Shigeru Terashima

The behavior of gold in unaltered island arc tholeiitic rocks from Izu-Oshima, Fuji, and Osoreyama volcanic areas, Japan

Abstract High Au concentrations (4–25 ppb) in titanomagnetite concentrates from basalts and basaltic andesites of the Izu-Oshima and Fuji volcanic areas in the East Japan arc indicate that Au is strongly partitioned into titanomagnetite relative to the bulk rock. The concentration of An in volcanic rocks increases during the early stages of fractional crystallization, dominated by olivine, pyroxenes, and plagioclase, but decreases with subsequent fractional crystallization involving titanomagnetite. A delay in titanomagnetite formation during fractional crystallization subsequently results in comparatively high concentrations of Au in island arc tholeftes. Wide variations in the Au concentration are observed in rocks derived from primary magmas across a single volcanic area, and these can be positively correlated with the concentrations of incompatible elements such as Rb and Zr. These features are inherited from a S-depleted mantle source, since variable degrees of partial melting do not significantly affect the Au concentrations of the primary magmas.

Local variability of manganese nodule facies on small abyssal hills of the Central Pacific Basin

Abstract Local variability of manganese nodule facies and associated sediments were studied around abyssal hills ( 60 × 60 km 2 ) located in the equatorial zone of the Central Pacific Basin. Small-scale variations in nodule occurrence, mineralogy, chemistry, and internal structure are discussed in relation to seismic records and sediment lithology. The slow sedimentation rate from the late Tertiary to the Quaternary appear to have promoted formation of nodules around the abyssal hills, although the Plio-Pleistocene hiatus at depths below sediment surface suggests no clear evidence of increased growth of nodules during the period. In the area of rapid and continuous sedimentation away from the hill areas, nodules are rare probably because of insufficient lifting force for nodules. Mineralogical and chemical studies reveal that the two principal nodule morphologies (smooth and rough surfaces) result from preferential deposition of hydrogenous or diagenetic manganese minerals in relation to early diagenesis of surface siliceous sediments. Regional variability of nodule-surface morphology and composition suggests a very local change in sedimentary conditions in space, and internal mineral variation suggests changes in sedimentary conditions with time. The geological factors controlling the small-scale variability of nodule facies seem to be similar to those controlling regional-scale variations.

Deposition of hydrogenetic and hydrothermal manganese minerals in the Ogasawara (Bonin) Arc Area, Northwest Pacific

A widespread distribution of hydrothermal and hydrogenetic manganese deposits is described in the results of the Hakurei‐Maru cruises conducted in the Bonin Arc areas of the West Pacific from 1984 to 1989. Manganese deposits occur in the active volcano chains, back‐arc basins, remnant back‐arc ridges, and oceanic seamounts. The hydrogenetic iron‐manganese deposits commonly form earthly black crusts and nodules on the topographic highs of inactive ridges and old seamounts, sometimes as thick as 10 cm. They are always composed of the iron‐manganese mineral vernadite. Co and Ni contents are relatively high in the crusts from the seamounts in the open Pacific Ocean (up to 1.1% Co and 1.0% Ni). Two generations of distinct chemistry and texture are typical of these crusts, which can be compared to reported thick crusts from the Central Pacific seamounts. The hydrothermal deposits, characterized by dense, submetallic, and gray appearance, are dominant in the recent and also in past submarine volcanoes of the isl...

Regional geochemical mapping in eastern Japan including the nation's capital, Tokyo

ABSTRACT For use in a nationwide geochemical mapping project, 496 stream sediment samples were collected in eastern Japan including the nations capital, Tokyo. The 51 elements were determined using ICP-AES and ICP-MS; their spatial distribution maps were created using geographic information system (GIS) software. Spatial distribution patterns of elemental concentrations in stream sediments are consistent with the distribution of surface geology. Heavy metals and toxic elements are highly elevated in the sediments associated with mineral occurrences, but their distributions are limited to a small area. Surveying for nationwide geochemical mapping avoids collection of samples near contamination sources. Nevertheless, high concentrations of P (P2O5), Cr, Ni, Cu, Zn, Sn, Sb, Hg, and Pb were found in populated areas. Results of statistical analyses – analysis of variance and a multiple comparison test – suggest that the enrichments are not coincidental phenomena, rather these sediment samples accurately represented contamination.

