Magnetic properties of basalts at DSDP Hole 83-504B

We have studied the magnetic properties of 22 samples from DSDP Leg 83 to determine the origin of remanence and its relationship to such problems as the tectonic and chemical evolution of the section, the depth of the magnetized layer, and the applicability of magnetic properties of ophiolites to the marine crust. The magnitude of natural remanence has fairly typical values in the uppermost part of the section, falls two to three orders of magnitude in the transition zone, and returns to values slightly less than the upper part in the dike complex. This behavior reflects, for the most part, variations in the amount of magnetic minerals present. Directional behavior is highly variable throughout the section and often shows complexity even on the level of a single sample. Curie temperature measurements and preliminary opaque petrography indicate that the remanence is chemical in origin and probably involves a resetting of the original thermal remanent magnetization (TRM) direction. Selective destructive demagnetization of four breccia samples shows that the remanence of the clasts was acquired prior to consolidation and did not change significantly thereafter. There are also indications that some of the remanence may be carried by secondary magnetic phases. A comparison of these samples with comparable ophiolite rocks is equivocal, with similarities in remanence characteristics but differences in magnetic mineralogy. As for magnetic anomalies, the transition zone is too weakly magnetized to contribute significantly. The available data on the dike complex are inconclusive and their contribution is still open to debate.本研究针对深海钻探计划（Deep Sea Drilling Project, DSDP）第83航次采集的22件样品开展磁性特征研究，旨在明确剩余磁化（remanence）的成因，及其与剖面构造-化学演化、磁化层厚度、蛇绿岩（ophiolite）磁性特征对洋壳（marine crust）的适用性等科学问题的关联。天然剩余磁化强度（natural remanence）的幅值在剖面最上部呈现典型数值区间，在过渡带骤降2~3个数量级，而后在岩墙群（dike complex）中回升至略低于上部层位的水平。该特征整体上主要反映了样品中磁性矿物含量的变化。剖面内的磁化方向特征具有高度变异性，即便在单块样品尺度上也常表现出复杂性。居里温度测试与初步的不透明矿物岩石学（opaque petrography）分析结果表明，该剩余磁化属于化学成因，且可能发生了原始热剩余磁化强度（thermal remanent magnetization, TRM）方向的重置。对4件角砾岩（breccia）样品开展的选择性退磁实验显示，碎屑的剩余磁化是在固结作用发生前获得的，且在固结后未发生显著变化。另有迹象表明，部分剩余磁化可能由次生磁性矿物相承载。将本研究样品与同类蛇绿岩样品对比后，结果尚无定论：二者剩余磁化特征具有相似性，但磁性矿物组成存在差异。针对磁异常而言，过渡带磁化强度过弱，无法产生显著贡献。目前关于岩墙群的可用数据仍不充分，其对磁异常的贡献仍存在争议。