 15 m) and that the B concentrations and B isotopic compositions of hydrothermal plumes can be used to describe the variation of chemical compositions of hydrothermal plumes in the seawater environment. In addition, the pH, boron concentrations, and δ11B values of the hydrothermal plumes in the Kueishantao hydrothermal field are similar from the surface to ~5 m depth (Fig. 2), largely because of mixing of the hydrothermal plumes with seawater (Fig. 4B, 4C).

5. Discussion 5.1. Sources of boron Since B in seawater is isotopically distinct from B in andesite, it is possible to distribute the B in hydrothermal fluids into two source components: seawater and andesite. In the Kueishantao hydrothermal field, the δ11B value of ambient seawater is 40.05 ± 0.01‰, with a boron concentration of 3.81 mg/L, and the andesite rocks have an average boron content of 15.3 ppm (Table 2). In the Kamchatka arc, the southern Washington Cascades, and the Hunga Ha'apai, the andesitic rocks have a very significantly large range of δ11B values, from −8.01 to 15.3‰, with boron concentrations of 3.8–36.3 ppm (Tonarini and Leeman, 1998; Ishikawa et al., 2001; Leeman et al., 2004).

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The δ11B values of hydrothermal plumes are similar to those of the ambient seawater in the Kueishantao hydrothermal field, which highlights the significance of seawater as the main source of boron. The B concentrations of the hydrothermal plumes and fluids show enrichments relative to seawater, and the δ11B values are lower. A simple two end-member mixing model (δ11Bmix = X × δ11Bseawater + (1 − X) × δ11Bandesite; where X is the amounts of seawater component; δ11Bmix, δ11Bseawater (40.05‰), and δ11Bandesite (from −8.01‰ to 15.3‰) are the boron isotopic compositions of hydrothermal plumes and fluids, seawater, and andesite, respectively.) shows that approximately 73–99% of the boron is from seawater. The B concentrations in hydrothermal fluids are significantly higher than they are in the hydrothermal plumes at sea level, where the temperature is low. However, the high B concentrations and low δ11B values cannot result from local seawater contributions alone. The significantly lower δ11B values of the hydrothermal fluids may reflect an additional, lighter, boron source from andesitic basement rocks. Seawater–andesite interaction would lead to fluids that have a higher exchangeable B concentration. The high boron concentrations (7.6–21.8% above seawater) of the hydrothermal fluids reflect, in part, constraints imposed by the composition of the andesite with which the source fluids have reacted. These B-enriched fluids with low δ11B in both springs can be explained best in terms of interaction between seawater and andesite. They illustrate the difference between Kueishantao and mid-ocean ridge hydrothermal systems, and the δ11B values of hydrothermal fluids in the Kueishantao hydrothermal field are significantly higher (by 24.5‰) than they are in hydrothermal fluids from the Valu Fa ridge in the Lau basin (Fig. 3), which also result from the interaction of andesitic basement rocks with seawater under greenschist- facies conditions (Fouquet et al., 1991; Mottl et al., 2011). Hydrothermal fluids and plumes display a very regular array of data points in a δ11B–B diagram (Fig. 5), suggesting that the boron of hydrothermal fluids and plumes comes mainly from seawater, rarely from andesite. The B concentrations and δ11B values of the hydrothermal fluids in both springs are significantly lower than they are in seafloor hydrothermal fluids from mid-ocean ridges (the Juan de Fuca Ridge, the Axial Seamount, the Endeavour Segment, the East Pacific Rise near 11°N, 13°N, and 21°N, the Galapagos Spreading Center, the TAG hydrothermal field, the Guaymas Basin, the Escanaba Trough) and higher than they are in back-arc basins (Lau Basin, Mariana Trough, Okinawa Trough) (Fig. 3), also implying that the interaction between subseafloor fluids and andesite at the Kueishantao hydrothermal field is of short duration.

Fig. 4. (A) pH vs boron concentration and (B) boron concentration vs δ11B value for fluids and plumes in the white spring and the yellow spring.

