沉积地层中的黄铁矿形态及同位素特征初探——以华南埃迪卡拉纪深水相地层为例

地质历史时期新元古代大气氧含量普遍较低。在硫酸盐还原细菌作用下，作为海洋重要的氧化性离子，陆源硫酸根离子有效促进了深层海水的氧化进程。在此过程中，硫元素在硫酸根和黄铁矿之间发生显著同位素分馏，其分馏程度可反推当时古海洋的氧化还原状态。沉积地层中的黄铁矿普遍具有多种形态，不同形态黄铁矿的形成环境多有不同。如草莓状黄铁矿多形成于底层缺氧水体或沉积物的浅表面，而大颗粒单晶黄铁矿或脉状黄铁矿则多沉积于成岩早期的沉积物孔隙或形成于成岩后期的热液改造。与草莓状黄铁矿不同，大颗粒单晶或脉状黄铁矿的硫同位素组成并不能反映沉积时期的古海洋氧化还原条件。判定沉积地层中不同形态的黄铁矿及形成过程，是获得有效反映海洋沉积环境硫同位素组成特征的基本前提。简要总结了地质历史时期沉积地层中的黄铁矿类型及矿物形成过程，并以华南埃迪卡拉纪蓝田组岩芯样品为例，识别出各个样品中的黄铁矿形态组成特征，对比分析了全岩黄铁矿与样品中大颗粒黄铁矿硫同位素组成差异。研究结果表明:不同岩性样品中黄铁矿的形态种类及含量均存在差异。页岩样品保存有更好形态的自形晶以及草莓状黄铁矿；碳酸盐岩样品中具有较多自形晶以及他形晶黄铁矿，并且其中的少量草莓状黄铁矿遭受后期成岩作用而发生不同程度的晶体蚀变。样品中大颗粒黄铁矿的硫同位素值（δ34S_L-pyr）通常显著高于全岩黄铁矿的硫同位素值（δ34S_T-pyr），最大差值可达48.5‰。在利用黄铁矿的硫同位素组成来反推当时古海洋环境时，需要区分不同形态黄铁矿，仔细剔除大颗粒黄铁矿，降低成岩期黄铁矿对样品中硫同位素组成的影响。更细致的微区黄铁矿硫同位素分析工作将依赖于SIMS分析测试手段进行。

Morphologic and Isotopic Characteristics of Sedimentary Pyrite: A case study from deepwater facies,Ediacaran Lantian Formation in South China

In the low-oxygen atmospheric conditions during the Neoproterozoic, continental sulfate was a major oxidant in anoxic deep ocean water. Bacterial sulfate reduction (BSR) caused significant isotopic fractionation between seawater sulfates and sedimentary pyrite, which reveals the paleo-ocean redox conditions. Sedimentary pyrite occurs in a variety of forms in sedimentary rocks, each indicating its depositional process and microenvironment. For example, most framboidal pyrite was deposited from the euxinic bottom water or in surface sediments overlain by oxic or suboxic bottom water. Large-grained single-crystal pyrite mainly formed in pore-water during early diagenesis, and pyrite veins were formed by late hydrothermal activity; neither of these indicate paleo-ocean redox conditions. Identifying sedimentary pyrite morphologies and determining their formation processes are fundamental for determining the sulfur isotopes of the ancient seawater. In this study, sedimentary pyrite morphological types and formation processes are briefly summarized, along with morphological observations and sulfur isotope analyses for pyrite in fresh drill-core samples of Ediacaran Lantian Formation (deep-water facies) in the southern Anhui Province, South China. The morphologies and contents of the pyrite were found to vary in different lithological samples. Euhedral crystal and framboidal pyrites preserved in Lantian Formation black shales have better morphology than in carbonates, indicating that less crystal alteration took place during late diagenesis in the black shales. The sulfur isotope content in large-grained pyrite (δ34S_L-pyr) is generally greater than in bulk rock samples (δ34S_T-pyr), with differences up to 48.16‰. The study suggests that analysis of sedimentary pyrite with different morphologies is needed to obtain reliable sulfur isotope values indicative of seawater redox conditions. High-resolution analysis requires secondary-ion mass spectrometry (SIMS) measurements on individual pyrite crystals and framboids.