Selective destructive demagnetization of breccias from DSDP Hole 83-504B

The magnetization of four breccia samples from the Leg 83 section of DSDP Hole 504B was analyzed by selective destructive demagnetization in order to study the origin and stability of hydrothermally altered basalts. The NRM directions of the clasts for three of the four samples are randomly oriented and much more strongly magnetized than the bulk sample. Clasts which were individually demagnetized show two or more components of magnetization, but neither are coincident with those of the bulk sample, indicating that NRM was probably acquired prior to the consolidation of the breccia and suggesting that any overprint (VRM or otherwise) can be removed by AF demagnetization to at most 50 Oe. Reflected light microscopy and electron microprobe analysis of two samples show that the unexpectedly high NRM of the matrix regions is apparently the result of secondary magnetic phases precipitated from hydrothermal solutions.

为探究热液蚀变玄武岩的成因与稳定性，本研究采用选择性破坏性退磁技术，对深海钻探计划（Deep Sea Drilling Project, DSDP）504B钻孔第83航次剖面的4件角砾岩样品的磁化特性展开了分析。4件样品中3件的岩屑天然剩余磁化强度（Natural Remanent Magnetization, NRM）方向呈随机分布，且磁化强度远高于整体样品。对单颗岩屑进行退磁处理后发现，其存在2个及以上的磁化分量，但这些分量均与整体样品的磁化分量不重合，这表明天然剩余磁化强度大概率是在角砾岩固结之前获得的；同时也说明，任何重磁化（粘滞剩余磁化（Viscous Remanent Magnetization, VRM）或其他类型）均可通过不超过50奥斯特（Oe）的交变退磁（Alternating Field, AF）予以清除。对2件样品开展反光显微镜分析与电子探针测试后发现，基质区域异常高的天然剩余磁化强度，实为热液流体沉淀形成的次生磁性矿物相所致。