Jai-Woon Moon

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

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 (ph) for axial and wall casts (ph=0.30, 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 ultraslow-spreading 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.

Geochemistry and petrogenesis of mafic-ultramafic rocks from the Central Indian Ridge, latitude 8°–17° S: denudation of mantle harzburgites and gabbroic rocks and compositional variation of basalts

This study investigates the formation of lower oceanic crust and geochemical variations of basalts along the Central Indian Ridge (CIR, lat. 7°45′–17°10′ S). Harzburgites, various gabbroic cumulates, medium- to fine-grained oxide gabbros, diabases, and pillow basalts were recovered by dredging from segment ends such as ridge-transform intersections (RTIs), non-transform discontinuities (NTDs), and transform offset areas. The occurrence of both harzburgites and gabbroic rocks with minor basalts at all segments ends, and leucogabbro intrusive into harzburgite at the 12°45′ S NTD indicates that oceanic crust at segment ends exposes mantle-derived harzburgites and gabbroic intrusions with a thin basaltic cover due to sparse magmatic activity. Basalts collected along the entire ridge show wide compositional variations between N (normal)- and E (enriched)-mid-ocean ridge basalt (MORB). T (transitional)-MORBs with enriched affinities are more prominent than N-MORBs. There is no tendency of enrichment towards specific directions. (La/Sm)N variations in MORB along the CIR (8°–21°S) fluctuates at a regional scale with local high positive anomalies reflecting compositional heterogeneity of the sub-CIR mantle domain.

Characteristics of Seafloor Morphology and Ferromanganese Nodule Occurrence in the Korea Deep-sea Environmental Study (KODES) Area, NE Equatorial Pacific

Abstract Seafloor morphology and ferromanganese nodule occurrence were studied using a multibeam side scan sonar (SeaBeam, 2000) and a deep-sea camera system in the Korea Deep-sea Environmental Study (KODES) area, northeast equatorial Pacific. Seafloor morphology and nodule abundance are highly variable even in this small study area. The NNE-SSW oriented hills are parallel and about 100–200 m high. Valleys are very flat-floored, while hilltops are rugged with depressions of tens of meters. Cliffs to about 100 m bound the valleys and the hills. The study area can be classified into three types based both on nodule occurrence and seafloor morphology, mostly G- and B-types and some M-type. G-type is characterized by high nodule abundance, ubiquitous bioturbation, and flat seafloor morphology, while B-type is characterized by irregular-shaped nodules, variable nodule abundance, occurrence of giant nodules and sediment lumps, rugged bottom morphology with depressions, and white calcareous surface sediments. Medium nodule abundance and a generally flat seafloor characterize M-type. G-type occurs mostly in the valley regions, while B-type is on the hilltop areas. M-type is located between the hilltop and the valley. Tectonic movement of the Pacific plate resulted in the elongated abyssal hills and cliffs. The rugged morphology on hilltops resulted from erosion and redistribution of surface siliceous sediments on hilltops by bottom currents, outcropping of underlying calcareous sediments, and dissolution of the carbonate sediments by corrosive bottom water undersaturated with CaCO3. Sediment eroded from the hills, which is relatively young and organic-rich, is deposited in the valleys, and diagenetic metal supply to manganese nodules in the valley area is more active than on the hills. We suggest that tectonic movement ultimately constrains morphology, surface sediment facies, bottom currents and sediment redistribution, bioturbation, thickness of the sedimentary layer, and other conditions, which are all interrelated and control nodule occurrence. The best potential area for mining in the study area is the G-type valley zones with about 3–4 km width and NNW-SSE orientation.

Textural and geochemical characteristics of Fe−Mn crusts from four seamounts near the Marshall Islands, western Pacific

Textural and geochemical properties of ferromanganese crust (Fe−Mn crust) samples from four adjacent seamounts near the Marshall Islands were investigated to delineate the paleoceanographic condition on their growth history. The Fe−Mn crust samples of this study show four distinct layers (layers 1 to 4 from top to bottom). The uppermost layer 1 is massive and black, and is enriched in hydrogenetic elements such as Mn, Co, Ni, and Mo. The next layer 2 is porous and filled with sediment. Detrital (Al, Rb, and Ti) and biogenic (Cu, Zn, and Ba) elements are enriched in layer 2. The layers 3 and 4 are phosphatized layers which are impregnated with carbonate fluorapatite (CFA), and therefore their primary mineralogy and geochemistry were not preserved. The property of layer 2 suggested that this layer had grown under the condition of high biogenic and detrital flux. Such a condition may be met in the Inter-tropical Convergence Zone (ITCZ) where the northeast and southeast trade winds meet. Considering the present location and paleotracking of seamounts, layer 2 appears to have formed when these seamounts were beneath the ITCZ. On the other hand, layer 1 may have started to grow after the seamounts moved out of the ITCZ with the northwestward movement of the Pacific plate. Our study indicates that the Fe−Mn crusts can be used to trace the paleolocation of ITCZ when precise age determination and information on the plate movement are provided.

