Water of Eastern Taiwan Mud Volcanoes. Part I. H, Triple O, Triple Sr Isotopes and Trace Elements of Lo-Shan Mud Volcano

Mud volcano (MV) is one of the most important passageways for deep seated volatile materials to migrate back to Earth’s surface in sedimentary basins and subduction zones. Waters of MV fluid emitted from 18 mud pools in MV Luo-Shan (LS) in eastern Taiwan were sampled from the year 2002 to the year 2021. Major and trace components as well as H, triple O (δ18O and Δ17O) and triple Sr isotopes (87Sr/86Sr and δ88Sr) were measured. The results show that major components of water are Cl-, Na and Ca. Compared with seawater, water of MV LS reveals similar chemical characteristics with low-temperature ridge-flank hydrothermal spring and marine pore water in reducing condition. Limited spatial and temporal variation of major components as well as H, triple O and 87Sr/86Sr indicates waters emitted by mud pools come from the same source regionally. Slightly radiogenic 87Sr/86Sr at southern mud pools and before the year 2003 denotes different fluid reservoir from northern ones. Small 87Sr/86Sr variation in waters of northern mud pools indicates near surface mixing from 2 fluid reservoirs. The correlation among all components reveals sediment component addition is the major factor and evaporation is the key factor for conservative elements. In summary, waters expelled by MV LS mud pools originate from the same regional source, and their trace element composition such as Mg, K, Sr as well as 87Sr/86Sr slightly varies, depending on the location of the reservoir they are <Ac cept ed M anu scrip t> hosted. A stable source with small vibration of fluid reservoir of MV LS is indicated during the 19-years investigation period.

The rest of the MVs expel mixed gases (Etiope et al., 2009). Generally speaking, the water expelled by MVs originates from marine sedimentary pore fluids. It may have experienced important diagenesis and/or have been influenced by clay dehydration and water-rock interaction. Additionally, it may pass through halide dissolution and may mix with groundwater, surface runoff, and meteoric water near land surface (Bray and Karig, 1985;Dia et al., 1999;Dahlmann and de Lange, 2003;You et al., 2004;Mazzini et al., 2009). The solid matter is mainly derived from the ambient sediments surrounding the fluid reservoirs or the migration channel. They are mostly clay minerals, such as smectite, illite, kaolinite and chlorite as well as other minerals like quartz and calcite (Shih, 1967;Dia et al., 1999;Kopf and Deyhle, 2002;Farhadian Babadi et al., 2019). The origination of all three phases can be decoupled from each other (Sun et al., 2010;Mazzini et al., 2018).
Waters discharged by eastern Taiwan MVs have different chemical characteristics < A c c e p t e d M a n u s c r i p t > from those expelled by western ones. The evidence of major elements as well as 87 Sr/ 86 Sr show signature of rock-rock interaction with igneous rocks (You et al., 2004;Chao et al., 2013). The geological sedimentary structure emits waters containing geothermal signal. This hybrid characteristic matches the sediment-hosted geothermal system (Procesi et al., 2019). Although large 87 Sr/ 86 Sr variation in different mud pools are reported in nearby MV Lei-Gong-Huo (LGH; Chao et al., 2013;Chao et al., 2021), Sr isotopes of the water in MV Luo-Shan show consistent igneous characteristics for all mud pools collected in this area (You et al., 2004;Chao et al., 2013), implying different source condition between the 2 MVs.
In this study, water samples from 18 mud pools at MV LS were collected from the year 2002 to the year 2021. The major as well as trace ions/elements, hydrogen, triple oxygen, and triple strontium isotopic compositions were measured. The systematic measurements with 19-year investigations will help to understand the source of the fluid, underground fluid reservoirs as well as chemical and isotopic variation of MV LS.

