VARIATIONS IN OCEANIC CRUSTAL MAGNETIZATION - SYSTEMATIC CHANGES IN THE LAST 160 MILLION YEARS

The amplitudes of marine magnetic anomalies show a clear worldwide pattern of systematic variation when viewed as a function of age. Globally, the amplitudes decrease with age over the first 20 to 30 million years, a phenomenon that has been attributed to the low-temperature oxidation of the magnetic mineral titanomagnetite in the upper extrusive rocks. In oceanic crust older than 40 million years, however, the amplitudes of the magnetic anomalies increase with increasing age and remain at elevated levels for the entire period between 80 and 160 million years. In order to examine the processes responsible for this elevated crustal magnetization in older oceanic crust, we compiled all of the existing rock magnetic data from relevant Deep Sea Drilling Project and Ocean Drilling Program sites that sampled extrusive rocks from ''normal'' ocean crust. This compilation shows that the laboratory measurements of the magnetization of the upper basement rocks closely reflect that of the anomaly amplitudes, both in the decrease due to oxidation in the first 20 to 30 million years and in the increase in magnetization in older (>80 million years) crust. This positive correlation between anomaly amplitudes and upper crustal magnetization supports the argument that a major source of the marine magnetic anomalies is in the upper extrusive volcanic rocks. Further, the Curie temperature data show that oxidation of the magnetic minerals in oceanic basalts occurs largely within the first 30 million years, and does not increase significantly beyond that point. Finally, the strong positive correlation between the intensity of the magnetization of the drill core samples and the saturation magnetization argues that the higher magnetic anomaly amplitudes over older crust are related to a change in an intrinsic property of the upper crust. Specifically, we propose that the observed elevated magnetization of older ocean crust is due primarily to an increased abundance of magnetic FeTi oxides in the older crustal rocks. This large increase in abundance of the FeTi oxides may be related to a systematic increase in bulk FeTi content in older tholeiitic basalts, or, more likely, is due to a difference in the partitioning of the iron and titanium content between the silicate and oxide phases.