The downhole variation of the rock-magnetic properties of DSDP Leg 73 Holes

The magnetic stability and mean intensity of the natural remanent magnetization (NRM) of Leg 73 sediments (Holes 519 to 523) decreases with the age of the sediment. We demonstrate that these variations are linked with physical and chemical changes in the magnetic grains themselves. Alteration of the magnetic component occurs most rapidly shortly after deposition. A significant magnetic alteration over the topmost few meters of the sediments is thought to be the result of oxidation. The modification of the NRM characteristics through the partial dissolution of the carbonate is largely accounted for by the effects of concentraion of the magnetic minerals. We apply the techniques of rock-magnetism and X-ray fluorescence analysis to clarify the physical and chemical mechanisms that affect the magnetic character of the sediment.第73航次（Leg 73）519至523号钻孔沉积物的天然剩余磁化强度（natural remanent magnetization, NRM）的磁稳定性与平均强度，随沉积物沉积年代的增加而降低。我们证实，上述变化与磁性颗粒自身的物理、化学变化密切相关。磁性组分的蚀变作用在沉积后初期发生最为迅速。研究认为，沉积物顶层数米内发生的显著磁性蚀变，系氧化作用所致。由碳酸盐部分溶解所引发的天然剩余磁化强度特征改变，在很大程度上可归因于磁性矿物浓度变化的影响。我们采用岩石磁学（rock-magnetism）与X射线荧光分析（X-ray fluorescence analysis）技术，阐明了影响沉积物磁性特征的物理与化学机制。