Abstract

Gold (Au), as one of the most precious metal resources that is used for both industrial products and private ornaments, is a global investment target, and mining companies are making huge investments to discover new Au deposits. Here, we report in situ Au adsorption in an acidic hot spring by a unique adsorption sheet made from blue-green algae with a high preferential adsorption ability for Au. The results of in situ Au adsorption experiments conducted for various reaction times ranging from 0.2 h to 7 months showed that a maximum Au concentration of 30 ppm was adsorbed onto the blue-green algal sheet after a reaction time of 7 months. The Au concentration in the hot spring water was below the detection limit (< 1 ppt); therefore, Au was enriched by preferential adsorption onto the blue-green algal sheet by a factor of more than ~ 3 × 10⁷. Thus, our gold recovery method has a high potential to recover Au even from an Au-poor solution such as hot spring water or mine wastewater with a low impact on the environment.

The error appeared in calculations of the sulfur isotopic fractionation correction.

Abstract

The Carnian Pluvial Episode (CPE) was a short interval of extreme rainfall in the Late Triassic that caused significant changes in marine ecosystems. Global warming induced by Wrangellia volcanism is thought to have resulted in oceanic anoxia during the CPE, but the global extent, duration, and severity of anoxia, and its effects on major marine taxa, remain unclear. To address this, we examined an equatorial record of conditions in the Panthalassa Ocean during the CPE, focusing on marine Os isotope data, redox conditions, and conodont and radiolarian biostratigraphy. The results show that Wrangellia volcanism peaked in the latest Julian (early Carnian), coinciding with development of reducing conditions in the deep-sea Panthalassa. A strong conodont turnover occurred during the period of oceanic anoxia, whereas radiolarians were less affected and their diversity increased after the recovery from anoxia. The increased radiolarian diversity during the early Tuvalian (late Carnian) can be attributed to chemical weathering and enhanced nutrient fluxes associated with global warming and the more humid climate of Pangea.

Abstract

Better understanding metallogenesis in oceanic crust depends on costly sea-floor drilling projects in areas where metal-bearing deposits, such as sea-floor massive sulfide deposits, are currently forming. In 2018, International Ocean Discovery Program (IODP) Expedition 376 recovered drill cores from an active hydrothermal field at Brothers volcano, in the Kermadec arc. These provide insight into the formation of mineral deposits along arcs, the structure and permeability of hydrothermal sites, and the relationship between the discharge of magmatic fluids and the deep biosphere. We report whole-rock major and trace element compositions and the Re-Os isotope geochemistry of hydrothermally altered volcanic rocks in a core from Hole U1530A, extending 453 m beneath the sea floor, and unaltered volcanic rocks in cores from four other drilling sites, and interpret these data to better understand subseafloor mixing of hydrothermal fluids and ambient seawater. The core exhibits more radiogenic 187Os/188Os values than typical basal values in four intervals. We propose two causal mechanisms of these radiogenic values: (1) mixing between seawater and hydrothermal fluid, associated with abundant deposition of sulfide or sulfate minerals; and (2) ingress of seawater with radiogenic 187Os/188Os values, associated with abundant chlorite and high porosity. Extreme Os enrichments up to 61.5 ppb are interpreted as the result of mobilization of Os as OsO4 or OsF6 and transport by volcanic gas, which also affected the Re-Os geochemistry of the rocks from the other Expedition 376 holes. Mobilization and transport of Os by volcanic gas may be an appreciable factor in the influx of unradiogenic Os into the ocean.

Abstract

The hydrothermally active Brothers volcano on the Kermadec arc, New Zealand, hosts two geochemically distinct hydrothermal systems within a single caldera. At the NW Caldera, metal-sulfide–rich black smoker spires form on the caldera wall. In contrast, Fe-rich crusts and native sulfur-rich chimneys occur at the resurgent central Upper Cone. Previous studies have revealed that the contrasting styles of hydrothermalism relate to the variable contribution of magmatic volatiles between these sites, with the Upper Cone experiencing relatively higher amounts of magmatic volatile influx. We present results of a study of the hydrothermal alteration within Brothers volcano based on core samples to a depth of 453 meters below sea floor (mbsf) from both the Upper Cone (Site U5128) and NW Caldera sites (Site U1527 and U1530), drilled by the International Ocean Discovery Program. The dacitic to rhyolitic breccias that make up the volcano are variably altered to alteration mineral assemblages consisting of chlorite + quartz, illite + pyrophyllite, natroalunite + pyrophyllite, and smectite-rich assemblages. The distribution and textures of the alteration minerals within and between different sites at Brothers volcano reflect variations in temperature, fluid pH, and fluid flux. We find that natroalunite only occurs at the Upper Cone, while alteration at the NW Caldera is more diverse and is characterized by both chlorite and pyrophyllite-rich alteration, indicating that seawater-derived hydrothermal fluids overprinted earlier magmatic volatile-influenced alteration. Our data indicate that in magmatic volatile-dominated systems, the alteration mineralogy transitions from natroalunite to pyrophyllite-rich with increasing age or maturity. This is accompanied by a distinct change in sample texture from dominantly bleached selvages to a more massive, equigranular texture.

We observed temperature variations over 10 months within a Kuroko ore (hydrothermal sulfide) cultivation apparatus installed atop a 50-m-deep borehole drilled in the Noho hydrothermal system in the mid-Okinawa Trough, southwestern Japan, for monitoring of hydrothermal fluids and in situ mineral precipitation experiments. Temperature and pressure in the apparatus fluctuated with the tidal period immediately after its installation. Initially, the average temperature was 75–76°C and the amplitude of the semidiurnal tidal temperature modulation was ∼0.3°C. Four months later, the amplitude of tidal temperature modulation had gradually increased to 4°C in synchrony with an average temperature decrease to ∼40°C. Numerical modeling showed that both the increase in tidal amplitude and the decrease in average temperature were attributable to a gradual decrease in inflow to the apparatus, which promoted conductive cooling through the pipe wall. The reduced inflow was probably caused by clogging inside the apparatus, but we cannot rule out a natural cause, because the drilling would have significantly decreased the volume of hot fluid in the reservoir. The temperature fluctuation phase lagged the pressure fluctuation phase by ∼150°. Assuming that the fluctuations originated from inflow from the reservoir, we conducted 2D numerical hydrothermal modeling for a poroelastic medium. To generate the 150° phase lag, the permeability in the reservoir needed to exceed that in the ambient formation by ∼three orders of magnitude. The tidal variation phase can be a useful tool for assessing the hydrological state and response of a hydrothermal system.

We determined the K-Ar ages of phengite occurring as a gangue silicate mineral in the massive sulfide ores from the Kotsu and Iyo Besshi-type volcanogenic massive sulfide (VMS) deposits in the Sanbagawa high-P/T metamorphic belt, southwest Japan. The K-Ar ages of phengite are dated to be ca. 45 or 85 Ma and these ages correspond to the timing of initial or terminal stages of regional retrograde metamorphism. Contrastingly, the Re-Os isochron ages of sulfide mineral aggregates from the Kotsu and Iyo Besshi-type VMS deposits are ca. 145 Ma, which is much older than those of phengite K-Ar ages. Since exactly the same ore samples were used for both K-Ar and Re-Os geochronology, it was confirmed that the K-Ar isotope system was reset due to the regional metamorphism up to lower epidote-amphibolite facies, although the Re-Os isotope system retained the original geochemical and isotopic composition even though the regional (Sanbagawa high-P/T) metamorphism.

Properly calibrated discrete dynode SEM ion-counting detectors and minimization of blank Os by an improved vial-cleaning method enables Os isotope analysis at the 1–15 pg level with high accuracy and precision.

<title>Abstract</title>Seafloor massive sulphide (SMS) deposits, modern analogues of volcanogenic massive sulphide (VMS) deposits on land, represent future resources of base and precious metals. Studies of VMS deposits have proposed two emplacement mechanisms for SMS deposits: exhalative deposition on the seafloor and mineral and void space replacement beneath the seafloor. The details of the latter mechanism are poorly characterised in detail, despite its potentially significant role in global metal cycling throughout Earth’s history, because in-situ studies require costly drilling campaigns to sample SMS deposits. Here, we interpret petrographic, geochemical and geophysical data from drill holes in a modern SMS deposit and demonstrate that it formed via subseafloor replacement of pumice. Samples from the sulphide body and overlying sediment at the Hakurei Site, Izena Hole, middle Okinawa Trough indicate that sulphides initially formed as aggregates of framboidal pyrite and matured into colloform and euhedral pyrite, which were replaced by chalcopyrite, sphalerite and galena. The initial framboidal pyrite is closely associated with altered material derived from pumice, and alternating layers of pumiceous and hemipelagic sediments functioned as a factory of sulphide mineralisation. We infer that anhydrite-rich layers within the hemipelagic sediment forced hydrothermal fluids to flow laterally, controlling precipitation of a sulphide body extending hundreds of meters.

