黑海西北部冷泉碳酸盐岩的沉积岩石学特征及氧化还原条件的稀土元素地球化学示踪

黑海西北部罗马尼亚大陆架(水深120m)和乌克兰陆坡(水深190m)发育冷泉碳酸盐岩结壳。XRD测试表明此结壳主要由高镁方解石和文石组成。结壳中与渗漏系统微生物活动有关的凝块和葡萄状文石等特殊的沉积组构非常发育。冷泉碳酸盐岩酸可溶部分(碳酸盐岩相矿物)的稀土元素含量很低(0.068×10-6~2.817×10-6),稀土元素页岩标准化配分模式显示罗马尼亚大陆架冷泉碳酸盐岩具有明显的Ce负异常,乌克兰陆坡冷泉碳酸盐岩具Ce的正异常,表明它们是分别在氧化和还原环境中沉积的。稀土元素和V、Cd和U等微量元素的含量在泥晶中最高,亮晶中最低,可能反映成岩过程对元素含量有控制作用。

Petrographic Characterization and Rare Earth Elements as Geochemical Tracers for Redox Condition of Seep Carbonates from Northwestern Black Sea

Seep carbonate crusts described here were collected on the Romanian(RO)shelf(120 m water depth)and Ukrainian(UKR)slope(190 m water depth)of the Black Sea.X-ray diffraction investigations show that high-Mg-calcite and aragonite are the dominant minerals.Peloids and botryoidal aragonite in the carbonate crust suggest a genesis induced by bacterial metabolism at the seep sites.The content of rare earth elements(REE)of the 5% HNO3-treated solution of the carbonates are very low(0.068×10-6 to 2.817×10-6).The shale-normalized REE patterns of RO carbonate show significantly negative Ce anomalies,while that of UKR carbonate shows no Ce anomalies,indicating that RO and UKR seep carbonates were deposited in oxic and anoxic environments,respectively.The contents of REE and trace elements(e.g.V,Cd and U)are highest in microcrystalline,lowest in sparite,and intermediate in microspar,suggesting that the elements concentration of seep carbonate is a function of diagenesis.