西藏南部晚白垩世厚壳蛤的锶同位素年龄标定

黄思静; 石和; 张萌; 武文慧; 沈立成; 黄成刚

西藏南部晚白垩世厚壳蛤的锶同位素年龄标定

成都理工大学油气藏地质及开发工程国家重点实验室四川成都 610059

基金项目:

国家自然科学基金项目 40272059

国家重点基础研究发展规划项目前期研究专项 2001CCA01800

详细信息

中图分类号: P53
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出版历程
- 收稿日期: 2004-11-29
- 刊出日期: 2005-07-25

Strontium Isotope Age Calibration of Rudist Bivalves from Late Cretaceous Section in Southern Tibet

State Key Laboratory of Oil /Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China

摘要

摘要: 沉积岩样品年龄的直接标定是沉积学与地层学研究的难点之一.由于锶在海水中的残留时间(≈106a) 大大长于海水的混合时间(≈103a), 因而同一时间全球海水的锶元素在同位素组成上是均一的, 并造成地质历史中海水的锶同位素组成是时间的函数, 这是锶同位素地层学(SIS) 的基本原理和利用锶同位素地层学进行海相地层定年的理论基础.本文根据锶同位素地层学的基本原理, 测试了西藏南部岗巴剖面上白垩统宗山组上段地层中厚壳蛤化石的锶同位素组成, 尝试对这些化石进行了年龄标定, 4个样品分别位于剖面累计厚度381, 362, 358和296m处, 其⁸⁷Sr/⁸⁶Sr比值分别为0.707832, 0.707769, 0.707768和0.707695, 年龄的标定结果分别是65.68, 69.34, 69.39和72.32Ma, 定年的平均误差为±1Ma左右.研究结果表明, 锶同位素地层学在海相地层定年方面具有潜在价值.
- 西藏南部 /
- 晚白垩世 /
- 厚壳蛤 /
- 锶同位素地层学 /
- 海相地层定年
Abstract: The direct numerical age calibration for sedimentary rocks is more difficult in sedimentology and stratigraphy. The long oceanic residence time of Sr (≈106 a) and the rapid mixing rate of the oceans (≈103 a) have caused the strontium isotope ratio of seawater to be globally homogeneous at any given time, as documented by identical ⁸⁷Sr/ ⁸⁶Sr ratios for coeval marine carbonates, and therefore, the ⁸⁷Sr/⁸⁶Sr ratios are the function of geological time which is the fundamental principle of strontium isotope stratigraphy (SIS) and the theoretical basis of dating marine sediments using SIS. In this paper, ⁸⁷Sr/ ⁸⁶Sr ratios of 4 rudist bivalves collected from a sedimentary section of the Late Cretaceous in the southern Tibet were measured. Based on the very low Mn/Sr ratio (average 0.01) of the samples, it is thought that they contain information on the original seawater strontium isotope composition. The ages of the fossils were calibrated according to the principle of strontium isotope stratigraphy. The 4 samples are located at 381, 362, 358 and 296 m, respectively, with the accumulation thickness in the section. Corresponding ⁸⁷Sr/ ⁸⁶Sr ratios are 0.707 832, 0.707 769, 0.707 768 and 0.707 695, respectively, and the ages calibrated are 65.68, 69.34, 69.39 and 72.32 Ma, respectively. The average error for dating by strontium isotope stratigraphy is approximately ±1 Ma. The research shows a potential value of SIS in dating marine rocks.
- southern Tibet /
- Late Cretaceous /
- rudist bivalve /
- strontium isotope stratigraphy /
- dating marine sediment

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图 1 样品在剖面中的位置

1.微晶灰岩; 2.颗粒灰岩或微晶颗粒灰岩; 3.颗粒微晶灰岩或含颗粒微晶灰岩; 4.厚壳蛤灰岩; 5.有孔虫灰岩; 6.白云石化灰岩; 7.泥灰岩

Fig. 1. Column of Gamba Section showing the sampling locations

下载: 全尺寸图片幻灯片

图 2 厚壳蛤横截面磨光面(样品C取自编号1处)

Fig. 2. Polished cross section of the rudist bivalve

下载: 全尺寸图片幻灯片

图 3 厚壳蛤不同壳层以及体腔充填物和围岩的X射线衍射曲线

1~3.与图 2对应的厚壳蛤的不同壳层; 4.体腔充填物, 成分为微晶灰岩; 5.厚壳蛤样品周围的微晶灰岩

Fig. 3. XRD profiles for different shell layers, celom filling and surrounding rock of the rudist bivalve

下载: 全尺寸图片幻灯片

图 4 a) 65~75 Ma海水的锶同位素演化曲线, 由Howarth and McArthur (1997) 的锶同位素比值-年龄数据表作出, A, B, C, D为本文所讨论的4个厚壳样品的编号; (b) 图(a) 中样品B和C所在部位的局部放大, 1为年龄值置信区间下限值, 2为最佳年龄值, 3为年龄值置信区间上限值

Fig. 4. (a)Variations in ⁸⁷Sr/⁸⁶Sr values of sea water for 65 to 75 Ma(the data are from the look-up table version 8 /96 of Howarth and McArthur (1997). A, B, C and D are the sample numbers for the 4 rudist bivalves studied here.(b) The magnification of the location of samples B and C. 1 is lower confidences limit on age, 2 is the best fit on age, and 3 is upper confidences limit on age

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表 1 样品的采样位置, Mn、Sr含量, Mn/Sr比值, ⁸⁷Sr/⁸⁶Sr比值和对应的年龄值

Table 1. Sampling locations, concentrations of Mn and Sr, ratios of Mn/Sr and⁸⁷Sr/⁸⁶Sr, and corresponding ages of the samples

表 2 研究样品锶同位素定年误差的评估

Table 2. Evaluation for error of dating fossil samples studied here by strontium isotope age

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