Geological context and vents morphology in the ultramafic-hosted Tianxiu field, Carlsberg Ridge
Abstract: The Tianxiu hydrothermal field (TXHF) located on Carlsberg Ridge is one of the few active ultramafic-hosted venting systems known in the Indian Ocean. Despite numerous investigations, there is limited understanding of its sulfide structure morphology, and the factors controlling the formation of TXHF are poorly understood. In this study, we conducted detailed seafloor mapping using visual data obtained by dives using the human-occupied vehicle (HOV) Jiaolon g. The TXHF is found to be an active, off-axis, ultramafic-hosted, high-temperature hydrothermal area in which serpentine peridotite is exposed. Two main hydrothermal sites were identified, i.e., P and Y, both of which feature a complex of chimneys and beehive diffusers constituting a “chimney jungle” and isolated large steep-sided structures developed on flat-lying sulfide mounds. In addition, some sporadic inactive chimneys and outcrops of hydrothermal deposits were noted. The chimneys are rich in Fe and Zn sulfide, and lack the central fluid channel formed by focused high-temperature fluid flow. Hydrothermal venting at TXHF is likely related to low-angle detachment faults that focus and transport hydrothermal fluids away from a heat source along the valley wall. Our results complement and expand upon previous works concerning sulfide chimney morphology and their corresponding mineral paragenesis in ultramafic-hosted hydrothermal systems in the Indian Ocean and further our understanding of modern seafloor hydrothermal systems.
Figure 1. Bathymetric map of the Tianxiu hydrothermal field on the Carlsberg Ridge (CR) (the bathymetric data were collected by multi-beam surveys of Chinese Dayang Cruise in 2013). The red dashed line indicates the ridge axis. CIR: Central Indian Ridge; SWIR: Southwest Indian Ridge; SEIR: Southeast Indian Ridge. OCC: Oceanic Core Complex.
Figure 2. Seafloor morphology of the Tianxiu hydrothermal field. Bathymetric map of the study area showing track of the submersible Jiaolong’s dives in Chinese Dayang Cruise 72nd, the white box shows the range of the side scan (the bathymetric data were collected by multi-beam surveys of Chinese Dayang Cruise 72nd in 2022) (a); colored and corresponding backscatter image showing the main area of the P site (white and red dashed lines, respectivelys) (b, c); scattered serpentine peridotite fragments (d); fault scratch and steps (e); mylonitized outcrop, white dotted line indicates the approximate strike of the fault (f); accumulation of hydrothermal deposits can be seen at the landslides (g); sulfide distributed in lines (h); pelagic sediments showing a certain thickness of more than 50 cm (the square pit is caused by box sampler) (i). All photos or videos were taken in Chinese Dayang Cruise 72nd in 2022.
Figure 3. Representative chimney morphology in Tianxiu hydrothermal field. Mosaic of the Y chimney (similar in shape to the Chinese classical folk instrument “Yu”) (a); focused fluid flow emitting for the top of the Y structure (b); beehive structures at the middle of the Y structure showing dominance of alvinocaridid shrimps (c); the boundary between the brownish sulfide base and the dark-gray active chimneys (d); massive sulfide and chimney fragments at the foot of the Y site (e); inactive chimneys complex in P site (similar in shape to the Chinese classical folk instrument “Pan Flute” P) (f), showing small flanges with ~10 cm in width (g); lined distributed chimneys in P site showing white shells of mussels indicating waning stage (h); beehive diffusers shimmering light smoke surrounded by toppled chimney clusters in P site with yellowish-brown oxide on the surface (i), showing anemones and Alviniconcha marisindica snails (j). All photos or videos were taken in Chinese Dayang Cruise 72nd in 2022.
Figure 4. Representative chimney fragments in Tianxiu hydrothermal field. a. Toppled chimney clusters in P site with yellowish-brown oxide on the surface, partly in dark gray (b), indicating status of still active. c. The extinct “chimney jungle” in P site, showing absence of large fluid conduit (d). e. Chimney fragment collected from Y site showing multi fluid conduits wall mainly composed of pyrite and sphalerite. f. Chimney fragment collected from Y site showing major fluid conduit wall mainly composed of pyrite and sphalerite, showing reddish-brown pyrrhotite rich layer. g. Another chimney fragment showing major reddish-brown pyrrhotite rich layer; h. Inactive chimney fragment collected from P site, showing sphalerite rich and multi micro fluid conduits. i. Chimney fragment of focused fluid flow, rich in chalcopyrite and pyrite composing the fluid conduit walls, collected from other high-temperature hydrothermal area is taken here for comparison. py: pyrite; sp: sphalerite; po: pyrrhotite; ccp: chalcopyrite. The scale bar represents 5 cm.
Table 1. Ultramafic-hosted or related hydrothermal fields reported in the Indian Ocean
Site Location Latitude Longitude Water
Activity Host rock Tectonica control Deposit of
Source Tianxiu CR 3.70°N 63.83°E 3 350 24.5 active ultramafic rocks detachment fault chimney, mound this study Onnuri CIR 11.42°S 66.42°E 2 000 34.4 diffuse ultramafic rocks detachment fault breccia Lim et al. (2022) Cheoeum CIR 12.67°S 66.20°E 3 100 36.7 diffuse ultramafic rocks oceanic core complexes chimney, mound Choi et al. (2021) Yokoniwa CIR 25.27°S 70.07°E 2 500 47.2 inactive ultramafic rocks non-transform offset small sulfide chimneys Fujii et al. (2016) Kairei CIR 25.32°S 70.03°E 2 450 47.5 active basalt detachment fault mound, chimeny,
massive sulfide, breccia
Nakamura et al. (2009)
Wang et al. (2018)
Tianzuo SWIR 27.95°S 63.53°E 3 630 12 inactive ultramafic rocks detachment fault breccia, mound Ding et al. (2021) Longqi SWIR 37.78°S 49.65°E 2 700 14 active basalt detachment fault mound, chimeny,
massive sulfide, breccia
Tao et al. (2014) Yuhuang SWIR 37.94°S 49.26°E 1 500 14 inactive basalt detachment fault massive sulfide, breccia Yu et al. (2021) Note: CR: Carlsberg Ridge; CIR: Central Indian Ridge; SWIR: Southwest Indian Ridge.
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