Hot Spring Gas Geochemistry in Western Sichuan Province, China After the Wenchuan Ms 8.0 Earthquake

The chemical components, He, Ne, and C isotropic ratios of gas samples collected thrice from 32 hot springs in western Sichuan Province, southwestern China in June and October 2008 and June 2009 were investigated in order to discuss the relationship between hot spring gas geochemistry and the Wenchuan Ms 8.0 earthquake. The data showed that the 3He/4He and C 13 CO2 d values in spring gases in the Longmenshan fault (LMSF), Xianshuihe fault (XSHF), and Minjiang fault (MJF) zones increased obviously after the great earthquake. It was estimated that up to 62% of mantle helium contributed to the spring gas in the Kangding region based on the maximum 3He/4He (7.42 × 10-6) in June 2008. Over time the mantle derived fluid contribution to the hot springs gradually decreased, but the crustal gas components: CO2 and CH4 derived from organic matter and radiogenic He increased. The gas geochemical data suggested that more mantle fluids migrated into the crust in western Sichuan Province after the Wenchuan Ms 8.0 earthquake.


INTRODUCTION
The Wenchuan Ms 8.0 earthquake occurred on 12 May 2008 in the Longmenshan fault (LMSF) zone at the eastern margin of Tibet plateau.The aftershocks were distributed along the NE strike surface rupture zone of about 270 km long (Fig. 1) (Li et al. 2010).
Earthquakes are usually accompanied with geochemical variations of gases from hot springs.The obvious variations of gas geochemistry related to the great earthquakes have been found around the world, especially in active fault zones in which permeability was enhanced (Du et al. 2008a;Bräuer et al. 2009).Gas components, such as CO 2 , He, H 2 , Rn, N 2 , gaseous hydrocarbons, etc. in the lithosphere can be expelled to the surface during the earthquake genetic process (Sano et al. 1998;Umeda et al. 2007;Du et al. 2008b;Italiano et al. 2009).For example, geochemical anomalies coinciding with seismic events in Umbria (central Apennines, Italy) and Irpinia (Southern Apennines) were not only related simply to the seismic shocks, but also to the entire seismogenic processes (Favara et al. 2001;Italiano et al. 2009).Consequently, geothermal zones are usually characterized by higher heat flow value, with active faults have acted or are acting since 1000 ka before now, with the seismic zones overlapped (Du et al. 2008b).
He and C isotopic compositions in geothermal fluids carry useful information about the fluid origins and migration processes.Therefore, He and C isotopic geochemistry has been widely utilized in monitoring earthquakes and volcanoes (Sano et al. 1998;Tedesco and Scarsi 1999;Bräuer et al. 2005;Umeda et al. 2007;Yang et al. 2009).The C 13 CO2 d and 3 He/ 4 He values studied in the hot springs in western Sichuan Province varied significantly in the different earthquake zones.The He and C isotopic ratios of hot spring gases in the Kangding and Luding regions, western Sichuan Province indicated that a lot of mantle derived fluids mixed with fluids originating from the crust (Du et al. 2006;Shen et al. 2007).
The study aims at correlating hot spring gas geochemistry with the Wenchuan Ms 8.0 earthquake.

