Geochemistry and mineralogy of sediments from the Atlantis II Fracture Zone

Thirteen sediment samples, including calcareous ooze, sandy clay, volcanic sand, gravel, and volcanic breccia, from Ocean Drilling Program (ODP) Sites 732B, 734B, 734G and Conrad Cruise 27-9, Station 17, were examined. Contents of major and trace elements were determined using XRF or ICP (on samples <0.5 g). Determinations of rare earth elements (REE) were performed using ICP-MS. Mineralogy was determined using XRD.
On the basis of the samples studied, the sediments accumulating in the Atlantis II Fracture Zone are characterized by generally high MgO, Cr, and Ni contents compared with other deep-sea sediments. A variety of sources are reflected in the mineralogy and geochemistry of these sediments. Serpentine, brucite, magnetite, and high MgO, Cr, and Ni contents indicate derivation from ultramafic basement. The occurrence of albite, analcime, primary mafic minerals, and smectite/chlorite in some samples, coupled with high SiO2, Al2O3, TiO2, Fe2O3, V, and Y indicate contribution from basaltic basement. A third major sediment source is characterized as biogenic material and is reflected primarily in the presence of carbonate minerals, and high CaO, Sr, Pb, and Zn in certain samples. Kaolinite, illite, quartz, and some chlorite are most likely derived from continental areas or other parts of the ocean by long-distance sediment transport in surface or other ocean currents. Proportions of source materials in the sediments reflect the thickness of the sediment cover, slope of the seafloor, and the nature of and proximity to basement lithologies. REE values are low compared to other deep-sea sediments and indicate no evidence of hydrothermal activity in the Atlantis II Fracture Zone sediments. This is supported by major- and trace-element data.

本研究对取自大洋钻探计划（Ocean Drilling Program, ODP）732B、734B、734G站位以及Conrad航次27-9第17站位的13件沉积物样品展开了分析，样品类型涵盖钙质软泥、砂质黏土、火山砂、砾石及火山角砾岩。采用X射线荧光光谱法（XRF）与电感耦合等离子体发射光谱法（ICP，仅针对质量小于0.5g的样品）测定了常量与微量元素含量，利用电感耦合等离子体质谱法（ICP-MS）分析了稀土元素（Rare Earth Elements, REE）的含量，并通过X射线衍射法（XRD）确定了沉积物的矿物组成。基于本次研究的样品，亚特兰蒂斯II断裂带（Atlantis II Fracture Zone）的沉积产物相较于其他深海沉积物，普遍具有较高的MgO、Cr及Ni含量。这类沉积物的矿物学与地球化学特征反映了多种物源贡献：蛇纹石、水镁石、磁铁矿的产出以及高MgO、Cr、Ni含量特征，指示其物源来自超镁铁质基底岩石；部分样品中产出的钠长石、方沸石、原生铁镁矿物以及蒙脱石/绿泥石，结合高SiO₂、Al₂O₃、TiO₂、Fe₂O₃、V及Y含量特征，表明其物源来自玄武质基底岩石；第三类主要沉积物物源为生物成因物质，主要体现为部分样品中产出碳酸盐矿物，且具有高CaO、Sr、Pb及Zn含量。高岭石、伊利石、石英以及部分绿泥石，极有可能通过表层洋流或其他海洋环流的长距离输运，来自大陆区域或大洋其他区域。沉积物中各物源组分的占比，反映了沉积物覆盖层厚度、海底坡度以及基底岩石的类型与临近程度。相较于其他深海沉积物，该区域沉积物的稀土元素含量偏低，表明亚特兰蒂斯II断裂带的沉积物中未发现热液活动迹象，这一结论得到了常量与微量元素数据的佐证。