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Fig. 5. Boron concentration vs δ11B value for hydrothermal fluids and plumes. Seawater data from this work. Andesitic rock data from Ishikawa et al. (2001), Rose et al. (2001), and Leeman et al. (2004).

In a plot of δ11B versus Bsw / Bh, where Bsw is the boron concentration of seawater, and Bh is the boron concentration of hydrothermal fluids and plumes, the data form a linear array (Fig. 6). This indicates that the boron of hydrothermal fluids and plumes is the result of the mixing of two components of constant isotopic composition. Extrapolation to Bsw / Bh values of zero and unity gives the isotopic composition of the two end-member components. In the yellow spring, the intercepts are at δ11B values of 6.99‰ (andesite end member) and 40.31‰ (seawater end member), whereas in the white spring, the intercepts are at δ11B values of 18.90‰ and 40.08‰, respectively, these are similar to the values found for andesite rock and seawater (Spivack and Edmond, 1987; Tonarini and Leeman, 1998; Ishikawa et al., 2001; Leeman et al., 2004; Foster et al., 2010). Therefore, the boron of the hydrothermal fluids and plumes in the Kueishantao hydrothermal field can be explained as a mixture of seawater boron with andesite boron extracted from the rocks with no resolvable boron isotopic fractionation. 5.2. Seawater–rock interaction Boron behaves conservatively as evidenced by its behavior during basalt alteration experiments (Seyfried et al., 1984). The geochemistry

of boron in mid-ocean ridge hydrothermal systems has been extensively investigated and may be used to constrain subseafloor fluid–rock interaction processes (Spivack and Edmond, 1987). In the mid-ocean ridges, the high-temperature fluids have reacted with fresh basalt (Von Damm et al., 1985; Spivack and Edmond, 1987). Boron is thought to be quantitatively extracted from basalt during hydrothermal reactions without subsequent reequilibration with secondary mineral phases (Von Damm et al., 1985; Spivack and Edmond, 1987). The relative boron concentrations in the hydrothermal fluids (after correction for water loss from hydration of the crust and original seawater concentrations) should reflect those in the basalt from which they were leached (Campbell et al., 1988). Previous studies of subaerial geothermal fields indicate that thermal waters often have δ11B values similar to those of the adjacent rocks, suggesting leaching of boron from these local rocks or the local hydrologic reservoir (Musashi et al., 1988; Palmer and Sturchio, 1990). Even at high temperatures, isotope fractionation between vapor and fluid does not change the δ11B values of the fluid significantly (Kasemann et al., 2004). Studies of vapor/liquid systems have shown that the effect of this fractionation is less than 3% (Kanzaki et al., 1979; Nomura et al., 1982). Therefore, the boron isotope composition of hydrothermal fluids can identify different source rocks in a hydrothermal system. Boron is very sensitive to reactions with altered basalts because it is present in seawater in relatively high concentrations and is rapidly taken up during low-temperature (b150 °C) weathering of oceanic crust (Spivack and Edmond, 1987). Data from the EPR were modeled in terms of little or no removal of boron from seawater on the cold down-welling limb of the hydrothermal convection cell followed by quantitative extraction of boron from fresh mid-ocean ridge basalt during high-temperature water–rock reactions (Spivack and Edmond, 1987). Application of the same model to the Kueishantao hydrothermal fluids suggests that at least 7–26% of the boron is removed during reactions and requires that the fresh andesite undergoing interactions contains 3–36 ppm boron, which is consistent with the B contents (average 15.30 ppm, n = 29) of the andesite. In the Kueishantao hydrothermal field, boron is enriched relative to seawater concentration. This is in contrast to basalt-hosted hydrothermal systems where B loss from entraining seawater during the formation of low-temperature alteration minerals is followed by mobilization of B in the high-temperature reaction zone (Seyfried et al., 1984), commonly leading to slight B enrichments in high-temperature hydrothermal fluids. The Kueishantao hydrothermal system is generally characterized by B concentrations similar to those in systems from the fast-spreading East Pacific Rise. At both locations, water–rock interaction is weak resulting in lesser B removal during hydrothermal fluid circulation. A distinct enrichment of B in the hydrothermal fluids relative to seawater, however, is a characteristic of andesite-hosted hydrothermal systems, since trace B has been leached by seawater. Water/rock ratios for the fluid-rock reaction can be calculated as follows: 11

