Abstract
Continuous tow investigations have shown that the present vent field inventory along fast to intermediate spreading ridges may be underestimated by at least 3–6 times, while the limited towed line investigations of venting sites along slow to ultra-slow spreading ridges make it impossible to determine their distribution. The Chinese Dayang cruise has conducted detailed towed line surveys of hydrothermal activity on segment 27 of the ultra-slow spreading southwest Indian ridge in 2015. The results have identified as many as 9 hydrothermal fields along 85-km-long segment, including one confirmed hydrothermal field, three inferred hydrothermal fields and five suspected fields. Hydrothermal activities are not only limited along-axis but also found approximately 10 km away from the axis. These vent fields are likely powered by a seismically identified axial magma chamber, including melt migration along normal faults to flank areas. The calculated hydrothermal activity frequency on segment 27 is approximately 3.6–8 times higher than that calculated from the Interridge database, suggesting that careful system exploration can reveal more hydrothermal activities even on ultra-slow spreading ridges effected by hotspot.
Highlights
Hydrothermal activity is widespread on the seafloor, such as at ocean spreading ridges (OSRs) and arc volcanoes[1]
Seismic tomography and full waveform inversion of ocean bottom seismometer data indicate the presence of a large low-velocity anomaly (LVA) approximately ~4–9 km below the seafloor interpreted as an axial magma chamber (AMC) in the lower crust[23]
An NTU anomaly is a plume spread more than 1 km and rise height above 100 m with the value above 0.01NTU, and an oxidation-reduction potential (ORP) anomaly is identified by a dE/dt value more negative than −0.04 mV/s for consecutive measurements with an overall decrease >2 mV3
Summary
Hydrothermal activity is widespread on the seafloor, such as at ocean spreading ridges (OSRs) and arc volcanoes[1]. Our survey of hydrothermal activity on the ultra-slow SWIR which interacts with the Crozet hotspot shows obvious differences from previous studies. Seismic tomography and full waveform inversion of ocean bottom seismometer data indicate the presence of a large low-velocity anomaly (LVA) approximately ~4–9 km below the seafloor interpreted as an axial magma chamber (AMC) in the lower crust[23]. Magmatism in this area is related to the Crozet hotspot, which is located 1000 km south of the ridge axis[19]. The hydrothermal field has an area of approximately 200 × 125 m25
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