Geochemical Characteristics and Circulation Conceptual Model of Geothermal Fluid in the Shenzao Coastal Hot Springs in Guangdong Province
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摘要: 广东神灶温泉出露于海水之中,揭示其流体循环机制对地热资源的可持续开发利用具有重要意义.通过采集神灶温泉区地热水、地热气体和海水样品,测试其流体地球化学组成及主要同位素组成,得到以下认识:神灶温泉水化学类型为Cl-Na-Ca型,主要由大气降水补给;水中盐分主要来自硅酸盐矿物溶解和现代海水混入,海水混入比例为29%~32%.利用化学温度计估算热储温度为130 ℃,地热水循环深度约4 km.地热气体以大气起源N2为主要组分,CO2、CH4为壳内有机沉积物的热变质产物.此外,He同位素指示幔源组分占比不足5%,研究区大地热流值为67~69 mW/m2.综上,神灶温泉区是以壳内放射性生热为主要热源的中温对流型地热系统.Abstract: Shenzao hot springs are exposed in seawater and it is significant to reveal the circulation mechanism of geothermal fluids for the sustainable development and utilization of geothermal resources. By collecting samples of geothermal water, geothermal gas and the seawater, and testing the chemical and isotopic compositions of the samples, this study draws the following conclusions: the hot spring water is of Cl-Na-Ca type, recharged by the precipitation; the dissolved constituents are mainly derived from the dissolution of silicates and mixture with the seawater, with a mixing ratio of 29%-32%. According to the hydrochemical geothermometers, the reservoir temperature is 130 ℃, implying a circulation depth of 4 km. The atmospheric-derived N2 is the dominant component in geothermal gas and the CO2, CH4 are thermogenic products of the crustal organic sediments. In addition, the helium isotopic compositions suggest that the mantle contribution is less than 5% and the local heat flow value is about 67-69 mW/m2. Therefore, the Shenzao geothermal system is a medium-temperature convective system mainly heated by the crustal radiogenic heat source.
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图 1 全球地热带分布(a);中国大陆东南沿海温泉分布(b)(修改自汪集旸等,1996);神灶温泉附近地形(c);神灶温泉采样点分布(d)
Fig. 1. Distribution of global geothermal belts (a); distribution of hot springs in southeast coastal of Chinese mainland (b; modified from Wang et al., 1996); topographical map of the area near the Shenzao hot spring (c); sampling sites at the Shenzao hot spring (d)
图 2 水中主要阳离子(a)、阴离子(b)相对含量三角图
文献中数据来源于Wang et al.(2018)
Fig. 2. Triangle plot of relative content of main cations (a) and ions (b) in water samples
图 3 水同位素δ2H、δ18O关系
图中即墨海水同位素数据引自Hao et al.(2020),广东海水数据引自Chen et al.(2016),地热水数据引自Wang et al.(2018),当地降水线袁建飞(2013)
Fig. 3. Plot of δ2H versus δ18O values
图 5 稀土元素NASC标准化配分图
NASC据Taylor and Mclennan(1985)
Fig. 5. NASC-normalized REE patterns for water samples in this study
表 1 水样的基本特征、主量化学组成、同位素组成及基于SiO2浓度的海水混入比例
Table 1. Major characteristics, chemical compositions and isotopic compositions of geothermal water samples, and the mixing ratios of seawater calculated based on the SiO2 contents
编号 样品类型 水温
(℃)pH TDS
(mg/L)Na
(mg/L)K
(mg/L)Mg
(mg/L)Ca
(mg/L)Cl
(mg/L)SO4
(mg/L)HCO3
(mg/L)SiO2
(mg/L)EI (%) 水化学类型 δ18O
(‰)δD
(‰)混合比
(%)SZ01 地热井 80 7.0 8 730 2 433.9 124.7 9.8 852.4 5 101.5 211.5 71.3 101.2 1.0 Cl-Na∙Ca -6.0 -36.9 0.32 SZ02 地热井 61 7.3 8 840 2 585.6 133.9 10.5 922.2 5 297.3 206.9 63.8 102.5 2.6 Cl-Na∙Ca -5.5 -34.9 0.31 SZ03 地热井 75 7.3 8 110 2 386.7 123.9 9.2 808.9 4 691.1 202.3 63.8 102.3 3.7 Cl-Na∙Ca -5.8 -35.8 0.31 SZ04 地热井 71 7.2 8 130 2 520.4 121.3 8.9 790.0 4 867.3 207.9 73.8 104.7 3.4 Cl-Na∙Ca -5.9 -35.7 0.29 SZ05 海水 26 7.8 28 200 7 594.4 272.7 831.5 350.6 13 144.0 1 821.1 635.9 12.6 0.5 Cl-Na -5.2 -17.3 0.32 表 2 水样部分微量元素、锶同位素比值、摩尔含量比值和稀土元素含量(μg/L)