SEISMOGEOLOGICAL SETTING
On 12 May 2008 (06:28:01 UTC) the devastating Wenchuan earthquake (Ms 8.0) struck at the center of the LMSF zone.This fault zone presents a complex structure including several large thrust faults with 250 -300 km lateral extension (Fig. 1).The shock was characterized by its long time duration, extensive fault surface rupture and shallow hypocenter depth (~16 km) The shock epicenter was about 90 km west of Chengdu, the largest city and capital of Sichuan province in southwest China (Wang et al. 2011).Five major faults exist in western Sichuan Province, named the LMSF, Minjiang fault (MJF), Xianshuihe fault (XSHF), Anninghe fault (ANHF), and Zemuhe fault (ZMHF) (Fig. 1) (Tang and Han 1993;Wen et al. 2008).The LMSF zone with a NE strike began to develop in the Triassic period.Due to episodic movements during the Mesozoic and Cenozoic Eras, especially since the Late Cenozoic Era, accompanied by eastward extrusion of the Qinghai-Tibet Plateau, the middle and southern segments of the LMSF zone have undergone strong compression.Strata in the fault zone are predominantly Palaeozoic and early Mesozoic flysch and limestone, interbeded with volcanic rocks (Tang and Han 1993;Burchfiel et al. 1995).The NS strike MJF developed along the Min-Jiang River on the western margin of the Min Mountains, of which the southern end is connected with the central Longmen Mountains (Tang and Han 1993).The XSHF presents a strike-slip feature and trends 40°N -50°W with a steep southwest dip.It extends from Ganzi, crosses through the towns of Luhuo, Daofu, Kangding to Shimian (Fig. 1).The XSHF zone has a long history of highly ductile shear zones consisting of mylonite, mylonitized, and magmatic rocks.The XSHF left-laterally offsets a huge Pre-Cambrian (700 -800 Ma) metamorphic complex belt for 90 -100 km with an emplaced granodiorite body (Tang and Han 1993).The NS strike ANHF from Shimian to Xichang is characterized by severe activity in the Holocene period.The length of fault is about 160 km with prominent left-lateral slip (Ran et al. 2008).The northern ZMHF segment has a NNW strike connected to the ANHF, and its southern end joins the Daliangshan fault (Fig. 1).The basement along the ZMHF zone consists mainly of metamorphic rocks derived from Pre-Cambrian granite and rhyolite, along with Permian basalt and Triassic granite (Fig. 1).Cenozoic strata (Late Tertiary to Early Quaternary) consist mainly of interbedded mudstones and sandstones with thin layers of coal (Ren et al. 2010).

METHODS
A total of 84 samples of free and dissolved gases were collected repeatedly from 32 hot springs and wells in western Sichuan Province (Fig. 1).Samples were collected in June, October 2008, and June 2009, respectively.Samples were collected only from the free gas phase, using vessels (500 ml) made of soda-lime glass as this type of glass has very low helium permeability.The vessels were first filled with spring water with gas bubbles collected using a funnel, replacing the water in the vessel.The vessels were sealed with a rubber plug (Du et al. 2006).The dissolved gases were extracted from water samples stored in vessels utilizing a technique based on partitioning equilibrium of gas species between the liquid and gas phases.Duplicate samples were collected; one for gas composition measurement and the second for analyzing isotope (He and C) ratios.
The spring water temperatures were measured with a thermometer having an accuracy of 0.1°C.All gas samples were analyzed at the Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences.The gas sample compositions were analyzed with a Finnigan MAT-271 mass spectrometer, with a precision of ±0.1%.The MAT-271 instrument parameters were adjusted to an ion source pressure of 8.0 kV, an emission current of 40 μA and vacuum maintained less than 1.0 × 10 -7 Pa (Reber and Cordes 1995;Cao et al. 2011).He and Ne isotopes in the gas samples were measured using the MM5400 mass spectrometer.The minimum heat blanks of the MM5400 mass spectrometer are 1.1 × 10 -14 for 4 He and 1.82 × 10 -14 for 20 Ne, respectively (Ye et al. 2007).The measurements were normalized to standard atmospheric value (Ye et al. 2001).Carbon isotope compositions of CH 4 and CO 2 in the gas samples were measured with the GC-IRMS analytical system gas chromatography (Agilent 6890)-stable isotope ratio mass spectrometer (Thermo-Fisher Scientific Delta Plus XP), coupled with an online sample preprocessor (Li et al. 2007).Methane and carbon dioxide were separated on a GC for stable carbon isotope analyses.The methane was combusted over hot copper oxide (940°C), and the carbon dioxide produced by the system was transmitted to the mass spectrometer (Li et al. 2012).The GC parameters were as follows: an Agilent 6890 GC equipped with a fused silica capillary column coated with carbon molecular sieve stationary phase (25 m × 0.53 mm × 20 μm, CP-Carbobond, Varian, U.S.A.) was used to separate the samples with a helium carrier gas (purity ≥ 99.9999%) at a constant flow of 4 mL min -1 .The GC split ratio was 4:1.The GC temperature programming were as follows: the initial temperature was set to 35°C for 3 minutes, followed by heating to 280°C at a rate of 15°C min -1 for 10 minutes.The values of C 13 d are reported relative to PDB in per mill, and had a precision of ±0.5%.

