Geochemical Journal, Vol. 39, pp. 469 to 480, 2005
*Corresponding author (e-mail: tyyang@ntu.edu.tw ) Copyright © 2005 by The Geochemical Society of Japan.
Gas compositions and helium isotopic ratios of fluid samples around Kueishantao,
NE offshore Taiwan and its tectonic implications
TSANYAO FRANK YANG,1* TEFANG FAITH LAN,1 HSIAO-FEN LEE,1 CHING-CHOU FU,1 PEI-CHUAN CHUANG,1 CHING-HUA LO,1 CHENG-HONG CHEN,1,2 CHEN-TUNG ARTHUR CHEN3 and CHAO-SHING LEE4
1Department of Geosciences, National Taiwan University, P.O. Box 13-318, Taipei 106, Taiwan 2National Applied Research Laboratories, Taipei 106, Taiwan
3Institute of Marine Geology and Chemistry, National Sun Yat-Sen University, Kaohsiung 804, Taiwan 4Institute of Applied Geophysics, National Taiwan Ocean University, Keelung 202, Taiwan
(Received March 20, 2005; Accepted July 20, 2005)
Kueishantao is a Holocene volcanic islet (<7,000 yrs) located at NE offshore Taiwan, and tectonically is part of west-ern extension of the Okinawa Trough. Magmatic activities are considered very active around this area on the basis of the fact that on-land fumaroles and submarine hydrothermal systems are prevailing currently. Representative bubble gas sam-ples from submarine hydrothermal vents were collected for gas composition and helium isotope analysis. The gases show similar compositions of low temperatre fumaroles in the world, i.e., with high CO2 and H2S but low SO2 and HCl contents. They exhibit consistent high 3He/4He ratios (7.3–8.4 R
A, where RA is the 3He/4He ratio of air), probably the highest 3He/
4He values of gases ever reported in active hydrothermal areas of the western Pacific region. Meanwhile, seawater
sam-ples around Kueishantao and other fluid samsam-ples from I-Lan Plain, the land area closest to the Kueishantao and also the southernmost part of the Okinawa Trough, show a significant excess of 3He compositions as well. This indicates that the
mantle component plays an important role for their gas sources, and implies that the mantle fluids may have invaded into I-Lan Plain. The westward opening of the Okinawa Trough may have caused thinning of the continental crust and pro-duced deep normal faults and hence, the primordial 3He is able to degas from mantle source region without significant
crust contamination.
Keywords: 3He/4He ratios, Okinawa Trough, Kueishantao, hot springs, Taiwan
canic activities are considered to be non-dormant by Song et al. (2000).
Another area in northern Taiwan that subjected to the continuous subsidence is the I-Lan (IL) Plain (Fig. 1C). GPS data demonstrated that the west-bounding extension rate of this plain is 126 mm/y (Liu, 1995). A major thrust fault, the Li-Shan (LS) Fault, divides the Central Range and IL Plain in NE Taiwan and runs through the KST to the Okinawa Trouth (Sibuet et al., 1998). Tsai et al. (1975) reported a subsurface vertical fault less than 20 km deep, which runs from the IL Plain to KST, extending further to the Okinawa Trough in a NE direction. Recently, more than 70 active submarine volcanoes have been identified in the SPOT area (Lee et al., 1998). Those volcanoes, including KST, are located ~100 km above the Wadati-Benioff zone (Kao et al., 1998). In addition to the differ-ences of structural and sedimentary events (Hsu et al., 1996; Park et al., 1998) recorded in SPOT and middle-northern part of the Trough, Chung et al. (2000) suggested that the SPOT is not a simple backarc basin but instead an embryonic rift zone in which early arc volcanism oc-curs as a result of the Ryukyu subduction. The estimated lithospheric structure based on calculation of isostatic INTRODUCTION
Kueishantao (KST) is a young volcanic island located at the conjunction of the extension of the major fault sys-tem of northeastern Taiwan and the southernmost part of the Okinawa Trough (SPOT). Tectonically, this trough extends from SW Kyushu to NE Taiwan (Figs. 1A and B) and has been considered as an intracontinental backarc basin of the Ryukyu arc-trench system owing to subduc-tion of the Philippine Sea plate underneath the Eurasian plate (Lee et al., 1980; Letouzey and Kimura, 1986; Sibuet et al., 1998). Due to the southpropagating of west-ern edge of subducting Philippine Sea plate, the collision between Luzon arc and Eurasian plate was moving west-ward and induced spreading of the Okinawa Trough and consequently, caused the mountain collapse in northern Taiwan (Teng, 1996). Wang et al. (1999, 2004) proposed that the post-collisional extension in northern Taiwan could have triggered the Plio-Pleistocene volcanism of the Tatun Volcano Group (TVG in Fig. 1C), and its
vol-equilibrium in the region also supported the conclusion (Hsu, 2001).
