Slow cooling of middle and lower oceanic crust inferred from multicomponent magnetizations of gabbroic rocks from the Mid‐Atlantic Ridge south of the Kane fracture zone (MARK) area

Abstract

[1] The remanent magnetization of gabbroic material of the Mid-Atlantic Ridge south of the Kane fracture zone (MARK) area provides constraints on both the thermal structure and tectonic history of the lower crust in this slow spreading environment. The remanence of these gabbroic samples is often complex, with the juxtaposition of intervals of apparently normal and reversed polarity rocks over small spatial scales (tens of centimeters to a few meters). Moreover, several samples when thermally demagnetized have a reversed polarity magnetization component between higher and lower stability normal polarity components. Given the nominal age ($1 Ma) of the crust, we suggest that this pattern of normal/reversed-normal polarity most plausibly reflects emplacement and/ or cooling through three successive polarity intervals, Jaramillo normal polarity interval (1.07-0.99 Ma), a portion of the Matuyama reversed polarity interval (0.99-0.78 Ma), and the Brunhes normal polarity interval (0.78 Ma to present). A small number of samples with three well-defined magnetization components have magnetic characteristics compatible with a remanence carried by fine-grained, possibly single domain, magnetite. Laboratory unblocking temperatures in these samples therefore allow estimation of lower crustal temperatures at the time of the Jaramillo/Matuyama (0.99 Ma) and Matuyama/ Brunhes (0.78 Ma) polarity transitions. Together with depth estimates derived from fluid inclusion studies these results suggest that middle and lower crustal temperatures remained as high as $350°-475°C for a minimum of 0.21 m.y. after emplacement. We suggest that continued injection of liquid, in the form of sills or small magma bodies, over a broad region (half width of $3 km) is responsible for this slow cooling. In addition, inclinations of the highest stability component from these drill sites are remarkably similar to that expected from an axial geocentric dipole, suggesting that little, if any, resolvable tilt occurred during uplift of these rocks to the seafloor.

The oceanic crust in 3D: Paleomagnetic reconstruction in the Troodos ophiolite gabbro

“…This domain has a width of 6 km and probably represents the lower oceanic crust beneath the neo-volcanic zone, a zone that does not suffer significant rotational deformation (e.g., [40,41]). The width of the inferred lower crust neo-volcanic zone stands in agreement with the prediction of Gee and Meurer [34] for 6 km width of the region where magma is re-supplied into the lower crust at the Mid-Atlantic Ridge, south of Kane fracture zone.…”

“…These studies show that the characteristic remanent magnetization (ChRM) is primary and reside in magnetite. In addition, studies of the Oman ophiolite [31] and of the present-day oceanic crust (for example see [32][33][34]) have shown that the primary magnetic carrier is mostly low Ti-magnetite that acquired its magnetization during the initial intrusion time. According to their interpretation, the high temperature component of magnetization can be used to reconstruct the rigid body rotations of the lower crust.…”

Oceanic lithosphere magnetization : marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments

“…The early one layer models assume that the upper crust extrusive lavas make the strongest magnetic contribution, due to the high remanent magnetization of basalts, and have vertical polarity boundaries Irving et al, 1970;Talwani et al, 1971]. More recently, arguments have been made that the lower crust (gabbros) and upper mantle (serpentinized peridotites) rocks can have high natural remanent magnetizations (NRM) and represent significant magnetic sources in slow spreading ridge environments Gee and Meurer, 2002; Tivey and Tucholke, 1998]. Two-layer models frequently assume that polarity boundaries in the second layer, analogous to the gabbros in many cases, follow the curve of a conductively cooling isotherm that results in delayed acquisition of magnetization at depth [e.g.…”

“…Fresh peridotites in the upper mantle are weakly ferromagnetic (they are paramagnetic), but become strongly magnetic upon alteration to serpentinized peridotites. It has been assumed that serpentinized peridotites acquire their magnetization too slowly to contribute constructively to the to 75 % of the overall magnetic anomaly [Fox and Opdyke, 1973;Gee and Meurer, 2002; Tivey et al, 1998a; Tivey and Tucholke, 1998;Zhao et al, 2006]. Models of the magnetic source layer have consequently evolved into a two-layer approximation with an upper basalt layer and a lower intrusive layer [Dyment et al, 1997;.…”

“…In contrast, our understanding of how the lower lithologic units, including diabase dikes, gabbros and serpentinized peridotites, relate to the anomaly signal is more uncertain. The manner in which they acquire remanent magnetization and their contribution to the magnetic source layer are also poorly defined in comparison [Dunlop and Prevot, 1982a;Gee and Meurer, 2002; Johnson and Salem, 1994; Kent et al, 1978;Tivey, 1996].Certain magnetic characteristics must be considered for a rock to contribute to the magnetic source layer. The rock must be able to record the polarity of the geomagnetic field with reasonable fidelity and preserve this record over geologically significant time periods .…”