Evidence for the Rapid Low-Temperature Alteration of Oceanic Crust on the Pacific-Antarctic Ridge from the Amplitude Envelope of Magnetic Anomalies

According to conventional wisdom there is a systematic decay in the amplitude of marine magnetic anomalies that starts at the ridge axis and which continues until the crust is several million years old. The decay in anomaly amplitudes is generally attributed to a systematic decrease of NRM values caused by the low temperature alteration of titanomagnetites to titanomaghemite. However, although it is often assumed that the effects of low temperature alteration take place over several million years, there is actually a wide range of values cited in the literature for this phenomenon, ranging from tens of years to tens of millions of years. We have analyzed the amplitude envelope of a recently acquired sea surface magnetic profile that crossed the moderately fast spreading (85 mm/yr full rate) Pacific-Antarctic ridge about 100 km north of the Menard Fracture Zone. This profile was collected for a distance of 3000 km along a spreading flowline that maintained a constant distance from the Menard Fracture Zone and which avoided major seamounts and other significant tectonic irregularities. Contrary to expectation, there is no decay in anomaly amplitudes over the first 8 Ma. Modeling of the anomaly profile incorporating the observed bathymetry shows that any exponential decay in magnetization either must have taken place very quickly (20 kyr or less) or else involved only a very small portion of the source layer. Anomaly amplitudes over crust between 10 and 25 Ma in age, normalized for bathymetry, are about 60% of the amplitudes of the younger crust, but spreading rates in this time period were slower than for the last 8 Ma. Together, these observations suggest that the effects of low temperature alteration take place rapidly while some of the more prominent changes in anomaly amplitude off axis are related to changes in spreading rate.