KST is a volcanic island mainly composed of andesitic lava flows and pyroclastic deposits (Hsu, 1963; Chen, 1990). A siltstone xenolith sample has been dated to be ~7,000 yrs by thermoluminescence dating technique (Chen et al., 2001). Based on the Sr-Nd-O isotopic data, Chen et al. (1995) suggested that more than 30% of crustal sediments get involved in the KST magma genesis. Chung et al. (2000) compared the geochemical characteristics of KST lavas with those from Okinawa basin and Ryukyu arc and found that they are similar to those of pre-backarc rifting volcanic rocks from the central Ryukyu arc, and different from those of backarc basin lavas from the mid-dle Okinawa Trough and the post-backarc rifting Ryukyu
arc volcanics. Hence, they concluded that they are the products of arc magmatism instead of rifting products. Recently, Chu (2005) reported some high-Mg andesites in the KST and concluded that they can be result from partial melting of subducting sediments and subsequently melt-mantle interaction.
The helium isotopic ratios of terrestrial materials can be identified as deriving from four components. They are: (1) Air: its 3He/4He ratio is very homogeneous (1.39 × 10–6) and is commonly used as global standard (1 R
A);
(2) Crust: the helium isotopic ratios are much lower than air (0.1~0.01 RA) because abundant 4He gases are
pro-duced by the radiogenic elements in the crust; (3) Upper mantle: as represented by the mid-ocean ridge basalts (MORB), which show narrow range of helium isotopic Fig. 1. Location of the studied area. (A) Simplified tectonic map around Taiwan. (B) Bathymetric map of Okinawa Trough. The triangular symbol indicates the location of Kueishantao islet offshore NE Taiwan. The marked rectangle is enlarged shown (C), which shows several identified submarine active venting sites, labeled as AV and marked as stars in southernmost part of the Okinawa Trough (Lee et al., 1998). At least four submarine hydrothermal venting sites (AV5–AV8) around Kueishantao islet have been found. Note that Kueishantao islet is smaller than the symbol size. The Tatun Volcano Group (TVG) in northern Taiwan is also hydrothermally active. Open circles are the on-land sampling sites in this study.
ratios (8 ± 1 RA); (4) Lower mantle: hot spot basalts are
believed to have trapped the primordial 3He, and usually exhibit much higher 3He/4He ratios (>30 RA) (e.g., Lupton,
1983; Farley and Neroda, 1998; Ozima and Podosek, 2002; Porcelli et al., 2002). Hence, the helium isotopic ratio is widely used as a useful tool to trace of relevant samples source domain (e.g., Yang et al., 2003b; Fu et al., 2005), especially in the magma activity (e.g., Sano et al., 1984; Sano and Wakita, 1985; Poreda et al., 1988; Hilton et al., 1995, 2002; Van Soest et al., 1998, 2002; Yang et al., 1999, 2003a; Notsu et al., 2001). Poreda and Craig (1989), who measured 3He/4He ratios of volcanic gases from Circum-Pacific active volcanic arcs, showed that the dominant source of helium was the mantle wedge rather than subducted oceanic crust or sediment, both of which are rich in radiogenic 4He. Combined with other geochemical data, Hilton et al. (1995) and Van Soest et al. (1998, 2002) used the helium isotopic results to re-solve the sediment subduction and crustal contamination for the genesis of an arc magma.
A cluster of over 30 vents, at a water depth of about 10–20 m offshore the northeastern Taiwan, emits hydro-thermal fluids and volcanic gases (Chen et al., 2005a, b). Many hot and cold springs also occur in IL Plain, where is the land area closest to the KST and also is the southernmost part of SPOT. Those fluid and bubble sam-ples are believed can provide important information for the magma sources of KST and regional tectonic evolu-tion. Therefore, representative samples, including those from on-land IL Plain and, sea surface and also subma-rines around KST, have been collected for gas composi-tion and helium isotopic measurements in this study. Their results will be presented and used for further discussion on the tectonic implications of the area.
SAMPLINGAND MEASUREMENTS
Five bubble gas samples on sea surface around KST and three seawater samples from different water depths at the sites of AV1, AV2 and AV5 (Fig. 1C) were col-lected during the ORII cruised in June-July, 1999. Rest ten KST bubble gas samples were collected from the sub-marine venting sites by diving into the water depths of 10–20 m from 2000 to 2003. The sampling sites in IL Plain are shown in Fig. 1C. Bubbling samples are from Su-Au (SA) cold springs, Yuan-Shan (YS) bubbles, Ching-Suei (CS) hot springs, Fan-Fan (FF) hot springs, and Ren-Tzer (RZ) hot springs. Only water samples were collected from Jiao-Shi (JS) hot springs. Natural gases in Wu-Yuan (WY) were also collected. In contrast to most samples with CO2 as major composition, WY gases are the only CH4-dominant samples in this study.
The pre-evacuated low permeability glass bottles with two evacuated stopcocks at both ends were used for
col-lecting the representative sea water and hot springs sam-ples and, also the bubbling gas samsam-ples from hot and cold springs for the measurement of their gas compositions. The seawater samples were transferred to the glass bot-tles on board right after collected by Niskin botbot-tles at dif-ferent depths from 40 m to 1200 m during the OR II cruise in June, 1999. Bubbling samples were collected by the Fig. 2. Three component plots of the Kueishantao (KST) hot spring bubble gases. (A) In the CO2-Stotal-HCl plot, most KST samples fall in the range of volcanic hydrothermal gases and are similar with those TVG gases but show distinct deviations from the gases from other active volcanoes. (B) In the N2 -He-Ar plot, KST samples fall in the corner of the compositions of divergent plate and hot spot gases, which are significantly dif-ferent from those from TVG and convergent plate gases. TVG gas data are from Lee et al. (2005); other volcanoes data are from Table 1. Gas boundaries are those described by Delmelle and Stix (2000).
method of water replacement using a funnel covering on the top of bubbling sites. Those submarine bubble sam-ples around KST showed strong sulfur smells. The so-called Giggenbach method, alkaline solution in the pre-evacuated Giggenbach bottles, was used to collected those acidic gases and then for further gas analysis (Giggenbach, 1975; Lee et al., 2005).
