Delayed magnetization of the deeper kilometer of oceanic crust at Ocean Drilling Project Site 504

Abstract

The Fe‐Ti magnetic carriers of the dike‐rich second kilometer of oceanic crust at Ocean Drilling Program Site 504 are shown to be secondary in origin, that is, their formation is likely to considerably postdate initial igneous crustal formation. An important consequence of the delay in formation of carriers is that the polarity of their magnetization has a significant probability of opposing that of the overlying, little‐altered extrusives. If this is the case, the linear magnetic anomalies in the area have sources crust, which, at least locally, is magnetized with successive layers of opposite polarities. The magnitude of the contribution of deeper layers to anomalies will depend both on their thickness and average magnetization, which are not as yet well known. Two types of secondary magnetite, SMI and SMII, separated spatially and by the steps leading to their formation, are recognized in the dikes of the second kilometer of the crust at Site 504. Both were formed in conditions of hydrothermal greenschist metamorphism. Since all newly formed oceanic crust is thought to experience this type of alteration at an early stage, polarity reversal with depth as a result of the delayed formation of magnetic carriers is likely to be widespread.