Sediments are actively subducted in virtually every arc worldwide. However, quantifying their contributions to arc lavas and thereby establishing budgets of how sediments participate in slab-mantle interaction is challenging. In this contribution we use thallium (Tl) abundances and isotopic compositions of lavas from the Ryukyu arc (including south Kyushu) and its back-arc basin, Okinawa Trough, to investigate the influence of sediments from arc to back-arc. We also present extensive geochemical data for sediments and altered oceanic crust (AOC) outboard of the northern (DSDP Sites 296, 442B, 443 and 444) and central (DSDP Sites 294 and 295) part of the Ryukyu arc. The Tl isotopic compositions of sediments change systematically from lighter outboard of northern Ryukyu arc to heavier outboard of central Ryukyu arc. The feature reflects the dominance of terrigenous material and pelagic sedimentation outboard of the northern and central Ryukyu arc, respectively. Central and northern sections of Ryukyu arc and Okinawa Trough display larger range of Tl isotopic variation than southern section, which is consistent with more pelagic provenance for sediments outboard of central and northern Ryukyu arcs than that of expected sediments outboard of southern Ryukyu arc. Identical Tl, Sr, Nd and Pb isotope variations are found when comparing arc and back arc lavas, which indicates that sediments fluxes also account for the Tl isotopic variations in the Okinawa Trough lavas. Two-end-member mixing models of Tl with Pb, Sr and Nd isotopes require sediment inputs of< 1%, 0.1–1% and 0.3–2% by weight to the depleted mantle source to account for all these isotopic compositions of lavas from northern, central and southern portion of the Ryukyu arc and Okinawa Trough. Bulk mixing between mantle and sediment end members predict very similar sediment fluxes when using Tl, Sr, Nd and Pb isotopes, which indicates that fractionation of these elements must have happened after mixing between mantle and sediments. This conclusion is corroborated by model calculations of mixing between sediment melts with fractionated Sr/Nd ratios and mantle wedge, which show that no arc lava plot on such mixing lines. Thus bulk sediment mixing, rather than sediment melt, is required for the generation of the lavas from the Ryukyu arc and Okinawa Trough. The requirement of bulk sediment mixing occurring before trace element fractionation in the sub-arc mantle is consistent with models where mélange layers form at the top of the slab and are the principle source material for arc lavas. In addition, the fact that sediment components observed in the Ryukyu arc and Okinawa Trough lavas are similar, suggests that transport of mélange material to the source regions of the arc and back arc is equally efficient. This feature is most readily explained if mélange material is transported from the slab as diapirs.

•Th and Th/Nb in Okinawa Trough lavas gradually decrease as distance increases from the Ryukyu Trench.•The basaltic magma source in the Okinawa Trough include DM, EMI material, subducted sediment and AOC.•The subducted sediment joined into the mantle wedge in form of aqueous fluid and bulk physical mixture.•AOC joined into the mantle wedge in form of aqueous fluid.The Okinawa Trough (OT) is a back-arc, initial continental marginal sea basin located behind the Ryukyu Arc-Trench System. Formation and evolution of the OT have been intimately related to subduction of the Philippine Sea Plate (PSP) since the late Miocene; thus, the magma source of the trough has been affected by subduction components, as in the case of other active back-arc basins, including the Lau Basin (LB) and Mariana Trough (MT). We review all the available geochemical data relating to basaltic lavas from the OT and the middle Ryukyu Arc (RA) in this paper in order to determine the influence of the subduction components on the formation of arc and back-arc magmas within this subduction system. The results of this study reveal that the abundances of Th in OT basalts (OTBs) are higher than that in LB (LBBs) and MT basalts (MTBs) due to the mixing of subducted sediments and EMI-like enriched materials. The geochemical characteristics of Th and other trace