Magnetic study of serpentinized harzburgites from the Islas Orcadas Fracture Zone

Magnetic properties and bulk densities of 27 serpentinized harzburgite samp les from the Islas Orcadas Fracture Zone, located in the vicinity of the Bo uvet Triple Junction, have been measured and analyzed. Polished sections we re examined using reflected light and scanning electron microscopy to chara cterize the size and geometric arrangement of opaque minerals. The relation ship between the saturation magnetization (I-S) and remanent coercive force (H-R) is considered in terms of the amount of ferrimagnetic material and m aghemitization. A suite of continental serpentinites from Canada is offered as contrast, to consider the role of weathering and maghemitization. Magne tite in the Islas Orcadas serpentinites is variably maghemitized, whereas c ontinental serpentinites do not appear to contain maghemitized oxides. We v erify this with optical microscopy, thermomagnetic analyses and cryogenic t emperature cycling of saturation remanence. Maghemitization serves to reduc e initial magnetic susceptibility, and introduce error in the use of I-S to evaluate the magnetic mode of magnetite. The presence of maghemite and the existence of a three dimensional vein network for magnetite geometry would suggest that magnetic hysteresis parameters can not reliably indicate grai n size. Magnetic hysteresis ratios fall in a restricted range regardless of coercivity. The apparent grain size configured in a three dimensional vein network plus maghemitization might be responsible for this observation. Ma ghemitization does not affect thermal magnetic stability and enhances the g eophysical importance of remanence in serpentinites. Paleomagnetic data sug gest that important information about the geologic circumstances for oceani c rock serpentinization is embodied in the paleomagnetic records. This obse rvation may be very important for generation of long wavelength aeromagneti c and possibly even satellite magnetic anomalies.