巴丹吉林沙漠石英δ18O值及其物源意义

巴丹吉林沙漠是世界上沙丘最高大的沙漠，其沙源研究对认识沙漠形成、高大沙山发育和防沙治沙工程有重要意义。石英是沙漠沉积物中常见的矿物，其氧同位素值可示踪物源。采集沙漠西北部、东部、东南部高大沙山、丘间低地与湖泊以及雅布赖山前的表层沉积物，测定了样品不同粒级的石英δ18O值。结果表明:①石英δ18O值随粒级减小有增大趋势，同一样品不同粒级石英δ18O值存在较大差异，相同粒级石英δ18O值也有变化。②石英δ18O值介于9.4‰~19.3‰，均值为13.3‰（n=55）；其中沙丘沙的石英δ18O值介于9.5‰~16.6‰，均值为12.9 ‰（n=39）；湖相沉积物的石英δ18O值介于9.4‰~19.3‰，均值为14.2‰（n=16）。③区域内，<16 μm粒级的石英δ18O值与16~64 μm、125~154 μm、200~250 μm、> 300 μm粒级的石英δ18O值都存在显著差异，但200~250 μm与 > 300 μm粒级的石英δ18O值没有显著差异；经区域对比，巴丹吉林沙漠 < 16 μm粒级的石英δ18O值与柴达木盆地沙漠、蒙古戈壁风成沉积物 < 16 μm石英δ18O值无显著差异，但巴丹吉林沙漠16~64 μm粒级的石英δ18O值与蒙古戈壁风成沉积物16~64 μm石英δ18O值存在显著差异；这似乎暗示研究区的细颗粒物质可能是远源的。巴丹吉林沙漠沉积物的石英δ18O值位于火成岩石英、砂岩和变质岩石英δ18O值分布阈值内，受区域地质条件、物源混合、粒级效应等因素的影响，砂粒级的石英δ18O值所指示的母岩成份特征与祁连山区岩石的岩性有较好吻合。

Characteristics of Oxygen Isotopic Compositions of Quartz in the Badain Jaran Desert and Its Implications for Sand Provenances

Badain Jaran Desert is the second largest area of shifting sands in China and features the tallest dunes on Earth. Identifying provenance of aeolian deposits in the Badain Jaran Desert is critical to understand the formation and development of mega-dunes. Quartz is the most common mineral of Chinese desert sediments, and oxygen isotopic composition of quartz has been regarded as a useful tracer of provenance. Oxygen isotopic compositions were measured by isotopic techniques in various size fractions of quartz, which were extracted from aeolian and lacustrine sediments collected on northwestern, eastern, and southeastern part in the Badain Jaran Desert. The results indicate that the δ18O values increase with decreasing grain size in quartz from aeolian and lacustrine sediments, and the δ18O composition of quartz in various size fractions has notable variations ranging from 3.1‰ to 9.9‰. The δ18O values in quartz in all size fractions are between 9.4‰ and 19.3‰, in which aeolian sands range from 9.5‰ to 16.6‰, and lacustrine sediments vary from 9.4‰ to 19.3‰. In the study area, the δ18O values of fine-grained (<16 μm) quartz show significant differences between different size fractions of the grains (i.e., 16~64 μm,125~154 μm, 200~250 μm, >300 μm), whereas two coarser fractions (200~250 μm and >300 μm) exhibit no significantly different in the δ18O values. Furthermore, when isotopic results are compared with available findings at regional scale, the δ18O values of fine-grained (<16 μm) quartz have no significant difference between Badain Jaran Desert, Qaidam Basin Desert, and Mongolian Gobi Desert, but that of silt-grained (16~64 μm) quartz exist notable difference between the Badain Jaran Desert and Mongolian Gobi Desert. From these observations, it appears likely that the fine-grained components in each sample probably have a more distal origin. A large range of δ18O values, from 9.4‰ to 19.3‰ is present within the aeolian dune and lacustrine sediments, fall within the range of values considered to be indicative of igneous, metamorphic, and sedimentary rocks. The variable δ18O values seem to result from the mixing ratio of igneous, sedimentary and metamorphic rocks. These differences in δ18O values can be related to regional geological setting, mixing ratio of various detrital components, and grain size dependence so on. In the dominant size fraction (125~154 μm, 200~250 μm) of sediments in Badain Jaran Desert, δ18O values are mostly between 11.0 ‰ and 14.9‰, but extend down to 9.4‰. It suggests that quartz detritus is mostly derived from sedimentary and metamorphic rocks, which is considered as mostly originating from Qilian Shan mountainous areas although the local source cannot be ruled out.