All the gas samples were analyzed by a gas chroma-tography system, which is equipped with two thermal conductivity detectors and one flame ionic detector, for determining their gas compositions first. The acidic gas samples collected with Giggenbach bottles need to do solution analysis for dissolved gases, including CO2, HCl, H2S and SO2. The detailed analysis procedure and errors discussion has been described by Lee et al. (2005).
The analyzed samples were then sent for further he-lium isotopes measurement. 3He/4He and 4He/20Ne ratios have been measured with the Micromass 5400 noble-gas mass spectrometer with dual collectors in the Department of Geosciences, National Taiwan University. The system includes a two-stage purification line and a cryogenic pump with charcoal trap. The gas sample first passes through the first-stage purification line, including U cold-trap with liquid nitrogen, Cu-CuO furnace at 400°C, Ti-sponge furnace at 700°C, and charcoal trap with liquid nitrogen to remove most active gases (including H2O, CO2, N2, O2, H2, hydrocarbon and sulfur gases etc.) and heavy noble gases (Ar, Kr, and Xe). Then the sample is allowed to enter the second-stage purification line for further purification. It includes Ti-sponge furnace, char-coal trap with liquid nitrogen, and SEAS Ti-Zr getters. At this stage all the active gases should be totally removed then, the purified gas can be trapped into a cryogenic pump at 15°K. At last, helium and neon are released by step-wisely increasing temperature, at 34°K and 70°K, respectively, to sequentially admit into the mass spectrometry for isotope measurement. Air is routinely run as a standard for calibration. A 20 RA pure helium gas
standard (Matsuda et al., 2002) is also prepared and run as working standard to reduce the analytical errors. In general, the total errors on the ratios are less than 2% and 5% for 3He/4He and 4He/20Ne, respectively. Details of the measurements have been given by Yang (2000).
ANALYTICAL RESULTS
The compositions of KST submarine bubble gases are shown in Table 1. Gases from TVG and other volcanoes in the world are also listed for comparison. Except for samples 001125-GSD-1 and -2 having significant air con-tamination with high N2, O2, Ar, most KST samples ex-hibit similar composition with TVG gases, i.e., high CO2 (90–99%) and H2S (0.8–8.4%) but low SO2 (<0.03%) and HCl (<50 ppm) contents. It indicates that they have
simi-T
able 1. Dry gas compositions of bubbles fr
om Kueishantao submarine hot springs and fumar
olic gases fr
om other volcanoes
*Data fr
om (1) this study; (2) Chen et al. (2005b); (3) Lee et al. (2005); (4) Chiodini et al. (2001); (5) Giggenbach and Matsu
o (1991); (6) Giggenbach et al. (2001).
Note:
All gas compositions shown as the unit of mmoles (10
–3
mole) per mole.
AC = air contaminated; b.d.l. = below detection limit; n.d. = not determined.
Sample No. Temp. (° C) CO 2 H2 SS O2 HCl H e H2 Ar O2 N2 CH 4 C O Data source* N o te Kueishantao (KST) 0 0 1 1 2 5 -G S D -1 110 538 34.2 0.052 n.d. 0.024 25.5 3.50 56.0 338 5.0 n.d. (1 ) A C 0 0 1 1 2 5 -G S D -2 43 258 2.3 0.008 n.d. 0.040 6.90 6.10 108 618 1.6 n.d. (1 ) A C 030622-GSD-AVJ 55 978 20.4 0.271 0.028 0.028 0.114 0.009 0.195 0.871 b.d.l. 0.0004 (2 ) 030622-GSD-VA 48 976 21.0 0.226 0.049 0.025 0.0007 0.025 0.032 2.231 0.335 0.0008 (2 ) 030814-GSD-VA 56 992 8.46 0.073 0.023 0.015 0.021 0.001 0.011 0.108 0.035 <0.0001 (2 ) 030814-GSD-VAK 107 987 12.6 0.009 0.023 0.007 0.003 0.0004 0.007 0.043 0.007 <0.0001 (2 ) 030815-GSD-VAL 78 916 84.0 0.051 0.030 0.005 0.002 0.0002 0.002 0.024 0.030 <0.0001 (2 ) Tatun Volcano Group, N . Taiwa n Da-you-keng fumarole 1 0 2 9 4 2 37.3 6.74 0.01 0.007 0.16 0.12 1.46 11.4 0.40 b.d.l. (3 ) She-huang-ping hot springs bubbles 99.5 938 46.3 0.69 0.01 0.006 0.01 0.11 0.84 11.3 2.27 0.07 (3 ) Other volcanoes in the world Solfatara, Italy 9 7 9 9 2 2.99 b.d.l. b.d.l. 0.010 0.78 0.004 n.d. 3.65 0.14 < 0.001 (4 )
White Island, New
Zealand 1 1 1 8 0 8 4.47 168 3.60 0.002 0.20 0.03 n.d. 9.8 8.9 n.d. (5 ) Mt. Usu, Japan 690 575 29.3 52.2 68.0 n.d. 294 n.d. n.d. 16.0 0.90 0.08 (5 )
Merapi, Gendol, Indonesia
8 0 3 4 8 9 13.0 95.4 53.8 0.004 44.3 4.29 1.59 319 n.d. 1.08 (6 )
lar gas compositions to those of typical low temperature fumaroles in the world and, are also consistent with the measured temperatures (43–110°C) at the venting sites.
