海相碳酸盐岩碳同位素地层学研究中存在的问题及建议——以二叠系研究为例

PROBLEMS AND PROSPECTS OF CARBON ISOTOPIC STRATIGRAPHY OF MARINE CARBONATES: EXAMPLES FROM THE PERMIAN SYSTEM

以二叠系碳同位素地层学研究为例,在有条件控制的实验模拟基础上发现:1)采集到的地质样品一般都是不同碳酸盐矿物的混合样品,这些不同碳酸盐矿物具有不同的碳氧同位素比值(δ13C和δ18O);2)自然露头和采石场剖面样品的无机碳氧稳定同位素组成会在可预见的时间内发生足以影响研究结果的淋滤交代作用;3)利用地层中碳酸盐样品的δ13C研究地层划分和对比时,δ13C的数值和变化趋势受到该地层沉积相等因素的限制;4)利用δ13C和δ18O的相关性判断样品的可靠性时受严格的条件限制。为解决碳同位素地层学研究中的这些困难,通过研究和实验模拟及相关应用尝试,有条件地提出了以下解决方案:1)混合样品的单矿物分析方法;2)通过相差逼近法和模拟交代法判断样品的可靠性,并对其原始δ13C值进行逼近并讨论其可能的应用。对碳同位素比值不能简单用于地层划分与对比的观点进行了讨论并提出了初步的解决方案,同时初步限定了碳同位素地层学研究中的应用及推测的条件。

Laboratory controlled alteration and leaching simulation experiments and the authors＇ empirical data from Permian isotopic chemostratigraphy indicated the following findings,1） nearly all kinds of geological sample for isotopic chemostratigraphic analysisy include a mixture of carbonate minerals,with different minerals having different isotopic fractionations and different δ13C and δ18O values;2） δ13C and δ18O values of carbonate samples collected from natural outcrops and fresh quarries show evidence leaching and alternation in the time scale of years;3） δ13C chemostratigraphic division and correlation is limited by facies control,,δ13C values of carbonate samples are controlled by deposition facies and/or carbonate mineral composition;4） detection of diagenetic alteration via δ13C vs δ18O crossplots has its limitation.To address these problems,this paper propose the following suggestions： 1） δ13C measurements should focus on a single carbonate mineral separated from carbonate rock;2） laboratory simulation of carbon isotopic alteration and leaching experiment should be used to screen samples and to identify least altered δ13C values.The authors also reviewed criticism of possibility of δ13C for worldwide stratigraphy correlation,and propose suggestions for carbon isotopic chemostratigraphy methods.