Geological background and site description
The Taiwan mountain belt formed as a result of arc-continent collision between the Philippine Sea Plate and the Eurasian Plate and one of the possible suture zones is the Longitudinal Valley, situated between the Central Range and the Coastal Range (Li, 1976;Teng, 1990). The Luzon Arc moves northeast toward the Asian continent at an average rate of 7 cm per year (Yu et al., 1997). Across the Longitudinal Valley Fault (LVF), the movement decreases dramatically to 2 cm per year (Ching et al., 2011). The highly deformed Lichi Mélange formation, which was the forearc basin and composed of chaotic mudstone mixed with fragments of oceanic crust (Chang et al., 2000), may be the substance that absorbs the movement (Ching et al., 2011) and provides the suitable environment for the formation of MVs.
There are 3 MVs located along the Longitudinal Valley in eastern Taiwan (Fig. 1a), which are Luo-Shan (previously also named Yencheng by Shih, 1967), Shih-Men-Wai, and Lei-Gong-Huo from north to south (Shih, 1967). All the MVs are located on Lichi Mélange as well as the hanging wall of Lichi Fault and the foothill of the Coastal Range (Fig. 1b). This area happens to be extensional co-seismically (Ching et al., 2007;Lin et al., 2010) and the channels of MV fluids are in unclamping conditions < A c c e p t e d M a n u s c r i p t > (Bonini et al., 2016;Bonini, 2021), enhancing the eruption activities (Jiang et al., 2011).
MV LS is the northernmost MV in eastern Taiwan. Dozens of swamp-like mud pools are distributed linearly in the area approximately 1 km in length and 200 m in width with the orientation of N20〫E (NNE-SSW), parallel to the strike of the major faults. Based on the location of the mud pools, six groups, A to F, are classified from north to south (Fig. 1c). Group A has one long-lasting mud pool, LS-A1, while others are short-lived ones. Mud pools in group B all exist temporally. Group C is in a private yard, and thus not sampled in this study. Another long-lived mud pool is in group D, LS-D1. LS-E1 has existed since the 2008 investigation, and the path to group F was broken after 2015, as a result, only the samples collected before 2008 are discussed in this study.

Sampling
MV samples have been collected since October 2002. More intense sampling were performed from October 2015 to July 2016 monthly and from January 2017 to 2021 yearly. The fluid samples were collected right below the gas bubbling area using four 50 cm 3 pre-weighted polypropylene (PP) centrifuge tubes. The temperature, pH, and oxidation-reduction potential (ORP) values were obtained on site with a WalkLAB® TI9000 temperature compensation pH meter. Flux of expelling gas is measured by inverting a volumetric PP beaker in the bubbling area and slurry flux is measured by a volumetric PP beaker under the slurry overflowing incision The time to fill the beaker was determined by a stopwatch. The average flux was achieved through 7 measurements on site. Field samples were shipped back to the laboratory at low temperature. The samples were filtered with 0.45 μm nylon membrane filters after being centrifuged by 2560 ×g relative centrifugal force (RCF) for 30 minutes. The residual solids were dried in the oven at 50 ℃ overnight. The weight of dried mud along with the centrifuge tube was measured and the percentage of mud weight was obtained by the dry weight over raw weight after the weight of the centrifuge tube was deducted. Half of the filtered solutions were acidified with purified concentrated nitric < A c c e p t e d M a n u s c r i p t > acid to pH < 2 for the determination of major and trace elements, and Sr isotopes.
Unacidified samples were preserved for the measurement of anion concentrations, total alkalinity, and H and triple O isotopes. All samples were kept at 4 ℃ in the refrigerator for later analysis.

Chemical composition in the fluids
Dissolved anions (Cl -) were determined using ion chromatography (Dionex® ICS-3000) with the precision better than 5 %. Quality assurance was obtained via diluted international seawater standard IAPSO. Chloride concentration of diluted IAPSO was calculated based on the salinity and the equation established by Millero et al. (2008). Major and some trace elements (B, Ba, Ca, Fe, K, Li, Mg, Mn, Na, S, Si and Sr) were measured using Agilent 5100 inductively coupled plasma optical emission spectrometry (ICP-OES) with a precision of better than 3 %. Other trace elements (Al, As, Br, Co, Cs, Cu, Ge, I, Mo, Ni, Pb, Rb, Sb, Ti, U, and Zn) were measured with Agilent 7500cx quadrupole inductively coupled plasma mass spectrometer (ICP-Q-MS). Samples before 2008 only had trace elements Br, I, Rb, U measured with high resolution inductively coupled plasma mass spectrometer (HR-ICP-MS, Element II, Thermo Fisher Scientific). Chemical composition of the < A c c e p t e d M a n u s c r i p t > year 2008 samples were published (Chao et al., 2011) but analyzed again in this study. in the samples, matrix-matched calibration standards were prepared, and the quality assurance was obtained through 3 reference materials,  The results fall in the range of 20 % certified values except As, Co, and Ni. Arsenic is seriously overprinted by isobar 35 Cl 40 Ar + while 59 Co and 60 Ni have isobaric interferences by calcium oxide. Since the intensity of the signals was too low to be quantified in collision mode, numerical normalization is applied. The 43/59 and 43/60 ratios of samples spiking with concentration matching Cl and Ca were monitored before and after each 10 samples run. The intensity and ratios were applied to remove the potion from the interferences. The intensity of m/z 75 was deduced by intensity of m/z 77 and multiplying by 35 Cl/ 37 Cl ratio to correct the contribution from 35 Cl 40 Ar + . Before Cl correction, the intensity of m/z 77 contributed by Se was removed by monitoring m/z 82. After numerical normalization, the results of reference materials fall in 20 % range of certified values for the three elements.
Total alkalinity (TA) was measured by the acid titration (Metrohm® 905 Titrando) with the precision better than 1 %, estimated by repeating analyses of the sample (n = < A c c e p t e d M a n u s c r i p t > 3) and in-house prepared bicarbonate standard.