RESULTS
The chemical and isotopic compositions of both bubbling and dissolved presented wider ranges and varied in the different fault zones both in the same period and different periods (Table 1).

Chemical Composition of the Spring Gases
CO 2 concentrations of the bubbling gas samples ranged from 1.2 -98.8 vol%, of which the majority was more than 85 vol%.(Weiss 1971).See Hilton (1996) for further details of the correction protocol.d: R m /Ra is measured 3 He/ 4 He ratio divided by the 3 He/ 4 He in air = 1.4 × 10 -6 .e: R c /Ra is the air corrected He isotope ratio = [(R m /Ra × X) -1]/(X -1).presented wide ranges of 0.9 -94 and 0.1 -10.1%, respectively.The He concentrations varied from 1 -32100 ppm (Table 1).Nitrogen was predominant in the dissolved gases (29.1 -95.8 vol%).The oxygen concentrations had a range of 0.5 -20 vol%.Those values indicate that N 2 and O 2 were mainly derived from air with consumption of O 2 during water circulation.CO 2 concentrations ranged from 0.1 -64.7 vol%, higher values were measured in the samples Nos.7, 14, and 24 from the XSHF zone, but low values (0.2 -3%) in the samples from the ANHF zone (Table 1).CO 2 in the gas phase is positively correlated with the water temperature.The higher the water temperature is, the less CO 2 is dissolved in water (Table 1), and the more CO 2 is in the gas phase.

Isotopic Ratios of He and CO 2
The measured 3 He/ 4 He ratios (R m ) of the samples have a ranged of 0.02 -5.3 Ra (Ra is atmospheric 3 He/ 4 He = 1.4 × 10 -6 ).The R m values of the free gas samples were mostly greater than 1 Ra.Nevertheless, the R m values of the dissolved gas samples were found to be between 0.04 -1 Ra.The R m ratios of the hot springs in the Kangding and Luding regions at the southeastern segment of the XSHF were higher than those in other locations (Fig. 2 ) in the spring gases ranged from -58.8 -19‰ and were mostly around -20‰ (Fig. 2, Table 1).

Origins of 4 He, CO 2 , and CH 4
Origins of gaseous components in the spring gases were discussed in order to calculate the contribution of mantlederived He and CO 2 after the Wenchuan Ms 8.0 earthquake.

