Hydrothermal Activity on ultraslow Spreading Ridge: new hydrothermal fields found on the Southwest Indian ridge

Abstract:

Ultraslow spreading ridge makes up about 25% of global mid-ocean ridge length. Previous studies believed that hydrothermal activity is not widespread on the ultraslow spreading ridge owing to lower magma supply. Southwest Indian ridge (SWIR) with the spreading rate between 1.2cm/a to 1.4cm/a, represents the ultraslow spreading ridge. In 2007, Chinese Cruise (CC) 19^th discovered the Dragon Flag deposit (DFD) on the SWIR, which is the first active hydrothermal field found on the ultraslow spreading ridge. In recent years, over 10 hydrothermal fields have been found on the SWIR between Indomed and Gallieni transform faults by the Chinese team. Tao et al. (2012) implied that the segment sections with excess heat from enhanced magmatism and suitable crustal permeability along slow and ultraslow ridges might be the most promising areas for searching for hydrothermal activities. In 2014, CC 30^thdiscovered five hydrothermal fields and several hydrothermal anomalies on the SWIR.

• Dragon Horn Area (DHA). The DHA is located on the southern of segment 27 SWIR, with an area of about 400 km². The geophysical studies indicated that the DHA belongs to the oceanic core complex (OCC), which is widespread on the slow spreading ridges (Zhao et al., 2013). The rocks, such as gabbro, serpentinized peridotite, and consolidated carbonate were collected in the DHA, which provide the direct evidence with the existence of the OCC. However, all rock samples gathered by three TV-grab stations are basalts on the top of the OCC. A hydrothermal anomaly area, centered at 49.66°E，37.80° S with a range of several kms, is detected in the DHA. It is probably comprised of several hydrothermal fields and controlled by a NW fault.
• New discovery of hydrothermal fields. From January to April 2014, five hydrothermal fields were discovered on the SWIR between 48°E to 50°E during the leg 2&3 of the CC 30^th, which are the Su Causeway field (48.6°E, 38.1°S), Bai Causeway field (48.8°E, 37.9 °S), Dragon Well West field (49.6°E, 37.8°S), Dragon Well East field (49.8°E, 37.8°S), and Landing Stage field (51.0°E, 37.5°S), respectively.