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

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

The role of late sulfide saturation in the formation of a Cu- and Au-rich Magma: Insights from the Platinum Group Element Geochemistry of Niuatahi-Motutahi Lavas, Tonga Rear Arc

Magmas enriched in Cu and Au are likely to be the most prospective for magmatic–hydrothermal deposits of these metals. However, the mechanism that leads to the formation of metal-rich magmas is not well constrained. We report major, trace and platinum group element (PGE) data for the Niuatahi–Motutahi lavas, Tonga rear arc, with the aim of studying their petrogenesis with special emphasis on the evolution of chalcophile elements during magmatic differentiation. Major and trace element contents, including S and Cl, were also measured in glasses, phenocrysts and melt inclusions to complement the whole-rock data. The Niuatahi–Motutahi lavas are associated with Cu and Au mineralization and therefore provide an opportunity to investigate the characteristics of an ore-associated magmatic system. They show a wide compositional variation from basalts to dacites with MgO contents that vary between ∼14 and 1 wt %. The dacites can be subdivided into high-K and low-K dacites. Geochemical modeling and their mineralogy suggest that the low-K dacites are comagmatic with the basalts and evolved by fractional crystallization. Copper, Au and Pd behave incompatibly in the basalts and andesitic glasses, whereas they become compatible in the low-K dacite. In contrast, Pt, Rh, Ru and Ir are moderately compatible in the basalts and correlate negatively with MgO, but become strongly compatible in the dacites. The Cu, Au and PGE trends can be explained by fractional crystallization of a Pt-rich alloy in the basalts and andesitic glasses, followed by sulfide saturation in the low-K dacites. The high-K dacites may have evolved from a slightly different magma source; however, their chalcophile elements fractionated in a similar manner. Chlorine and S data for melt inclusions and glasses from the basalts and dacites suggest that water-rich volatile saturation occurred during evolution of the dacites. The Niuatahi–Motutahi dacites were enriched in Cu from 80 to 200 ppm and in Au from <1 to 7 ppb by fractional crystallization as the magma evolved from basalt through andesite to dacite. We suggest that late sulfide saturation allowed Cu and Au to become enriched by a factor of two in the Niuatahi–Motutahi dacites before volatile saturation and that this played an important role in the formation of the Cu- and Au-rich mineralization. This sulfide saturation history contrasts with that of the Honshu arc volcanic rocks, Japan, where sulfide saturation occurred early. The Honshu arc volcanic rocks are not associated with porphyry Cu or Au deposits, which we suggest is due to loss of most of the Cu and Au to an early immiscible sulfide phase.This research was supported by an Australian Research Council Discovery grant to Ian Campbell and a fund from the Ministry of Oceans and Fisheries of Korea (PM57580). Three-month stipend for Jung-Woo Park was supported by Australian Research Council grant to Oliver Nebel in 2012

The Role of Late Sulfide Saturation in the Formation of a Cu- and Au-rich Magma: Insights from the Platinum Group Element Geochemistry of Niuatahi-Motutahi Lavas, Tonga Rear Arc

Magmas enriched in Cu and Au are likely to be the most prospective for magmatic–hydrothermal deposits of these metals. However, the mechanism that leads to the formation of metal-rich magmas is not well constrained. We report major, trace and platinum group element (PGE) data for the Niuatahi–Motutahi lavas, Tonga rear arc, with the aim of studying their petrogenesis with special emphasis on the evolution of chalcophile elements during magmatic differentiation. Major and trace element contents, including S and Cl, were also measured in glasses, phenocrysts and melt inclusions to complement the whole-rock data. The Niuatahi–Motutahi lavas are associated with Cu and Au mineralization and therefore provide an opportunity to investigate the characteristics of an ore-associated magmatic system. They show a wide compositional variation from basalts to dacites with MgO contents that vary between ∼14 and 1 wt %. The dacites can be subdivided into high-K and low-K dacites. Geochemical modeling and their mineralogy suggest that the low-K dacites are comagmatic with the basalts and evolved by fractional crystallization. Copper, Au and Pd behave incompatibly in the basalts and andesitic glasses, whereas they become compatible in the low-K dacite. In contrast, Pt, Rh, Ru and Ir are moderately compatible in the basalts and correlate negatively with MgO, but become strongly compatible in the dacites. The Cu, Au and PGE trends can be explained by fractional crystallization of a Pt-rich alloy in the basalts and andesitic glasses, followed by sulfide saturation in the low-K dacites. The high-K dacites may have evolved from a slightly different magma source; however, their chalcophile elements fractionated in a similar manner. Chlorine and S data for melt inclusions and glasses from the basalts and dacites suggest that water-rich volatile saturation occurred during evolution of the dacites. The Niuatahi–Motutahi dacites were enriched in Cu from 80 to 200 ppm and in Au from <1 to 7 ppb by fractional crystallization as the magma evolved from basalt through andesite to dacite. We suggest that late sulfide saturation allowed Cu and Au to become enriched by a factor of two in the Niuatahi–Motutahi dacites before volatile saturation and that this played an important role in the formation of the Cu- and Au-rich mineralization. This sulfide saturation history contrasts with that of the Honshu arc volcanic rocks, Japan, where sulfide saturation occurred early. The Honshu arc volcanic rocks are not associated with porphyry Cu or Au deposits, which we suggest is due to loss of most of the Cu and Au to an early immiscible sulfide phase.OAIID:oai:osos.snu.ac.kr:snu2015-01/102/2015000769/1ADJUST_YN:YEMP_ID:A079985DEPT_CD:3345CITE_RATE:4.424DEPT_NM:지구환경과학부SCOPUS_YN:YCONFIRM:

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

<div><p>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.</p></div

Seabed Mapping Using Shipboard Multibeam Acoustic Data for Assessing the Spatial Distribution of Ferromanganese Crusts on Seamounts in the Western Pacific

Cobalt-rich ferromanganese crusts (Fe&ndash;Mn crusts), potential economic resources for cobalt, nickel, platinum, and other rare metals, are distributed on the surface of seamounts, ridges, and plateaus. Distribution of Fe&ndash;Mn crust deposits and their geomorphological characteristics are prerequisites to selecting possible mining sites and to predicting the environmental impact of deep-sea mining activity. Here, we map the spatial distribution of Fe&ndash;Mn crust deposits on seamount summits and flanks in the Western Pacific using shipboard multibeam echo sounder (MBES) data and seafloor images from a deep-towed camera system (DCS) and evaluate the relationship between acoustic backscatter variations and the occurrence of Fe&ndash;Mn crusts. We find a positive correlation between high backscatter intensity, steep seabed slope gradients, and the occurrence of Fe&ndash;Mn crusts. However, our analysis was not effective to distinguish the spatial boundary between several seabed types that occur over small areas in mixed seabed zones, particularly where transition zones and discontinuous seabed types are present. Thus, we conclude that MBES data can be a valuable tool for constraining spatial distribution of Fe&ndash;Mn crust deposits over a large exploration area

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 Rèunion plume mantle, except for He isotopes. This suggests that the southern segments have incorporated a contribution of the fossil Rèunion 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 Rèunion 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 Rèunion 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