Hydrogen and triple oxygen isotopes
where λ = 0.528 and γ is generally assumed to be 0.

Triple Sr isotopes
The separation and purification of Sr were performed using an extraction chromatography technique, Sr Spec ® resin (Eichrom Technologies, USA).
Approximate 120 ng Sr from the sample is needed for Sr isotope analysis.

Chemical and isotopic characteristic of waters
In general, waters expelled by MVs reveal similar chemical characteristics to deep marine pore fluids. They originate from ancient seawater and are altered by early diagenesis, clay dehydration, and water-rock interaction (e. g. Dia et al., 1999;Dählmann and de Lange, 2003;Hensen et al., 2004;You et al., 2004;Mazzini et al., 2007;Mazzini et al., 2009;Ray et al., 2013;Farhadian Babadi et al., 2019;Chen et al., 2020). Seawater is an important reference for chemical and isotopic composition of MV waters and chloride is one of the most important dissolved components to decipher the source of the fluid besides seawater and the behavior of the elements.
Both marine and terrestrial MVs generally contain lower chloride concentration, lower Mg/Cl, Ca/Cl, δD ratios with higher Na/Cl, B/Cl, Li/Cl, Ba/Cl, Br/Cl, I/Cl, δ 18 O ratios compared with seawater (e. g. Dia et al., 1995;Lavrushin et al., 2003;Aloisi et al., 2004;Chao et al., 2011;Farhadian Babadi et al., 2019;Chen et al., 2020). MV LS reveals similar chemical and isotopic features except reverse relationship between Na/Cl and Ca/Cl (Table A1). The characteristic of higher Ca/Cl but lower Na/Cl ratios relative to seawater is usually indicated as low temperature (< 80 ℃) water-rock interaction with volcanic ashes or oceanic crust (Garlick and Dymond, 1970; Seyfried < A c c e p t e d M a n u s c r i p t > and Bischoff, 1979;Lawrence and Gieskes, 1981;Henderson, 1982) and has been observed in marine pore water (e.g. Gieskes et al., 1975;Perry et al., 1976;Lawrence and Gieskes, 1981;Gieskes et al., 1990). The low 87 Sr/ 86 Sr ratio of the waters further supports this observation (  (Table A1).
The hydrothermal experiment (up to 350℃) of seawater and pelagic sediment indicates release of As, Cs, and Rb into the aqueous phase at high temperature (You et al., 1996). However, As/Cl shows slight higher, Cs/Cl shows similar, and Rb/Cl shows lower ratios than those of seawater, possibly resulting from lower formation temperature than that of hydrothermal experiments. The formation temperature of MV LS fluid is between 79 and 98 ℃, determined by various chemical geothermometers (Chao et al., 2011). The field investigation of low-temperature (62 to 64℃) ridge-flank hydrothermal springs reveals that the concentrations of Co, Mn, Mo, Ni, and Zn are < A c c e p t e d M a n u s c r i p t > greater in the hydrothermal fluid than in bottom seawater while alkalinity, Cu, K, Li, Mg, Na, Rb, and U are lower (Wheat et al., 2002;Wheat and Mottl, 2000). The low-temperature deep sea springs show more similar characteristic to MV LS. Due to elevated temperature, the greater water-rock interaction results in higher Al/Cl, Ba/Cl, Ge/Cl, Si/Cl, and Sr/Cl ratios but lower K/Cl, Mg/Cl, Na/Cl, and Rb/Cl in the water.
Cs is more mobile than Rb. That may explain a higher Cs/Cl ratio than Rb/Cl compared to that of seawater. Besides water-rock interaction, the temperature enhances organic matter decomposition and leads to high B/Cl and I/Cl ratios (e.g. , 1983;You et al., 1993). The high B/Cl ratio is also contributed by desorption of B from clays (e.g. You et al., 1993;Chao et al., 2011). Mn and U are caused by the change of redox condition. The low ORP (Table 1) indicates reducing condition in MV waters, thus, releases Mn into but precipitates U out of the water.

