Tectonic and magmatic control of hydrothermal activity along the slow‐spreading Central Indian Ridge, 8°S–17°S

Abstract

The complex geology and expansive axial valleys typical of slow-spreading ridges makes evaluating their hydrothermal activity a challenge. This challenge has gone largely unmet, as the most undersampled MOR type for hydrothermal activity is slow spreading (20-55 mm/yr). Here we report the first systematic hydrothermal plume survey conducted on the Central Indian Ridge (CIR, 8 S-17 S), the most extensive such survey yet conducted on a slow-spreading ridge. Using a combined CTD/Miniature Autonomous Plume Recorder (MAPR) package, we used 118 vertical casts along seven segments of the CIR ( 700 km of ridge length) to estimate the frequency of hydrothermal activity. Evidence for hydrothermal activity (particle and methane plumes) was found on each of the seven spreading segments, with most plumes found between 3000 and 3500 m, generally <1000 m above bottom. We most commonly found plumes on asymmetric ridge sections where ultramafic massifs formed along one ridge flank near ridge-transform intersections or nontransform offsets. The estimated plume incidence (p h ) for axial and wall casts (p h 50.30, 35 of 118 casts) is consistent with the existing global trend, indicating that the long-term magmatic budget on the CIR is the primary control on the spatial frequency of hydrothermal venting. Our results show that the tectonic fabric of the CIR strongly determines where hydrothermal venting is expressed, and that using only near-axial sampling might underestimate hydrothermal activity along slow-spreading and ultraslowspreading ridges. Serpentinization is a minor contributor to the plume inventory, based on 15 profiles with methane anomalies only, predominantly at depths above the local valley walls.

Discovery of Active Hydrothermal Vent Fields Along the Central Indian Ridge, 8–12°S

“…Active venting was expected on Segment 2 based on ORP anomalies observed during previous work, although particle and methane anomalies were weak (Son et al, 2014). ORP anomalies were mapped more thoroughly during CTD tows in 2018, and two off-axis abyssal hill locations near 9°47.5′S, 66°41.9′E (~6 km west of the axis) and 9°48.9′S, 66°40.6′E (~9 km west of the axis) were identified as the likely source locations.…”

“…The OVF is located more than 800 km from previously known vent fields on the CIR, though the plume surveys of Son et al (2014) suggest there are additional active sites on the MCIR segments south (12°S to 17°S) of Segment 3, and some may be very similar to OVF. Considering its isolated location, relatively shallow depth, and metal-poor/methane-rich vent fluid, the vent fauna of the OVF adds valuable information to understand the relatively unknown ecology and biogeography of Indian Ocean hydrothermal vents.…”

“…Thus, these earlier results provided priority target sites for seafloor observations during the 2018 survey to locate hydrothermal vent sources. Contour plots showing (a) particle anomaly (ΔNTU) and (b) dissolved methane (CH 4 , nM) along Segments 1-3 on the CIR (modified after Son et al, 2014). Yellow and white stars mark the locations of discovered vent fields and plume-only site, respectively.…”

“…Here we present the results from the hydrothermal expedition by RV ISABU in 2018 that targeted select sites on Segments 1 through 3 (Pak et al, 2017) based on the previous plume-only surveys (Son et al, 2014). Four new active hydrothermal vent fields were located and characterized by seafloor observations, plume chemistry, and rock type.…”

Where are the undiscovered hydrothermal vents on oceanic spreading ridges?

“…2A, Supplementary information). Surveys at slow MORs also reflect recent interests in ultramafic-hosted vents and the association of hydrothermal activity with oceanic core complexes and detachment faults (Snow and Edmonds, 2007;Escartin et al, 2008;German et al, 2010;McCaig and Harris, 2012;Son et al, 2014).…”