<title>Abstract</title>The solid earth plays a major role in controlling Earth’s surface climate. Volcanic degassing of carbon dioxide (CO₂) and silicate chemical weathering are known to regulate the evolution of climate on a geologic timescale (&gt; 10⁶ yr), but the relationship between the solid earth and the shorter (&lt; 10⁵ yr) fluctuations of Quaternary glacial–interglacial cycles is still under debate. Here we show that the seawater osmium isotope composition (¹⁸⁷Os/¹⁸⁸Os), a proxy for the solid earth’s response to climate change, has varied during the past 300,000 years in association with glacial–interglacial cycles. Our marine Os isotope mass-balance simulation reveals that the observed ¹⁸⁷Os/¹⁸⁸Os fluctuation cannot be explained solely by global chemical weathering rate changes corresponding to glacial–interglacial climate changes, but the fluctuation can be reproduced by taking account of short-term inputs of (1) radiogenic Os derived from intense weathering of glacial till during deglacial periods and (2) unradiogenic Os derived from enhanced seafloor hydrothermalism triggered by sea-level falls associated with increases of ice sheet volume. Our results constitute the first evidence that ice sheet recession and expansion during the Quaternary systematically and repetitively caused short-term (&lt; 10⁵ yr) solid earth responses via chemical weathering of glacial till and seafloor magmatism. This finding implies that climatic changes on &lt; 10⁵ yr timescales can provoke rapid feedbacks from the solid earth, a causal relationship that is the reverse of the longer-term (&gt; 10⁶ yr) causality that has been conventionally considered.

The Carnian Pluvial Episode (CPE) was a global environmental change and biotic crisis that occurred during the Carnian (Late Triassic). The climate during the CPE was characterized by a short-lived period of extreme rainfall, and an extinction of marine taxa is known to have occurred during the latest Julian (i.e. early Carnian). Although these events are considered to have been caused by the Wrangellia Flood Basalt (FB) volcanism, existing studies have found little direct evidence to support this. We investigated the temporal relationship between the eruption of Wrangellia FB and CPE using high-resolution microfossil biostratigraphy and paleo-seawater Os isotope data of an Upper Triassic bedded chert succession from an accretionary complex in Japan, which accumulated in a pelagic deep-sea environment in an equatorial region of the Panthalassa Ocean. Our biostratigraphic analysis, based on conodonts and radiolarians, and osmium isotope data show: (i) a continuous decline of initial Os isotope ratios (Os-187/Os-188(i)) in the early Julian; (ii) low Os-187/Os-188(i) ratios during the late Julian; and (iii) an abrupt increase in Os-187/Os-188(i) ratios at the end of the Julian. The decrease in Os-187/Os-188(i) ratios throughout the Julian suggests an increased input of unradiogenic Os from the eruption of the Wrangellia FB into the ocean. Moreover, redox-sensitive elements, such as V and U, increased abruptly at the end of the Julian, which is the first evidence of reducing conditions during the CPE within the pelagic deep-sea Panthalassa Ocean. Marine anoxic event in the late Julian has been recognized from widespread deposition of black shales and organic-rich marls in intermediate to shallow water Tethyan sections. Thus, oxygen-depleted conditions occurred at the Tethyan shallow continental margin, as well as in the pelagic deep-sea Panthalassa Ocean, at the end of Wrangellia FB volcanism.

Artificial hydrothermal vents, created by boreholes that discharge hydrothermal fluids and useful for observing secular changes in mineral precipitates and the chemical compositions of hydrothermal fluids, are periodically cleaned of scale deposits. Here, we report petrographic and geochemical features of hydrothermal scale with a concentric structure and extreme enrichment in Zn, recovered as an intact plug from an artificial hydrothermal vent pipe in the Okinawa Trough. The scale consists of sphalerite with accessory galena and chalcopyrite, and minor cotunnite (PbCl2), barite, an unidentified Zn sulfate, and Bi-rich minerals. It comprises at least five concentric layers alternating between thin, reddish-brown porous layers composed of relatively Fe-rich sphalerite accompanied by galena and chalcopyrite, and coarse-grained, dark gray layers dominated by relatively Fe-poor sphalerite. Cotunnite occurs only in the innermost reddish-brown layer, and barite occurs only in the uppermost and innermost layers. The scale is composed of > 50 wt% Zn, several wt% Fe and Pb, and < 1 wt% Cu, Mn, and Cd, a composition consistent with the solubility of Zn and Cu in a hydrothermal fluid at 304–311 °C and pH 4.7–5.0. We suggest that the concentric layers are related to periodic scale removal operations through which (1) mostly clogged pipe conditions and weak hydrothermal discharge alternate with (2) fully open pipe conditions and vigorous hydrothermal discharge, thus affecting sulfur fugacity and [Cl−] and [SO42−] activities in the hydrothermal fluid and leading to the formation of concentric layers of precipitated minerals.

Seafloor hydrothermal deposits form when hydrothermal fluid mixes with ambient seawater, and constituent sulfide minerals are usually interpreted to precipitate abiogenically. Recent research drilling at Izena Hole and Iheya North Knoll in the middle Okinawa Trough (East China Sea), combined with secondary ion mass spectrometry determinations of δ34S in pyrite grains, provides compelling evidence that the initial stage of subseafloor sulfide mineralization is closely associated with microbial sulfate reduction. During the sulfide maturation process, pyrite textures progress from framboidal to colloform to euhedral. Pyrite δ34S has highly negative values (as low as –38.9‰) in framboidal pyrite, which systematically increase toward positive values in colloform and euhedral pyrite. Sulfur isotope fractionation between seawater sulfate (+21.2‰) and framboidal pyrite (–38.9‰) is as great as –60‰, which can be attained only by microbial sulfate reduction in an open system. Because framboidal pyrite is commonly replaced by chalcopyrite, galena, and sphalerite, framboidal pyrite appears to function as the starting material (nucleus) of other sulfide minerals. We conclude that framboidal pyrite, containing microbially reduced sulfur, plays an important role at the initial stage of subseafloor sulfide mineralization.

© 2020, The Author(s). An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Deep-sea sediments with total rare-earth elements and yttrium (ΣREY) concentrations exceeding 400 ppm, which are termed REY-rich mud, are widely distributed in the world oceans. Specifically, deep-sea sediments within the Japanese exclusive economic zone (EEZ) surrounding Minamitorishima Island in the western North Pacific have attracted significant attention as a new REY resource, because they contain REY-enriched layers exceeding 2000 ppm of ΣREY. However, neither the sediments deeper than 15 m below the seafloor (mbsf) nor those outside the Minamitorishima EEZ have ever been studied. Recently, a number of distinct geochemical features which are aligned in stratigraphic order were recognized in these sediments, based on multi-elemental composition data. Chemostratigraphy enables us to laterally correlate three REY peaks among apparently featureless pelagic clays. Here, we apply chemostratigraphic correlation to 19 new cores collected from the northern Pigafetta Basin and several small basins within the Marcus-Wake Seamounts. This study revealed that the REY-enriched layers occur at greater depths than the piston core length in a wider area than previously investigated (e.g., ~20.5 mbsf at Ocean Drilling Program Site 801A). This finding suggests that the depositional environments in these areas were basically similar, although local geographic conditions could have affected the continuity of REY peaks.

© 2020, The Author(s). The deep-sea clay that covers wide areas of the pelagic ocean bottom provides key information about open-ocean environments but lacks age-diagnostic calcareous or siliceous microfossils. The marine osmium isotope record has varied in response to environmental changes and can therefore be a useful stratigraphic marker. In this study, we used osmium isotope ratios to determine the depositional ages of pelagic clays extraordinarily rich in fish debris. Much fish debris was deposited in the western North and central South Pacific sites roughly 34.4 million years ago, concurrent with a late Eocene event, a temporal expansion of Antarctic ice preceding the Eocene–Oligocene climate transition. The enhanced northward flow of bottom water formed around Antarctica probably caused upwelling of deep-ocean nutrients at topographic highs and stimulated biological productivity that resulted in the proliferation of fish in pelagic realms. The abundant fish debris is now a highly concentrated source of industrially critical rare-earth elements.

© 2020 The Author(s) This study describes a method to quantify the chemical composition of deep-sea hydrothermal deposits in situ using laser-induced breakdown spectroscopy (LIBS). Partial least squares (PLS) regression analysis is applied to spectra obtained using a long laser pulse with a duration of 150 ns. The number of measurements needed to address the spatial heterogeneity of samples is determined through high-resolution mapping of the elemental distribution in rock samples. PLS applied to laboratory measured seawater-submerged samples achieved an average relative error (RE) of 25% for Cu, Pb, and Zn compared to benchmark concentration values in cross-validation and validation studies, where both the benchmark concentration values and LIBS spectral data are made available with this publication. The PLS model was applied to LIBS signals obtained in situ from hydrothermal deposits at 1000 m depth in the ocean. The results show that target inhomogeneity limits the accuracy of the surface LIBS measurements compared to benchmark values from bulk analysis of samples. Making multiple measurements with small position offsets at each location improves the accuracy of estimates compared to an equivalent number of measurements at a single position. Maps of element distribution generated using quantified in situ data demonstrate how chemical survey outputs can be generated by combining LIBS with multivariate analysis. This enables real-time chemical feedback during deep-sea operations and chemical surveys in situations or with platforms where sample recovery is not possible.

© 2019, © 2019 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. The increasing global demand for industrially critical metals has driven the development of deep-sea mineral resources. In the past, acoustic exploration has been utilised as an efficient technique for investigating deep-sea mineral resources. However, a practical method applicable to a wide distribution of resources has not yet been established. We show that the acoustic backscatter intensity of the seafloor determined using a vessel-equipped multi-narrow beam echo sounder provides an inherent threshold that bounds different types of geology, e.g. barren pelagic sediment and dense ferromanganese nodules. We found that multiple datasets from different sounders and/or the observation periods could be integrated on the basis of the thresholds that indicate the same geological boundaries. Furthermore, the integrated data and threshold provided visualisation and quantitative estimations regarding the distribution of ferromanganese deposits covering thousands of square kilometres. Our method is applicable after minimal data processing and ground-truth observations, and provides expeditious and exhaustive reconnaissance exploration.

