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

[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 (similar to1 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 similar to350degrees-475degreesC 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.