On the Rock Magnetic Properties of Some Serpentinized Peridotites in the Southern Mariana Forearc

We have studied the magnetic properties of 12 peridotite samples recovered during the JAMSTEC Cruise of the R/V Yokosuka YK10-12 and collected on Shinkai 6500 dive 1234 in the Shinkai See Field (SSF) located in the deep (~5700 m) outer forearc (11°39.10'N, 143°02.94'E). We conducted remanence and induced experiments on the samples to determine degree of serpentinization. Stepwise alternating field (AF) and thermal demagnetization experiments from 2.5 to 70 mT and from 28 to 575°C, respectively, yield univectorial diagrams showing the removal of secondary components (e.g., VRM, IRM, CRM etc) by isolating a Characteristic Remanent Magnetization (ChRM) at low fields and temperatures. The normalized intensity of demagnetization J/Jo shows the decrease of the magnetization of the specimens where about 50% of the original magnetization is lost at about 5 mT and 100°C (i.e. Median Destructive Field). The stereograms show magnetic stability of the specimens by determining the directional behavior after 4 demagnetization steps (from 7.5-10 mT fields and low temps). Induced magnetization such as magnetic granulometry tests, sIRM's, hysteresis saturation loops and back-fields were performed. Diagnostic values of Mrs/Ms and Hrc/Hc determine the domain structure of a magnetic sample. The magnetic grain sizes were determined using the protocol of Dunlop [2002 a and b]. Most of the samples are distributed over the Pseudo-Single Domain (PSD) range with a certain degree of clustering. Curie points were obtained by measuring their low-field susceptibility vs. temperature (k-T) from 28 °C up to 700 °C in an Argon atmosphere showing a minimum of 1-4 magnetic mineral phases with temperatures ranging from ~100°C up to 640°C, which are predominantly Ti-poor and Ti-rich magnetites and magnetite. Samples recovered by the Shinkai 6500 show appreciable variation in bulk susceptibility (22.3 x 10-3 to 142 x 10-6 SI units). The samples appear to be modestly to moderately serpentinized based on their magnetic properties (magnetic stability and pseudo single-domain character) and they do not show the exponential increase of NRM, magnetic susceptibility and magnetite content during serpentinization (implying a major change in alteration conditions) as is the case of other areas such as the Mid-Atlantic Ridge (MAR).