The Magnetic Transition From low Temperature to Hydrothermal Alteration Within the Oceanic Crust at IODP Site 1256

The interplay between magnetic properties and alteration processes was investigated on oceanic basalt, dyke and gabbro samples collected along a continuous section of the oceanic crust drilled at IODP site 1256 during ODP 206 and IODP 309 and 312 expeditions. Observations using optical and electronic microscopes and chemical analyses made using electron microprobe and SEM-EDX apparatus indicate progressive transformation of the primary magnetic mineral phases controlled by changes in alteration conditions. Rock magnetic measurements such as Curie temperature, hysteresis parameters, susceptibility and low- temperature magnetic measurements allow more precise description and lead to the determination of three distinct magnetic zones. From the top of the volcanic section to 660 mbsf, measurements indicate a progressive maghemitization of the primary titanomagnetites. From 660 to 1030 mbsf, a threshold occurs which feature presence of highly altered titanomagnetites, and local depletion of titanium relatively to iron in the magnetic phases, this layer was named magnetic transition zone (MTZ). Petrological observations show that this 'magnetic transition zone' corresponds to a interval of secondary non- magnetic phase rich in titanium: hydroschorlomite. Below 1030 mbsf, through sheeted dike complex and gabbros, magnetite is the main magnetic carrier, ilmenites and titanites are often observed in close association with it.