Three‐dimensional inversion of marine magnetic anomalies on the equatorial Atlantic Ridge (St. Paul Fracture Zone): Delayed magnetization in a magmatically starved spreading center?

Abstract

[1] The St. Paul Fracture Zone (FZ) in the equatorial Atlantic is interrupted by three intratransform ridge (ITR) spreading centers. A detailed magnetic survey, corrected for the diurnal variations using a moored magnetic station, six submersible dives, and three bottom-towed video camera tracks provide data on the most eastern ITR (0°37 0 N, 25°27 0 W). Visual observations and submersible sampling displayed a high ultramafic/ volcanic ratio, supporting the assumption that the ITR is in a magmatically starved state. Volcanics were mainly found on the rift valley floor from 4700 to 4000 m and as a thin cap (<160 m) on the top of the eastern rift crest (2700 m). Most of the rift walls consist essentially of serpentinized peridotites and gabbros. The magnetic data show a welldefined ridge centered anomaly. A generalized inversion method was applied to the field data to calculate the crustal equivalent magnetization, assuming that the seafloor is broken down into elementary cells of 1 Â 1 Â 0.5 km 3 which fit the topography. The average of absolute value of equivalent magnetization is 2.7 A m À1 . The width of the central normal polarity (Brunhes epoch) is wider (at least 34 km) than that indicated by the NUVEL-1 kinematic model (24.5 km). This 40% excess is believed to be significant and is thought to be the result of prolonged chemical remanent magnetization acquired during the serpentinization of peridotites. In a magmatically starved accretion segment, we suggest that peridotites could continue to acquire magnetization as long as tectonic activities facilitate the circulation of seawater in the upper mantle.

Focused magmatism versus amagmatic spreading along the ultra‐slow spreading Southwest Indian Ridge: Evidence from TOBI side scan sonar imagery

“…For example, beneath the smaller tectonized area between segment #11 and #10 where seawater has to cross ∼2 km thicker crust before reaching the upper mantle, the serpentinization process may be less intense producing no moderate magnetization anomaly but weak magnetization values relative to the segment centers. Serpentinization of peridotites has already been put forward by Sichler and Hékinian [2002] on a comparable amagmatic ridge section of the MAR. These authors explain a wider central magnetic anomaly than that expected by the Nuvel‐1 kinematic model by such a weathering process in a magmatically starved ridge section of the equatorial MAR which is thought to be related to a thermal minima in the upper mantle [ Bonatti , 1996].…”

Magnetic structure of an oceanic core complex at the southernmost Central Indian Ridge: Analysis of shipboard and deep‐sea three‐component magnetometer data

“…In addition, the magnetization from serpentinized peridotite is also a candidate for marine magnetic anomalies. Some observational studies have suggested that induced magnetization in serpentinized peridotite can also contribute to observed magnetic anomalies [e.g., Tivey and Tucholke , 1998; Sichler and Hékinian , 2002; Hosford et al , 2003; Okino et al , 2004]. Experimental and field studies of rock magnetism also show that magnetite is crystallized through extensively serpentinized peridotite with high natural remanent magnetization (NRM) (4–10 A/m) and magnetic susceptibility (∼0.07, average), giving an indication of induced magnetization [ Oufi et al , 2002].…”

Ridge segmentation and the magnetic structure of the Southwest Indian Ridge (at 50°30′E, 55°30′E and 66°20′E): Implications for magmatic processes at ultraslow‐spreading centers

“…The magnetization solution of survey box B shows a shift toward more positive magnetization values toward the distal part of the flanks. As such a long wavelength signal in the result of the inversion is always suspect, we have also calculated the crustal equivalent magnetization using a generalized inversion method [e.g., Sichler and Hékinian , 2002] which generates no spurious long wavelength component. This method is based on a linear discrete inversion taking as unknowns the magnetization of parallelepiped shaped prisms which fit the topography.…”