Geophysical and Geological Exploration of Cobalt-rich Ferromanganese Crusts on a Seamount in the Western Pacific

Co-rich ferromanganese crusts (Fe-Mn crusts) distributed on the seamounts in the western Pacific are potential economic resources for cobalt, nickel, platinum, and other rare metals in the future. Regulations for prospecting and exploration of Fe-Mn crusts in the Area, which enables the process to obtain an exclusive exploration right for blocks of the fixed size, were enacted recently by the International Seabed Authority, which led to public attention on its potential for commercial development. Evaluation and selection of a mining site can be established based on abundance and grade of Fe-Mn crusts in the site as well as topography that should be smooth enough for mining efficiency. Therefore, acquisition of shipboard echo-sounding and acoustic backscatter data are prerequisite to select potential mine sites in addition to visual and sampling operations. Acoustic backscatter data can be used to locate crust-covered areas in a regional scale with the understanding of acoustic properties of crust through its correlation with visual and sampling data. KIOST had collected the topographic and geologic data to assess the resources potential for Fe-Mn crusts in the west Pacific region from 1994 to 2001. However, they could not obtain acoustic backscatter data that is crucial for the selection of prospective mining sites. Therefore, additional exploration surveys are required to carry out side scan sonar mapping combined with seafloor observation and sampling to decide the blocks for application of an exclusive exploration right.

Geophysical investigation of seamounts near the Ogasawara fracture zone, western Pacific

This paper provides an analysis of multi-channel seismic data obtained during 2000–2001 on seamounts near the Ogasawara Fracture Zone (OFZ) northwest of the Marshall Islands in the western Pacific. The OFZ is unique in that it is a wide rift zone that includes many seamounts. Seven units are delineated on the basis of acoustic characteristics and depth: three units (I, II, and III) on the summit of seamounts and four units (IV, V, VI, and VII) in basins. Acoustic characteristics of layers on the summit of guyots and dredged samples indicate that the seamounts had been built above sea level by volcanism. This was followed by reef growth along the summit margin, which enabled deposition of shallow-water carbonates on the summit, and finally by subsidence of the edifices. The subsidence depth of the seamounts, estimated from the lower boundary of unit II, ranges between 1,550 and 2,040 m. The thick unit I of the southern seamounts is correlated with proximity to the equatorial high productivity zone, whereas local currents may have strongly affected the distribution of unit I on northern seamounts. A seismic profile in the basin around the Ita Mai Tai and OSM4 seamounts shows an unconformity between units IV and V, which is widespread from the East Mariana Basin to the Pigafetta Basin.

Chemical Speciations of Elements in the Fe-Mn Crusts by Sequential Extraction

Abstract : Sequential extraction was carried out on twenty two subsamples of three ferromanganese crustsfrom three seamounts (Lemkein, Lomilik, and Litakpooki) near the Marshall Islands in the western Pacific.The extraction was designed to fractionate Fe-Mn crust forming elements into four defined groups: (1)exchangeable and carbornate, (2) Mn-oxide, (3) Fe-oxyhydroxide, and (4) residual fraction. X-raydiffraction result shows that target material were well removed by each extraction step except for CFA inphosphatized crusts generation. According to chemical analysis of each leachate, most of elements in theFe-Mn crusts are bound with two major phases. Mn, Ba, Co, Ni, Zn, (Fe, Sr, Cu, and V) are stronglybounded with Mn-oxide (δ-MnO 2 ) phase, whereas Fe, Ti, Zr, Mo, Pb, Al, Cu, (V, P, and Zn) show chemicalaffinity with Fe-oxyhydroxide phase. This result indicates that significant amount of Al, Ti, and Zr can notbe explained by detrital origin. Ca, Mg, K, and Sr mainly occur as exchangeable elements and/or carbonatephase. Outermost layer 1 and innner layer 2 which are both young crusts generations are similar in chemicalspeciations of elements. However, some of Fe-oxyhydroxide bounded elements (Pb, Y, Mo, Ba, Al, and V)in phosphatized innermost layer 3 are released during phosphatization and incorporated into phosphate (Pb,Y, Mo, and Ba) or Mn-oxide phase (Al and V). Our sequential extraction results reveal that chemicalspeciations of elements in the hydrogenetic crusts are more or less different from interelemental relationshipcalculated by statistical method based on bulk chemistry.Key words : (sequential extraction), (Fe-Mn crusts), (phosphatization), (hydrogenetic elements)

