## 2 Materials and Methods
### Hydrological Background of the Longqi Hydrothermal Field  
The SWIR is a typical ultra-slow spreading ridge [22; 23]. Figure 1a shows that the SWIR has obvious segmental property, and the SWIR is the best place to study the relationship between magmatic activity, tectonics and hydrothermal circulation of mid-ocean ridges [24]. The Longqi hydrothermal field is located at the junction of the ridge axis and non-conversion discontinuity in the 28 th segment of the SWIR, on a hump of the southeast slope of the central axis rift, with a water depth of about 2750 m [25]. Among them, the Longqi-1 hydrothermal field is on the southern side of SWIR segment 28 [6]. It can be seen from Figure 1 that the terrain in the middle and eastern section of the ridge is relatively gentle, the ridge rift is shallow and narrow, the central axis rift in the western section of the ridge is wide and deep, and the water depth changes greatly. The topographic structure with fissures provides channels for seawater infiltration into the deep crust and upwelling of hydrothermal fluids and provides favorable topographic environment for hydrothermal activities. Figure 1: Bathymetric map of the Longqi-1 hydrothermal field. (**a**) Overview map of the Southwest Indian Ridge (SWIR). The red line of the Earth in the upper left represents an overview of SWIR. (**b**) Bathymetric map of the Longqi hydrothermal field in the range of 49.4\({}^{\circ}\) E-50.2\({}^{\circ}\) E, 37.6\({}^{\circ}\) S–38.0\({}^{\circ}\) S. The location of the Longqi-1 hydrothermal field is shown as a red five-pointed star. (**c**) Detailed bathymetric map of the Longqi-1 hydrothermal field (S Zone). The seven active events in the S zone are indicated by red dots. The six inactive events in the S zone are indicated by black dots. The seven CTD stations are indicated by green triangles. The information of CTD stations is derived from the 30 IV-Swir-S024. Topographic data are derived from the ETOPO1 database and 30 IV-SWIR-S024-CTD voyage data. The first active hydrothermal field was discovered in the 28 th ridge segment of SWIR during the 19 th Chinese Oceanic voyage in 2007, and it was named the Dragon Flag Hydrothermal Field (DFHF, 49.65\({}^{\circ}\) E, 37.78\({}^{\circ}\) S) [25; 26; 27]. The Longqi hydrothermal field has three hydrothermal vent zones: the N zone, the M zone and the S zone. During the 35 Ocean voyages of China from 2014 to 2015, Jiaolong was used to make detailed near-bottom observations of the hydrothermal field of Longqi, and it was determined that the Longqi hydrothermal field is one of the large-scale hydrothermal vents of the mid-ocean ridge [6; 22; 28]. Two active hydrothermal zones about 500 m apart, called the S and the M zones, have been found at depths of 2750-2780 m in the Longqi-1 hydrothermal field [25]. Observations made by the researchers in 2016 using a HOV (Human-Occupied Vehicle) indicated that there are six high-temperature vents in the M zone and three in the S zone. At the same time, five large diffuse areas were found in the S zone [28]. [PERSON] et al. investigated the hydrothermal zone of Longqi-1 (49.645\({}^{\circ}\) E-49.650\({}^{\circ}\) E, 37.780\({}^{\circ}\) S-37.785\({}^{\circ}\) S) and found three active spouts (DFF3, DFF5 and DFF20) in the S zone (Figure 1b and Table 2) [6; 28; 29]. It was recently found that a melt zone in the lithospheric mantle \(\sim\)13 \(\pm\) 2 km below the sea floor is the most likely source of heat for the Longqi-1 hydrothermal field, and the geochemical composition of the Longqi-1 hydrothermal fluids implied that the path of the Longqi-1 hydrothermal circulation may be longer than any other hydrothermal system studied to date [6]. Figure 1c shows the distribution of CTD stations marked by our team on the fourth leg of the 30 th voyage. Among them, the hydrothermal plume near the CTD 05-2 station is the research object of this paper. Miniature Automatic Plume Record (MAPR) is a tool used in ocean observations to find hydrothermal vents.