The study area falls in the segment S2 of CIR w.r.t. RTJ between 70°54′E, 25°14′S and 70°50′E, 24°41′S spanning a length of ~ 65 km (Fig. 1). The studied sediment samples are collected from the study region using a cylindrical pipe of ~15 cm diameter and ~ 1 meter in length attached to a dredge (Fig. 1, Table S1) during the scientific cruises onboard R/V Akademik Nikolaj Strakhov (ANS- leg 4) and R/V Sagar Kanya (SK– 300). The samples are collected precisely from four principal locations: the first location (B-3-16) is in the northern part of segment S2 and lies west of the neo-volcanic mount (Table S1, Fig. 1). The other three locations are close to the transform fault between segments S1 and S2. B-4-7 location lies in the transition zone of the two segments, and B-05 location lies in the northeastern flank of the 25°S OCC (oceanic core complex)35. Sample B-04 is collected from the termination point of the slope of an older axial ridge, terminating at the transform fault between segments S1 and S2. A detailed bathymetry map of the regions is analyzed before sample collection, and the regions with topographic lows in and around the significant mounts are selected explicitly for sampling. The studied sediments are distinguished based on their appearance, which is dark to pale brown in appearance, and the sediments are mostly coarse-grained (Table S1). The majority of the sediment samples consist of pelagic sediments with calcareous fossils and foraminiferal oozes.
All four sediment samples are taken for identifying potential samples with magnetic particle enrichment. Samples are first washed for the removal of superfluous matter using distilled water (3 times). Then sediment samples were wet sieved using 60 mesh (250 microns) sieve and dried overnight in an oven at 600C. Samples with > 250-micron fraction are taken for hand-magnet separation for separating magnetic grains from non-magnetic grains. These magnetic grains were then taken under a binocular microscope to pick potential hydrothermally derived Fe-rich components and spherical grains. Sediment sample B-3-6 having higher spherule density (Table S1) in the >250 µ fraction is taken for component-level analysis, i.e., back-scattered imaging of whole-grain and polished grains, and EPMA spot analysis.
Back-scattered electron imaging of the handpicked magnetic grains is conducted on a JEOL-JSM 6360LV Scanning Electron Microscope (SEM) at the National center for polar and ocean research (NCPOR), Goa. The grains, including spherules, are mounted on double-sided carbon tape attached to 1 cm diameter aluminum stubs and sputter-coated with carbon. Mounted minerals are taken for back-scattered electron imaging (BSEI) of the whole grain. An accelerating voltage of ~15 kV and a beam current of ~ 4 nA are used with a focused beam.
Following whole-grain imaging, extracted spherules from sample B-3-6 are taken for BSE imaging of the polished section and quantitative spot analysis using EPMA. Magnetic particles, including spherules mounted on aluminum stubs from sample B-3-6, are mounted on epoxy resin, grinded, polished and carbon-coated. In total, ~ 19 magnetic particles, including spherules, are then taken for back-scattered electron imaging (BSEI) and spot analysis on an SX five electron probe microanalyzer using wavelength-dispersive spectroscopy (EPMA-WDS) installed at NCPOR-Goa. All the spherules are imaged at an accelerating voltage of 15 kV, beam current 4nA, and a working distance of 15 mm. A focused beam is used for the elemental analysis of the spherules (Major elements: Al, Si, Ca, Fe, Mg, Mn; and minor elements: S, Ti, Cr, V, Co, Ni, Zn, Cr, Pb, and Cu). A defocused beam of 10 µm diameter is used for analyzing sensitive elements (Na, K, and P). In addition, sensitive elements Na and K are analyzed in an integrated approach of sub-counting using four steps of 5s. The counting time is 20 sec for the peak and 10 sec for the background. The overlap corrections of Ti, Cr, V, Fe, and Zn are employed on V, Mn, Cr, Co, and Na, respectively. All the spherules are analyzed at an accelerating voltage of 15 kV, beam current 20nA, and a working distance of 15 mm. For calibration of the instrument following standards are used, Na - Albite, Mg - Peridot, K - Orthoclase, Ca - Diopside, Ti - TiO2, Fe &S - Pyrite, V- V-metal, Cr – Cr2O3, Pb – Pyromorphite, Al – Al2O3, Si – Wollastonite, P – Apatite, Co – Co-metal, Ni – Ni-metal, Zn – ZnS, Mn – Rhodonite, Cu – Cu-metal. Several spots are analyzed on each spherule for gathering better information on the spherules. The detection limit (DL) and standard deviation (STD) for all the elements analyzed are below 500 ppm and ±200 ppm, and data points of any element of any particular analysis having values greater than DL & 3*STD of the respective element analyzed is only considered.
The analytical totals and stoichiometry of Fe-oxide are examined to evaluate the quality of WDS analyses. The analysis of small phases or phases proximal to cavities and grain boundaries showed matrix overlap from different materials or analytical total < 100%. The ideal analytical total of magnetite based on oxygen bound to Fe2+ is 93 wt% 3, and when stoichiometrically converted to Fe3O4 (magnetite), will give a total of ~ 100 wt%. The analytical total of 93 ± 3wt% is assumed to be accurate for magnetite compositions. However, the Fe-O phase relation in wustite may result in stoichiometry with excess oxygen of 1.0–4.1 atomic% (0.7 2.9 wt%) because of vacancies3,36. Analytical totals between ~96 and ~100 wt% are considered ideal for wustite. The spots where the analytical total is < 90% are considered less representation of the total owing to overlapping cavities or grain boundaries. However, proper interpretation is drawn for other altered Fe-phases with < 90% total together with morphological features.
Geological setting
CIR, whose rift valley trends towards NNW of RTJ (25°30′S) is categorized as a slow-spreading ridge with an average full opening rate of ~47.5 mm/yr37. The CIR axis is marked by an axial valley of 5–35 km wide and 500–1800 m deep, having neo-volcanic ridges ranging in height from 100-600 m at some places38. An oceanic core complex (OCC) lies ~15 km east of the Kairei Hydrothermal Field (KHF)35. The southern part of the CIR is marked by the presence of one inactive and two active hydrothermal vent fields, namely, MESO, Edmond, and Kairei at 23°23.56' S, 69°14.53' E, 23°52.68' S, 69°35.80' E, and 25°19.23' S, 70°02.42' E, respectively39,40. The study area falls in segment S2 of CIR w.r.t. RTJ, where the Axial valley depth ranges from 3600-4200 m with Axial valley width of 25 Km and consists of a significant neo-volcanic ridge with a height of ~ 300m38.