Magnetic structure of a slow spreading ridge segment:Insights from near-bottom magnetic measurementson board a submersible

摘要

Near-bottom magnetic measurements on board submersible Nautile were carried out onthe Mid-Atlantic Ridge 21 °40'N segment, and deep-sea geomagnetic vector anomaliesalong 19 dive tracks were obtained by applying the processing method for shipboard three-component magnetometer data. A forward modeling technique using short-wavelengthcomponents of the anomalies arising from local topography and vertical motion of thesubmersible was designed to estimate the absolute magnetization intensity of the seafloor.In the vicinity of the spreading axis a considerable number of magnetization estimations arereliably confirmed by the high correlation between observed and modeled anomalies,whereas less reliable estimations are obtained off-axis, probably because the sediment buriesthe basement topography. The natural remanent magnetization (NRM) measured onbasalt samples collected during these dives is compared with the magnetization fromanomalies. Though both results give a similar range of magnetization intensity, nocorrelation is confirmed between them, possibly because the magnetization from anomaliesrepresents laterally averaged seafloor magnetization, whereas the NRM has variations atthe scale of individual pillow or lava pile. Equivalent magnetization inverted from thesea-surface magnetic anomalies shows axial magnetization increases significantly fromthe segment center to the segment ends. However, the results of eight dives conducted nearthe spreading axis at different locations along the segment show much less variation inmagnetization intensity along the axis. We ascribe the high equivalent magnetization atsegment ends to preferential serpentinization of peridotite near the segment ends and theassociated formation of magnetite. The results of three across-axis transects composed of15 dives running in the spreading direction can be consistently interpreted as recordinggeomagnetic paleointensity variations during the Brunhes epoch. Although magnetizationlows are generally correspondent to periods of low paleointensity, they show deeper dropthan predicted from the paleointensity variation.