Meteorite impacts have caused catastrophic perturbations to the global environment and mass extinctions throughout the Earth's history. Here, we present petrographic and geochemical evidence of a possible impact ejecta layer, dating from about 11 Ma, in deep-sea clayey sediment in the Northwest Pacific. This clay layer has high platinum group element (PGE) concentrations and features a conspicuous negative Os isotope anomaly (Os-187/Os-188 as low as similar to 0.2), indicating an influx of extraterrestrial material. It also contains abundant spherules that include pseudomorphs suggestive of porphyritic olivine as well as spinel grains with euhedral, dendritic and spherical forms and NiO contents as great as 23.3 wt%, consistent with impact ejecta. Osmium isotope stratigraphy suggests a most plausible depositional age of similar to 11 Ma (Miocene) for this layer, as determined by fitting with the seawater evolution curve. No large impact crater of this age is known on land, even within the relatively large uncertainty range of the relative Os age. Thus, we suggest that an unrecognised impact event in the middle or late Miocene produced the impact ejecta layer of the Northwest Pacific.

<title>Abstract</title>Hitachiite, Pb₅Bi₂Te₂S₆, is a new mineral discovered in the Hitachi mine, located in the Ibaraki Prefecture of Japan. The mean of 21 electron microprobe analyses gave: Pb 52.01, Bi 23.06, Fe 0.69, Sb 0.17, Te 13.74, S 9.71, Se 0.54, total 100.04 wt.%. The empirical chemical formula based on 15 apfu is (Pb_4.75Fe_0.23)_Σ4.98(Bi_2.09Sb_0.03)_Σ2.12Te_2.04(S_5.73Se_0.13)_Σ5.86, ideally Pb₅Bi₂Te₂S₆. Synchrotron single-crystal X-ray diffraction experiments indicated that hitachiite has trigonal symmetry, space group <italic>P</italic><inline-formula><alternatives><inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0026461X19000458_inline1" /><tex-math>${\bar 3}$</tex-math></alternatives></inline-formula><italic>m</italic>1, with <italic>a</italic> = 4.2200(13) Å, <italic>c</italic> = 27.02(4) Å and <italic>Z</italic> = 1. The four strongest diffraction peaks shown in the powder X-ray pattern [<italic>d</italic>, Å (<italic>I</italic>)(<italic>hkl</italic>)] are: 3.541(35)(012), 3.391(59)(013), 3.039(100)(015) and 2.114(56)(110). The calculated density (<italic>D</italic>_calc) for the empirical chemical formula is 7.54 g/cm³.

The crystal structure of hitachiite has been refined using synchrotron single-crystal X-ray diffraction data, to <italic>R</italic> = 7.38% and is based on <italic>ABC</italic>-type stacking of 15 layers (five Pb, two Bi, two Te, and six S layers) along the [001] direction, and with each layer ideally containing only one kind of atom. The stacking sequence is described as Te–Bi–S–Pb–S–Pb–S–Pb–S–Pb–S–Pb–S–Bi–Te. The discovery of hitachiite implies that the minerals of the Bi₂Te₂S–PbS join might form a homologous series of Bi₂Te₂S·<italic>n</italic>PbS.

The Pb isotope compositions of galena in hydrothermal deposits obtained by drillings from two hydrothermal fields in the middle Okinawa Trough were studied. One of the study fields was the Iheya North field located on a volcanic complex and the other was the Izena field located in the sediment-filled caldera structure. LA-MC-ICP-MS was applied to directly measure Pb isotope compositions in individual galena grains which highlighted variations not only in regional scale, but also in microscopic scale so that changes of Pb isotope compositions within a galena grain can be tracked. Homogeneous Pb isotope compositions were found within the same hydrothermal site, irrespective of the mineral assemblage, texture and sampling depth beneath the seafloor. In contrast, the isotope compositions varied significantly between the two hydrothermal fields. The Pb isotope composition from the Iheya North field was isotopically close to the volcanic rocks of the Okinawa Trough, whereas that from the Izena field was more radiogenic with values intermediate between sediments and volcanic rocks of the Okinawa Trough. Within the Iheya North field, intra-field variation of Pb isotope compositions was recognized between two active sites 2.5 km apart from each other. The intra-field variation was recognized also in the Izena field. The sub-seafloor massive sulfide layer has a more sediment-like Pb isotope composition, compared to the inactive sulfide mound. These results illustrate that the Pb-mineralizing hydrothermal fluids originate from their local host rocks with/without sediment and that the isotope compositions of the galena grains reflect their metal sources; either the volcanic rocks and/or the sediments via water-rock interactions.

We observed the initial release rate of metals from four fresh (i.e., without long time exposure to the atmosphere) hydrothermal sulfide cores into artificial seawater. The sulfide samples were collected by seafloor drilling from the Okinawa Trough by D/V Chikyu, powdered under inert gas, and immediately subjected to onboard metal-leaching experiments at different temperatures (5°C and 20°C), and under different redox conditions (oxic and anoxic), for 1-30h. Zinc and Pb were preferentially released from sulfide samples containing various metals (i.e., Mn, Fe, Cu, Zn, Cd, and Pb) into seawater. Under oxic experimental conditions, Zn and Pb dissolution rates from two sulfide samples composed mainly of iron disulfide minerals (pyrite and marcasite) were higher than those from two other sulfide samples with abundant sphalerite, galena, and/or silicate minerals. Scanning electron microscopy confirmed that the high metal-releasing sample contained several galvanic couples of iron disulfide with other sulfide minerals, whereas the low metal-releasing sample contained fewer galvanic couples or were coated by a silicate mineral. The experiments overall confirmed that the galvanic effects with iron disulfide minerals greatly induce the initial release of Zn and Pb from hydrothermal sulfides into seawater, especially under warm oxic conditions.

Potential risks of supply shortages for critical metals including rare-earth elements and yttrium (REY) have spurred great interest in commercial mining of deep-sea mineral resources. Deep-sea mud containing over 5,000 ppm total REY content was discovered in the western North Pacific Ocean near Minamitorishima Island, Japan, in 2013. This REY-rich mud has great potential as a rare-earth metal resource because of the enormous amount available and its advantageous mineralogical features. Here, we estimated the resource amount in REY-rich mud with Geographical Information System software and established a mineral processing procedure to greatly enhance its economic value. The resource amount was estimated to be 1.2 Mt of rare-earth oxide for the most promising area (105 km2 × 0-10 mbsf), which accounts for 62, 47, 32, and 56 years of annual global demand for Y, Eu, Tb, and Dy, respectively. Moreover, using a hydrocyclone separator enabled us to recover selectively biogenic calcium phosphate grains, which have high REY content (up to 22,000 ppm) and constitute the coarser domain in the grain-size distribution. The enormous resource amount and the effectiveness of the mineral processing are strong indicators that this new REY resource could be exploited in the near future.

The commercial use of metals such as copper, lead, and zinc has markedly increased in recent years, resulting in increased interest in deep-sea mining of seafloor hydrothermal sulfide deposits. However, the full extent of the impact of deep-sea mining at hydrothermal field deposits on the environment remains unclear. In addition to impacting the deep sea, the leaching of heavy metals from extracted sulfide mineral may also affect the upper ocean zones as the sulfide rock is retrieved from the seafloor. Here, we used a delayed fluorescence-based bioassay using the marine cyanobacterium Cyanobium sp. NIES-981 to evaluate the toxicity of three sulfide core samples obtained from three drill holes at the Izena Hole, middle Okinawa Trough, East China Sea. Leaches from two of the cores contained high concentrations of zinc and lead, and they markedly inhibited delayed fluorescence in Cyanobium sp. NIES-981 compared with control. By examining the toxicity of artificial mixed-metal solutions with metal compositions similar to those of the leaches, we confirmed that this inhibition was a result of high zinc and lead concentrations into the leaches. In addition, we conclude that this delayed fluorescence-based bioassay is a viable method for use by deep-sea mining operations because it is quicker and requires less laboratory space and equipment than the standard assay.

© 2018 The Authors Scandium is a critical element in state-of-the-art, green technologies. However, it is also extremely expensive due to its scarcity. Recently, some deep-sea sediments that contain significant amounts of rare-earth elements that are indispensable to modern industries have been discovered in the western North Pacific Ocean, and the concentration of Sc in these sediments is comparable with that of land-based mines. A simple leaching experiment has demonstrated that up to 70% of Sc in these sediments, contained within biogenic calcium phosphate particles, can be recovered in 5 min, using dilute acids. On the basis of available core samples, we propose that a promising target area to mine Sc is along a west–east transect between longitudes 153°30′ E and 154°00′ E, at an approximate latitude of 21°58′ N. Depending on the depth of the most enriched sediment layer, just 1 km2 of the sediment could potentially yield 3.3–6.6 times the current global annual consumption of Sc. Because these sediments contain an unusually high concentration, at commercial quantities, of industrially important, heavy rare-earth elements, apart from Sc, they could be a productive source for the long-term extraction of these critical metals.

