北大西洋ODP1049C孔Aptian-Albian期高频旋回大洋红层的成因:矿物学证据

北大西洋ODP171B航次1049C孔Aptian-Albian期沉积以出现大洋红层与灰色、白色沉积物高频旋回为特征。为了探讨大洋红层的成因,本文进行了矿物学、地球化学、沉积学等方面的研究。漫反射光谱、磁化率和活性铁数据表明,赤铁矿、针铁矿的出现是导致样品由白色向红色转变的矿物学原因。棕色和橙色样品中出现赤铁矿和针铁矿的特征峰,FeR/FeT平均值分别为0.23和0.24,明显区别于其他颜色的样品。磁化率与红层成良好的正相关关系,说明铁氧化物矿物含量的变化是导致磁化率变化的原因。X射线衍射结果表明,不论颜色如何,样品中均含伊利石、高岭石、蒙脱石、绿泥石等粘土矿物,其分布与样品颜色没有直接关系,很可能反映当时物源区气候稳定。ODP1049C孔岩芯沉积物出现橙色、棕色、白色、灰色等颜色的高频变化,颜色过渡接触界线清晰,说明导致红色变化的赤铁矿和针铁矿是沉积期低温氧化的产物。棕色样品中CaCO3含量最低,推测红层所对应的氧化条件是由于较低的有机质堆积速率造成的。

Origin of the Aptian Albian High Cyclic Oceanic Red Beds in the ODP Hole 1049C, North Atlantic:Mineralogical Evidence

The Aptian-Albian sediments in the ODP Leg 171B Hole 1049C are characterized by a high frequency of cyclothems between oceanic red beds and grey, or white depositions. To better understand the origin of the red beds, research on the mineralogy, geochemistry and sedimentology was carried out by way of Diffuse Reflectance Spectrometry (DRS), magnetic susceptibility and reactive iron analysis. Our data show that hematite and goethite are the minerals responsible for the red color of the red beds. Characteristic peaks of hematite and goethite occur in brown and orange samples, and their averaged FeR/FeT values are 0.23 and 0.24 respectively, obviously different from those of other samples. The good positive correlation between magnetic susceptibility and red beds suggest that the magnetic susceptibility changes due to the variety of the hematite or maghemite composition. X-ray Diffraction results reveal that all of the samples, no matter what color they are, contain illite, kaolinite, chlorite and montmorillonite. Their distribution has no correlation with the color, which probably reflects stable climate of the source area at that time when deposited. The samples from Hole 1049C show a high frequency cyclothems in orange, brown, white, gray, and the interface of different colors is clear, which indicates that hematite and goethite are the results of oxygenation at a low temperature. Brown samples with a low carbonate content probably indicates that oxidizing environment to which red beds are corresponding, results from a low rate accumulation of organic materials.