11

11

11

W=R ¼ ððδ Bh −δ Bandesite Þ  Bandesite Þ=ððδ Bsw −δ Bh Þ  Bsw Þ

Fig. 6. δ11B vs Bsw / Bh for hydrothermal fluids in the Kueishantao hydrothermal field. Seawater data from this work. Bsw and Bh are the boron concentrations of seawater and hydrothermal fluid respectively. Gray zone indicates the possible range of δ11B values of andesitic basement rocks in the Kueishantao hydrothermal field.

ð1Þ

where δ11Bh, δ11Bandesite, and δ11Bsw (40.05‰) are the values in hydrothermal fluid, andesite, and seawater, respectively, and Bandesite (15.3 ppm) and Bsw (3.81 mg/L) are the andesite and seawater boron concentrations, respectively. The water/rock ratios found for δ11Bandesite = 6.99‰ and 18.90‰ are listed in Table 3. They range from 3.58 to 3.63 and 1.96 to 2.18, respectively. These water/rock ratios are within the range (0.28–3) on the East Pacific Rise at 21° and 13°N reported by Spivack and Edmond (1987). The difference between these results and those from fluids from the slow-spreading midocean ridges can be ascribed to a shorter time of seawater–rock interaction and/or shorter fluid pathways. The results from the Kueishantao hydrothermal field are consistent with those from the fast-spreading mid-ocean ridges. The fluids at both have shorter reaction paths

Z. Zeng et al. / Marine Chemistry 157 (2013) 59–66 Table 3 Estimated water/rock ratios and boron fluxes in the Kueishantao hydrothermal field. Spring fluid

Yellow spring fluid, out, bottle Yellow spring fluid, in, bottle Yellow spring fluid, out, tube Yellow spring fluid, in, tube White spring fluid, out, bottle White spring fluid, in, bottle White spring fluid, out, tube White spring fluid, in, tube

Water/rock ratio

Boron flux (mol/yr)

δ11Bandesite = 6.99‰

δ11Bandesite = 18.90‰

3.63

2.18

1.29 × 105

3.62

2.15

1.32 × 105

–

–

1.17 × 105

– 3.62

– 2.15

1.22 × 105 6.69 × 104

3.58 3.63 3.63

1.96 2.16 2.17

7.17 × 104 6.87 × 104 6.96 × 104

“–” no data.