4 He
Helium in the hot springs had multiple origins that are illustrated by the R m /R A plot and 4 He/ 20 Ne (Fig. 3), in which most data scattered in the mixture region of mantle, crustal and atmospheric origins.This is concordant with the previous results (Du et al. 2006).For estimating the proportions of mantle-derived helium (He m ), it was proposed that the R m values should be corrected by the atmospheric value of 4 He/ 20 Ne ratio in order to exclude the atmospheric contamination (Duchkov et al. 2010).The R m values of the samples collected in June 2008 cannot be corrected because of lacking Ne isotopic compositions.The differences between the corrected (R c ) He isotopic ratios and the R m values in the free gas samples were mostly less than 5% in October 2008 and June 2009 (Table 1), indicating the atmospheric He contamination in the free gas samples can be neglected.The R c values are greater than the typical crustal 3 He/ 4 He (R/Ra = 0.02) (Andrews 1985), indicating the contribution of mantle-derived helium.Therefore, the proportion of mantle-derived helium in the samples can be estimated using the two-end member model for mantle and crustal helium.The maximum of the estimated He m values is 62% at Longtougou in June 2008 (Table 2).
The mantle-derived helium may come to the Earth's surface along the permeable high-angle XSHF (Fig. 1), without significant dilution by the crustal radiogenic helium, resulting in the emanation of high 3 He hot spring gases in Kangding and Luding regions.It is generally accepted that faults and fractures play a major role in the localization and evolution of hydrothermal systems.The XSHF extends for 350 km in a northwest-southeast direction.The width of the XSHF zone is variable from dozens of meters to several kilometers.The XSHF has a steep dip near the surface (70 -80°), however, the dip direction is variable (Xu et al. 2003).In comparison with XSHF, LMSF is unfavorable to emanate mantle-derived 3 He.The 30 -50 km-wide Longmen Shan region consists of several large napes separated by a number of nearly parallel, NW-dipping thrust faults (Xu et al. 2003).These thrust faults have been identified as three major listric thrust fault zones, the Wenchuan-Maowen Fault, the Yingxiu-Beichuan Fault, and the Guanxian-Jiangyou Fault.These faults may merge downward with the Guanxian-Jiangyou Fault at depth (Chen and Wilson 1996).

CO 2
The C 13 d values of CO 2 , combined with R m , indicate that CO 2 has multiple crust and mantle origins.It is considered that CO 2 with C 13 d value more negative than -14‰ could originate from organic matter (Bergfeld et al. 2001).Dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) is composed of dissolved carbonate species (H 2 CO 3 , HCO 3 -, and CO 3 2-) in temperature-and pH-dependent equilibrium with one another.Isotopic fractionation occurs during conversion from one species to another and dissolution of CO 2 gas.The equilibrium carbon fractionation during CO 2 gas dissolution in water is -1.31 ± 0.06‰ at 5 -25°C (Zhang et al. 1995) .Therefore, C 13 CO2 d values of less than -22‰ from the hot springs in the LMSF zone indicated that CO 2 was predominantly derived from sedimentary organic matter (Table 1).The C 13 CO2 d values of the Permian-Triassic limestone samples ranged from -1.5 -2.7‰ in the western Sichuan (Cui et al. 2009).The free gas sample from the Chuanpanqiao hot spring occurred in a limestone area enriched in CO 2 , of which the C 13 d value was -1.9‰ in June 2008, indicating CO 2 originated from the dissolution of limestone (Fig. 2, Table 1).The C

CO2
d values of about -6‰ indicated different origins of mixing marine limestone, metamorphic rocks and mantle-derived gases in MJF and XSHF zones (Table 1).
For determining relative contributions from the gas sources, C 13 d values of the gas samples were plotted against their CO 2 / 3 He ratios together with those of the three assumed sources (Fig. 4).The CO 2 / 3 He ratios of the free gas and dissolved gas (Fig. 2, Table 1) in the hot springs have a wider range (3 × 10 5 ~ 1 × 10 12 ) (Table 1).CO 2 / 3 He ratios of the bubble gas samples are concordant with or greater than that of the MORB (2 × 10 9 ) (Marty and Jambon 1987).The contributions of the three sources to the hot spring gases in western Sichuan Province were estimated using the method from the literature (Sano and Marty 1995) (Fig. 4, Table 2).The CO 2 / 3 He ratios of the samples Nos. 8,10,13,16,17,18,19,and 23 at Chuanpanqiao, Mounigou, and in the Kangding region were larger than 10 9 , and had shown an increasing trend from June 2008 to June 2009 (Fig. 4).The crustal marine limestone contributions to the hot spring gases in western Sichuan Province were assessed to be larger than 80% of the total CO 2 budget in the free gases.The mantle CO 2 contribution was found to be less than 8.7%.For example, more than 80% of the spring CO 2 in the Kangding area was derived from limestone because the sedimentary basement of the Kangding region is mainly comprised of Mesozoic limestone (Qi et al. 2011).The highest mantle CO 2 contribution (8.7%) was found in the Guanding Spring with higher 3 He/ 4 He values (3.4 Ra).Sedimentary organic carbon is a minor contributor, as indicated by widespread travertine deposition throughout the region (Zhang and Hu 2000).