Ullman and Aller
The characteristic of Cu shows conflicting results between low-temperature deep sea springs and MV LS. The concentration of Cu in the springs is lower than that of bottom seawater and inferred as being precipitated into sulfide phase (Wheat et al., 2002;Wheat and Mottl, 2000). The concentration of S in MV LS is low due to strong anaerobic oxidation of methane (AOM; Chang et al., 2012). Low S may result in higher Cu content in the MV water.
The results of δD, δ 18 O, and Δ 17 O are slightly lower than present seawater (Table   < A

< A c c e p t e d M a n u s c r i p t >
sediments (Fig. 3). The preservation of ancient seawater is a possible source of the rise in 87 Sr/ 86 Sr as well as Sr content in the water. However, seawater composition does not fall on the extrapolating mixing line (Fig. 3). Additionally, δ 88 Sr has negative correlation with 87 Sr/ 86 Sr (Fig. 4). Seawater has δ 88 Sr as high as 0.38 ‰ presently.
Considering the age, the reported ancient seawater of maximum deposition age 5 to 6 Ma ( River water and groundwater have 87 Sr/ 86 Sr, Sr/Cl and Mg/Cl ratios higher than MV LS waters and is a possible source if they mix with MV waters at near surface. However, LS-D1_2003, which shows chloride concentration 33 % lower than other MV waters, is diluted by surface runoff from nearby rice paddy ( Pearson's correlation between 87 Sr/ 86 Sr and other chemical parameters (Table A2) is able to further determine the elements which are dominated by the source. The LGH (Chao et al., 2021). However, the source also causes great variation of major elements, Na and Ca, in LGH waters, but they have only minor variation and are considered conservative elements in MV LS. This discrepancy probably results from the proportion of the mixing. Sedimentary water contributes more to MV LGH than to MV LS. More contribution leads to variation of major elements. The contribution to MV LS is small and can only cause the trace elements and Sr isotopes, which are more sensitive to the source, to vary.

< A c c e p t e d M a n u s c r i p t >
The second cluster of correlated elements is chloride related elements. Chloride has high positive correlation with B, Br, Ca, Na, moderate positive correlation with δ 18 O and temperature, high negative correlation with d-excess (Table A2)

Temporal variation
The temporal variation of 87 Sr/ 86 Sr was notably different in mud pools at groups E and F from which samples were collected before the year 2004 ( Fig. 6).
LS-F1_2002 had the highest 87 Sr/ 86 Sr ratio among all samples, and the ratio dropped sharply down to a similar value to group D before the year 2008 investigation. Mud

< A c c e p t e d M a n u s c r i p t >
pool LS-E2 was dead before the year 2003 investigation; therefore, it is not meaningful to discuss the variation. Group F is a swamp-like mud volcano and located on a boggy mudstone hill. The formation is loose, the hill is fracture rich (Chao et al., 2010), and the location is at the southernmost of the MV field. These characteristics make group F more sensitive to geological stress variation or additional fluids than other groups. A major earthquake, the Chengkung earthquake Mw = 6.8, LS-A2_2008 has lower 87 Sr/ 86 Sr, Mg, K but higher Li and closer to LS-A1_2002 and LS-A1_2015-1 than LS-A1_2008, indicating the source of LS-A2_2008 has taken over LS-A1. The small variation of 87 Sr/ 86 Sr, K, Mg and Li may not represent the change of the source but the vibration of the reservoir. Mg shows more variation (250 %) than K (20 %) and other major elements such as Cl, Na, and Ca show no variation (Table 1 and A1). During the fluid migration, Mg has shorter re-equilibrium time than K and Na (Giggenbach, 1988;Verma et al., 2008). If the fluid has stopped in the underground reservoir on a longer timescale, Mg concentration will fluctuate owing to < A c c e p t e d M a n u s c r i p t > new condition of water-rock interaction such as temperature or lithology. Therefore, the increase in Mg, K and 87 Sr/ 86 Sr may have resulted from the move of reservoir or the change of fluid residence time. Neither gas nor slurry fluxes show significant difference during the entire study period (Table 1). Although flux measurement did not conduct on the year 2008, the photo indicates similar gas bubbling area and mud overflow condition comparing to other period (Fig. 1d). The vibration of the reservoir location or the size is the plausible mechanism for the chemical and isotopic variation of LS-A1.
By comparing the major mud pool in group A, LS-A1, with satellite mud pools (i.

Conflicts of Interest statement
Author In-Tian Lin is employed by CPC Corporation. The remaining authors declare that the research was conducted in the absence of any commercial or financial < A c c e p t e d M a n u s c r i p t > relationships that could be construed as a potential conflict of interest.          Chen et al. (1990) and Bentahila et al. (2008). Variation of the ancient seawater since 5 Ma is smaller than the symbol.     Chang et al. (2000). The horizontal scale is twice the vertical scale.