accompanied by weaker water–rock interaction, and the isotopic fractionation decreases with increasing water/rock ratio (Shanks et al., 1995; Bach and Humphris, 1999; Shanks, 2001). 5.3. Boron flux Hydrothermal circulation causes extensive chemical and isotopic exchange depending on the fluid temperature. For boron, the direction of reaction changes from uptake by the rock at low temperature to release at high temperature (Spivack and Edmond, 1987). The boron concentrations and δ11B values of the yellow spring (108 °C) are similar to those of the white spring (51 °C), suggesting that the magnitude of boron exchange between seawater and the andesite in the yellow spring is consistent with that in the white spring. In calculating the B flux, it was assumed that the flow rates of the yellow spring and white spring are stable and that only andesites are present in the reaction zone, and the measured boron concentrations of hydrothermal fluids were used. The flux of boron from the yellow-spring vent into the oceans is between 1.17 × 105 mol/yr and 1.32 × 105 mol/yr, and from the white-spring vent it is between 6.69 × 104 mol/yr and 7.17 × 104 mol/yr (Table 3). It is clear that the hydrothermal flux of boron from the yellow-spring vent is slightly larger than that from the white-spring vent. If there are more than 30 vents in the Kueishantao hydrothermal field (Chen et al., 2005a), the hydrothermal flux of boron is approximately 3 × 106 mol/yr, very much lower than the hydrothermal flux of 0.4–0.8 × 109 mol/yr at 21° and 13°N on the East Pacific Rise (Spivack and Edmond, 1987). The difference suggests that the subseafloor fluid–andesite interaction at the Kueishantao hydrothermal field has a short lifetime and/or that the reaction paths are short. In addition, the subseafloor fluid temperature is lower than it is on the East Pacific Rise. 6. Conclusions In the Kueishantao hydrothermal field, the boron concentrations and δ11B values of the hydrothermal fluids and plumes are less variable than those of vent fluids from the Eastern Lau Spreading Center. The δ11B values of fluids vary from 33.27 ± 0.22 to 36.84 ± 0.11‰, and the boron concentrations range from 4.10 to 4.64 mg/L. In the plumes, the δ11B values vary from 37.56 ± 0.01 to 40.37 ± 0.21‰, and the boron concentrations range from 3.94 to 4.17 mg/L. From hydrothermal fluids at the seafloor to hydrothermal plumes at sea level, the boron concentration tends to decrease, whereas δ11B tends to increase. The B concentrations of the hydrothermal fluids and plumes of both springs are slightly elevated over the ambient seawater concentration of 3.81 mg/L, and the δ11B values of the hydrothermal fluids of both springs are significantly lower than that of ambient seawater. The B concentration and isotopic variation in the hydrothermal plumes and fluids from the Kueishantao

