南海不同沉积环境黄铁矿的矿物学及原位微区地球化学研究^*

A Study on mineralogical and in-situ geochemical characteristics of pyrite under different sedimentary environments in the South China Sea

摘要

黄铁矿是沉积物和沉积岩中的常见矿物,其地球化学特征能够明确指示周围的沉积环境。然而,黄铁矿地球化学特征的差异性及形成过程中的控制因素尚不清楚。本研究利用扫描电镜和激光剥蚀电感耦合等离子体质谱对南海不同沉积环境黄铁矿形貌和原位微区主微量元素特征进行了研究。结果表明,SH-CL38站位黄铁矿含量与 δ³⁴S 值呈镜像关系,代表了正常海洋沉积环境下形成的黄铁矿受有机碎屑硫酸盐还原作用的控制; 而F站位碳酸盐颗粒极低的δ¹³C值(-45.55‰)表明该站位发育甲烷渗漏,黄铁矿的形成与硫酸盐驱动的甲烷厌氧氧化过程有关。2个站位黄铁矿微区的地球化学分布明显存在差异: SH-CL38站位黄铁矿的Mn、Co、Ni、Mo和Sb含量相对于F站位含量高,可能是铁锰(氢)氧化物的还原性溶解导致的; 而F站位黄铁矿的Cu、V、As和Cd含量相对于SH-CL38站位高,可能是受到有机质矿化作用的影响。黄铁矿颗粒的Ca和Mg含量表明F站位的甲烷渗漏条件下由于优先沉淀了低Mg高Ca的自生方解石,导致后期沉淀的黄铁矿具有高Mg低Ca特征。2种不同沉积环境中黄铁矿的形貌特征和微量元素含量的差异表明黄铁矿的矿物学和地球化学特征可以用来识别甲烷渗漏。

Abstract

Pyrite is a common mineral in sediments and sedimentary rocks,and its geochemical characteristics can clearly indicate the surrounding sedimentary environment. However,the differences in geochemical characteristics of pyrite and the controlling factors of its formation are still unclear. This study uses scanning electron microscope and laser ablation inductively coupled plasma mass spectrometry to study the morphology and in-situ geochemical characteristics of pyrite in different sedimentary environments in the South China Sea. The results show that the contents of pyrite and δ³⁴S values at site SH-CL38 exhibit a mirror-image relationship,representing the pyrite formed in a normal marine sedimentary environment,which is controlled by organoclastic sulfate reduction;The extremely low δ¹³C value(-45.55‰)of authigenic carbonate particles at site F indicates methane seepage,and the formation of pyrite is related to sulfate-driven anaerobic oxidation of methane process. There are significant differences in the geochemical distribution of pyrite micro-areas between the two sites: the contents of Mn,Co,Ni,Mo,and Sb in pyrite at site SH-CL38 are higher than those at site F,which may be caused by the reducing dissolution of iron and manganese(hydrogen)oxide. In contrast,the content of Cu,V,As,and Cd in pyrite at site F is higher than that at site SH-CL38,which may be influenced by organic matter mineralization. The Ca and Mg content of pyrite particles indicate that,under methane seepage condition at the site F,authigenic calcite with low Mg and high Ca was preferentially precipitated,resulting in the later precipitated pyrite having high Mg and low Ca characteristics. The differences in morphology and trace element content of pyrite in two different sedimentary environments indicate that the mineralogical and geochemical characteristics of pyrite can be used to identify methane seepage.

关键词

南海 / 黄铁矿 / 微晶形貌 / 原位微区分析 / 成因机制 / 激光剥蚀电感耦合等离子体质谱

Key words

South China Sea / pyrite / microcrystalline morphology / in-situ analysis / formation mechanism / laser ablation inductively coupled plasma mass spectrometry

基金资助

*国家自然科学基金项目(编号: 42121005),中央高校基本科研业务费专项(编号: 202172002,202172003)和青年泰山学者项目(编号: tsqn202211069)联合资助