Table 2. Compositions of some trace elements, molar ratios, isotopic composition of Sr and rare earth elements in water samples (μg/L)
编号 Li B Al Br Sr 87Sr/86Sr Br/Cl Na/Cl La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu SZ01 2 759 517 42.2 12 500.0 17 548.2 0.712 0 0.001 1 0.74 0.9 0.2 1.0 1.1 0.4 37.1 0.4 2.4 0.3 2.9 2.1 6.0 1.6 4.2 SZ02 2 733 479 31.6 11 944.4 17 678.3 0.712 1 0.001 0 0.75 0.2 0.1 0.6 0.3 0.4 25.0 2.7 2.4 2.4 2.9 0.6 8.0 3.5 8.3 SZ03 2 741 571 25.1 11 926.6 15 594.4 0.712 2 0.001 1 0.79 0.3 0.3 0.3 1.2 1.6 8.1 2.9 2.4 2.6 6.7 2.4 6.0 7.4 6.3 SZ04 2 724 466 20.2 11 834.9 15 679.6 0.712 2 0.001 1 0.80 0.4 0.1 0.8 0.8 3.2 16.1 3.8 3.5 1.2 7.7 3.2 4.0 5.5 25.0 SZ05 328 2 675 107.0 25 321.1 5 021.4 0.710 0 0.000 9 0.74 0.4 0.1 1.4 1.1 1.6 1.6 2.5 4.7 1.0 22.1 2.4 4.0 12.6 10.4 表 3 地热气体组分、同位素比值及相关计算
Table 3. The component and isotopic compositions of geothermal gas and correlated calculation
编号 气体组分 碳同位素 氦氖同位素及相关计算 N2
(%)CO2
(%)CH4
(%)Ar
(%)O2
(%)He
(10-6)δ13CCO2
(‰)δ13CCH4
(‰)3He/4He
(10-7)4He/20Ne R/Ra q
(mW/m2)qc/q
(%)T40 km
(±40 ℃)T50 km
(±50 ℃)SZ01 94.34 0.89 0.31 1.54 2.45 2 221 -16.9 -45.3 6.17 132 0.43 69 51.62 848 966 SZ04 79.86 0.12 0.02 0.83 19.09 120 -13.9 -42.5 5.17 5.8 0.36 67 52.83 826 941 表 4 使用地球化学温度计所得的热储温度(℃)及相应公式
Table 4. The results of reservoir temperatures and the corresponding equations
Chalcedony a Chalcedony b Quartz a Quartz b Na-K a Na-K b Na-K c Na-K d Na-K-Ca K-Mg 20SZ08 111 110 138 138 166 184 129 135 159 135 20SZ09 112 110 139 139 166 185 130 136 160 136 20SZ10 112 110 138 138 167 185 130 136 160 136 20SZ11 113 112 140 140 161 180 124 131 158 136 注:Chalcedony a相应公式为$ \frac{1\mathrm{ }032}{4.69-\mathrm{l}\mathrm{g}\mathrm{ }\left(\mathrm{S}\mathrm{i}{\mathrm{O}}_{2}\right)}-273.15 $,据Fournier(1977);Chalcedony b相应公式为$ \frac{1\mathrm{ }112}{4.91-\mathrm{l}\mathrm{g}\mathrm{ }\left(\mathrm{S}\mathrm{i}{\mathrm{O}}_{2}\right)}-273.15 $,据 Arnórsson et al.(1983) ;Quartz a相应公式为$ \frac{1\mathrm{ }309}{5.19-\mathrm{l}\mathrm{g}\mathrm{ }\left(\mathrm{S}\mathrm{i}{\mathrm{O}}_{2}\right)}-273.15 $,据Fournier(1977);Quartz b相应公式为$ \frac{1\mathrm{ }315}{5.205-\mathrm{l}\mathrm{g}\mathrm{ }\left(\mathrm{S}\mathrm{i}{\mathrm{O}}_{2}\right)}-273.15 $,据Truesdell(1976);Na-K a相应公式为$ \frac{1\mathrm{ }217}{1.483+\mathrm{l}\mathrm{g}\mathrm{ }(\mathrm{N}\mathrm{a}/\mathrm{K})}-273.15 $,据Fournier and Potter(1979);Na-K b相应公式为$ \frac{1\mathrm{ }390}{1.750+\mathrm{l}\mathrm{g}\mathrm{ }(\mathrm{N}\mathrm{a}/\mathrm{K})}-273.15 $,据Giggenbach(1988);Na-K c相应公式为$ \frac{833}{0.780+\mathrm{l}\mathrm{g}\mathrm{ }(\mathrm{N}\mathrm{a}/\mathrm{K})}-273.15 $,据Tonani(1980);Na-K d相应公式为$ \frac{933}{0.993+\mathrm{l}\mathrm{g}\mathrm{ }(\mathrm{N}\mathrm{a}/\mathrm{K})}-273.15 $,据Arnórsson et al.(1983) ;Na-K-Ca相应公式为$ \frac{1\mathrm{ }647}{\mathrm{l}\mathrm{g}\mathrm{ }(\mathrm{N}\mathrm{a}/\mathrm{K})+\beta \mathrm{l}\mathrm{g}(\mathrm{C}{a}^{0.5}/\mathrm{N}\mathrm{a})+2.24}-273.15 $,据Fournier and Truesdell(1973),单位为mol/kg,t > 100 ℃且lg(Ca0.5/Na) < 0时,β=1/3;K-Mg相应公式为$ \frac{4\mathrm{ }410}{14.0-\mathrm{l}\mathrm{g}\mathrm{ }({\mathrm{K}}^{2}/\mathrm{M}\mathrm{g})}-273.15 $,据Giggenbach(1988). 上述公式中,除特殊说明外,水化学组分浓度单位为mg/L. -
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