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hydrothermal field can be explained by mixing of the hydrothermal plumes and fluids with seawater in the water column. Boron in fluids is derived from different sources, depending on the subseafloor hydrothermal circulation system and the local country rocks. One significant boron source is the ambient seawater. Another is the Kueishantao andesitic basement rocks. The B concentration and isotopic composition of seawater-derived hydrothermal fluid are affected slightly by weak seawater–rock interaction in subseafloor reaction zones. The interaction is characterized by leaching of B, without fractionation, from andesite. The hydrothermal flux of boron from the Kueishantao hydrothermal field is about 3 × 106 mol/yr. The low flux compared with the fluxes in other hydrothermal field may be due to lower temperatures of formation, higher seawater/andesite ratios (1.96–3.63), shorter lifetime of seawater–andesite interaction and/or shorter fluid paths. Acknowledgments We thank Bing-Jye Wang and Sea-watch Company for sampling the hydrothermal fluids and plumes. We are most grateful for the detailed and constructive comments and suggestions provided by two anonymous reviewers and journal editorial assistant Dr. Marcie Henderson, which significantly improved the content of this paper. This work was supported by National Key Basic Research Program of China (Grant no. 2013CB429700), National Natural Science Foundation of China (Grant no. 40376020, 40830849, 40976027, 40906029), and Shandong Province Natural Science Foundation of China for Distinguished Young Scholars (Grant no. JQ200913). References Bach, W., Humphris, S.E., 1999. Relationship between the Sr and O isotope compositions of hydrothermal fluids and the spreading and magma-supply rates at oceanic spreading centers. Geology 27, 1067–1070. Berndt, M., Seyfried Jr., W.E., 1990. Boron, bromine, and other trace elements as clues to the fate of chlorine in mid-ocean ridge vent fluids. Geochim. Cosmochim. Acta 54, 2235–2245. Butterfield, D.A., Massoth, G.J., 1994. Geochemistry of north cleft segment vent fluids: temporal changes in chlorinity and their possible relation to recent volcanism. J. Geophys. Res. 99 (B3), 4951–4968. Campbell, A.C., Palmer, M.R., Klinkhammer, G.P., Bowers, T.S., Edmond, J.M., Lawrence, J.R., Casey, J.F., Thompson, G., Humphris, S., Rona, P., Karson, J.A., 1988. Chemistry of hot springs on the Mid-Atlantic Ridge. Nature 335, 514–519. Chen, Y.-G., Wu, W.-S., Chen, C.-H., Liu, T.-K., 2001. A date for volcanic eruption inferred from a siltstone xenolith. Quat. Sci. Rev. 20, 869–873. Chen, C.T.A., Zeng, Z.G., Kuo, F.W., Yang, T.F., Wang, B.J., Tu, Y.Y., 2005a. Tide-influenced acidic hydrothermal system offshore NE Taiwan. Chem. Geol. 224, 69–81. Chen, C.T.A., Wang, B.J., Huang, J.F., Lou, J.Y., Kuo, F.W., Tu, Y.Y., Tsai, H.S., 2005b. Investigation into extremely acidic hydrothermal fluids off Kueishantao islet, Taiwan. Acta Oceanol. Sin. 24, 125–133. Ellis, A.J., Mahon, W.A.J., 1964. Natural hydrothermal systems and experimental hot water/rock interactions. Geochim. Cosmochim. Acta 28, 1323–1357. Ellis, A.J., Mahon, W.A.J., 1967. Natural hydrothermal systems and experimental hot water/rock interactions (part II). Geochim. Cosmochim. Acta 31, 519. Foster, G.L., Pogge von Strandmann, P.A.E., Rae, J.W.B., 2010. Boron and magnesium isotopic composition of seawater. Geochem. Geophys. Geosyst. 11 (8), Q08015. http://dx.doi.org/10.1029/2010GC003201. Fouquet, Y., Von Stackelberg, U., Charlou, J.L., Donval, J.P., Erzinger, J., Foucher, J.P., Herzig, P., Mühe, R., Soakai, S., Wiedicke, M., Whitechurch, H., 1991. Hydrothermal activity and metallogenesis in the Lau back-arc basin. Nature 349, 778–781. Ishikawa, T., Tera, F., Nakazawa, T., 2001. Boron isotope and trace element systematics of the three volcanic zones in the Kamchatka arc. Geochim. Cosmochim. Acta 65 (24), 4523–4537. James, R.H., Elderfield, H., Palmer, M.R., 1995. The chemistry of hydrothermal fluids from the Broken Spur site, 29°N Mid-Atlantic Ridge. Geochim. Cosmochim. Acta 59 (4), 651–659. Kanzaki, T., Yoshida, M., Nomura, M., Kakihana, H., Ozawa, T., 1979. Boron isotopic composition of fumarolic condensates and sassolites from Satsuma Iwo-Jima, Japan. Geochim. Cosmochim. Acta 43, 1859-1859. Kasemann, S.A., Meixner, A., Erzinger, J., Viramonte, J.G., Alonso, R.N., Franz, G., 2004. Boron isotope composition of geothermal fluids and borate minerals from salar deposits (central Andes/NW Argentina). J. S. Am. Earth Sci. 16, 685–697. Klochko, K., Kaufman, A.J., Yao, W., Byrne, R.H., Tossell, J.A., 2006. Experimental measurements of boron isotope fractionation in seawater. Earth Planet. Sci. Lett. 248, 261–270. Kuo, F.W., 2001. Preliminary Investigation of Shallow Hydrothermal Vents on Kueishantao Islet of Northeastern Taiwan. (MS Thesis) Institute of Marine Geology and Chemistry, National Sun Yat-Sen University (81 pp. (in Chinese)).