Measured 3He/4He ratios of KST bubble samples range from 5.60 to 11.38 × 10–6 and 4He/20Ne ratios range from 0.62 to 46.4 (Table 2). If we assume that all the 20Ne con-centration of the sample comes from the atmosphere, then we can correct its helium ratio for atmospheric helium contamination by Eq. (1) (Poreda and Craig, 1989). (3He/4He)cor = [(3He/4He)m – (3He/4He) × r]/(1 – r) (1)
r = (4He/20Ne)air/(4He/20Ne)m.
Where the subscript cor is the corrected value; m is the measured value; air is the value of air.
The measured 3He/4He ratios are calibrated against atmospheric standard gas and are expressed relative to RA, where RA is the air 3He/4He ratio of 1.39 × 10–6. All
the KST samples, including bubbles from both sea sur-face and submarine, exhibit consistent high helium
iso-topic ratios after air correction, i.e., 7.35–8.39 RA. It
should be noted that samples 990906-KST-1 and -2, may not be properly corrected due to serious air contamina-tion indicating by the very low 4He/20Ne ratios which close to the ratio of air (0.319) (Table 2). The high 3He/ 4He ratios, up to 8.4 R
A, are the highest values obtained
so far from Taiwan, and probably are also the highest 3He/ 4He values of gases ever reported in active hydrothermal areas of the western Pacific region.
Assuming the measured sample is the mixture of man-tle and crust components with the 3He/4He ratios of 8.0 and 0.05 RA, respectively, then we can calculate the
per-centage of each component following Eq. (2).
Rc = 8.0 × XM + 0.05 × (1 – XM). (2) Where the subscript c is the corrected 3He/4He ratio; XM is the percentage of mantle component involved in this sample. Percentage of crust component will be (1 – XM). It is clear that mantle component, XM = 91–100%, is the dominant source for KST gas samples (Table 2). Note that Table 2. Helium isotopic compositions around Kueishantao, NE offshore of Taiwan
1. The affix of the sample name (990620-) is the sampling date (99/06/20). 2. R: 3He/4He ratio of measured samples; R
A: ratio of air (=1.39 × 10–6).
3. Rc: air corrected helium isotopic ratio, assuming all the Ne derived from the air (Poreda and Craig, 1989). 4. XM: percent of mantle component based on the calculation of Eq. (2).
5. X: Rc may not be properly corrected due to the He/Ne ratio (<1) is close to the ratio of air (0.319). 6. ∆He = [(R – RA)/RA] × 100. Sample1 3He/4He (×10– 6) ± 1σ (%) [R/RA]2 4He/2 0Ne [Rc/RA]3 [He] (ppm) XM4 Note5
Bubble gas samples collected on sea surface
990620-KST-1 10.29 0.59 7.40 6.03 7.76 ± 0.19 — 97.0 990622-KST-1 11.18 0.26 8.04 13.2 8.22 ± 0.21 — 100 990622-KST-2 10.62 0.22 7.64 16.4 7.77 ± 0.19 — 97.1 990706-KST-1 5.60 0.30 4.03 0.62 7.26 ± 0.18 — 90.7 X 990706-KST-2 6.75 0.34 4.86 0.81 7.38 ± 0.18 — 92.2 X
Bubble gas samples collected at submarine venting sites 000516-GSD-2 11.30 0.27 8.13 9.13 8.39 ± 0.25 46.8 100 001125-GSD-1 10.17 0.37 7.31 5.78 7.68 ± 0.19 23.7 96.0 010324-GSD-1 10.07 0.18 7.25 19.0 7.35 ± 0.18 34.0 91.8 010324-GSD-2 10.45 0.32 7.52 21.1 7.62 ± 0.22 35.7 95.2 030622-GSD-AVJ 10.38 0.57 7.47 28.3 7.54 ± 0.12 24.6 94.2 030622-GSD-VA 10.38 0.33 7.47 28.4 7.54 ± 0.11 28.0 94.2 030814-GSD-VA 10.20 0.70 7.34 40.8 7.39 ± 0.13 15.0 92.3 030814-GSD-VAK 10.41 1.41 7.49 46.4 7.53 ± 0.16 6.66 94.1 030815-GSD-VAL 10.43 1.16 7.51 27.2 7.58 ± 0.15 5.18 94.7 030815-GSD-VAM 10.28 1.41 7.40 32.6 7.46 ± 0.15 5.10 93.2
Sea water samples ∆He6
990621-ST1-200 1.40 3.70 1.01 0.26 0.72 — — 990621-ST2-40 1.30 7.80 0.93 0.23 –6.47 — — 990621-ST5-1200 3.20 3.78 2.30 0.23 130.2 — —
two samples, 990622-KST-1 and 000516-GSD-2, have corrected 3He/4He ratios larger than 8.0. We treat them being derived from 100% mantle component contribution. The seawater sample, 990621-ST5-1200, shows sig-nificant excess 3He with 3He/4He ratio of 2.30 R
A in the
dissolved gas (Table 2). It implies that mantle fluids are degassing into the sea from the active submarine volca-noes, although rest two seawater samples do not show excess 3He.