element ratios indicate that the OTB originated from a more enriched mantle source (compared to N-mid-ocean ridge basalt, N-MORB) and was augmented by subducted sediments. Data show that the magma sources of the south OT (SOT) and middle Ryukyu Arc (MRA) basalts were principally influenced by subducted aqueous fluids and bulk sediments, which were potentially added into magma sources by accretion and underplating. At the same time, the magma sources of the middle OT (MOT) and Kobi-syo and Sekibi-Syo (KBS+SBS) basalts were impacted by subducted aqueous fluids from both altered oceanic crust (AOC) and sediment. The variable geochemical characteristics of these basalts are due to different Wadati-Benioff depths and tectonic environments of formation, while the addition of subducted bulk sediment to SOT and MRA basalts may be due to accretion and underplating, and subsequent to form mélange formation, which would occur partial melting after aqueous fluids are added. The addition of AOC and sediment aqueous fluid to MOT and KBS+SBS basalts is therefore the result of cold subducted slab dehydration combined with a rapid subduction rate (82 mm/a), leading to the migration of fluids into the mantle wedge. The presence of these attributes is likely because the OT was a back-arc, initial continental marginal sea basin.Download high-res image (212KB)Download full-size image

•Zn, Mo, and Pb contents in clam tissues vary by tissue type.•V/As and Fe/Cr ratios of mussel and clam shells trace seawater composition.•Mussels and clams are a sink of LREEs from hydrothermal fluids.•δ13C values of shells are heavier for mussels than for clams in the study areas.Studies of the chemical characteristics of mussels and clams in seafloor hydrothermal fields are important for understanding mass fluxes and elemental partitioning from hydrothermal vents into the biosphere, metal bioaccumulation of seafloor hydrothermal ecosystems, and the sources and sinks of biogeochemical and fluid cycles. We are the first to measure the mineral, major, trace and rare earth element, and carbon and oxygen isotope compositions of mussels (Bathymodiolus platifrons) and clams (Conchocele bisecta) from the Tangyin and Yonaguni Knoll IV hydrothermal fields in the southwestern Okinawa Trough. Mineralogical analysis shows that the carbonate shells of the mussel and clam samples are mainly composed of calcite and aragonite. Metal elements exhibit linear correlations in the shells (e.g., V and U) and tissues (e.g., Li and Rb) of the mussels and clams, suggesting that not all positive correlations of elements in tissues are inherited by the shells. V/As, Ca/Sr, and Fe/Cr ratios in the mussels and clams are close to those in the seawater, indicating that element ratios of seawater might be inherited by the mussels and clams. In addition, the Fe/Cr ratio of the shells of both mussels and clams can be used to trace the local seawater composition.The total LREE concentrations of mussel and clam tissue samples are higher than those of the mussel and clam shell samples, are similar to the hydrothermal fluids, exhibit LREE enrichment (LaCN/NdCN ratios = 1.86-32.1), and no or only slightly negative Eu anomalies, indicating that benthic animals are a sink of LREEs from hydrothermal fluids, and that the Eu/Eu* ratios of fluids change when fluids are incorporated into the tissues of the mussels and clams. In addition, the δ13C values of mussel shell samples are heavier than those of the clam shell samples in the hydrothermal field, indicating that more than one carbon source may be involved in defining the δ13C compositions of the shells. The majority of the δ18O values of clam shell samples fall in the range of δ18O values of the mussel shell samples, and are close to the hydrothermal fluid δ18OH2O values, implying that the δ18O values of mussel and clam shell carbonate is influenced by the hydrothermal environment (magmatic water and fluid dilution with seawater).Download high-res image (154KB)Download full-size image

