A reduced crustal magnetization zone near the first observed active hydrothermal vent field on the Southwest Indian Ridge

Inversion of near‐bottom magnetic data reveals a well‐defined low crustal magnetization zone (LMZ) near a local topographic high (37°47′S, 49°39′E) on the ultraslow‐spreading Southwest Indian Ridge (SWIR). The magnetic data were collected by the autonomous underwater vehicle ABE on board R/V DaYangYiHao in February‐March 2007. The first active hydrothermal vent field observed on the SWIR is located in Area A within and adjacent to the LMZ at the local topographic high, implying that this LMZ may be the result of hydrothermal alteration of magnetic minerals. The maximum reduction in crustal magnetization is 3 A/M. The spatial extent of the LMZ is estimated to be at least 6.7 × 10 4 m 2 , which is larger than that of the LMZs at the TAG vent field on the Mid‐Atlantic Ridge (MAR), as well as the Relict Field, Bastille, Dante‐Grotto, and New Field vent‐sites on the Juan de Fuca Ridge (JdF). The calculated magnetic moment, i.e., the product of the spatial extent and amplitude of crustal magnetization reduction is at least −3 × 10 7 Am 2 for the LMZ on the SWIR, while that for the TAG field on the MAR is −8 × 10 7 Am 2 and that for the four individual vent fields on the JdF range from −5 × 10 7 to −3 × 10 7 Am 2 . Together these results indicate that crustal demagnetization is a common feature of basalt‐hosted hydrothermal vent fields at mid‐ocean ridges of all spreading rates. Furthermore, the crustal demagnetization of the Area A on the ultraslow‐spreading SWIR is comparable in strength to that of the TAG area on the slow‐spreading MAR.