四川黄龙沟天然水中的深源CO_2与大规模的钙华沉积

在有大规模钙华沉积的四川黄龙沟中,使用化学成分数据以及碳氧稳定同位素组成对其水文地球化学特征进行了分析研究。研究区钙华沉积的地表溪流水质基本上受到两种水混合的制约,即断层泉水和山区的融雪(冰)水。泉水中含有高浓度的经由断层提供的CO2,结果高浓度的溶解CO2使得其溶解的碳酸盐岩比普通的岩溶泉溶解的碳酸盐岩高得多,同时也导致硅酸盐岩的溶解。黄龙沟中上游的泉水相对于方解石接近于平衡。溶解无机碳(DIC)的浓度和它们的δ13C值是大约由c=0.02mol·L1δ13C=-3‰的CO2(aq)与含有δ13C=+3‰的碳酸盐岩在封闭系统条件下反应的结果。估计这些CO2中约有70%来自上地幔。所有泉水的水化学数据均落在高岭石稳定域内,但对Na长石和Ca长石具有侵蚀性。由于这些长石矿物的溶解速率太慢,所以水中的化学成分远离长石稳定域。地表溪流的DIC种类之间达到同位素平衡,在不同观测点发现的δ13CDIC变化主要是由于从水中释放出的CO2的程度不同引起的。水样的δ18O值与其采集点的海拔高度之间存在线性关系;研究区的地表溪流的氧同位素组成受到蒸发的制约。在流经钙华沉积物的地表溪流中白天和夜晚的水化学及pH的日变化表明生物作用促进了碳酸盐的沉积,尽管作用不显著。据估计研究区碳酸钙的日沉积速率是4778kg·km2,即约1mm·a1。

DEEP-SOURCE CO2 IN NATURAL WATERS AND ITS ROLE IN EXTENSIVE TUFA DEPOSITION IN THE HUANGLONG RAVINES, SICHUAN, CHINA

The aqueous geochemistry of Huanglong Ravine, Sichuan, China, where tufa is being deposited extensively, is discussed using chemical composition data together with the carbon and oxygen stable isotope ratios. The water quality of tufa-depositing surface streams in this area is basically controlled by mixing of two sources of water - fault-bounded spring water, and snow and/or glacier molten water. The spring water has high concentrations of CO_2 supplied via faults: the resulting high concentration of dissolved CO_2 gives rise to much greater dissolution of carbonate rocks than usual in karst springs and also to the dissolution of silicate rocks. Upstream spring waters in the Ravine are close to equilibrium with respect to calcite. The concentrations of dissolved inorganic carbon (DIC) and their δ~(13)C values result from interaction between about 0.02 mol·L~(-1) CO_2(aq) of δ~(13)C -3 ‰ with carbonate rocks of δ~(13)C +3 ‰ under closed system conditions. About 70 % of the CO_2 is estimated to be supplied from the upper mantle. The data for all the springs fall in the kaolinite stability field, in agreement with observed dissolutional aggressiveness towards Na- and Ca-feldspar, and that the dissolution rates of these minerals are too slow to move the composition data of these waters into the feldspar stability field. The DIC species in the surface streams are in isotopic equilibrium; variations in δ~(13)C_(DIC) found at different sites are mainly due to differences in the degree of CO_2 degassing from the water. There is a linear relationship between δ~(18)O values and the altitude at which samples were collected; the oxygen isotopic composition of surface stream water in this area is controlled by evaporation. Water chemistry in daytime and nighttime and daily change of pH in the surface water flowing on the tufa deposition indicate that biological activities contribute to the carbonate precipitation, although their extent is not great. The daily precipitation rate in this area is estimated to be 4 778 kg as CaCO_3 d~(-1) over a surface area of 1 km~2, i.e., 1 mm· a~(-1)