海相石膏的硫同位素

海水硫酸盐参与许多发生在海洋水中和海相沉积物中的氧化还原作用,并且是多种沉积物和矿床中硫的来源。海水硫酸盐的硫同位素组成与海相环境中各种含硫化合物的硫同位素组成有着直接或间接的成因联系。在很大程度上,海水硫酸盐的硫同位素组成提供了海相环境中硫同位素演变的起点。找到这一起点才能正确阐明同时代海相沉积物中硫同位素之间的关系,进而探寻其演变规律。目前,对确定古海洋硫同位素组成最方便的研究对象是海相石膏。

SULFUR ISOTOPES OF SOME MARINE GYPSUM

In this paper,we report the determination of sulfur isotope ratios of 89 marine gypsum (and/or anhydrate) samples,among which 40 samples were collected from Triassic,38 from Ordovician,10 from Cambrian and one from Sinian strata.The results available are shown as follows:1)The δS³⁴ -value variance of marine gypsum or anhydrate from a single layer less than 40 m in thickness is usually no more than 1.5‰,as shown in Fig 1.2)The δS³⁴ - value of marine gypsum or anhydrate from several (about 6-8) layers in a gypsum-bearing formation vary also within a range of 1.5‰,as shown in Fig 2.3)The δS³⁴ -value variance of marine gypsum,in a contemporaneous gypsum-bearing formation is about 2‰(Fig 3) for two localities being at a distance of 130km away.4)The δS³⁴ - value variance of marine sulfates in a single gypsum-bearing cyclothem vary within a range of 2‰(Fig 4).5)The δS³⁴ - values of marine sulfates in a single salt- bearing cyclothem vary within a range of 2‰ (Fig 5).From the results mentioned above,it may be preliminarily concluded that in a sedimentary basin the contemporaneous marine gypsums or anhydrates are very smilar in the sulfur isotope compositions,with the variation no more than 1.5‰.6)The sulfur isotope analyses of 40 samples from Triassic marine deposits indicate that Lower and Middle Triassic marine anhydrates have in a range of δS³⁴-value from 14.9 to 32.4‰ with a variation of 17.5‰,i.e 28.3-32.4‰ and 14.9-20.9‰ respectively.Sulfur isotope composition of the Lower Triassic marine gypsum is enriched in S34 by about 13‰ relative to the Middle Triassic one.(Fig 6).7)According to our data,theδS³⁴ - value is 18.8‰ for the Sinian gypsum with a geologic age of 600-640 m.y.,for the.Lower Cambrian it is 25.3-29.8‰ and for Middle Ordovician it is 21.2-27.7‰,as,shown in Fig 7.This trend of variation iu δS³⁴- value can be comparable with that described by Holser (1977).