Near-bottom magnetic observation of the first active hydrothermal vent field at the Southwest Indian Ridge

Near-bottom magnetic field data were collected by the Autonomous Benthic Explorer (ABE) of Woods Hole Oceanographic Institution during the second Chinese cruise to the Southwest Indian Ridge (SWIR) on board R/V DaYangYiHao in February-March 2007. Inversion of these data for crustal magnetization reveals a well- defined, circular, low magnetization zone (LMZ) located at a local volcanic topographic high (37°47'S, 49°39'E). The LMZ has a radius of about 80 m and a maximum reduction of 3A/m compared with adjacent terrain. A zone of active hydrothermal activity was discovered directly adjacent to this LMZ approx. 20 meters away on the flank of the volcanic high. It is unknown at this time if the summit of the volcanic high has hydrothermal deposits, however, we suggest that the LMZ is a result of hydrothermal alteration whereby corrosive hydrothermal fluids have rapidly demagnetized magnetic minerals within oceanic crust through chemical alteration. The flank located hydrothermal system may have recently been established and may not show a magnetic low because the hydrothermal fluids could be transported laterally from the LMZ through fractures or faults which would produce little or no magnetic effect due to the vertical crustal averaging inherent in magnetic studies. We discount crustal structure, such as an axial dike zone for example, as a source of the LMZ anomaly because other volcanic topographic highs in the area produce magnetization highs. There are, however, additional zones of low magnetization, which suggest the existence of more hydrothermal vents, both active and inactive, near the LMZ, an observation that can be tested by near-bottom observations in future research expeditions.