Mantle heterogeneity in the source region of mid-ocean ridge basalts along the northern Central Indian Ridge (8°S-17°S)

The northern Central Indian Ridge (CIR) between 8°S and 17°S is composed of seven segments whose spreading rates increase southward from ∼35 to ∼40 mm/yr. During expeditions of R/V Onnuri to study hydrothermal activity on the northern CIR in 2009–2011, high-resolution multibeam mapping was conducted and ridge axis basalts were dredged. The major and trace element and Sr-Nd-Pb-He isotopic compositions of basaltic glasses dredged from the spreading axis require three mantle sources: depleted mantle and two distinct enriched mantle sources. The southern segments have Sr, Nd, and Pb that are a mix of depleted mantle and an enriched component as recorded in southern CIR MORB. This enrichment is indistinguishable from Reunion plume mantle, except for He isotopes. This suggests that the southern segments have incorporated a contribution of the fossil Reunion plume mantle, as the CIR migrated over hot-spot-modified mantle. The low 3He/4He (7.5–9.2 RA) of this enriched component may result from radiogenic 4He ingrowth in the fossil Reunion mantle component. Basalts from the northern segments have high 206Pb/204Pb (18.53–19.15) and low 87Sr/86Sr (0.70286–0.70296) that are distinct from the Reunion plume but consistent with derivation from mantle with FOZO signature, albeit with 3He/4He (9.2–11.8 RA) that are higher than typical. The FOZO-like enriched mantle cannot be attributed to the track of a nearby mantle plume. Instead, this enrichment may have resulted from recycling oceanic crust, possibly accompanied by small plume activity.

Rock-magnetic Properties of Chimneys from TA25 Seamount in the Tofua Arc, Southwest Pacific

To identify rock-magnetic properties of volcanogenic hydrothermal sulfide deposits, chimneys were obtained from the Tofua Arc in Southwest Pacific, using a remotely operated vehicle (ROV) and Grab with AV cameras (GTVs). Three different types of chimneys used in this study are a high-temperature chimney with venting fluid-temperature of about 200 o C (ROV01), a low-temperature chimney of about 80 o C (GTV01), and an inactive chimney (ROV02). Magnetic properties of ROV01 are dominated by pyrrhotite, except for the outermost that experienced severe oxidation. Concentration and grain-size of ROV01 pyrrhotite are relatively low and fine. For GTV01, both magnetic concentration and grain-size increase from interior to margin. Pyrrhotite, dominant in the core, becomes mixed with hematite in the rim of the chimney due to secondary oxidation. High concentration and large grain-size of magnetic minerals characterize the ROV02. Dominant magnetic phases are pyrrhotite, hematite and goethite. In particular, the outermost rim shows a presence of magnetite produced by magnetotactic bacterial activity. Such distinctive contrast in magnetic concentration, grain-size and mineralogy among three different types of chimney enables the rock-magnetic study to characterize an evolution of hydrothermal deposits.

Sulfur Isotope Composition of Seafloor Hydrothermal Vents in the Convergent Plate Boundaries of the Western Pacific: A Role of Magma on Generation of Hydrothermal Fluid

Seafloor hydrothermal system occurs along the volcanic mid-ocean ridge, back-arc spreading center, and other submarine volcanic regions. The hydrothermal system is one of the fundamental processes controlling the transfer of energy and matter between crust/mantle and ocean; it forms hydrothermal vents where various deepsea biological communities are inhabited and precipitates metal sulfide deposits. Hydrothermal systems at convergence plate boundaries show diverse geochemical properties due to recycle of subducted material compared to simple systems at mid-ocean ridges. Sulfur isotopes can be used to evaluate such diversity in generation and evolution of hydrothermal system. In this paper, we review the sulfur isotope composition and geochemistry of hydrothermal precipitates sampled from several hydrothermal vents in the divergent plate boundaries in the western Pacific region. Both sulfide and sulfate minerals of the hydrothermal vents in the arc and backarc tectonic settings commonly show low sulfur isotope compositions, which can be attributed to input of magmatic SO2 gas. Diversity in geochemistry of hydrothermal system suggests an active role of magma in the formation of seafloor hydrothermal system.