© 2018 Elsevier Ltd We performed X-ray adsorption fine structure (XAFS) analyses of Y, Ce, Fe, and P for rare earth elements and yttrium-rich muds (REY-rich muds) from the two regions of the Pacific Ocean: the eastern South Pacific and the central North Pacific. Yttrium K-edge XAFS revealed that calcium phosphate (apatite) is the common host of REY even in the hydrothermal sediments near the East Pacific Rise (EPR), as it is in REY-rich muds in the other regions including the western North Pacific and the Indian Ocean. In contrast, Ce L3-edge XANES revealed a systematic difference in the major oxidation states of Ce between the central North Pacific, where Ce(III) is dominant (60–77%), and the eastern South Pacific, where Ce(IV) is dominant (55–83%). The dominance of Ce(IV) indicates a strong hydrothermal influence on REY behavior in the eastern South Pacific. Based on inter-elemental relationships, it has long been considered that hydrothermal Fe precipitates control the REY concentration in the sediments of the eastern South Pacific. However, on the basis of these spectroscopic findings, we propose that the initial uptake of REY by rapid precipitation of hydrothermal Fe-Mn (oxyhydr)oxides is followed by eventual fixation of REY to apatite via REY redistribution in the oxic sediments. In contrast, REY accumulation in the central North Pacific can be explained by primary seawater-mineral interactions with apatite and small contributions of Mn oxides. Phosphorous and Fe K-edge XANES for the eastern South Pacific sediments both show transitions in their major species with distance from the EPR, from P adsorbed on goethite to apatite and goethite to montmorillonite, respectively. In particular, apatite appears coincident with the occurrence of the REY-rich muds, even though the Fe species still suggests intensive deposition of hydrothermal goethite. The correlation between the mass accumulation rates (MAR) of Fe and P together with these results implies the existence of two types of the apatite: biogenic apatite (i.e., fish debris) and authigenic one derived from P species adsorbed on hydrothermal Fe-Mn (oxyhydr)oxides. This result implies that authigenic apatite forms uniquely in the geological setting around the EPR, and also that biogenic apatite is responsible for the widespread formation of the REY-rich muds. We found that apatite is of specific importance as the eventual host of REY within the REY-rich muds, but various geochemical processes account for the origin of apatite and its accumulation of REY.

This study investigates methods to analyze Laser-induced breakdown spectroscopy (LIBS) signals generated from water immersed deep-sea hydrothermal deposits irradiated by a long pulse (&gt
100 ns) that are analyzed using Artificial Neural Networks (ANNs). ANNs require large amounts of training data to be effective. For this reason, we propose methods to preprocess full-field spectral signals into an appropriate form for ANNs artificially increase the amount of training data. The ANN was trained using a dataset of signals from immersed pelletized hydrothermal deposit samples that were preprocessed using the proposed method. The proposed method improved the accuracy of identification from 82.5% to 90.1% and significantly increased the speed of learning. The result shows that the ANN can be used to construct a generic method to identify hydrothermal deposits by long pulse underwater LIBS signals without the need for explicit peak detection.

The Proterozoic Bryah and Yerrida basins of Western Australia contain important base and precious metal deposits. Here we present microtextural data, trace element and S isotope analyses of massive sulphide mineralisation hosted in Palaeoproterozoic subvolcanic rocks (dolerite) recently discovered at Red Bore. The small-scale high-grade mineralisation, which extends from the sub-surface to at least 95 m down-hole, is dominated by massive chalcopyrite and contains minor pyrite and Bi-Te-(Se) phases. Massive sulphide mineralisation is surrounded by discontinuous brecciated massive magnetite, and a narrow (&lt
2 m) alteration halo, which suggests very focussed fluid flow. Laser ablation ICP-MS analyses indicate that chalcopyrite contains up to 10 ppm Au and in excess of 100 ppm Ag. Sulphur isotope analyses of pyrite and chalcopyrite indicate a narrow range of δ34SVCD (− 0.2 to + 4.6 ‰), and no significant mass-independent fractionation (− 0.1 &lt
Δ33S &lt
+ 0.05 ‰). Re-Os isotope analyses yield scattered values, which suggests secondary remobilisation. Despite the geographical proximity and the common Cu-Au-Ag association, the mineralisation at Red Bore has significant differences with massive sulphide mineralisation at neighbouring DeGrussa, as well as other massive sulphide deposits around the world. These differences include the geometry, sub-volcanic host rocks, extreme Cu enrichment and narrow δ34S ranges. Although a possible explanation for some of these characteristics is leaching of S and metals from the surrounding volcanic rocks, we favour formation as a result of the release of a magmatic fluid phase along very focussed pathways, and we propose that mixing of this fluid with circulating sea water contributed to sea floor mineralisation similar to neighbouring VHMS deposits. Our data are permissive of a genetic association of Red Bore mineralisation with VHMS deposits nearby, thus suggesting a direct connection between magmatism and mineralising fluids responsible for VHMS deposition at surface. Therefore, the Red Bore mineralisation may represent the magmatic roots of a VHMS system.

Hydrogenous ferromanganese (Fe-Mn) crusts can provide records of long-term environmental changes during the Cenozoic. To understand the paleoceanographic conditions in the southwestern Atlantic Ocean, we investigated depth profiles of major- and trace-element concentrations as well as Os and Pb isotopic compositions in a Fe-Mn crust collected from the southern flank of the São Paulo Ridge in the southwestern Atlantic. Major and trace element data plotted on ternary Mn–Fe–10×(Ni+Co+Cu) and rare-earth element plus yttrium (REY) discrimination diagrams indicate that the analyzed sample is a typical hydrogenous Fe-Mn crust. The obtained 187Os/188Os data were matched to the Cenozoic seawater Os isotope evolution curve reconstructed from pelagic sediments. The result suggests that the Fe-Mn crust has accreted over ~30 Myr with growth rates of 0.5–3 mm/Myr, although the sample likely grew in two directions during the early stage of its growth. We found no evidence of growth hiatus in the sample, which may contrast with the growth histories of many Pacific Fe-Mn crusts. Hence, the conditions favorable for the accretion of hydrogenous Fe-Mn crusts were likely to have developed on the São Paulo Ridge over the past ~30 Myr. The Pb isotopic compositions show very limited ranges (e.g., 206Pb/204Pb=18.80–18.85), and are similar to those of pre-anthropogenic seawater in the Southern Ocean. As the São Paulo Ridge is located near the Vema Channel, which is presently a major path of Antarctic Bottom Water, we suggest that a strong northward bottom current has continuously swept detrital and biogenic sediments from the ridge, and played a vital role in the Fe-Mn crust formation since ~30 Ma.

Whale carcasses (whale falls) deposited on the deep seafloor are associated with a distinctive biotic community. A fossil whale bone recovered from Sao Paulo Ridge, South Atlantic Ocean, during cruise YK13-04 Leg 1 of R/V Yokosuka was covered by a ferromanganese (Fe-Mn) crust approximately 9 mm thick. Here, we report an age constraint for this fossil bone on the basis of Os isotopic stratigraphy (Os-187/Os-188 ratio) of the Fe-Mn crust. Major-and trace-element compositions of the crust are similar to those of Fe-Mn crusts of predominantly hydrogenous origin. Rare earth element concentrations in samples of the crust, normalized with respect to Post-Archean average Australian Shale, exhibit flat patterns with positive Ce and negative Y anomalies. These results indicate that the Fe-Mn crust consists predominantly of hydrogenous components and that it preserves the Os isotope composition of seawater at the time of its deposition. Os-187/Os-188 ratios of three Fe-Mn crust samples increased from 0.904 to 1.068 in ascending stratigraphic order. The value of 1.068 from the surface slice (0-3 mm depth in the crust) was identical to that of present-day seawater within error (similar to 1.06). The value of 0.904 from the basal slice (6-9 mm) equaled seawater values from ca. 4-5 Ma. Because it is unknown how long the bone lay on the seafloor before the Fe-Mn crust was deposited, the Os stratigraphic age of ca. 5 Ma is a minimum age of the fossil. This is the first application, to our knowledge, of marine Os isotope stratigraphy for determining the age of a fossil whale bone. Such data may offer valuable insights into the evolution of the whale-fall biotic community.

Submarine hydrothermal deposits are one of the promising seafloor mineral resources, because they can store a large amount of metallic minerals as sulfides. The present study focuses on the electrical properties of active modern submarine hydrothermal deposits, in order to provide constraints on the interpretation of electrical structures obtained from marine electromagnetic surveys. Measurements of resistivity and spectral induced polarization (IP) were made using drillcore samples taken from the Iheya North Knoll and the Iheya Minor Ridge in Okinawa Trough, Japan. These hydrothermal sediments are dominantly composed of disseminated sulfides, with minor amounts of massive sulfide rocks. The depth profiles of resistivity and spectral IP properties were successfully revealed to correspond well to layer-by-layer lithological features. Comparison with other physical properties and occurrence of constituent minerals showed that resistivity is essentially sensitive to the connectivity of interstitial fluids, rather than by sulfide and clay content. This suggests that, in active modern submarine hydrothermal systems, not only typical massive sulfide rocks but also high-temperature hydrothermal fluids could be imaged as low-resistivity anomalies in seabed surveys. The spectral IP signature was shown to be sensitive to the presence or absence of sulfide minerals, and total chargeability is positively correlated with sulfide mineral abundance. In addition, the massive sulfide rock exhibits the distinctive IP feature that the phase steadily increases with a decrease of frequency. These results show the effective usage of IP for developing and improving marine IP exploration techniques.