Samples from IL Plain, in contrast to KST samples, show a wide range of 3He/4He ratios from 0.22 to 1.94 × 10–6 (Table 3). The ratios are from 0.09 to 1.55 R
A after
air correction. They can be divided into two major groups. First group, including WY, SA and CS samples, shows significant mantle input, XM = 14–19%. Second group, FF and RZ samples, has only very small amount of man-tle contribution, XM < 1%. JS and YS samples are not
belonging to above two groups. They do not show clear mantle contributions but also have the elevated 3He/4He ratios, i.e., falling in the range between above groups with XM = 4–5%.
DISCUSSION
Gas sources and potential magma reservoir in the region Similar to TVG gases, KST gases have very low HCl content but high CO2/Stotal ratios. Therefore, most KST data fall into the range of volcanic-hydrothermal gas com-position in the plot of CO2-Stotal-HCl (Fig. 2A). Never-theless, KST samples show higher He/Ar and lower N2/ He ratios and, fall in the composition of divergent plate and hot spot gases of the N2-He-Ar plot (Fig. 2B). It is different from the TVG gases, which fall in the range of convergent plate gases. It implies that the KST and TVG Table 3. Helium isotopic compositions of fluid samples from I-Lan Plain, NE Taiwan
1. G = nature gas; W = water; BG = bubbling gas. 2. R: 3He/4He ratio of measured samples; R
A: ratio of air (1.39 × 10–6).
3. Rc: air corrected helium isotopic ratio, assuming all the Ne derived from the air (Poreda and Craig, 1989). 4. XM: percent of mantle component based on the calculation of Eq. (2).
Sample No. Sample type1 4He/2 0Ne 3He/4He [R/R
A]2 [Rc/RA]3 ± 1σ [He]
(ppm) XM4
Wu-Yuan natural gas
50309-WY-1 G 4.55 1.93 × 10– 6 1.39 1.42 ± 0.03 7.14 17.2
50403-WY-2 G 5.59 1.94 × 10– 6 1.40 1.42 ± 0.02 6.34 17.2 Su-Au cold springs
90728-SA-2-2 BG 3.59 1.61 × 10– 6 1.16 1.17 ± 0.03 27.7 14.1 20223-SA-2-1 BG 7.33 1.77 × 10– 6 1.27 1.29 ± 0.03 33.4 15.6 20710-SA-1-1 BG 4.32 1.60 × 10– 6 1.15 1.17 ± 0.03 64.7 14.1 30227-SA-1 BG 14.8 1.70 × 10– 6 1.22 1.23 ± 0.04 36.7 14.8 30314-SA-1 BG 11.5 1.80 × 10– 6 1.29 1.30 ± 0.04 38.0 15.7 30404-SA-1 BG 15.5 1.71 × 10– 6 1.23 1.23 ± 0.03 42.1 14.8 Jiao-Shi hot springs
90408-JS-2 W 7.75 0.56 × 10– 6 0.41 0.39 ± 0.01 — 4.3 30221-JS-2 W 8.73 0.54 × 10– 6 0.39 0.37 ± 0.01 61.8 4.0 30404-JS-2 W 5.09 0.57 × 10– 6 0.41 0.37 ± 0.01 41.2 4.0 30418-JS-1 W 9.94 0.54 × 10– 6 0.39 0.37 ± 0.01 39.4 4.0 Yuan-Shan bubbles 30213-YS-2 BG 0.69 0.93 × 10– 6 0.67 0.39 ± 0.01 11.4 4.3 30519-YS-2 BG 0.63 1.02 × 10– 6 0.73 0.45 ± 0.01 11.5 5.0 30811-YS-2 BG 0.68 0.94 × 10– 6 0.67 0.39 ± 0.01 11.3 4.3 31009-YS-2 BG 0.82 0.92 × 10– 6 0.66 0.44 ± 0.01 17.2 4.9 Ching-Suei hot springs
20223-CS-2 BG 0.73 1.82 × 10– 6 1.31 1.55 ± 0.04 2.27 18.9
20710-CS-2 BG 0.64 1.54 × 10– 6 1.11 1.22 ± 0.31 1.50 14.7 Fan-Fan hot springs
30206-FF-1 BG 2.65 0.31 × 10– 6 0.22 0.12 ± 0.00 8.17 0.88 Ren-Tzer hot springs
00819-RZ-2-1 BG 4.34 0.22 × 10– 6 0.16 0.09 ± 0.01 6.34 0.5
gases may be derived from different tectonic environ-ments. If those gases were derived from similar magmatic sources for lavas, then the result is consistent with the traditional tectonic model that the TVG samples were the product of Ryukyu arc subduction system; whereas, KST were the products of rifting of the Okinawa Trough (e.g., Teng, 1996).