Correlation Between Gas Geochemistry and Seismic Activity
In the Longmenshan seismic zone the increase in R m /Ra and C 13 CO2 d values in the spring gases at Wenchuan Seismostation after the Wenchuan Ms 8.0 earthquake indicated that more mantle-derived fluid migrated to the surface (Fig. 5).The He m was up to 25.8% (Table 2) in the sample collected in October 2008 at Wenchuan Seismostation at the hypocenter area.The geophysical data in LMSF also indicated that the lower velocity bodies under the main shock area and aftershocks in western Sichuan Province can be related to mantle fluid upwelling or partial melting (Lei and Zhao 2009;Zhang et al. 2009;Wang et al. 2010Wang et al. , 2011;;Bai et al. 2011).Gas samples collected near the Gofukuji fault and its surrounding active faults are characterized by an increase in post seismic 3 He/ 4 He ratios in the main shock epicenter after the M w 5.4 central Nagano earthquake (Umeda et al. 2013).
In the XSHF zone, the evident increase of R m /Ra ratios and C values for the gas samples of June 2008 in the Kangding regions (Table 1) indicated there was a percentage of mantle-derived fluid in the hot spring.The proportions of CO 2 mantle in the samples of June 2008 at Longtougou, Erdaoqiao, and Guanding were 8.1, 8.6, and 8.7%, respectively (Table 2).In addition, the calculated high Poisson's ratio (> 0.30) of lithosphere in Kangding region where is the junction of the LMSF, XSHF, and ANHF zones could be attributed to mantle fluid contribution (Wang et al. 2010(Wang et al. , 2011)).
The R m /Ra ratios and C 13 d values in samples from the MJF in June 2008 were clearly higher than data obtained in June 2009, which might be attributable to the Wenchuan Ms 8.0 earthquake (Fig. 2).The proportions of He m in the samples of June 2008 at Chuanpanqiao and Mounigou were 19.4 and 25.0%, respectively (Table 2).
From June 2008 to June 2009, the R m /Ra ratios and C 13 d values from 5 springs with epicentral distances from 270 -445 km in the ANHF and ZMHF zones significantly fluctuated, but did not show clear trends related to the Wenchuan Ms 8.0 earthquake and aftershocks.This indicated the effect of the great earthquake at distance was weaker.
The R m /Ra and C 13 CO2 d values of some hot springs in the LMSF, XSHF, and MJF zones showed a decreasing tendency after Wenchuan Ms 8.0 earthquake, as the aftershocks decreased (Fig. 5c).Over time, contribution of the mantle fluids to the hot springs gradually decreased, but the crustal gas components, radiogenic helium and CO 2 and CH 4 from organic origin increased relatively (Tables 1, 2).4711 aftershocks with Ms ≥ 3.0 were recorded during 12 May 2008 to 30 September 2009 in the LMSF zone, and magnitudes the aftershocks reduced gradually (Fig. 1).In addition, the magnitudes of the He and H 2 anomalies of soil gas declined significantly with decreasing strength of the aftershocks with time in the seismic fault zone produced by the 12 May 2008 Wenchuan Ms 8.0 earthquake (Zhou et al. 2010).
The relationship between the mantle-derived fluid and seismic activity distribution were tangled and depend on active fault motions in western Sichuan Province.The deformation in western Sichuan Province is governed by interactions among three crustal blocks (Songpan, Chuandian, and South China) of distinctive rheological properties under the tectonic framework that eastward growth of the "soft" Eastern Tibet is blocked by the "hard" lithosphere of the South China block (Zhang 2013).The left-lateral XSHF continues to the north-south trending fault system without crustal shortening to form a bounding fault that limits the northern extend of the magnificent clockwise rotation of crustal material around the Eastern Himalaya Syntax.The relative motions among the brittle upper crustal blocks cause strain accumulations among their bounding faults to generate large earthquakes (Wang et al. 2011;Zhang 2013).The boundary faults (LMSF, XSHF, MJF, ANHF, ZMHF) among these three blocks in western Sichuan Province play an important role in generating devastating earthquakes with more than 80% of historical earthquakes having magnitude over 7.0 occurring along the boundary faults (Wen et al. 2008;Zhang 2013).There were several scenarios for upwelling mantle-derived fluids and the large earthquake in boundary faults in western Sichuan Province: (1) large earthquake induced the upwelling of more mantle-derived fluid into the crust and mixed with crustal fluid.(2) The emission of mantle-derived fluids through the crust towards the surface accompanied by an increase in temperature in the deeper reservoirs, which produced an increase in stress in the crust (Polyak et al. 1994;Gilat and Vol 2012).Consequently, the accumulation of thermal energy in the crust might induce deformation and trigger large earthquakes.(3) the permeability barriers in the reverse fault keep the permanent mantle fluid in flux in the upper mid-crust, resulting in highly over pressured fluids in the hypcentral area which trigger large earthquakes by weakening the rock strength over the accumulation and/or explode when the fluid pressure is larger than the strength threshold (Terakawa et al. 2012;Chen et al. 2013).The gas geochemical data in XSHF, MJF in June 2008 suggests the Wenchuan Ms 8.0 earthquake induced the upwelling of more mantle-derived fluid into the crust and mixed with crustal fluid, whereas, in LMSF, the relationship between the Wenchuan Ms 8.0 earthquake and mantlederived fluid need more evidence to confirm.