We report major element and trace element compositions of ferromanganese crusts (Fe-Mn crusts) on Takuyo Daigo Seamount, northwestern Pacific Ocean. Highly enriched elements in these crusts, such as Co (6500 ppm), Ni (4000 ppm), Mo (520 ppm), Te (39 ppm), W (92 ppm), Pt (0.19 ppm), and rare earth elements (REEs; Sigma REE = 1700 ppm), exhibit strong positive correlations with either Fe or Mn concentrations. Chemical analyses of the crusts show that the Fe/ Mn ratio decreases toward shallower water and older deposits. Thus, deeper or younger crust is more promising as a REE (other than Ce) resource, and shallower or older crust has a greater potential as Co, Ni, Ce, and Pt resources. REE profiles of Fe-Mn crusts with less than 0.5 wt% P, normalized to post-Archean average Australian shale, are flat with positive Ce anomalies whereas samples with more than 0.5 wt% P are enriched in heavy REE, and four samples from deep in the crusts with more than 2.2 wt% P do not exhibit positive Ce anomalies. These differences in REE patterns can be explained by mixing of Fe-Mn oxide and Ca-phosphate minerals (apatite). Assuming that Fe-Mn crusts with 5 cm thickness and 1.29 g/cm(3) dry density are homogeneously distributed at water depths from 1000 to 3000 m along the 500 km perimeter of Takuyo Daigo Seamount, the possible reserve represented by Fe-Mn crusts is roughly 65 Mt of ore. Based on the average concentrations obtained here, resource amounts at Takuyo Daigo Seamount are 420 x 10(3) t Co, 110 x 10(3) tREO, 2500 t Dy, and 440 t Tb, comparable to the known reserves of major mineral producing nations. Thus, Fe-Mn crusts in the northwestern Pacific Ocean have a notable potential as a source of Co.

We have discovered deep-sea mud that is extremely enriched in rare-earth elements and yttrium (together called REY) in the Japanese Exclusive Economic Zone around Minamitorishima Island, in the western North Pacific Ocean. The maximum total REY concentration reaches approximately 7000 ppm, which is much higher than that reported for conventional REY deposits on land and other known potential REY resources in the ocean. The extremely REY-rich mud is characterized by abundant phillipsite and biogenic calcium phosphate. In addition, the stratigraphic layer with the highest REY concentration occurs just similar to 3 m beneath the seafloor. The shallow burial of these strata together with the high REY content, especially those of heavy rare-earth elements, suggest that the newly discovered extremely REY-rich mud may be a promising REY resource.

We report detailed lithological and chemical characteristics of deep-sea sediments, including rare-earth elements and yttrium-rich mud (REY-rich mud), in the Japanese Exclusive Economic Zone (EEZ) around Minamitorishima Island. Three research cruises obtained fourteen sediment cores collected by piston coring. Based on the visual descriptions and geochemical analysis of the sediment cores, we confirm the presence of REY-rich mud containing more than 400 ppm total REY (Sigma REY) in the southern and northwestern areas of the Minamitorishima EEZ. The REY-rich mud layers are characterized by abundant grains of phillipsite, biogenic calcium phosphate, and manganese oxides, and are widely distributed in relatively shallow depths beneath the seafloor. In contrast, relatively thick, non-REY-rich mud lies near the seafloor in the northern areas of the EEZ. In the three cores from the southern part of the EEZ, we also confirm the presence of highly/extremely REY-rich mud layers. Further accumulation of geochemical data from the sediments will be required to constrain the extent of the highly/extremely REY-rich mud layers.

Seafloor massive sulphide deposits are potential resources for base and precious metals (Cu-Pb-Zn +/- Ag +/- Au), but difficulties in estimating precise reserves and assessing environmental impacts hinder exploration and commercial mining. Here, we report petrological and geochemical properties of sulphide chimneys less than 2 years old that formed where scientific boreholes vented hydrothermal fluids in the Iheya-North field, Okinawa Trough, in East China Sea. One of these infant chimneys, dominated by Cu-Pb-Zn-rich sulphide minerals, grew a height of 15 m within 25 months. Portions of infant chimneys are dominated by sulphate minerals. Some infant chimneys are sulphide-rich similar to high-grade Cu-Pb-Zn bodies on land, albeit with relatively low As and Sb concentrations. The high growth rate reaching the 15 m height within 25 months is attributed to the large hydrothermal vent more than 50 cm in diameter created by the borehole, which induced slow mixing with the ambient seawater and enhanced efficiency of sulphide deposition. These observations suggest the possibility of cultivating seafloor sulphide deposits and even controlling their growth and grades through manipulations of how to mix and quench hydrothermal fluids with the ambient seawater.

In the late Miocene the Mediterranean Sea experienced a salinity crisis and thick sequences of evaporites precipitated across the deep and marginal basins. In this study we report Os isotopic records from Deep Sea Drilling Project and Ocean Drilling Project cores in the Mediterranean: the Balearic Sea (Site 372), the Tyrrhenian Sea (Site 654), the Ionian Basin (Site 374), and the Florence Rise (Sites 375-376), as well as Integrated Ocean Drilling Project Site U1387 in Gulf of Cadiz, North Atlantic. Pliocene-Pleistocene sediments at all sites show Os-187/Os-188 values close to that of the coeval ocean water, indicating that the Mediterranean was connected to the North Atlantic. Evaporitic sediments deposited during the latest Miocene, however, have Os-187/Os-188 values significantly lower than coeval ocean water values. The offset of the Mediterranean evaporite Os-187/Os-188 is attributed to limited exchange with the North Atlantic during the Messinian salinity crisis. The source of unradiogenic Os is likely to be weathering of ultramafic rocks (ophiolites) cropping out in the Mediterranean's drainage basins. Based on a box model we estimated the amount of unradiogenic Os and the Atlantic-Mediterranean exchange rate to explain this offset. Os isotopic ratios of the pre-evaporite sediments in the western Mediterranean are almost identical to that of the coeval ocean water. In contrast, equivalent sediments from the Florence Rise have significantly lower Os-187/Os-188 values. The offset in the Os isotopic ratio on the Florence Rise is attributed either to limited water exchange between eastern and western Mediterranean or to local effects associated with exhumation of the Troodos ophiolites (Cyprus).

Besshi-type deposit is a Cu–Zn (±Ag±Au) volcanogenic massive sulfide deposit occurring worldwide in the accretionary complexes, which is derived from seafloor mineralization related with a mid-ocean ridge volcanism. There are numerous Besshi-type deposits including the type locality in the Japanese accretionary complexes and they are one of the main Cu producers for the Japanese industry until 1980s. Despite of the long research history over 100 yr, the depositional environment and geological reason why Besshi-type deposits are unevenly distributed in the Japanese accretionary complexes are still uncertain due to the lack of direct age constraints for the timing of Besshi-type sulfide deposition on a paleo-seafloor. Here, we report the Re–Os isochron ages of Besshi-type deposits in the Sanbagawa, Northern Shimanto and Hitachi Belts. Eleven typical Besshi-type deposits in the Sanbagawa Belt yielded Re–Os isochron ages around 150 Ma (148.4±1.4 Ma from the composite isochron) in the Late Jurassic time. Since the depositional site of these Besshi-type deposits is truly pelagic, voluminous massive sulfide deposition/preservation in the Late Jurassic was considered to be triggered by intense hydrothermal activity with a concomitant Late Jurassic oceanic anoxic event. The three Besshi-type deposits in the Northern Shimanto Belt have been dated to be Late Cretaceous to Paleogene, which was closely associated with the Kula Ridge subduction beneath the Japanese Island. The Re–Os isochron age of the Fudotaki deposit in the Hitachi Belt was 533±13 Ma, indicating that the Hitachi Fudotaki deposit is the oldest ore deposit in the Japanese Island and was produced by bimodal volcanic activity on a back-arc area between the China block and the subduction belt of the Panthalassa Ocean.

There has been an increasing interest in seafloor exploitation such as mineral mining in deep-sea hydrothermal fields, but the environmental impact of anthropogenic disturbance to the seafloor is poorly known. In this study, the effect of such anthropogenic disturbance by scientific drilling operations (IODP Expedition 331) on seabed landscape and megafaunal habitation was surveyed for over 3 years using remotely operated vehicle video observation in a deep-sea hydrothermal field, the Iheya North field, in the Okinawa Trough. We focused on observations from a particular drilling site (Site C0014) where the most dynamic change of landscape and megafaunal habitation was observed among the drilling sites of IODP Exp. 331. No visible hydrothermal fluid discharge had been observed at the sedimentary seafloor at Site C0014, where Calyptogena clam colonies were known for more than 10 years, before the drilling event. After drilling commenced, the original Calyptogena colonies were completely buried by the drilling deposits. Several months after the drilling, diffusing high-temperature hydrothermal fluid began to discharge from the sedimentary subseafloor in the area of over 20 m from the drill holes, 'artificially' creating a new hydrothermal vent habitat. Widespread microbial mats developed on the seafloor with the diffusing hydrothermal fluids and the galatheid crab Shinkaia crosnieri endemic to vents dominated the new vent community. The previously soft, sedimentary seafloor was hardened probably due to barite/gypsum mineralization or silicification, becoming rough and undulated with many fissures after the drilling operation. Although the effects of the drilling operation on seabed landscape and megafaunal composition are probably confined to an area of maximally 30 m from the drill holes, the newly established hydrothermal vent ecosystem has already lasted 2 years and is like to continue to exist until the fluid discharge ceases and thus the ecosystem in the area has been altered for long-term.

