贵州正安县奥陶系—志留系界线碳质泥岩Re-Os同位素精确厘定及其古环境反演

李欣尉; 李超; 周利敏; 赵鸿

doi:10.15898/j.cnki.11-2131/td.201907310116

贵州正安县奥陶系—志留系界线碳质泥岩Re-Os同位素精确厘定及其古环境反演

Accurate Determination of the Age of the Carbonaceous Mudstone of the Ordovician-Silurian Boundary in Zheng'an County, Guizhou Province by Re-Os Isotope Dating Method and Its Application in Paleoenvironmental Inversion

摘要

摘要: 我国上扬子地台地区在奥陶系—志留系之交广泛发育蕴含丰富页岩气资源的五峰组—龙马溪组富有机质沉积岩。采用Re-Os同位素体系对该套沉积地层进行研究，不仅能得到精确的地层沉积年龄，同时根据Re、Os元素的富集机制，对该时期沉积环境进行有效反演，可以为这一阶段发生的地球历史上第二大规模的生物绝灭事件的触发机制提供更合理的解释。本文在贵州正安县班竹1井岩心采集11件碳质泥岩样品，岩心样品连续且完整跨越奥陶系五峰组—志留系龙马溪组界线地层，通过对该样品开展高精度Re-Os同位素研究，获得了奥陶系—志留系地层界线Re-Os同位素年龄为443.68±6.24Ma2σ，n=7，（¹⁸⁷Os/¹⁸⁸Os）_i=0.699±0.019，MSWD=0.55，其结果与国际地层委员会发布的年龄（443.7±1.5Ma）高度一致，为奥陶系—志留系界线年龄提供了直接、准确的年龄依据。Os同位素特征反映了大量陆源碎屑参与成岩过程、多期火山活动的发生及冰期向间冰期的转换。连续沉积地层Re-Os同位素特征的变化反映了研究区奥陶系五峰组—志留系龙马溪组沉积环境经历富氧—缺氧—富氧的变化，指示赫南特期冰川事件和火山喷发共同造成了生物大绝灭并促进了有机质的富集，为五峰组—龙马溪组富有机质沉积岩提供了生烃潜力。

Abstract:
BACKGROUNDOrganic-rich sedimentary rocks of the Wufeng-Longmaxi Formation are widely developed in the Ordovician-Silurian boundary of the Upper Yangtze platform in China, which is rich in shale gas resources. Using the Re-Os isotope system to study this set of sedimentary formations, not only can the precise age of the formation be obtained, but also the sedimentary environment of this period based on the enrichment mechanism of Re and Os elements can be inferred. This provides a more reasonable explanation for the trigger mechanism of the second large-scale biological extinction event in Earth's history.
OBJECTIVESTo accurately constrain the age of carbonaceous mudstone and infer the conditions of the paleoenvironment.
METHODSThe 11 carbonaceous mudstone samples from dirll core of Banzhu No.1, Zheng'an County, Guizhou Province were studied. These dirll core samples were continuous across the boundary of the Ordovician Wufeng Formation-Silurian Longmaxi Formation. Through the high precision Re-Os isotopic dating of the 11 samples, the Ordovician-Silurian boundary stratigraphic age was obtained.
RESULTSThe Re-Os isotope age was calculated to be 443.68±6.24Ma2σ, n=7, (¹⁸⁷Os/¹⁸⁸Os)_i=0.699±0.019, MSWD=0.55. The results were highly consistent with the age (443.7±1.5Ma) published by the International Commission on Stratigraphy, which provided a direct and accurate age basis for the Ordovician-Silurian boundary. Os isotope characteristics showed that amounts of terrigenous detrital were involved in the diagenesis, the occurrence of multi-stage volcanic activities and the transition from glacial period to interglacial period. The Re-Os isotopic features of the continuous sedimentary strata reflected that the sedimentary environment of the Ordovician Wufeng Formation-Silurian Longmaxi Formation had undergone the change of oxygen enrichment-oxygen enrichment-rich oxygen enrichment in this study area.
CONCLUSIONSHirnantian glaciation events and volcanic eruption caused biological extinction and together promoted organic matter enrichment, providing hydrocarbon generation potential for the Wufeng Formation-Longmaxi Formation organic-rich sedimentary rocks.

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