CONCLUSION
The variations in the isotopic ratios of helium, carbon and gas compositions of gases in the hot springs of western Sichuan Province lead to the following conclusions.
The CO 2 / 3 He, CH 4 / 3 He, R m /Ra, C  The contribution of the mantle fluids to the hot springs gradually decreased with the decay of aftershocks in the LMSF, XSHF, and MJF zones, but the crustal gas components, radiogenic helium and CO 2 and CH 4 from organic

Fig. 1 .
Fig. 1.Location of the sampling sites and the main fault zones in western Sichuan Province together with the epicentres of the 2008 Wenchuan Ms 8.0 earthquake and aftershocks.
Fig. 2. Spatial-temporal variation of the temperature (a), R m /Ra (b), C 13 CO2 d Note: I: Sampling in June 2008, II: Sampling in October 2008, III: Sampling in June 2009.

Fig. 4 .
Fig. 4. Plot of CO 2 / 3 He vs. C 13 CO2 d . The trajectories for binary mixing between M and L, M and S, and L and S are shown in the diagram; the square stands for the samples of June 2008, circles for the samples of October 2008 and diamonds for the samples of June 2009; arrow indicates the increasing trend of CO 2 and 4 He concentrations and decreasing trend of 3 He concentration.CO 2 in gas samples collected from the Qingzhu River in Qingchuan County, one of the regions affected by the 2008 Ms 8.0 giant Wenchuan Earthquake have typical biogenic signatures, with high C 1 /(C 2 + C 3

Fig. 5 .
Fig. 5. Temporal variations of R m /Ra values, C 13 CO2 d 2008 in the Kangding region from 2000 -2009 indicated more mantle fluid contribution to the hot springs which were related to the enhanced degassing triggered by the Wenchuan Ms 8.0 earthquake (Figs.5a, b).The proportions of He m in the samples of June 2008 at Longtougou, Erdaoqiao, and Guanding were 62, 41.5, and 38.9%, respectively springs in the LMSF, XSHF, and MJF zones in June 2008 indicated that more mantle-derived fluids migrated into the crust and mixed with crustal fluid after the Wenchuan Ms 8.0 earthquake.The maximum proportions of the estimated He m values were 62% at Longtougou.The He m was up to 25.8% (