柴达木盆地白钠镁矾的矿物学和稳定同位素特征

MINERALOGICAL AND ISOTOPIC CHARACTERISTICS OF BLOEDITE IN QAIDAM BASIN

白钠镁矾是卤水演化过程中的次生硫酸盐矿物,含水蒸发盐矿物的镁等稳定同位素特征少有报道。实验选择柴达木盆地西部干盐湖钻孔SG-1中的白钠镁矾,对其进行了矿物学、结晶水氢氧同位素、镁同位素等的研究。柴达木盆地白钠镁矾的晶胞参数为a(nm)=0.553 76,b(nm)=0.824 7,c(nm)=1.111 7,β=100°45′。结晶水失水温度为135℃～180℃和200℃～298℃,Mg^(2+),Na^+和SO_4^(2-)含量分别为63.99×10^(-3)～82.8×10^(-3),134.33×10^(-3)～162.21×10^(-3),567×10^(-3)～641.62×10^(-3)。δ^(26)Mg为-1.163‰～-0.072‰,结晶水的δD为-12.10‰～3.72‰,δ^(18)O为13.07‰～15.34‰,浅部白钠镁矾的镁同位素及结晶水氢氧同位素值均比深部的值偏正,与研究区气候干旱化趋势一致,古湖水受到强烈的蒸发作用,Mg同位素的变化也具有环境指示意义。结晶水氢氧同位素线明显不同于大气降水线,也说明盐类矿物中氧同位素比该钻孔中石膏结晶水氧同位素明显偏正,说明白钠镁矾结晶水来源于大气降水,但受到强烈的蒸发作用影响。

Bloedite is precipitated from brine as a secondary sulfate mineral.Few report stable isotopic data as δ26Mg of salt minerals bearing hydration water.The bloedite of the text from a core SG-1 in Qaidam Basin was examined for its mineralogical and isotopic features.Its cell pa-rameters are:a=0.553 76nm,b=0.824 7nm,c=1.111 7nm and β=10.076 1nm.The dehydration temperature of bloedite ranged from 135℃ to 180℃,and from 200℃ to 298℃.The ion contents of Mg2+,Na+and SO42-are 63.99×10^-3~82.8×10^-3,134.33×10^-3~162.21×10^-3 and 567×10^-3~641.62×10^-3,respectively.Isotopic compositions of hydration water are δD=-12.10‰~3.72‰ and δ18O=13.07‰~15.34‰,respectively.The linear relationship between δ18O andδD is parallel to those of the whole meteoric water line,which indicated that meteoric water was the original source of the bloedite hydration water with strong evaporation.δ26 Mg ranges from -1.163‰ to -0.072‰.All of the above isotopic values at about 112m were higher than those at about 126m as a result of evaporation,it consistent with ariditification drift in the study area,and also sugguest the δ26Mg in the salt minerals can be used in the reconstruction of paleoenvironment and paleoclimate,as well as the δ18O and δD.