Figure 3 shows the average air-corrected helium topic ratios of fluid samples in this study. The helium iso-topic data of KST samples, including seawaters and bub-bles, can be explained very well by binary end compo-nents mixing of air and MORB in the plot of 3He/4He vs. 4He/20Ne (Fig. 4). However, samples from IL Plain fall in the range between MORB, air and crust. They need ternary components mixing to account for their helium isotopic composition. For FF and RZ samples, neverthe-less, they fall close to the mixing line between air and crust components. It indicates that only very few or even no MORB component is needed to account for their gas source.
The 20Ne/4He vs. 3He/4He ratio plot also shows the same result, and it is easier to estimate the relative pro-portions for each component, i.e., Air-Crust-Mantle, di-rectly from diagram (Fig. 5). The estimation for the pro-portion of mantle component is similar to the result of the calculation from Eq. (2). The contribution of crust component for KST samples are very small and can be ignored. The KST submarine bubbles are dominant with mantle component (90–100%) and higher than those for
TVG fumaroles (50–90%). For samples from IL Plain, in contrast, crust component clearly is their dominant source. YS and CS samples are mainly mixed by crust and air components, and with a little mantle component. Mean-while, SA and WY gases, which do not have clear air con-tamination, exhibit higher mantle component than rest samples from IL Plain (i.e., CS, JS, YS, FF, and RZ).
The magma activity and existence of magma chamber in northern Taiwan have been concerned and debated by local geologists for long time. Recently, more data sup-port that there may be existing magma chamber under-neath TVG (Song et al., 2000; Yang, 2000; Lin et al., 2005). Chen and Shen (2005) reviewed some claimed his-torical eruptions in northern Taiwan and suggested that there were three records may be related to the magmatism of northern Taiwan volcanic zone and Quaternary Ryukyu volcanic front. Recently, Lin et al. (2004) analyzed the earthquake data to determine the three-dimensional Vp and
Vs velocity structures in NE Taiwan and found a low Vs
but high Vp/Vs sausage-like body, ~30 km in diameter,
lies within the Eurasian mantle wedge, on the top of the most western part of Ryukyu slab. A low Vs but high Vp/
Vs channel rises obliquely from the sausage-like body at
a depth of 40 km in direction of KST. They further pro-posed that the H2O-rich component and/or melt, i.e., magma, thus can rise up from the sausage-like body to-ward the surface via veins and/or narrow conduits. Their observation can support the high helium isotopic data around KST and suggest that there is a magma source underneath KST.
Invading of mantle fluids into the I-Lan Plain
Based on the continuous GPS data, Liu (1995) con-cluded that the IL Plain is propagating westward in the speed of 126 mm/y. He further estimated that the eastern part of IL Plain is subsiding in the rate of 20 mm/y due to the westward extension of the Okinawa Trough. Many normal faults have been recognized in SPOT and may be related to the NE extension of the on-land major fault, LS Fault, in IL Plain (Sibuet et al., 1998). It is clear that mantle component is necessary for most samples from IL Plain based on their elevated helium isotopic composi-tions (Fig. 3). If the mantle fluids have invaded into the IL Plain as suggested by present elevated helium isotopic data, the LS Fault may provide the major pathway for the mantle fluids migrating toward the shallower surface.
JS hot springs and YS bubbles are located along the LS Fault and closer to KST than other sampling sites in IL Plain. It was expected for them to exhibit highest he-lium isotopic ratios if the mantle fluids invade mainly through the LS Fault zone. However, the higher 3He/4He ratios are distributed in the southern part of IL Plain, i.e., WY, SA and CS, rather than in JS and YS. It indicates that the LS Fault is not the major pathway for the migra-Fig. 3. Average helium isotopic compositions of fluid samples
around Kueishantao and I-Lan Plain, NE Taiwan. The data are the air-corrected ratios from Tables 2 and 3. WY: Wu-Yuan natu-ral gas; SA: Su-Au cold springs; JS: Jiao-Shi hot springs; YS: Yuan-Shan bubbles; CS: Ching-Suei hot springs; FF: Fan-Fan hot springs; RZ: Ren-Tzer hot springs.
tion of mantle fluids. Therefore, we can further suggest that there would be a blind fault or magma intrusion in southern IL Plain to offer either the invading path or pri-mordial 3He source. Since the IL Plain is subsiding and the crust is thinning, alternatively, although the estimated crust in SPOT is ~25-30 km (Hsu, 2001), it could be able to generate some deep normal faults, as reported by Tsai et al. (1975), due to the extension of Okinawa Trough in the region. Therefore, the primordial 3He is able to degas from mantle source region via the fault fractures. Crust contamination vs. source contamination for KST magma
Chen et al. (1995) presented the very low Nd isotopic values (εNd = –1.9 to –5.2), and very high Sr ratios (87Sr/ 86Sr > 0.705) and high δ18O (between 7 and 8‰) with strong continental signature on KST lavas. They further explained the data resulting from a MORB-like magma assimilation with about 30% local continental crust ma-terials and/or the thick overlying sediments during the onset of the rifting stage of the Okinawa Trough. How-ever, present KST data seem not support the model, if the magma source of KST is mainly derived from upper man-tle with the helium ratio of 8.0 RA. Alternatively, if the
KST magma is derived from a plume source with the he-lium isotopic ratio of 11 RA, and mixed with 30% of
con-tinental curst with the 3He/4He ratio of 0.05 R
A, then it is
able to generate the present KST average helium isotopic ratios of 7.68 RA. However, there no current plume or hot
spot source with high 3He/4He ratios (i.e., >10 R
A) have
Fig. 4. 3He/4He vs. 4He/20Ne plot for the fluid samples in this study. Acronyms are same as those in Fig. 3. All the KST samples, including bubbles and sea waters, exhibit higher 3He/4He ratios and fall in the mixing trend between Air component and MORB
source component. Nevertheless, samples far away from KST, including FF and RZ hot springs, show much lower ratios with significant crustal signature. Note that samples from I-Lan Plain cannot be well explained by simple binary mixing of any two components.