66

Z. Zeng et al. / Marine Chemistry 157 (2013) 59–66

Leeman, W.P., Tonarini, S., Chan, L.H., Borg, L.E., 2004. Boron and lithium isotopic variations in a hot subduction zone—the southern Washington Cascades. Chem. Geol. 212, 101–124. Morris, J.D., Leeman, W.P., Tera, F., 1990. The subducted component in island arc lavas: constraints from Be isotopes and B–Be systematics. Nature 344, 31–36. Mottl, M.J., Seewald, J.S., Wheat, C.G., Tivey, M.K., Michael, P.J., Proskurowski, G., McCollom, T.M., Reeves, E., Sharkey, J., You, C.-F., Chan, L.-H., Pichler, T., 2011. Chemistry of hot springs along the Eastern Lau Spreading Center. Geochim. Cosmochim. Acta 75, 1013–1038. Musashi, M., Nomura, M., Okamoto, M., Ossaka, T., Oi, T., Kakihana, H., 1988. Regional variation in the boron isotopic composition of hot spring waters from central Japan. Geochem. J. 22, 205–214. Nomura, M., Kanzaki, T., Ozawa, T., Okamoto, M., Kakihana, H., 1982. Boron isotopic composition of fumarolic condensates from some volcanoes in Japanese island arcs. Geochim. Cosmochim. Acta 46, 2403–2406. Pagani, M., Lemarchand, D., Spivack, A., Gaillardet, J.A., 2005. Critical evaluation of the boron isotope pH-proxy: the accuracy of ancient ocean pH estimates. Geochim. Cosmochim. Acta 69, 953–961. Palmer, M.R., Sturchio, N.C., 1990. The boron isotope systematics of the Yellowstone National Park (Wyoming) hydrothermal system: a reconnaissance. Geochim. Cosmochim. Acta 54, 2811–2815. Palmer, M.R., Spivack, A.J., Edmond, J.M., 1987. Temperature and pH controls over isotopic fractionation during adsorption of boron on marine clay. Geochim. Cosmochim. Acta 51, 2319–2323. Rollion-Bard, C., Blamart, D., Trebosc, J., Tricot, G., Mussi, A., Cuif, J.-P., 2011. Boron isotopes as pH proxy: a new look at boron speciation in deep-sea corals using 11B MAS NMR and EELS. Geochim. Cosmochim. Acta 75, 1003–1012. Rose, E.F., Shimizu, N., Layne, G.D., Grove, T.L., 2001. Melt production beneath Mt. Shasta from boron data in primitive melt inclusions. Science 293, 281–283. Salters, V.J.M., Stracke, A., 2004. Composition of the depleted mantle. Geochem. Geophy. Geosy. 5, Q05004. Schmidt, K., Koschinsky, A., Garbe-Sch nberg, D., de Carvalho, L.M., Seifert, R., 2007. Geochemistry of hydrothermal fluids from the ultramafic-hosted Logatchev hydrothermal field, 15°N on the Mid-Atlantic Ridge: temporal and spatial investigation. Chem. Geol. 242, 1–21. Schmidt, K., Garbe-Sch nberg, D., Koschinsky, A., Strauss, H., Jost, C.L., Klevenz, V., K niger, P., 2011. Fluid elemental and stable isotope composition of the Nibelungen hydrothermal field (8°18 S, Mid-Atlantic Ridge): constraints on fluid-rock interaction in heterogeneous lithosphere. Chem. Geol. 280, 1–18. Seyfried Jr., W.E., Janecky, D.R., Mottl, M.J., 1984. Alteration of the oceanic crust: implications for the geochemical cycles of lithium and boron. Geochim. Cosmochim. Acta 48, 557–569. Seyfried Jr., W.E., Seewald, J.S., Berndt, M.E., Ding, K., Foustoukos, D.I., 2003. Chemistry of hydrothermal vent fluids from the Main Endeavour Field, northern Juan de Fuca Ridge: geochemical controls in the aftermath of June 199 seismic events. J. Geophys. Res. 18 (B9), 2429. http://dx.doi.org/10.1029/2002JB001957. Shanks, W.C., 2001. Stable isotopes in seafloor hydrothermal systems: vent fluids, hydrothermal deposits, hydrothermal alteration, and microbial processes. Stable Isotope Geochemistry 43, 469–525.

