四川盆地五峰组—龙马溪组页岩U-Mo协变模式与古海盆水体滞留程度的判识  

U-Mo covariation in marine shales of Wufeng—Longmaxi Formations in Sichuan Basin, China and its implication for identification of watermass restriction

作　　者：李艳芳[1] 吕海刚[1] 张瑜[1] 张小龙[1] 邵德勇 闫建萍[1] 张同伟[1] 

机构地区：[1]兰州大学地质科学与矿产资源学院甘肃省西部矿产资源重点实验室,甘肃兰州730000

出　　处：《地球化学》2015年第2期109-116,共8页Geochimica

基　　金：国家重点基础研究发展计划(2012CB214701);国家自然科学基金(41202095)

摘　　要：古海盆海水的氧化还原条件和滞留状况可通过沉积物中微量元素的富集程度来指示。本文利用页岩中U、Mo微量元素含量,研究了五峰组和龙马溪组沉积时期四川盆地的水体滞留情况。龙马溪组页岩在厌氧环境中Mo/TOC平均值为15,接近于半滞留海盆。且随着还原程度的增强,U、Mo富集系数趋于增加,但海水中较低的Mo浓度限制了Mo/U比值的继续增加,稳定在1倍海水值左右,指示龙马溪期四川盆地属于半滞留海盆。五峰组页岩Mo/TOC平均值为4.7,接近于黑海(4.5)这样的强滞留环境。U、Mo富集系数明显小于龙马溪组,在缺氧的环境中未表现出随富集系数增大Mo/U比值增大的趋势,且Mo/U比值低于正常海水值,表明五峰组沉积时期海盆的滞留程度要比龙马溪组沉积时期强烈,属于强滞留环境。五峰组沉积时期强烈的海盆滞留导致水体交换速度缓慢,形成了有利于有机质保存的缺氧环境,而龙马溪组沉积时期海平面上升造成的缺氧环境控制了半滞留海盆有机质的富集。Redox conditions and degree of watermass restriction in a sedimentary basin can be inferred from enrichment anddepletion of trace elements in sediments. Based on uranium （U） and molybdenum （Mo） concentration of Wufeng-Longmaximarine shales, the degree of watermass restriction in Sichuan basin during late Ordovician-early Silurian period wasevaluated. The average Mo/TOC ratios of Longmaxi Formation shales under anoxic environment is 15, which is similar to theratios of moderately restricted basins. U and Mo enrichment factors tend to increase at more reducing redox conditions, butthe tendency toward high Mo/U ratios can be offset by low aqueous Mo concentrations, which would lead to constant Mo/Uratios being around two times of the ratios for sea water. These observations indicate that sedimentation during earlyLongmaxi period occurred under moderately restricted conditions. In contrast, the average Mo/TOC ratios of WufengFormation shales are 4.7, which is close to the ratios for extremely restricted Black Sea, while the U and Mo enrichmentfactors are low in relation to those for Longmaxi Formation shales. A lack of trend toward higher Mo/U ratio with increasingenrichment factors under anoxic environment, and Mo/U ratios being lower than that of normal sea water suggest that thesedimentation during Wufeng period was under strongly restricted conditions, which would lead to slow deep-water renewalrates, resulting in an anoxic environment which is favorable for the preservation of organic matter. As a result, it can be saidthat the anoxic environment caused by sea level rising resulted in the moderately restricted Longmaxi Formation andsubsequent accumulation of organic matter.

关 键 词：有机质富集 U、Mo富集系数 盆地滞留 页岩气 四川盆地 

分 类 号：P595[天文地球—地球化学]