Fig. 5. The A-C-M three-component plot for fluid samples in this study. A: air; C: crust; M: MORB components. KST sam-ples fall at the corner of MORB end component, indicating more than 90% of MORB source is necessary to account for their helium isotopic compositions. In addition to Air component, samples from I-Lan Plain (i.e., WY, SA, CS, and JS) also show elevated 3He/4He ratios and need both Crust and MORB com-ponents to explain their geochemical characteristics. Fumaroles from TVG, typical of arc volcanic affinity, are also plotted for comparison (data from Yang et al., 1999; Yang, 2000).
Fig. 6. Binary mixing curves of 3He/4He vs. 87Sr/86Sr end components. Assuming the compositions of the fumarolic gases from Kueishantao (KST) and Tatun Volcano Group (TVG) are similar with those of lavas. (A) MORB and subducted component mixing. The mixing curves represent an end-member scenario with the assumption of complete helium retention in the subducted compo-nents. The alternative scenario is presented by the dashed-line box extending from the MORB box towards higher 87Sr/86Sr,
labeled “sediment subduction without He addition”. This represents the case where no helium survives the subduction process and subduction addition is reflected only the Sr isotopes. Thick arrows indicate the trend of crust contamination. It is clear that KST samples, unlike TVG samples, do not show significant crust contamination, but can be explained by the mixing of MORB source with subducted sediments (either case with or without additional helium). (B) Potential magmas mixing with different crustal components. KST samples fall in the range of potential magma 2 (MA2), which is generated at upper mantle level by mixing the MORB source with 30% Sr of subducted sediments before enter the crustal level, and then experienced minimum crustal contamination. In contrast, TVG samples can be explained by magma-crust mixing with the potential magma 1 (MA1) and various degrees of crust input (2–5%). Note that MA1 is the composition of MORB source adding 10% Sr of subducted sediments. Compositions of all the mixing end members are listed in Table 4. KST helium data are from this study; Sr data, from Chen et al. (1995) and Chu (2005); and TVG helium data, from Yang et al. (1999) and Yang (2000); Sr data, from Wang et al. (2004).
source for the helium isotopic system.
People may argue that the present KST gas samples may be not derived from the same magma source for KST lavas. Before the last eruption occurred in KST 7,000 yrs ago (Chen et al., 2001), the thickness of the overlying ever been reported in this region, although Macpherson
et al. (1998) and Shaw et al. (2004) reported high 3He/ 4He ratios (up to 15 R
A) with a plume signature for lavas
form the Manus backarc basin. Thus, it is unlikely to have such big amount of crust component input the magma
a: The end components are defined as follows. MORB source: undergoing 10% of partial melting of upper mantle; GLOSS: global subducting sediments; Sub. Sed.: subducted sediments; Altered MORB: subducted altered oceanic crust; Sub. Sed. alt.: an alternative composition for subducted sediment with more extreme compositions; Potential magma 1: potential composition of magmas with Sr addition of 10% sediments and without He addition; Potential magma 2: potential composition of magmas with Sr addition of 30% sediments and without He addition; Continent crust-1, -2, -3: several potential compositions of crustal components. Most data are from Van Soest et al. (2002); except for the Sr concentrations and isotopic ratios of SS, potential magmas and the 87Sr/86Sr ratios of continent crust are from Chu (2005).
b: Mixing curves calculated for the binary mixing of subducted components with the MORB source. c: k = ([4He]/[Sr])
M/([4He]/[Sr])cont, where M represents the mixing components starting with an M, and cont represents the other mixing components.
d: Mixing curves calculated for the mixing of the magma as it enters the crust with potential crustal components.
continental crust might be thicker and could cause sig-nificant crustal contamination for both Sr-Nd and He sys-tematics. Due to the westward propagating extension of the Okinawa Trough, the thickness of crust underneath KST could become thinner and the mantle gas can degas quickly toward surface without significant crust contami-nation.
This argument can be against easily by calculating the difference of crustal thickness between present and 10 kyrs ago. Assuming the subsiding rate of the crust is keep-ing the same rate of 20 mm/y (Liu, 1995) in last 10 kyrs, the total amount of subsidence in this area is only 200 m. It will not make any significant changes for the crustal thickness.
