晚奥陶世碳酸岩锶同位素(^(87)Sr/^(86)Sr)以及元素(Sr,Mn,Fe)特征的成岩效应  

Diagenetic effects on strontium isotope(^(87)Sr/^(86)Sr)and elemental(Sr,Mn,and Fe)signatures of Late Ordovician carbonates

作　　者：胡东平 李丹丹 周炼[2] 孙立林 徐一伦 Dongping Hu;Dandan Li;Lian Zhou;Lilin Sun;Yilun Xu(School of Earth and Space Sciences,University of Science and Technology of China,Hefei 230026,China;State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Wuhan 430074,China)

机构地区：[1]中国科学技术大学地球和空间科学学院,安徽合肥230026 [2]中国地质大学(武汉)地质过程与矿产资源国家重点实验室,湖北武汉430074

出　　处：《中国科学技术大学学报》2023年第5期22-30,I0003,I0008,I0009,共12页JUSTC

基　　金：supported by the National Natural Science Foundation of China(42073075);the Fundamental Research Funds for the Central Universities(WK2080000147)。

摘　　要：详细评估成岩作用对碳酸岩锶同位素组成以及元素含量的影响是应用其揭示古环境变化的重要前提。本文探究来自美国Monitor Range的一个晚奥陶世剖面碳酸岩^(87)Sr/^(86)的主要控制因素。结果显示,在生物带D.ornatus的中上部,^(87)Sr/^(86)从0.70794升高至0.70830,显著高于已发表的同时期样品的比值(~0.7079)。这一异常高的^(87)Sr/^(86)比值指示了后期、局部过程对研究剖面碳酸岩原始锶同位素信息的改变,因而不能代表同时期海水的锶同位素组成。另外,^(87)Sr/^(86)与成岩作用指标(Mn、Fe含量,Mn/Sr与Fe/Sr比值)呈显著的正相关关系,表明成岩蚀变是所测异常^(87)Sr/^(86)的主控因素。通过成岩流体-岩石反应的数值模型计算,再次证实所测的锶同位素以及元素数据可用成岩作用造成的碳酸岩化学组成变化进行很好的解释。本文结果表明,即使碳酸岩样品满足“[Sr]>300 ppm,[Mn]<300 ppm,[Fe]<1000 ppm,Mn/Sr<0.2,Fe/Sr<1.6”这一严苛的锶同位素保存评判标准,后期成岩作用仍可能显著改变其原始锶同位素组成,提示在应用碳酸岩^(87)Sr/^(86)示踪古环境变化时需更加注意成岩作用的影响。Understanding the effect and extent of diagenesis on the isotopic compositions of Sr in marine carbonates is a critical prerequisite for their use to unravel past environments.Here,we explore the dominant controls on carbonate^(87)Sr/^(86)Sr of a Late Ordovician section from the Monitor Range,USA.Our results reveal a distinct increase in^(87)Sr/^(86)Sr from 0.70794 to 0.70830 in the mid-upper D.ornatus zone,which is markedly higher than the published datasets of contemporaneous samples with a relatively lower and stable^(87)Sr/^(86)Sr ratio of~0.7079.These elevated^(87)Sr/^(86)Sr ratios suggest a local and post-depositional overprint and cannot be interpreted to reflect the^(87)Sr/^(86)Sr of the coeval seawater.Furthermore,^(87)Sr/^(86)Sr exhibits statistically significant positive correlations with geochemical indicators for diagenesis([Mn],[Fe],Mn/Sr,Fe/Sr),indicating that diagenetic alteration is the principal control on the observed radiogenic^(87)Sr/^(86)Sr values.Using a numerical model of marine diagenetic fluid-rock interaction,we demonstrate that the observed Sr isotopic and elemental data can be best explained by the chemical variations in bulk carbonates associated with diagenetic alteration.Our results highlight that diagenesis may significantly alter the pristine^(87)Sr/^(86)Sr ratios of carbonates than previously thought,although the samples satisfy the stricter geochemical criteria of Sr isotope preservation([Sr]>300 ppm,[Mn]<300 ppm,[Fe]<1000 ppm,Mn/Sr<0.2,Fe/Sr<1.6),pointing to the need for more caution when using bulk carbonate^(87)Sr/^(86)Sr as a tracer of paleoenvironmental changes.

关 键 词：锶同位素 碳酸岩 晚奥陶世 成岩蚀变 

分 类 号：P597.2[天文地球—地球化学]