Spectroscopy is emerging as a technique that can expand the envelope of modern oceanographic sensors. The selectivity of spectroscopic techniques enables a single instrument to measure multiple components of the marine environment and can form the basis for versatile tools to perform in situ geochemical analysis. We have developed a deep-sea laser-induced breakdown spectrometer (ChemiCam) and successfully deployed the instrument from a remotely operated vehicle (ROV) to perform in situ multi-element analysis of both seawater and mineral deposits at depths of over 1000 m. The instrument consists of a long-nanosecond duration pulse-laser, a spectrometer and a high-speed camera. Power supply, instrument control and signal telemetry are provided through a ROV tether. The instrument has two modes of operation. In the first mode, the laser is focused directly into seawater and spectroscopic measurements of seawater composition are performed. In the second mode, a fiberoptic cable assembly is used to make spectroscopic measurements of mineral deposits. In this mode the laser is fired through a 4 m long fiber-optic cable and is focused onto the target's surface using an optical head and a linear stage that can be held by a ROV manipulator. In this paper, we describe the instrument and the methods developed to process its measurements. Exemplary measurements of both seawater and mineral deposits made during deployments of the device at an active hydrothermal vent field in the Okinawa trough are presented. Through integration with platforms such as underwater vehicles, drilling systems and subsea observatories, it is hoped that this technology can contribute to more efficient scientific surveys of the deep-sea environment. (C) 2014 The Authors. Published by Elsevier Ltd.

Os isotope compositions in ferromanganese crusts (Fe-Mn crusts) have been used for the dating of model ages from present to the Late Cretaceous. This dating method assumes that the Fe-Mn crusts preserve a paleo-seawater Os isotope composition at the timing of Fe-Mn crust deposition. However, available Os isotope data are limited to dredged samples without precise indications of water depths, and the Os isotope variation in relation to water depth remains uncertain. Here, we report on the Os isotope ratio data in the surface layer of Fe-Mn crusts from 956-2987 meters below sea level at the Takuyo Daigo Seamount in the northwestern Pacific Ocean collected by a remotely operated vehicle (ROV). Since the Re-187/Os-188 ratios of the surface layer samples exhibited low values ranging from 0.020 to 0.0078, the age correction of the Os-187/Os-188 ratios by subtracting radiogenic Os-187 from total Os-187 was not necessary for the Takuyo Daigo Fe-Mn crusts. Regardless of water depth, the surface layer samples possessed a narrow range of Os-187/Os-188 ratio (1.003-1.017). As their Os isotope ratios were very similar to or slightly lower than the present-day seawater value (similar to 1.06), the Fe-Mn crusts are inferred to preserve the modern seawater Os isotope composition at the investigated water depths. Therefore, Os isotope stratigraphy using Fe-Mn crusts is a powerful dating tool in paleoceanography.

Along with the progresses in analytical procedure and mass spectrometry, the number of papers using a Re–Os isotope system has rapidly increased, although Re and Os are trace elements whose typical concentrations in geological materials are ppt～ppb levels. Rhenium has two isotopes and ¹⁸⁷Re is a radio isotope which generates ¹⁸⁷Os through β- decay in the half-life of 41.6 Gyr. Since both Re and Os are highly siderophile and chalcophile elements, they are concentrated into sulfide minerals. Besides the ¹⁸⁷Re–¹⁸⁷Os decay system, Re is more incompatible than Os during a magma differentiation process, producing a large variety of the ¹⁸⁷Re/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios among various reservoirs on the Earth. The seawater ¹⁸⁷Os/¹⁸⁸Os ratio is mainly controlled by riverine flux having a high ¹⁸⁷Os/¹⁸⁸Os ratio (～1.4) and hydrothermal fluid/cosmic dust fluxes with low values (0.12～0.13). Thus, the Re–Os isotope system is a powerful tool for (1) geochronology of sulfide deposit, black shale and petroleum deposit using an isochron method, (2) geochronology of ferromanganese crust whose sedimentary age can be determined by fitting their ¹⁸⁷Os/¹⁸⁸Os ratios with the secular variation curve of the marine Os-isotope ratio, (3) decoding the trigger and processes of global climate change and impact event, and (4) unraveling the magma source and formation processes of volcanic rocks. Here, we review the recent geochemical study using the Re–Os isotope system and especially focus on a geochronology of the sulfide deposit.

Variations of U-238/U-235 ratio (delta U-238) in sedimentary rocks have been proposed as a possible proxy for the paleo-oceanic redox conditions, although the marine delta U-238 system is not fully understood. Here we investigate the spatial variation of delta U-238 in modern ferromanganese (Fe-Mn) crusts by analyzing U isotopes in the surface (0-3 mm depth) layer of 19 Fe-Mn crusts collected from 6 seamounts in the Pacific Ocean. delta U-238 values in the surface layers show little variation and range from -0.59 parts per thousand to -0.69 parts per thousand. The uniformity of delta U-238 values is consistent with the long residence time of U in modern seawater, although the delta U-238 values are lighter than that of present-day seawater by similar to 0.24 parts per thousand. The light delta U-238 values are consistent with the isotope offset observed in previously reported adsorption experiment of U to Mn oxide. These results indicate that removal of U from seawater to Mn oxide is responsible for the second largest U isotope fractionation in the modern marine system, and could contribute to isotopically heavy U to seawater. Depth profiles of U isotopes (delta U-234 and delta U-238) in two Fe-Mn crusts (MR12-03_D06-R01 and MC10_CB07_B), dated by Os isotope stratigraphy, were investigated to reconstruct the evolution of the oceanic redox state during the Cenozoic. The delta U-238 depth profiles show very limited ranges (-0.57 parts per thousand to -0.67 parts per thousand for MR12-03_D06-R01 and -0.56 parts per thousand to -0.69 parts per thousand for MC10_CB07_B), and have values that are similar to those of the surface layers of Fe-Mn crusts. The absence of any resolvable variation in the delta U-238 depth profiles may suggest that the relative amounts of oxic and reducing U sinks have not varied significantly over the past 45 Myr. However, the delta U-234 depth profiles of the same samples show evidence for the possible redistribution of U-234 after deposition. Therefore, the depth profile of delta U-238 in Fe-Mn crusts may have been also overprinted by later chemical exchange with pore-water or seawater, and may not reflect the paleo-oceanic environmental changes. To assess the potential role of U removal by Mn oxides on seawater delta U-238, we calculated seawater delta U-238 with different U sink fluxes into Mn oxides using a simple mass balance model. The results of these calculations suggest that seawater delta U-238 could have varied significantly throughout Earth's history due to changes in the accumulation rate of Mn oxides. (C) 2014 Elsevier Ltd. All rights reserved.

The Hitachi volcanogenic massive sulfide (VMS) deposit in the Hitachi belt is one of the largest Cu-Zn sulfide deposits in Japan. However, its primary formation age has not been precisely determined because regional and contact metamorphism have disturbed some radiometric isotope systems, e.g., K-Ar ages on wall rocks. Further metamorphism has greatly reduced the fossil record, which has limited the use of biostratigraphy. To establish the ore deposit age, we applied Re-Os isotope dating to samples from two representative sulfide deposits of the greater Hitachi VMS deposit: the Fudotaki and Fujimi deposits. Re-Os isotope data from the Fudotaki deposit yields a model 3 age of 533 +/- 13 Ma, with an initial Os-187/Os-188 ratio of 0.35 +/- 0.20. In contrast, Re-Os isotope data from the Fujimi sulfide ore did not exhibit a linear correlation, suggesting that the Re-Os isotope system there has been disturbed by regional metamorphism of higher grade than the Fudotaki deposit as well as contact metamorphism during intrusion of Cretaceous granitic rocks. Although the Fudotaki sulfide ores have been metamorphosed to lower epidote-amphibolite facies, the well-defined Re-Os isochron allowed us to interpret the timing of primary sulfide deposition on the paleoseafloor. Some orebodies of the greater Hitachi VMS deposit have been estimated to be Early Carboniferous based on the scarce fossil records and considered to be distributed in the Daioin Formation, but our results revealed that the Fudotaki deposit were formed during the Cambrian and occurs in the Upper Akazawa Formation, which strongly supports the idea that the Cambrian beds are widely distributed in the Hitachi belt and that a significant unconformity exists between the Cambrian and Early Carboniferous above the Fudotaki deposit. The Cambrian Re-Os date classifies the Hitachi VMS deposit as the oldest dated ore deposit in Japan, and in turn provides an insight to understand the geologic framework and genesis of the proto-Japanese islands and the East Asian region.

We have developed a protocol for Os isotope analysis employing a sparging method coupled with an enhanced-sensitivity multiple Faraday collector-inductively coupled plasma-mass spectrometry (MFC-ICP-MS) technique. The enhanced-sensitivity ICP interface with 10(12) Omega high-gain amplifiers allowed for the stable and precise isotopic ratio analysis of Os by sparging in a very wide concentration range of 15-4000 pg. The analytical reproducibilities of Johnson Matthey Chemical (JMC) Os standards at 50, 100, 200, 400, and 2000 pg Os were 0.8, 0.5, 0.2, 0.1, and 0.02% within two standard deviations (2SD), respectively. The low Os (50-200 pg) results compared with those obtained by sparging multiple-ion counter (MIC)-ICP-MS and high Os (400-2000 pg) results rivalled those of desolvating nebulisation MFC-ICP-MS and negative thermal ionisation mass spectrometry (N-TIMS). The analysed geological standards consisting of JCh-1 (chert; similar to 15 pg, n = 3), JMS-2 (marine sediment; similar to 150 pg. n = 5), UB-N (lherzoritic peridotite; similar to 4 ng, n = 4), and JP-1 (harzburgitic peridotite; similar to 3 ng, n = 5) showed Os-187/Os-188 = 0.657 +/- 0.065, 0.842 +/- 0.053, 0.12752 +/- 0.00016, and 0.12071 +/- 0.00069 (errors are in 2SD), respectively; these results are comparable with those obtained by MIC-ICP-MS and N-TIMS. The results showed that the sparging method coupled with enhanced-sensitivity MFC-ICP-MS is a strong tool for determining Os isotope ratios in natural samples over a wide range of Os concentrations. Simple sample digestion and low procedural blanks using Carius tube digestion alone without any further element separation provide an additional advantage for Os isotope analysis by the method.

