Marine lineated magnetic anomalies often exhibit anomalous skewness that indicates non-vertical boundaries between normally and reversely magnetized crust. The acquisition of secondary magnetization components well after formation of the crust has been suspected as one possible cause for anomalous skewness. A petrographic and rock magnetic study on sheeted dikes recovered from the lower part of the deepest drill hole in the oceanic crust, ODP (Ocean Drilling Program) Hole 504B, demonstrates that these rocks carry a secondary chemical remanent magnetization (CRM) and not a thermoremanent magnetization (TRM). Primary titanomagnetites have mostly been altered to non-magnetic phases while the remanent magnetization resides in secondary magnetite that formed below its Curie temperature (~600 ¿C at ambient pressure) mainly by the alteration of olivine. The natural remanent magnetization (NRM) has been compared with laboratory anhysteretic remanent magnetization (ARM) and TRM. The intensities relate NRM