湖泊无机碳酸盐矿物氧同位素组成差值及其单矿物同位素组成的确定

湖积物中不同无机碳酸盐矿物常常混杂在一起,其氧同位素组成(δ18O)差异会影响碳酸盐δ18O环境信息提取的可靠性。不同矿物之间δ18O差值明显且幅度不一。20~25℃时生成的白云石比共生的方解石富集18O可能为0‰~9‰不等,亦或方解石比白云石可能更富集18O达0‰~12.3‰。常温状态,相同条件下形成的文石δ18O值较方解石高出0‰~1‰,或者方解石较文石δ18O值高出0‰~4.47‰。镁方解石中MgCO3的mol百分含量每增加1%,其δ18O值相对于纯方解石δ18O值增加0.06‰~0.17‰。在利用碳酸盐δ18O进行气候及环境研究时,不能根据某种差值进行校正,而应进行单矿物测试。由此,对不同无机碳酸盐矿物的分离及同位素测试、推算方法进行了归纳和述评。

Evaluation of Oxygen Stable-Isotope Fractionation and Single Mineral Isotope Composition from Lacustrine Inorganic Precipitated Carbonates

Lacustrine inorganic precipitated carbonate minerals generally appear in the form of several types co-existing in the sediments.It is of importance to calibrate a possible disparity of oxygen isotope composition(δ~(18) O_c) between co-existing carbonates whilst using δ~(18) O_c as one of the environment proxies for the study of regional climatic evolution.The fractionation differences in δ~(18) O_c between inorganic precipitated dolomite and calcite can be 9‰ to 12.3‰ at 20～25℃,with marked fractionation differences ranging from 1‰ to 4.47‰ between aragonite and calcite at 20～25℃.As MgCO_3 molar content increases by 1% ,δ~(18) O of magnesian carbonate can be enriched by 0.06‰ to 0.17‰.While the δ~(18) O is impossible to be calibrated to one certain carbonate mineral based on a possible disparity,the testing of single carbonate mineral δ~(18) Oc becomes the best choice.The separating methods of different carbonate minerals,and the testing and calculating methods of single carbonate mineral δ~(18) Oc,proposed in the previous researches,are reviewed in the paper.