自生片钠铝石的碳氧同位素特征及其成因意义

Carbon-Oxygen Isotopic Characteristics of Authigenic Dawsonite and Its Genetic Significance

自生片钠铝石的碳氧同位素特征可以为片钠铝石的成因研究提供重要的地球化学依据。以海拉尔盆地乌尔逊凹陷和松辽盆地孤店构造片钠铝石碳氧同位素分析为基础,结合国内外已报道的自生片钠铝石的碳氧同位素数据,对自生片钠铝石的碳氧同位素特征及其成因意义进行了探讨。研究表明,海拉尔盆地乌尔逊凹陷和松辽盆地南部孤店CO2气田砂岩中片钠铝石的δ13C范围分别为-5.3‰^-1.5‰和-1.9‰^+0.3‰,均分布于含无机碳物质δ13C分布区间(-9.0‰^+2.7‰)内。计算出的海拉尔盆地和松辽盆地与片钠铝石平衡的CO2气的δ13C分布范围分别为-10.7‰^-7.0‰和-8.7‰^-6.9‰,表明片钠铝石绝大部分形成于无机CO2背景。实际地质观察中形成片钠铝石的CO2绝大多数为岩浆脱气来源,岩浆成因片钠铝石的δ13C分布范围为-5.5‰^+4.5‰。海拉尔盆地和松辽盆地的片钠铝石是岩浆成因CO2气运移、聚集的特征矿物。计算出的海拉尔盆地和松辽盆地片钠铝石沉淀时介质水的δ18O值范围为-14.3‰^-9.4‰,表现为轻同位素的特点,表明片钠铝石形成时地层水为大气降水。计算出的海拉尔盆地片钠铝石同位素温度为52.7~93.6℃,与样品所在深度处的古地温范围(65.4~87.6℃)基本吻合。

Carbon-oxygen isotopic characteristics of authigenic dawsonite can provide important geochemical evidence for research of its origin. Based on carbon-oxygen isotope analysis of dawsonite from Wuerxun Depression, Hailaer Basin and Gudian Structure in Songliao Basin, combined with carbon-oxygen isotopic data reported in domestic and overseas literature, the carbon-oxygen isotopic characteristics of authigenic dawsonite and its origin significance are discussed in this paper. It is determined that the range of dawsonite δ^13 values are -5.3‰ -1.5‰ and -1.9‰- ＋0.3‰, respectively in Wuerxun Depression of Hailaer Basin and Gudian Structure of Southern Songliao Basin. These values are within the range of inorganic carbonaceous substance δ^13 distribution （ -9. 0‰- ＋2.7‰）. Furthermore, the calculated δ^13 values of CO2 gas in isotope equilibrium with dawsonite range from - 10.7 to - 7.0‰ PDB and from - 8.7 to - 6.9‰, respectively in Hailaer Basin and Songliao Basin, which shows that the CO2 for forming dawsonite was originated under an inorganic background. In our case, CO2 for forming Dawsonite practically belonged to magmatic origin, with the carbon isotope range from - 5.5 to ＋ 4.5‰. The dawsonite in Hailaer Basin and Songliao Basin is a diagnostic mineral recording migration and accumulation of CO2 with magmatic origin. The calculated formation water δ^13 values during dawsonite deposition range from -14.3 to -9.4‰, representing the characteristics of meteoric water. The calculated isotopic temperature of dawsonite is between 52.7% and 93.6℃, generally in accordance with the palaeo-stratigraphic temperature （65.4 87.6% ）.