中国地下热水的基本类型和成因特征

GENETIC CHARACTERISTICS AND BASIC TYPES OF GEOTHERMAL WATER IN CHINA

循环型地下热水的δＤ和δ１８Ｏ具有类似于现代大气降水的线性关系和环境效应，其δＤ低于当地大气降水，据８０年代末检测其氛比值一般小于１，１４Ｃ年龄逾２００００ａＢ．Ｐ．（华北），这说明循环型地下热水起源于区域古大气降水，且补给期为冷湿的雨期。封存型地下热水中贫氘富钙的Ｃｌ－Ｃａ型烃矿床卤水，其δＤ和δ１８Ｏ接近初生水值域，４０Ａｒ／３６Ａｒ值接近海岭型值域，反映水的一级起源──上地幔的脱水排气作用。

Geothermal water in China can be divided, according to their isotopes of hydrogen and oxygen, into the cycling geothermal water and the fossil geothermal water.The cycling geothermal water is derived from hot reservoir, primarily from the deep aquifer underlying a folded belt or outer zone of a large artesian basin system, for example, the Sichuan Basin. It is mathly controlled by hydrostatic pressure. The predominantions of the cycling geothermal water are HCO3-Na, SO4-Na or Cl-Na; and the total dissolved solid (TDS) of the cycling geothermal water is≤30g/L, most of them being 1￣4g/L. Similar to meteoric water, there is a linear correlation be tween δD value and δ8O value of the cycling geothermal water, namely, most points of the cycling geothermal water are located along the average global meteoric water line. The δD values of the cycling geothermal water gradually decrease from the southeast part of China (≥-40‰, SMOW) to the north (≤- 109 ‰, SMOW) with rising latitude and to the west (≤-140‰, SMOW) with rising elevation. Besides,some of the genetic paramenters in the cycling geothermal water are quite similar to relevant values in meteoric water, for example: rNa / rCl value > 0.85, Cl / Br value > 300,40Ar /36Ar ratio≈300, etc.In addition to the temperature, the distinctions between the cycling geothermal water and the local meteoric water are as follows. (1) The δ18O value of the cycling geothermal water is a little higher than that of the local meteoric water, namely, the δ18O-shift value of the cycling geothermal water is quite small, ohly about 1.0‰, SMOW￣1.5‰, SMOW, so that δ8O value of the cycling geothermal water has changed with latitude and elevation.(2) be δD value of the cycling geothermal water is lower than that of the local meteoric water, with the former minus the latter being 7‰ SMOW ￣ 21‰SMOW, so that the cycling geothermal water has poorer 2H than the local meteoric water.(3) The trititum ratios of the cycling geothermal water is lower than that of the local meteoric water, most of the former being less than 1 and the latter 15-60 (measured in the end of 1980' s).It follows that the cycling geothermal water is an age-old water (pre-bomb water),namely, it has not recent recharge. In fact, the 14C-age of the cold ground water stored in the deep bed of aquifer of North China Plain is over 10 000 aB. P., about 10 000a younger than the 14C-age of the cycling geothemal water in the same aquifer. In that case, the 2H-poor characterastics of the Cycling geothermal water denotes that the palaeometeoric water was recharged at the chilly pluvial period of over 20 000aB. P.The fossil geothermal water in the inner zone of the large artesian system is controlled by earthstatic and earthdynamic pressure. The fossil geothermal water is a hot brine, which may be Cl-Ca type of Cl-Na type hot brines. Mostly, the hydrocarbon deposits and salt deposits are related to the Cl-Ca type and Cl-Na type hot brines respectively. The general features of the both are as follows.(1) There are no linear correlations between δD values and δ8O values, and between the δ values and the environmental temperature.(2) Both of them are rich in 18O, most of δ8O values being higher tham 0, and it varies from -2.0‰, SMOW to +6.0‰ SMOW.(3) The TDS is more than 50 (g/L) and even over 500 (g/L). (4) The Cl /Br ratio is less than 300, the rNa/rCl ratio less than 0.85, and the 40Ar /36Ar ratio more than 300.(5) The dating of them is by hundred million years.Also there are distinctions between the hot brines of Cl-Ca type and Cl-Na type.(1) The depth of the aquifer, the concentration of Ca2+ and the ratio of 40Ar /36Ar of the Cl-Ca type hot brine are higher than the relevant values of the Cl-Na type hot. brine.(2)- The ratios of 40Ar /36Ar of the Cl-Ca type brine range from 4440 to 9 255,being part of the submarine ridge's values, and the Cl-Na type brine about 1 000,being part of the continental values.(3) The δD values of the Cl-Ca type hot brine range from -100‰, SMOW to -70‰, SMOW, similar to that of the plutoni