Age‐dependent variation in the magnetization of seamounts

Abstract

The magnetization of Pacific seamounts shows a surprisingly large variation when viewed as a function of time. Similar to normal upper oceanic crust, the basalts that form the external shell of volcanic seamounts have a high average magnetization immediately after formation. This high magnetization decays rapidly with time, resulting in a factor of 5 reduction in intensity in less than 500,000 years. Because submarine volcanoes appear to consist of the same basalts that form the upper part of the seafloor, low temperature oxidation of the magnetic mineral titanomagnetite is assumed to be responsible for the reduction in magnetization for both types of crust. Some large oceanic volcanoes are of sufficient volume that the upper capping rocks erupt subaerially and submerge completely due to thermal subsidence only after several million years. These capping rocks can be locally abundant (i.e., Hawaii), are exposed to a different post formation environment, and can have a magnetic mineralogy that is different from submarine volcanoes. In spite of these differences, the magnetization of subaerially erupted rocks of the Hawaii/Emperor Chain also may show a similar rapid reduction in intensity with age. Finally, seamounts of Cretaceous age show a factor of 2 higher average magnetization when compared to Tertiary seamounts, similar to the previously observed increased magnetization of Cretaceous seafloor basalts recovered from Deep Sea Drilling Project/Ocean Drilling Project cores. This increase in Cretaceous magnetization appears to be reflected in the increased amplitude of magnetic anomalies in some, but not all of the ocean basins, prior and subsequent to the Cretaceous Normal Superchron. The processes potentially responsible for the elevated Cretaceous magnetization values include a high geomagnetic field intensity, large-scale variations in whole rock major element geochemistry of the extrusive rocks, or a systematic acquisition of a secondary magnetization during the Cretaceous Normal Superchron. [Johnson and Merrill, 1973; Marshall and Cox, 1971a and . to insure internal consistency, we limited our study to the 1971b; Johnson and Atwater, 1977]. h• addition to the examination of seamounts from the Pacific basin. progressive decrease of magnetization as young oceanic crust ages, normal seafloor basalts also show a anomalous high Seamount Magnetization Variability Unlike normal oceanic seafloor, seamounts have a wide range of initial formation conditions and are exposed to a variety of post formation environments. For simplicity, we divided these 13,701 13,702 JOHNSON ET AL.: PACIFIC SEAMOUNT MAGNETIZATION volcanoes into two categories:(1) those that formed entirely beneath the sea surface, and (2) those that are of sufficient size that the f'mal formation stages occur subaerially. Owing to the difference in eruptive environments, the relevant initial magnetic minerals for these two classes of seamounts are different, and the post formation alteration history for the two types of rocks is also likely to be distinct....

Temporal and spatial cyclicity of accretion at slow-spreading ridges-evidence from the Reykjanes Ridge

“…A decrease in magnetization intensity with age, attributed to low‐temperature oxidation of titanomagnetite, has been observed at all sampled ridges (e.g. Talwani et al 1971; Johnson & Atwater 1977; Lowrie 1977; Macdonald 1977; Johnson & Hall 1978) with a factor of five reduction in the natural remanent magnetization over the first 0.5 Myr (Johnson et al 1996). Variations in intensity across‐ and along‐axis may, therefore, provide a useful constraint on the relative timing of crustal accretionary processes and allow investigation of asymmetry in spreading that is suggested by the RMBA.…”

Crustal magnetization of the Reykjanes Ridge and implications for its along‐axis variability and the formation of axial volcanic ridges

“…One possibility is that magnetization reflects the time elapsed since the magmatic emplacement or the age of extrusive lavas. For instance, analyses of rock magnetization on Pacific seamounts have shown that initial magnetization intensity may be reduced by a factor of 5 in the first 500,000 years [Johnson et al, 1996] as a result of low-temperature oxidation. It would be unrealistic, however, to expect the regional variation in magnetization to be due to systematic variations in age, thus we suggest age-dependent effects may be important for explaining the local variations.…”

The intensity of the Earth's magnetic field over the past 160 million years

“…Although more difficult to demonstrate, some studies have suggested that a record of past geomagnetic intensity variations might be preserved in the magnetization of the ocean crust [28][29][30] . Several earlier investigations discounted the hypothesis that the intensity of the Earth's magnetic field played a significant role in the variations of the NRM on the basis of the lack of evidence for similar variations in the palaeointensity databases of the time.…”