Ultrafine-grained black fault rocks (BFRs) in the Pasagshak Point Thrust of the Kodiak accretionary complex are examples of fault rocks that have recorded seismicity along an ancient subduction plate boundary. Trace element concentrations and Sr-87/Sr-86 ratios of BFRs and surrounding foliated/non-foliated cataclasites were measured to explore the nature of fluid-rock interactions along a subduction thrust. Foliated and non-foliated cataclasites do not show significant geochemical anomalies, suggesting that they were formed by slowly distributed shear. BFRs are characterized by Li and Sr enrichment, Rb and Cs depletion, and a low Sr-87/Sr-86 ratio. These geochemical signatures can be explained by fluid-rock interactions at &gt;350 degrees C, which result in preferential removal of Rb and Cs and formation of plagioclase under the presence of fluids with high Li and Sr concentrations and low Sr-87/Sr-86 ratios. Geochemical anomalies recorded by the BFRs indicate both frictional heating and external fluid influx into the subduction thrust.

The Integrated Ocean Drilling Program (IODP) Expedition 331 investigated the Iheya North hydrothermal field in the Okinawa Trough. Several post-drilling underwater vehicle investigations were conducted over 2 years to identify post-drilling changes in fluid discharge pattern, mineral deposition, and fluid chemistry. Drilling-induced high-temperature hydrothermal fluid vents were identified at deep holes not only near the naturally occurring NBC hydrothermal fluid vent (Site C0016) but also at the seafloor similar to 450 m distal to the NBC vent (Site C0014), where no hydrothermal fluid discharge was observed prior to drilling. A chimney structure at Hole C0016A grew rapidly at the NBC mound crest, where only small chimneys had been found before drilling. A drilling-induced diffuse hydrothermal flow region spread at Site C0014, and this area was newly colonized by the galatheid crab. From a fluid chemistry perspective, the post-drilling hydrothermal fluids were enriched in Cl relative to seawater, although this fluid chemistry was not observed during the 12 years prior to drilling. The Cl-enriched fluid reservoir underlying the subseafloor impermeable layers, observed by IODP Expedition 331, is likely source for the Cl-enriched fluids discharging from the post-drilling vents. The drilling-induced physical disturbance of subseafloor hydrogeological structures would release such fluids to the seafloor. In turn, the rapid chimney growth at the NBC mound crest may also be attributed to highly turbulent fluid flow with the enlarged artificial vent of Hole C0016A, which can contribute to the retention of the fluid-seawater mixture for a sufficiently long period to precipitate sulfide/sulfate minerals on the seafloor.

Anomalously high platinum group element concentrations have previously been reported for Upper Triassic deep-sea sediments, which are interpreted to be derived from an extraterrestrial impact event. Here we report the osmium (Os) isotope fingerprint of an extraterrestrial impact from Upper Triassic chert successions in Japan. Os isotope data exhibit a marked negative excursion from an initial Os isotope ratio (Os-187/Os-188(i)) of similar to 0.477 to unradiogenic values of similar to 0.126 in a platinum group element-enriched claystone layer, indicating the input of meteorite-derived Os into the sediments. The timing of the Os isotope excursion coincides with both elevated Os concentrations and low Re/Os ratios. The magnitude of this negative Os isotope excursion is comparable to those found at Cretaceous-Paleogene boundary sites. These geochemical lines of evidence demonstrate that a large impactor (3.3-7.8 km in diameter) produced a global decrease in seawater Os-187/Os-188 ratios in the Late Triassic.

The Paleoproterozoic (2.5-2.0 Ga) is one of the most important periods in Earth's history, and was characterized by a rise in atmospheric oxygen levels and repeated (at least three) severe glaciations (the Huronian glaciations). In this study, we investigate redox conditions in the atmosphere and in shallow-marine environments immediately after the first Huronian glaciation based on the isotopic composition of Os, and the abundance of redox-sensitive elements (Os, Re, and Mo) in sedimentary rocks from the Huronian Supergroup, Canada. We found no significant authigenic enrichment of Os in the sedimentary rocks deposited during the first Huronian deglaciation. The initial isotopic composition of Os in the sediments was close to that of chondrite at the time of deposition (Os187/188Os=~0.11). These results suggest that atmospheric O2 levels were insufficient to mobilize radiogenic Os through continental weathering (pO2 &lt
10 -5-10 -3 present atmospheric level (PAL)). In contrast, we found enrichment of Re in the sedimentary rocks, which suggests the occurrence of oxidative weathering of Re under mildly oxidizing conditions (&gt
10 -8-10 -5 PAL). Despite the Re enrichment, low abundances of Mo imply possible non-sulfidic conditions in shallow-marine environments at the time of deposition. Together with the results of organic carbon and sulfur analyses, we suggest that atmospheric O2 remained at relatively low levels of around 10 -8-10 -5 PAL after the first Huronian deglaciation, which contrasts with proposed dramatic increases in O2 after the second and third Huronian deglaciations. These results imply that the second and third Huronian glaciations may have been global events, associated with climatic jumps from severe glaciations to super-greenhouse conditions and the subsequent blooming of photosynthetic cyanobacteria in the glacial aftermath. © 2013 Elsevier B.V.

The historically productive copper-bearing Besshi-type sulphide deposits in the Japanese accretionary complex were formed as volcanogenic massive sulphide deposits on the deep-sea floor of the Panthalassa Ocean. Here we report that eleven typical Besshi-type deposits yielded Re-Os isochron ages around 150 Ma (148.4 +/- 1.4 Ma from the composite isochron) in Late Jurassic time. This date coincides with the lowest marine Sr-87/Sr-86 ratio and highest atmospheric CO2 concentration of the past 300 million years. We infer that intense mid-ocean ridge hydrothermal and volcanic activity in the Late Jurassic produced huge sulphide deposits and large emissions of CO2 gas, leading to global warming and a stratified Panthalassa Ocean with anoxic deep seas that favored preservation of sulphides in the pelagic environment. The emergence of ocean anoxia triggered by seafloor volcanism is also consistent with a positive delta C-13 excursion and widespread deposition of petroleum source rocks and black shales.

A simple, rapid method for the determination of Re and Os concentrations and isotope compositions using isotope dilution multi-collector inductively coupled plasma-mass spectrometry (ID-MC-ICP-MS) combined with Carius tube digestion and sparging introduction of Os was developed. For Os measurement, four channeltron ion counters to detect different Os isotopes were used simultaneously, which led to a drastic reduction in the measurement time. Rhenium isotopes were measured by means of eight Faraday cups with solution nebulisation and an ultrasonic membrane desolvator. The representative 188Os count rate of an Os standard solution containing 50 pg of total Os was approximately 110000120000 cps at the onset of measurement; the Re intensity of our in-house 10 pg g-1 standard solution reached 1820 V/mu g g-1 with a sample uptake rate of 9599 mu l min-1. These values indicate that the sensitivity of the method was sufficient even for samples with low Re and Os concentrations, such as chert. As the temporal variations of the amplification efficiency of the ion counters differed from one another, we adopted a sample-calibrator bracketing method to correct the measured Re and Os isotope ratios. The Re and Os concentrations via the isotope dilution method and the 187Os/188Os ratios of two sedimentary rock reference materials (JMS-2 and JCh-1) on the basis of the isotope ratios determined by the MC-ICP-MS and by negative thermal ionisation mass spectrometry (N-TIMS) were comparable within their ranges. Based on Os isotope measurement of the IAG reference material [Durham Romil Os (DROsS)], the average difference from the recommended value and precision of Os isotope measurements by the sparging method in combination with multi-ion-counters were 0.72% and 0.76% [1RSD (%), n = 29], respectively. The precisions in the 187Os/188Os ratios [1RSD (%)] of JMS-2, JCh-1 and DROsS were 0.350.71, 1.563.31 and 0.991.28%, respectively, which depended on their Os ion intensities. No systematic difference was observed between the Re and Os geochemical compositions of JCh-1 and JMS-2 obtained by means of digestion with inverse aqua regia and CrO3-H2SO4 solutions, suggesting that either acid solution can be used for the sparging method of sedimentary rock samples. As CrO3-H2SO4 solution is believed to liberate predominantly the hydrogenous Re and Os fraction from organic-rich sediment, the sparging method combined with CrO3-H2SO4 digestion and multi-ion-counters in the mass spectrometry is expected to be a powerful tool for reconstructing the secular change in marine Os isotope compositions with high sample throughput.

