Composition of Eocene claystones of ODP Hole 105-647A

Authigenic carbonates were recovered in lower to middle Eocene claystones at Ocean Drilling Program Site 647 in the Labrador Sea. Detailed chemical, petrographic, and X-ray investigations reveal that these diagenetic carbonates have a complex mineralogical composition. At least five different carbonate phases are identified: calcium-rich rhodochrosite, rhodochrosite, manganosiderite, siderite, and calcite. Manganese carbonates are the dominant carbonate phases formed throughout the section. Textural analyses show two major generations of carbonate formation. Early cementation of micritic carbonate in burrow structures was followed by carbonate cementation forming microsparry to sparry crystals. At approximately 620 meters below seafloor (mbsf), three concretions of iron carbonates occur, which indicates a special pore-water chemistry. Thin section analyses from this level show (1) several generations of diagenetic carbonates, (2) widespread secondary cavity formation in burrow structures, and (3) various cement precipitations in voids. We suggest that this level represents a hiatus or highly condensed sequence, as indicated by (1) the low carbonate content in host sediments, (2) carbonate dissolution reflected by the high ratio of benthic to planktonic foraminifers, and (3) complex diagenetic alteration in the carbonate concretions. Iron and manganese enrichments observed in lithologic Unit IV may have been derived from a hydrothermal source at the adjacent, then active, Labrador Sea mid-ocean ridge. Authigenic smectites forming numerous pseudomorphs of siliceous microfossils are precipitated in burrow structures. We propose that diagenetic smectite formation from biogenic opal and iron oxyhydroxide (analogous to smectite formation in surface sediments of the East Pacific area) occurred in the Labrador Sea during the early and middle Eocene.在拉布拉多海大洋钻探计划（Ocean Drilling Program, ODP）647站位的始新世早-中期黏土岩中采获了自生碳酸盐岩。通过详细的化学、岩石学及X射线分析，研究人员发现这些成岩碳酸盐岩具有复杂的矿物组成。本次研究已识别出至少5种不同的碳酸盐相：富钙菱锰矿、菱锰矿、锰菱铁矿、菱铁矿及方解石。其中，锰碳酸盐是整个剖面中占主导的碳酸盐相。结构分析显示，该区域存在两期主要的碳酸盐形成作用：首先是潜穴构造内微晶碳酸盐的早期胶结，随后形成微亮晶至亮晶晶体的碳酸盐胶结物。 在海底以下约620米（meters below seafloor, mbsf）处，发育有3个铁质碳酸盐结核，这指示了该区域存在特殊的孔隙水化学环境。对该层位开展的薄片分析显示：（1）存在多期成岩碳酸盐作用；（2）潜穴构造内广泛发育次生孔洞；（3）孔隙内沉淀有多种胶结物。综合来看，该层位代表沉积间断或极薄沉积序列，依据如下：其一，围岩沉积物中的碳酸盐含量较低；其二，底栖有孔虫与浮游有孔虫的高比值反映了碳酸盐溶蚀作用；其三，碳酸盐结核内部存在复杂的成岩蚀变现象。 岩石地层单元IV中观测到的铁、锰富集现象，可能来源于当时处于活动状态的邻近拉布拉多海洋中脊的热液源。在潜穴构造内，可沉淀形成大量硅质微古生物假像的自生蒙脱石。研究团队提出，在始新世早-中期的拉布拉多海中，由生物成因蛋白石和羟基氧化铁形成成岩蒙脱石的过程（类似于东太平洋表层沉积物中的蒙脱石形成机制）曾广泛发生。