Identification and quantification of factors controlling hydrogen and oxygen isotopes of geothermal water: An example from the Guide Basin, Qinghai Province

doi:10.13745/j.esf.sf.2020.7.8

Abstract

Abstract:

Hydrogen and oxygen isotopes have been widely used in the study of geothermal fluid. However, due to the influences of multiple factors, the understanding of the processes controlling the hydrogen and oxygen isotopic compositions of geothermal water is often incomplete. Here, we conducted a case study on the geothermal water from the two faults along the margin of the Guide Basin. We collected water samples from five hot spring sites and compared the differences in hydrochemical components and geothermal reservoir temperatures of these samples. We found an approximate relationship between the degree of water-rock interaction and whether the sample deviates from the rainfall line. At the intensively studied Zhacangsi site, previous researchers have quantified the effect of cold water mixing in thermal water and determined the cold water mixing ratio. Based on this ratio, we estimated the magnitude of change in hydrogen and oxygen isotopes caused by cold water mixing and liquid-vapor separation. We also established the isotopic compositions of the geothermal water before water-rock interaction based on the effect of the interaction on the “oxygen shift”. The results were consistent with the isotopic compositions of geothermal water from sites with low degrees of water-rock interaction. The methods proposed in the current study can be applied to analyzing hydrogen and oxygen isotopes in other geothermal fields to enrich our understanding of the circulation process of geothermal fluid.

Key words: hydrogeochemical characteristics, hydrogen and oxygen isotopes, water-rock interaction, liquid-vapor separation, Zhacangsi

CLC Number:

P641.3

DAI Wan, JIANG Xiaowei, LUO Yinfei, ZHANG Hong, LEI Yude, TONG Jue. Identification and quantification of factors controlling hydrogen and oxygen isotopes of geothermal water: An example from the Guide Basin, Qinghai Province[J]. Earth Science Frontiers, 2021, 28(1): 420-427.

Figures/Tables 9

Fig.1 Geographical location of the study area and sampling sites of cold and geothermal waters

Table 1 Measurements of hydrochemical components and oxygen and hydrogen isotopes for thermal and cold water samples from the Guide Basin

类型	编号	地名	类别	井深/ m	pH	温度/ ℃	阴阳离子平衡误差/%	ρ_B/(mg·L^-1)										δ¹⁸O/ ‰	δD/ ‰
类型	编号	地名	类别	井深/ m	pH	温度/ ℃	阴阳离子平衡误差/%	C1^-	S $O 4 2 -$	C $O 3 2 -$	HC $O 3 -$	Na⁺	K⁺	Mg²⁺	Ca²⁺	TDS	SiO₂	δ¹⁸O/ ‰	δD/ ‰
地热水	HS1-QNH	贵德县曲乃亥	温泉		8.1	>93.0	8.70	1 079.9	205.2	0.0	111.0	632.0	48.3	0.2	38.7	2 059.9	182.1	-9.7	-79
	HS2-QNH	贵德县曲乃亥	温泉		8.3	54.0	3.15	916.3	222.2	6.4	91.4	621.0	48.0	0.5	38.3	1 898.8	169.5	-9.1	-78
	HS1-ZCS	贵德县扎仓寺	温泉		8.1	81.9	1.86	464.7	465.4	0.0	65.3	430.0	17.6	0.6	75.8	1 487.8	102.4	-9.7	-81
	HW1-ZCS	贵德县扎仓寺	地热井	110	8.4	>93.0	0.85	471.0	440.7	19.3	26.1	443.0	20.0	0.4	66.3	1 473.7	119.5	-10.2	-82
	HW2-ZCS	贵德县扎仓寺	地热井	100	8.6	52.7	1.11	448.7	469.0	19.3	26.1	441.0	19.7	0.2	65.4	1 476.6	100.2	-10.2	-83
	HS-XJ	贵德县新街	温泉		8.5	74.0	1.81	28.4	235.4	12.0	109.8	156.0	5.5	2.4	10.0	504.6	68.9	-12.8	-94
	HS-LC	同仁县兰采	温泉		8.7	87.0	0.74	42.5	148.9	30.0	73.2	138.0	4.0	0.5	5.2	405.8	110.7	-12.3	-90
	HS-XBS	同仁县西卜沙	温泉		8.8	46.6	1.06	31.9	158.5	18.0	48.8	118.0	2.0	0.5	5.2	360.7	65.5	-12.4	-89
冷水	R-QNH	贵德县曲乃亥	河水		8.5	30.8	-0.19	60.4	44.6	0.0	261.2	63.2	4.6	21.4	45.8	372.5	9.9	-8.4	-59
	CS-QNH	贵德县曲乃亥	冷泉		7.5	24.5	-4.77	351.2	207.5	0.0	417.9	345.0	5.2	47.7	81.4	1 247.2	14.6	-10.2	-74
	CS-ZCS^*	贵德县扎仓寺	冷泉		—	10.0	-1.50	96.3	494.9	-	219.7	272.2	—	34.4	48.6	1 172.0	—	-8.6	-59
	R-XJ	贵德县新街	河水		8.5	18.0	-5.86	4.7	12.3	6.4	169.8	13.7	1.3	5.8	53.9	183.7	7.8	-8.9	-57
	R-LC	同仁县兰采	河水		8.5	19.5	-2.90	5.8	11.4	12.8	150.2	6.1	1.4	6.3	53.2	173.1	11.2	-8.7	-59
	CS-LC	同仁县兰采	冷泉		8.2	23.4	-3.98	9.1	20.5	0.0	215.5	11.1	1.5	12.3	60.4	225.0	11.2	-9.4	-62
	R-XBS	同仁县西卜沙	河水		8.2	20.0	-9.55	5.7	14.4	0.0	189.4	7.8	1.8	12.1	58.5	196.1	7.0	-9.2	-61