Usually the phenocrysts (e.g., olivine and pyroxene) in the lavas can retain the primary helium isotopic com-positions of the magma and can be used for magma gen-esis discussion. Unfortunately, enough phenogrysts are not available due to the aphyric characteristics of KST lavas, although some olivine and pyroxene crystals can be observed under microscope. Nevertheless, recent study showed that the fluid samples usually exhibit similar he-lium isotopic compositions of phenocrysts in one area (Van Soest et al., 2002). Thus we can assume that the present KST gas samples share the same magma source for the KST lavas. Then, we need to explain the strong continental signatures of low Nd and high Sr isotopic position for KST lavas, however, high He isotopic com-position without crustal signatures for KST gas samples. Van Soest et al. (2002) used the plot of 3He/4He vs. 87Sr/ 86Sr to solve the processes between crustal
contamina-tion and subducted sediments contaminacontamina-tion. Figure 6A shows the binary mixing model with different end mem-bers (parameters shown in Table 4) for the He-Sr isotopic systematics. Assuming that the gas samples sharing the same magma source for the lavas, then we can plot KST and TVG composition into the diagram. It is clear that KST samples fall in the mixing lines of subducted sediments mixing with the MORB source, either with or without helium subduction. It is worthy to note that he-lium is generally believed to be released at very shallow depth during subducting process and will be not carried into the mantle source (e.g., Staudacher and Allegre, 1988; Hiyagon, 1994). Nevertheless, TVG samples are off the subducted sediments mixing line and fall in the trend of crustal contamination.
Figure 6B further plot the mixing lines for potential magmas with various continent crust components (Table 4). Note that the potential magma 1 (marked as MA1 in Fig. 6B) is the mantle source mixed with 10% Sr composition of subducted sediments without adding he-lium; similarly, the potential magma 2 (marked as MA2 in Fig. 6B) is mixed with 30% of subducted sediments. Then, the potential magmas mix with different continen-tal crust end members in cruscontinen-tal level. The TVG samples fall in the range of MA1 mixing with 2–5% continental crusts; nevertheless, KST samples exhibit similar com-positions of MA2 and do not need any crust input. This result is supported by the recent work of Chu (2005) that the KST magma results from the partial melting of subducted sediments and altered oceanic crust and then reacted with mantle component.
Table 4. End members for He-Sr mixing in the Fig. 6
End membersa [4He]
(ncm3STP/g)
RA [Sr]
(ppm)
87Sr/86Sr Subduction mixingb kc Crustal mixingd kc
MORB source (MS) 5000 8.00 9.0 0.7027 MS-AM 171 MA1-CC1 0.003
GLOSS (GL) 3000 0.05 327 0.7173 MS-SS 22 MA1-CC2 0.027
Sub. Sed. (SS) 5200 0.05 207 0.7136 MS-SSA 43 MA1-CC3 0.400
Altered MORB (AM) 390 0.05 120 0.7031 MS-GL 61 MA2-CC1 0.003
Sub. Sed. alt. (SSA) 5200 0.05 400 0.7200 MA2-CC2 0.031
Potential magma 1 (MA1) 20 8.00 256 0.7038 MA2-CC3 0.469
Potential magma 2 (MA2) 20 8.00 256 0.7060 Continent Crust-1 (CC1) 10000 0.05 400 0.7180 Continent Crust-2 (CC2) 1000 0.05 400 0.7180 Continent Crust-3 (CC3) 100 0.05 600 0.7180
CONCLUDING REMARKS
(1) Similar with TVG gas samples, the KST gas sam-ples exhibit typical compositions of low temperature fumaroles in the world, i.e., with high CO2 and H2S but low SO2 and HCl contents.
(2) KST gas samples fall in the range of compositions of divergent plate and hot spot gases; however, TVG gases belong to the compositions of convergent plate gases. It implies that they may be derived from different tectonic environments.
(3) KST gas samples exhibit consistent high helium isotopic compositions, ranging from 7.4 to 8.4 RA after
air correction. It indicates that mantle component play an important role for the gas source in the area. The high helium isotopic data around KST can further suggest that there is a magma reservoir underneath KST and can in-duce current active hydrothermal activity both on-land and submarine in the region.
(4) Samples from I-Lan Plain also show elevated 3He/ 4He ratios, estimated proportion of mantle components range from <1% to 19%. It implies that the mantle fluids may have invaded into I-Lan Plain via a blind fault or magma intrusion in southern part of I-Lan Plain.
(5) He-Sr binary mixing model demonstrates that KST samples experienced minimum crustal contamination; whereas, TVG samples need 2–5% continental crusts in-put the magma to explain their helium-strontium isotopic data.
Acknowledgments—We thank Messrs. N. T. Liu, H. H. Ho, K.
W. Wu, W. L. Hong, D. R. Hsiao, B. W. Lin and C. C. Wang at the Department of Geosciences of NTU for help with collect-ing and analyzcollect-ing samples. Drs. Y. Sano, S. L. Chung, China Chen, S. R. Song, K. L. Wang, C. H. Lin and S. Tsao gave valu-able suggestions during different stages of this work. Two anonymous referees reviewed the manuscripts and gave criti-cal comments and suggestions to improve the manuscript. The National Science Council (TFY/89-2116-M-002-040; 90-2116-M-002-009; 91-2116-M-002-017) and Central Geological Sur-vey of Taiwan financially supported this research.
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