台湾国姓地区中新世海相菱铁矿的成因

Genesis of Siderite in Miocene Marine Shale in Kuohsing Area,Taiwan

菱铁矿很好地记录了过去地质流体的信息,能够用于示踪生物地球化学反应相关的成岩作用带。台湾国姓地区中新世海相泥页岩中发育自生的菱铁矿结核,其成因尚未厘清。野外观察发现菱铁矿以不连续透镜体平行散布于泥页岩中,主要由自生碳酸盐菱铁矿(78.63%)等矿物组成。菱铁矿的稀土元素配分模式为轻稀土亏损、中稀土富集,无Ce异常,指示菱铁矿形成于弱氧化的沉积环境,弱氧化的环境促进了菱铁矿在次氧化带的沉淀。菱铁矿的δ^(13)C_(VPDB)和δ^(18)O_(VPDB)值分别为-3.69‰~+0.08‰和-1.09‰~+0.25‰,指示菱铁矿形成于次氧化带,碳源很可能是海水和有机质降解混合产生。研究表明自生菱铁矿能够被用于识别沉积物中的生物地球化学过程和指示成岩作用带。

Siderite records historical geological information about fluids,and thus can be used to trace diagenetic zones related to biogeochemical reactions.Siderite nodules have been found in Miocene marine shale in the Kuohsing area of Taiwan,but their genesis is not clear.Field observations show that the siderite is scattered in parallel discon⁃tinuous lenses in the shale,which consists mainly of authigenic siderite(78.63%)and other minerals.The REE dis⁃tributions of the siderite are characterized by MREE enrichment with no Ce anomaly.This indicates that the siderite was formed in weakly oxidizing conditions,facilitating the precipitation of siderite in a suboxic zone.The δ^(13)C and δ^(18)O values of this siderite are from-3.69‰to+0.08‰and from-1.09‰to+0.25‰,respectively,suggesting that it was precipitated in the suboxic zone with an admixture of seawater and organic matter.Studies show that authigenic siderite can be used to identify biogeochemical processes in sediments and indicate diagenetic zones.