摘要
西藏水热活动是青藏高原碰撞造山过程的产物 ,其成因类型、物质来源和时空分布与青藏高原的隆升过程密切相关 ,地热流体 (气、液相 )中携带有中上地壳乃至地幔物质的深部信息。西藏地热流体可以区分出 CO2 型和 N2 型两类气体 ,其中绝大多数的地热气体样品属于 CO2 型气体 ,而典型的 N2 型气体则较少。前者具有岩浆热源和深循环两种成因类型 ,后者都是深循环成因。西藏气体样品中的 He含量变化范围非常宽 ,最高的可达到 1.5%。在门士热泉 ,首次检测到地幔 He组分 ,这说明西藏地壳深处有地幔物质侵位。根据 He同位素组成推断 ,羊八井、谷露等处的地壳熔融体中约有 3 %的地幔组分。西藏地热气体中的 N2 和 Ar组分主要是大气成因 ,CO2 组分大多以海相碳酸盐岩成因为主 ,混有少量有机沉积物成因 CO2 。当 L og(H2 / Ar)处于 -0 .8～ 0 .3的区间时 ,H2 / Ar地热温度计可以良好地指示热储层的温度范围。实际调查表明 :西藏水热活动区大多分布在班公错—怒江缝合带以南地区 ,高温水热活动区主要出现在雅鲁藏布缝合带和那曲 -羊八井
Geothermal resources are abundant in the Tibetan Plateau. Two types of geothermal gases can be distinguished in the region, i.e. CO2- and N-2-rich gases. In most cases, CO2 is a dominant component of gases, usually higher than. 80% in volume and coexists with Cl-Na or HCO3-Na water. Cl-Na water generally appears in high-temperature geothermal systems associated with magma intrusions. HCO3-Na water presents in low- to medium-temperature geothermal systems warmed by high regional heat flow through meteoric water circulating at deep faults. In case of caverns appearance under sinter layers, CO2-rich gases will present pseudo-N-2-rich gases after fluid boiling, degassing and long-distance lateral movement under the layers. There are a few hot springs with N-2-rich dissolved gases in Tibet, in which N-2 content is more than 90% in volume. N-2-rich gases pre related to low-temperature geothermal systems and only coexist with HCO3-Na water. He content of gas samples is in a wide range, from under detection limit of instrument to 1.5% in volume. There is no relation found among He content, sampling temperature and genesis of hydrothermal systems. He-Ar-N-2 triangular diagram shows that nearly all of helium is generated by the decay of U and Th in the crust. N-2 and Ar are mostly of atmospheric origin. Low N-2/Ar ratio of the samples eliminates the possibility of N-2 and Ar escaping from mantle intrusions. On the basis of helium isotope ratios at well ZK4001 of the Yangbajain and at Gulu hot springs, it is inferred that there is about 3% of mantle component in partial melting layer of the crust in the region. Except the Yangbajain geothermal field and a few other hot springs most of CO2 originates from marine carbonates mixing with a little organic sediment. The relative abundances of geothermal gases can be applied to calculation of temperature in reservoirs. There are two H-2/Ar geothermometers proposed by different authors. When Log (H-2/Ar) value is in the range of -0.8 to 0.3, estimated temperatures from these ones are accordant with each other and close to the measured ones. However, if there is radioactive Ar adding in primordial gases or H-2 degassing during fluids' ascent, deviation of calculated temperature will increase. It is reasonable that H-2/Ar geothermometers can't be applicable for low temperature geothermal systems. In Tibet, the hydrothermal activities are mainly situated on the south side of the Banggong-Nujian suture. Only a few hot springs are located on the north side of this suture. Boiling, hydrothermal eruption and silica deposit manifestations associated with high-temperature geothermal systems are only distributed along the Yarlu-Zangbo suture and S-N striking Nagqu-Yangbajain-Yadong normal faults. On the Tibetan plateau, it is first time to detect the helium of mantle origin at Moincer hot spring. Therefore it indicates mantle component emplacement at the shallow part of the crust.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2002年第4期539-550,共12页
Acta Petrologica Sinica
基金
国家自然科学基金项目 (批准号 :4 96 72 1 6 8
4 9972 0 93)
国际原子能机构 ( CPR90 95/ R1 )联合资助
关键词
地热气体
碳同位素
水热活动
成因
西藏
geothermal gases
carbon isotope
hydrothermal activities
genesis
Tibet