Table 1 Measurements of hydrochemical components and oxygen and hydrogen isotopes for thermal and cold water samples from the Guide Basin

类型	编号	地名	类别	井深/ m	pH	温度/ ℃	阴阳离子平衡误差/%	ρ_B/(mg·L^-1)										δ¹⁸O/ ‰	δD/ ‰
类型	编号	地名	类别	井深/ m	pH	温度/ ℃	阴阳离子平衡误差/%	C1^-	S $O 4 2 -$	C $O 3 2 -$	HC $O 3 -$	Na⁺	K⁺	Mg²⁺	Ca²⁺	TDS	SiO₂	δ¹⁸O/ ‰	δD/ ‰
地热水	HS1-QNH	贵德县曲乃亥	温泉		8.1	>93.0	8.70	1 079.9	205.2	0.0	111.0	632.0	48.3	0.2	38.7	2 059.9	182.1	-9.7	-79
	HS2-QNH	贵德县曲乃亥	温泉		8.3	54.0	3.15	916.3	222.2	6.4	91.4	621.0	48.0	0.5	38.3	1 898.8	169.5	-9.1	-78
	HS1-ZCS	贵德县扎仓寺	温泉		8.1	81.9	1.86	464.7	465.4	0.0	65.3	430.0	17.6	0.6	75.8	1 487.8	102.4	-9.7	-81
	HW1-ZCS	贵德县扎仓寺	地热井	110	8.4	>93.0	0.85	471.0	440.7	19.3	26.1	443.0	20.0	0.4	66.3	1 473.7	119.5	-10.2	-82
	HW2-ZCS	贵德县扎仓寺	地热井	100	8.6	52.7	1.11	448.7	469.0	19.3	26.1	441.0	19.7	0.2	65.4	1 476.6	100.2	-10.2	-83
	HS-XJ	贵德县新街	温泉		8.5	74.0	1.81	28.4	235.4	12.0	109.8	156.0	5.5	2.4	10.0	504.6	68.9	-12.8	-94
	HS-LC	同仁县兰采	温泉		8.7	87.0	0.74	42.5	148.9	30.0	73.2	138.0	4.0	0.5	5.2	405.8	110.7	-12.3	-90
	HS-XBS	同仁县西卜沙	温泉		8.8	46.6	1.06	31.9	158.5	18.0	48.8	118.0	2.0	0.5	5.2	360.7	65.5	-12.4	-89
冷水	R-QNH	贵德县曲乃亥	河水		8.5	30.8	-0.19	60.4	44.6	0.0	261.2	63.2	4.6	21.4	45.8	372.5	9.9	-8.4	-59
	CS-QNH	贵德县曲乃亥	冷泉		7.5	24.5	-4.77	351.2	207.5	0.0	417.9	345.0	5.2	47.7	81.4	1 247.2	14.6	-10.2	-74
	CS-ZCS^*	贵德县扎仓寺	冷泉		—	10.0	-1.50	96.3	494.9	-	219.7	272.2	—	34.4	48.6	1 172.0	—	-8.6	-59
	R-XJ	贵德县新街	河水		8.5	18.0	-5.86	4.7	12.3	6.4	169.8	13.7	1.3	5.8	53.9	183.7	7.8	-8.9	-57
	R-LC	同仁县兰采	河水		8.5	19.5	-2.90	5.8	11.4	12.8	150.2	6.1	1.4	6.3	53.2	173.1	11.2	-8.7	-59
	CS-LC	同仁县兰采	冷泉		8.2	23.4	-3.98	9.1	20.5	0.0	215.5	11.1	1.5	12.3	60.4	225.0	11.2	-9.4	-62
	R-XBS	同仁县西卜沙	河水		8.2	20.0	-9.55	5.7	14.4	0.0	189.4	7.8	1.8	12.1	58.5	196.1	7.0	-9.2	-61

Table 2 Quartz saturation indices and geothermal reservoir temperatures of the five studied hot springs

地点	饱和指数	石英温标/℃	钻孔温度/℃
西卜沙	0.40	115
新街	0.19	118
兰采	0.20	143
扎仓寺	0.24	147	150
曲乃亥	0.46	174

Fig.2 Piper diagram of geothermal and cold waters

Fig.3 Relationship between Na/Cl ratio and Cl- content (a) or geothermal reservoir temperature (b) of geothermal water

Fig.4 The Na-K-Mg triangle diagram of geothermal water

Fig.5 Relationship between hydrogen and oxygen isotopes of geothermal and cold waters

Table 3 Results of calculated hydrogen and oxygen isotopic compositions of geothermal water in Zhacangsi

编号	实测数据		水汽分离前(冷水混合后)		冷水混合前(水岩反应后)		水岩反应(初始)
编号	δD/‰	δ¹⁸O/‰	δD/‰	δ¹⁸O/‰	δD/‰	δ¹⁸O/‰	δD/‰	δ¹⁸O/‰
HS1-ZCS	-81	-9.7	-81.8	-9.82	-92.5	-10.39	-92.5	-12.66
HW1-ZCS	-82	-10.2	-82.2	-10.25	-93.1	-11.02	-93.1	-12.73
HW2-ZCS	-83	-10.2	-83.2	-10.27	-94.6	-11.06	-94.6	-12.90

Fig.6 Schematic diagram showing the migration processes of hydrogen and oxygen isotopes in Zhacangsi

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