Besshi-type Cu deposits are strata-bound volcanogenic massive sulfide deposits usually associated with mafic volcanic rocks or their metamorphic equivalents. Although there are numerous Besshi-type deposits in the Sanbagawa metamorphic belt, Japan, their tectonic settings and depositional environments remain controversial because of a lack of depositional age constraints. We report Re-Os data for the Iimori deposit, one of the largest Besshi-type deposits in western Kii Peninsula, in order to examine the robustness of the Re-Os isotope system for dating sulfide minerals in the high-P/T metamorphic belt and to elucidate the primary depositional environment of the Iimori sulfide ores. An 11-point Re-Os isochron plot yields an age of 156.8 +/- 3.6 Ma. As this Re-Os isochron age is significantly older than the timing of the Sanbagawa peak metamorphism (110-120 or similar to 90 Ma) and a well-defined isochron was obtained, the Re-Os age determined here is most likely the primary depositional age. Despite high-grade metamorphism at up to 520 +/- 25 degrees C and 8-9.5 kbar, the Re-Os isotope system of the Iimori sulfides was not disturbed. Hence, we consider that the whole-rock Re Os closure temperature for the Iimori sulfide ores was probably higher than 500 degrees C. As the accretion age of the Sanbagawa metamorphic belt is considered to be 120-130 or 65-90 Ma on the basis of radiolarian and radiometric ages, we estimated the time from the Iimori sulfide deposition on the paleo-seafloor to its accretion at the convergent plate boundary to be greater than 25 Myr. Consequently, the depositional environment of the Iimori sulfide ores was not in the marginal sea, but was truly pelagic. (C) 2010 Elsevier Ltd. All rights reserved.

An improved alkali fusion method followed by HF-HNO3-HClO4 treatment is performed for simultaneous determination of 23 trace elements (Sr, Y, Zr, Nb, Ba, Hf, Ta, Th, U, and REE) by ICP-MS in rock reference materials: basaltic rocks (JB-2, JB-3) and granitic rocks (JG-1a, JG-2, JG-3). Our improved method offers several advantages including: (1) suppression of whitish precipitates probably composed of insoluble fluorides by addition of HClO4, (2) simple and reliable preparation procedure, (3) instrument calibration which enables straightforward simultaneous multi-elemental analysis, and (4) the very low background levels by using pure lithium tetraborate flux. We obtained the analytical results with a reproducibility of mostly &lt; 2% (1 sigma) for the basaltic rocks and &lt; 7% for the granitic rocks. The higher relative standard deviation (RSD) values for granitic rocks may be attributed to sample heterogeneity of coarse-grained granitic rocks. The analytical results of the granitic rocks demonstrate that Zr and Hf abundances are consistent with the compiled values and that REE concentrations agree well with recently published data, suggesting that the Li2B4O7 fusion method applied in the present study is suitable for the analysis of the granitic rocks.

Major and trace element contents are reported for Permian manganese ore and associated greenstone from the Ananai manganese deposit in the Northern Chichibu Belt, central Shikoku, Japan. The manganese deposit occurs between greenstone and red chert, or among red chert beds. Chemical compositions of manganese ore are characterized by enrichments in Mn, Ca, P, Co, Ni, Zn, Sr and Ba, and negative Ce and positive Eu anomalies relative to post-Archean average Australian Shale (PAAS). Geochemical features of the manganese ore are similar to those of modem submarine hydrothermal manganese deposits from volcanic arc or hotspot setting. In addition, geochemical characteristics of the greenstone closely associated with the Ananai manganese deposit are analogous to those of with-in plate alkaline basalt (WPA). Consequently, the Ananai manganese deposit was most likely formed by hydrothermal activity related to hotspot volcanism in the Panthalassa Ocean during the Middle Permian. This is the first report documenting the terrestrially-exposed manganese deposit that was a submarine precipitate at hotspot.

Whole-rock chemical compositions of the Besshi basic schist closely associated with the Besshi massive sulfide deposit from the Sanbagawa Belt are reported. Studied samples were collected from four outcrops around the Dozan-goe, central Shikoku. Common metamorphic mineral assemblage of the basic schist is albite + epidote + actinolite +/- chlorite +/- muscovite quartz. Major element contents are similar to those of typical tholeiitic basalts. Trace element patterns of the basic schist normalized to normal mid-ocean ridge basalt (N-MORB) are generally flat, although concentrations of highly mobile large-ion lithophile elements are quite variable. Chondrite-normalized rare earth element (REE) patterns are flat to slightly light REE-depleted patterns. In the Hf-Th-Ta and Nb-Zr-Y discrimination diagrams, basic schist samples closely associated with the sulfide deposit are plotted within the N-MORB field. The Th/Nb ratios of the basic schist are also comparable to those of N-MORB. These geochemical lines of evidence indicate that the protolith of the Besshi basic schist is N-MORB and the Besshi sulfide deposit was formed by hydrothermal activity in conjunction with MOR volcanism.

Rare earth, major and trace element geochemistry is reported for the Kunimiyama stratiform ferromanganese deposit in the Northern Chichibu Belt. central Shikoku, Japan. The deposit immediately overlies greenstones of mid-ocean ridge basalt (MORB) origin and underlies red chert. The ferromanganese ores exhibit remarkable enrichments in Fe, Mn, P, V, Co, Ni, Zn, Y and rare earth elements (excepting Ce) relative to continental crustal abundance. These enriched elements/Fe ratios and Post-Archean Average Australian Shale-normalized REE patterns of the ferromanganese ores are generally analogous to those of modern hydrothermal ferromanganese plume fall-out precipitates deposited oil MOR flanks. However in more detail, Mn and Ti enrichments in the ferromanganese ores are more striking than the modern counterpart, suggesting a significant contribution of hydrogenetic component in the Kunimiyama ores. Our results are consistent with the interpretation that the Kunimiyama ores were umber deposits that primarily formed by hydrothermal plume fall-out precipitation in the Panthalassa Ocean during the Early Permian and then accreted onto the proto-Japanese island arc during the Middle Jurassic. The presence of strong negative Ce anomaly in the Kunimiyama Ores may indicate that the Early Permian Panthalassa seawater had a more striking negative Ce anomaly clue to a more oxidizing oceanic condition than today.

We report whole-rock chemical data for the greenstones from the Kunimiyama area in the Northern Chichibu Belt and their implications on the tectonic setting of these rocks. The Kunimiyama greenstones are associated with stratiform ferromanganese deposits or bedded cherts in the northern part of the study area, but are closely associated with a thick limestone block or bedded cherts in the southern part. The constituent minerals of greenstones are albitized plagioclase, clinopyroxene, chlorite, calcite. epidote, pumpellyite, prehnite, quartz, celadonite, sericite and opaque minerals such as iron oxyhydroxide and hematite. These mineral assemblages, epidote + pumpellyite + chlorite and chlorite + pumpellyite + prehnite, suggest that the metamorphic grade of greenstones from the Kunimiyama area is prehnite-pumpellyite facies. The whole-rock chemical compositions of greenstones associated with ferromanganese deposits are generally similar to those of normal mid-ocean ridge basalt (N-MORB). In contrast, the chemical compositions of the greenstones associated with the limestone block are comparable to those of ocean island alkaline basalt. Greenstones associated with bedded cherts are of enriched MORB and ocean island basalt, as well as N-MORB origins, suggesting they probably formed as a result of plume-related MOR volcanism in file Panthalassa Ocean in Early Permian and by tectonic mixing of ocean island basalts with oceanic ridge crustal fragments during accretion/subduction processes. These geological and geochemical lines of evidence Suggest that the Kunimiyama greenstones are allochthonous blocks of accreted oceanic crust and seamounts. The ferromanganese deposits are frequently accompanied by reddish greenstones. Compared to common greenish greenstones, the reddish greenstones are characterized by high MnO and rare earth element contents and distinct negative Ce anomalies, implying a slight contribution of hydrothermal component forming the ferromanganese deposits.

The close association of basalts, bedded ferromanganese ore and red cherts is observable at the Kawame Quarry in the Hayachine Belt. Major, trace and rare earth element contents and chemical compositions of constituent minerals were determined for these three types of rocks. The basalts are composed of chlorite, albite, epidote, quartz, calcite, sphene, pumpellyite and actinolite, and locally preserve primary clinopyroxene relicts. The chemical compositions of these clinopyroxene relicts are generally similar to those of modern mid-ocean ridge basalt (MORB). In addition, bulk-rock chemical compositions of the basalts show that there is a general similarity between the Kawame basalts and modern MORB. The chondrite-normalized rare earth element (REE) patterns of the basalts are subtly light REE-enriched, suggesting the basalts appear to have an affinity for enriched MORB.<BR>The ferromanganese ores are dominated by hematite with minor amounts of quartz. They exhibit significant enrichment of Fe, Mn, P, V, Y and REE except for Ce. There is a gross positive correlation between Fe+Mn and these enriched elements. The average pattern of these elements/Fe ratios of the ferromanganese ores is similar to that of modem hydrothermal metalliferous (ferromanganese) sediments near MOR. The Post-Archean Average Australian Shale-normalized REE patterns of the ferromanganese ores are characterized by negative Cc anomalies. Their geochemical features mentioned above demonstrate consistently that the Kawame ferromanganese ores are ancient counterparts of modem MOR hydrothermal sediments. Red cherts comprise of quartz and hematite with trace calcite, rhodonite, stilpnomelane, apatite, actinolite and andradite. They are simple mixtures of ferromanganese ores and pelagic cherts in varying proportions. In conclusion, the Kawame basalts associated with ferromanganese ores and red cherts are most likely remnants of late Devonian oceanic crust.