Geochemical behavior of Pt and Pd in coastal marine sediments, southeastern margin of the Japan Sea

Abstract Eighty-one argillaceous marine sediments collected from the southeastern coastal margin of the Japan Sea were analyzed for Pt and Pd by atomic absorption spectrometry using a graphite furnace atomizer after solvent extraction separation. The average Pt (1.2 ± 0.5 ppb) and Pd(1.6 ± 0.9 ppb) contents are generally similar to those of the published crustal abundances. The Pt or Pd is generally enriched in the fine sediment fraction, and positively correlated with Cu, Pb and water depth. The inverse correlation between the Pt or Pd content and redox potential of the sediment suggests that significant amounts of Pt and Pd are supplied in their dissolved forms, and reduced to their metallic states. It is concluded that the Pt and Pd distributions in the coastal marine sediments are controlled by the geological characteristics of the studied area, grain size and redox potential of sediments, water depths, and dissolved O2 concentration of sea water.

Determination of trace amounts of platinum in manganese nodules by solvent extraction/graphite furnace AAS.

A rapid and precise method for the determination of Pt in manganese nodules and crusts is presented. The sample of 0.1 g was decomposed with 8 ml of aqua regia and filtered. The filtrate was evaporated to dryness, then dissolved in 20 ml of 20% tartaric acid with heating. After the addition of 0.5 ml iron (III) chloride solution (20 mg Fe/ml) and 0.5 ml of 1% 1, 3-diphenylthiourea in acetone, the mixture was heated for 5 min at about 100°C. The solution was extracted with 1 ml of 1, 2-dichloroethane for 5 min. Platinum in the organic phase was determined by graphite furnace AAS. The limit of detection is 0.05 ppm, and the relative standard deviation in the determination of 0.01 μg to 1.0 μg of Pt is 4 to 16%. The method is satisfactorily applied to USGS manganese nodule reference samples and the manganese nodules and crusts collected from the Central Pacific Ocean, Antarctic Ocean and Ogasawara Plateau.

Volcanic-hosted sulfide-barite deposit from the Kita-Bayonnaise submarine caldera, Izu-Ogasawara Arc, northwestern Pacific

Volcanic‐hosted sulfide and barite mineralization with high Au content (19.3 ppm) was discovered by submersible study (Shinkai 2000) on the eastern floor of the Kita‐Bayonnaise submarine caldera on the Shichito‐Iwojima Ridge, Izu‐Ogasawara Arc, northwestern Pacific. The hydrothermally mineralized rhyolitic tuff is partly coated by Mn oxyhydrox‐ide, and associated with hydrothermal Mn crusts and debris of mussel shells in the vicinity of the hydrothermally altered tuff‐breccias. The minearlized rhyolitic tuff occurs as a fragment of a large block (about 2 ×1 ×1 m) on talus, while the crusts occur on the caldera floor as a few thin patches several meters across or smaller. Ore‐mineral composition of the mineralized rhyolitic tuff consists mainly of sulfides and barite. Randomly interstratified seriate and montmorillonite is the only hydrothermal clay mineral from glass shards in the mineralized rhyolitic tuff. The mineralized tuff is significantly enriched in Zn, Fe, Cu, Ba, Au, Ag, and As, and has high Au ...

西南日本，岡山市の採石場に見られる還元型／酸化型花崗岩類

Two different granites in their oxygen fugacity during the formation, are seen in the drill cores from the northwestern part of Okayama City. One is coarse-grained granite having no or little rock-forming magnetite, while the other is fine-grained aplitic granite occurring in sheet-like form and containing coarse-grained magnetite in miarolitic aggregates of K-feldspar and quartz. The major and trace elements chemistry indicates that the aplitic granite is crystallized from fractionated melts of the coarse-grained granitic magma. It is suggested that the fractionated magma became rich in H2O which dissociated into O2 and H2 and the hydrogen diffused out to the roof rocks; then the oxygen fugacity was increased to form magnetite. Löllingite was discovered along cracks of the coarse-grained granite at one quarry, reflecting a low sulfur fugacity of the post-magmatic hydrothermal activity of these granites.