Shanks, W.C.I., Bohlke, J.K., Seal II, R.R., 1995. Stable isotopes in mid-ocean ridge hydrothermal systems: interaction between fluids, minerals and organisms. In: Humphris, S.E., Zierenberg, R.A., Mullineaux, L.S., Thomson, R.E. (Eds.), Seafloor Hydrothermal Systems: Physical, Chemical, Biological and Geological Interactions. American Geophysical Union, Washington D.C., pp. 194–221. Simon, L., Lécuyer, C., Maréchal, C., Coltice, N., 2006. Modelling the geochemical cycles of boron: implications for the long-term δ11B evolution of seawater and ocean crust. Chem. Geol. 225, 61–76. Spivack, A.J., Edmond, J.M., 1987. Boron isotope exchange between seawater and the oceanic crust. Geochim. Cosmochim. Acta 51, 1033–1043. Spivack, A.J., Palmer, M.R., Edmond, J.M., 1987. The sedimentary cycle of the boron isotopes. Geochim. Cosmochim. Acta 51, 1939–1949. Spivack, A.J., You, C.-F., Smith, H.J., 1993. The boron isotopic composition of seawater and foraminifera over the past 20 million years: implications for surface water pH. Nature 363, 149–151. Tonarini, S., Leeman, W.P., 1998. Boron isotopic systematics in primitive volcanic arcs. Goldschmidt Conference Toulouse, pp. 1515–1526. Von Damm, K.L., 1990. Seafloor hydrothermal activity: black smoker chemistry and chimneys. Annu. Rev. Earth Planet. Sci. 18, 173–204. Von Damm, K.L., Edmond, J.M., Grant, B., Measures, C.I., Walden, B., Weiss, R.F., 1985. Chemistry of submarine hydrothermal solutions at 21°N, East Pacific Rise. Geochim. Cosmochim. Acta 49, 2197–2220. Williams, L.B., Hervig, R.L., Holloway, J.R., Hutcheon, I., 2001. Boron isotope geochemistry during diagenesis: part I. Experimental determination of fractionation during illitization of smectite. Geochim. Cosmochim. Acta 65, 1769–1782. Xiao, Y.-K., Beary, E.S., Fassett, J.D., 1988. An improved method for the high-precision isotopic measurement of boron by thermal ionization mass spectrometry. Int. J. Mass Spectrom. Ion Proc. 85, 203–213. You, C.F., Spivack, A.J., Smith, J.H., Gieskes, J.M., 1993. Mobilization of boron in convergent margins: implications for boron geochemical cycle. Geology 21, 207–210. You, C.-F., Butterfield, D.A., Spivack, A.J., Gieskes, J.M., Gamo, T., Campbell, A.J., 1994. Boron and halid systematics in submarine hydrothermal systems: effects of phase separation and sedimentary contributions. Earth Planet. Sci. Lett. 123, 227–238. You, C.-F., Spivack, A.J., Gieskes, J.M., Rosenbauer, R., Bischoff, J.L., 1995. Experimental study of boron geochemistry: implications for fluid processes in subduction zones. Geochim. Cosmochim. Acta 59, 2435–2442. You, C.-F., Spivack, A.J., Gieskes, J.M., Martin, J.B., Davisson, M.L., 1996. Boron contents and isotopic compositions in pore waters: a new approach to determine temperature induced artifacts-geochemical implications. Mar. Geol. 129, 351–361. Zeebe, R., 2005. Stable boron isotope fractionation between dissolved B(OH)3 and B(OH)− 4 . Geochim. Cosmochim. Acta 69, 2753–2766. Zeng, Z.G., Liu, C.H., Chen, C.-T.A., Yin, X.B., Chen, D.G., Wang, X.Y., Wang, X.M., Zhang, G.L., 2007. Origin of a native sulfur chimney in the Kueishantao hydrothermal field, offshore northeast Taiwan. Sci. China D Earth Sci. 50, 1746–1753. Zeng, Z.G., Chen, C.-T.A., Yin, X.B., Zhang, X.Y., Wang, X.Y., Zhang, G.L., Wang, X.M., Chen, D.G., 2011. Origin of native sulfur ball from the Kueishantao hydrothermal field offshore northeast Taiwan: evidence from trace and rare earth element composition. J. Asian Earth Sci. 40, 661–671.