Integrated Ocean Drilling Program, United States

During Integrated Ocean Drilling Program Expedition 301, 235 m of upper igneous crust was cored in Hole U1301B, in an area of hydrothermal circulation on Juan de Fuca Ridge, providing an opportunity to better understand ocean crust magnetization. We studied the paleomagnetism of this section using a variety of techniques. Natural remanent magnetization (NRM) and alternating- field or thermal demagnetization measurements were made on 330 individual samples of igneous rock from both the working- half and archive-half cores. Rock magnetic characterization measurements were made on a subset of samples to better understand the source of the magnetization. These measurements included hysteresis parameters, Curie temperature, and isothermal remanent magnetization (IRM) acquisition. In addition, thin sections were examined for several samples in an effort to visually identify magnetic grains. Igneous samples from Hole U1301B display a range of NRM values between 1.3 and 73.0 A/m with a median value of 6.9 A/m, which are values typical of ocean crust basalts. IRM acquisition curves are consistent with titanomagnetite as the magnetic remanence carrier. Hysteresis parameters indicate single domain and pseudosingle domain behavior, despite large grain sizes seen in scanning electron microphotographs. The apparent paradox is resolved by observation of grain textures that indicate that large grains are split into small domains. Magnetization direction characteristics vary and have been divided into five types by demagnetization patterns. The many patterns suggest that the remanent magnetization of some samples has been altered by secondary processes, such as hydrothermal metamorphism.