Magnetic characteristics of the NTO massif between Central Indian Ridge segment 1 and 2 through the near-bottom magnetic survey

Near-bottom magnetic surveys have been carried out for many years and generally the signals may reflect the differences of rock types and/or degree of weathering, rather than magnetic polarity reversal patterns. In case of hydrothermal fields, hydrothermal processes can destroy magnetic minerals in volcanic rocks, leading to low magnetization, and create magnetic minerals by the serpentinization of ultramafic rocks, leading to high magnetization. But there is great diversity of tectonic settings. Therefore, to identify the magnetic source, combination of near-bottom magnetic survey and sampling is required at each site. A magnetic survey using an AUV called R2D4 was conducted during a Japanese scientific cruise in November 2010. Three-component magnetometer was installed in the front area of the AUV. Only one dive was done at the NTO (Non-transform offset) massif between the intermediate spreading Central Indian Ridge segment 1 and 2. The survey consisted of four north-south lines of approximately 6 km long and the line spacing of approximately 500m. The vehicle height varied between 40 and 200 m and the mean height was 80 m. A figure 8 turn was done before entering the survey line to calculate the vehicle magnetization coefficients. Total magnetic field was calculated from three-component magnetic fields after removing on the vehicle magnetization. The total magnetic anomaly was calculated by subtracting the IGRF value. Calculating the magnetization intensity, a new magnetic inversion method proposed by Honsho et al., (2012) was used. By changing the geometry of the magnetic layer such as constant layer, half-infinite layer etc., several possible cases of magnetization intensities were estimated. Magnetization intensity by a 100 m-thick magnetic layer show relatively positive magnetization at several distinct areas and the highest magnetization up to 30 A/m corresponds to a north-south trending small hill at the southeastern part of the survey area. Magnetization intensities of other geometry of magnetic layers show high magnetization only at the hill. Therefore the hill has relatively strong positive magnetization, regardless of the geometry of the magnetic layers. Sea surface magnetization distribution from previous studies shows that the NTO massif is located at the normally magnetized area. This suggests that the hill has a really strong magnetization. Basalt, peridotite and serpentinized peridotite were dredged at the western slope of the hill during the same cruise. These rocks suggest that the hill may be comprised of ultramafic rocks. In addition, dead-chimneys were observed at the top of the hill by a manned submersible Sinkai 6500 in 2009. The discoveries of ultramafic rocks and dead-chimneys suggest that the strong positive magnetization is attributable to serpentinization of ultramafic rocks, but the effect of volcanic rocks cannot be excluded at this point. Further analyses of rocks magnetism is required to clarify source of magnetization of the NTO massif.