蓟县下马岭组菱铁矿的成因及古海洋意义

Genesis of Siderites in the Xiamaling Formation of Jixian Section and Its Paleoceanic Significance

蓟县铁岭子村附近新出露的下马岭组下部黑色岩系中富含菱铁矿,对这些菱铁矿形成机制的认识直接关系到对下马岭期乃至中元古代古海洋氧化还原状态和地球化学性质的判断。文中利用碳同位素对其成因加以制约,进而反演该时期的古海洋环境。结果显示,研究区菱铁矿的δ~13 C的变化范围为-19.2‰^-7.8‰,平均约-15.0‰,明显富集碳的轻同位素。根据自然界中不同碳库的碳同位素组成特征,判定这些菱铁矿中的碳主要来源于有机碳。虽然海洋中大量有机质被氧化释放出富12 C的CO_2溶解于海水后可直接沉淀出轻同位素富集的碳酸盐岩,但这往往要求极端氧化的水体环境,而这种情况下并不能形成菱铁矿沉淀,因此认为研究区菱铁矿应是成岩过程中由有机质降解产生的CO_2与孔隙水中的Fe^(2+)结合的产物。海洋中的铁首先以氧化铁或氢氧化铁的形式沉淀下来,并与有机质一起埋藏于海底缺氧带中,后在成岩过程中发生氧化还原反应,三价铁作为氧化剂氧化有机质使之生成CO_2,有机质则作为还原剂将三价铁还原为二价铁,二者结合即构成菱铁矿。同时,有机质热脱羧反应提供的CO_2保证了更多的二价铁以菱铁矿的形式保存在地层中。海洋中铁的沉淀形式表明,至少在燕辽海盆,下马岭期古海洋已呈现广泛氧化状态,其氧化程度足以将海洋中的铁氧化为三价铁。同时,地层中的硫含量极低,说明该时期古海洋贫硫。然而下马岭期燕辽海盆广泛氧化、贫硫的古海洋特征不同于传统的分层海洋模式,对此有必要进行更深入系统的研究。

The new exposed Xiamaling Formation near Tielingzi village of Jixian are rich of siderites. Recognition of the siderite formation mechanism is crucially important for judging the redox state and geochemical nature of the ancient ocean during Xiamaling Period, even the whole Mesoproterozoic. In this paper, we used carbon isotope to constrain the genesis of these siderites, and then supported analysis of the paleoceanic environment during this period. The ~JaC values of these siderites range from --19.20//oo0 to --7.8~/000,with an average of ^--15.0~0, showing significantly enrichment of the lighter carbon isotope. Based on the carbon isotope compositions of different carbon sinks in nature, we can identify that the carbon in these siderites was mainly derived from organic carbon. Although the oxidation of organic matter in the ocean can provide much more ~zC into the seawater, and then precipitate carbonates rich in the lighter carbon isotope, that always needs a quit oxidizing water environment, and there can be no siderites precipitated in such a condition. So we can draw a conclusion that the siderites in our study area were products of combination of CO2 derived from organic matter and Fee＋ in pore water during the diagenetic process. Firstly, the iron precipitated from the ocean as iron oxides or hydroxides, and buried with organic matter in the seafloor hypoxia zone. Then during the early stage of diagenesis, organic matter was oxidized to COz by Fe3~,while Fea~ was reduced to Fe2＋ by organic matter, and siderites were formed by the combination of these resultants. Accordingly, thermally induced decarboxylation of organic matter offered a portion of CO2 to make more Fe2＋ exist in strata as siderites. The precipitation form of iron in the ocean indicates that in Yanliao basin, at least, the ancient ocean during Xiamaling Period had been wildly oxidic, and the degree of oxidation can make iron oxidized to ferric iron. Also the strata are low in sulfur, indicating that the ancient ocean was sulfur-poor. However,this wildly oxidic and sulfur-poor paleoceanic state in Yanliao basin is different from the traditional stratified paleoceanic model, so more in-depth and systematic research is needed to address this issue.