•New S and Pb isotope analyses were performed on sulfides from MORs and BAB.•Basalt contributed comparable amounts of sulfur and lead to sulfides in MORs.•Discrete degree or variation rate can be used to study S and Pb isotopic variations.•S and Pb sources and fluid processes jointly affect S and Pb isotopic compositions.Studies of sulfur and lead isotopic compositions in hydrothermal deposits are an important tool to determine the source and processes of both sulfur and lead, and to understand the origin of hydrothermal ore deposits. Here, the sulfur and lead isotopic compositions of sulfide minerals have been studied for different hydrothermal fields in the East Pacific Rise (EPR), Mid-Atlantic Ridge (MAR), Central Indian Ridge (CIR), Southwest Indian Ridge (SWIR), and North Fiji Basin (NFB). The sulfur isotopic compositions of the studied sulfide samples are variable (δ34S 0.0 to 9.6‰, avg. δ34S 4.7‰; n = 60), being close to the associated igneous rocks (~ 0‰ for, e.g., basalt, serpentinized peridotite), which may reflect the S in the sulfide samples is derived mainly from the associated igneous rocks, and a relatively small proportion (< 36%) of seawater sulfur incorporated into these sulfides during mixing between seawater (δ34S 21‰) and hydrothermal fluid. In contrast for a mixed origin for the source of S, the majority of the lead isotopic compositions (206Pb/204Pb 17.541 ± 0.004 to 19.268 ± 0.001, 207Pb/204Pb 15.451 ± 0.001 to 15.684 ± 0.001, 208Pb/204Pb 37.557 ± 0.008 to 38.988 ± 0.002, n = 21) of the sulfides possess a basaltic Pb isotopic composition, suggesting that the lead in the massive sulfide is mainly leached from local basaltic rocks that host the sub-seafloor hydrothermal systems in sediment-free mid-ocean ridges and mature back-arc basins. Furthermore, sulfide minerals in the super-fast and fast spreading mid-ocean ridges (MORs) exhibit less spread in their the δ34S values compared to sulfides from super-slow, and slow spreading MORs, which is most easily explained as a lesser degree of fluid-rock interaction and hydrothermal fluid-seawater mixing during hydrothermal ore-forming process. Additionally, the S and Pb isotope compositions of sulfides are controlled by the fluid processes for forming seafloor massive sulfide deposits. We demonstrate that the variable sulfur and lead isotopic compositions exhibit a relationship with the sulfur and lead sources, fluid–rock interaction, and fluid–seawater mixing.Download high-res image (144KB)Download full-size image

•Fluids in the Kueishantao hydrothermal field have variable B enrichments.•Boron in fluids and plumes is mainly from seawater, with only small contributions from andesite.•Using B of plumes, one can describe their diffusive processes.•Interaction of subseafloor fluids with andesite is of short duration, and the water/rock ratios are between 1.96 and 3.63.•The hydrothermal flux of boron is between 6.69 × 104 mol/yr and 1.32 × 105 mol/yr.Boron is a common element in vent fluids of seafloor hydrothermal fields, and it has been used to understand the hydrothermal flux and water–rock interaction in hydrothermal systems. We have measured the boron concentration and isotope composition of seawater, andesite, hydrothermal fluid and plume samples from the Kueishantao hydrothermal field. The δ11B value of ambient seawater near the field is 40.05 ± 0.01‰, and the boron concentration is 3.81 mg/L. Andesite rocks from the hydrothermal field have an average boron content of 15.3 ppm. The hydrothermal fluids from the yellow spring and white spring span a small range of δ11B values, from 33.27 ± 0.22 to 36.84 ± 0.11‰, and plumes from both springs also cover a small range, from 37.56 ± 0.01 to 40.37 ± 0.21‰.Hydrothermal fluids from both springs in the Kueishantao hydrothermal field have variable B enrichments relative to seawater between 7 and 21%. They have B concentrations (4.10–4.64 mg/L) that are slightly higher and δ11B values (33.27–36.84‰) that are lower than those of the hydrothermal plumes (3.94–4.17 mg/L, 37.56–40.37‰). Hydrothermal fluids and plumes display a very regular array of data points in a δ11B–B diagram, suggesting that the boron of hydrothermal fluids and plumes is mainly from seawater and that little of it is, from andesite. This implies that the interaction of subseafloor fluid and -andesite at the Kueishantao hydrothermal field is of short duration. In all the fluids, from springs to hydrothermal plumes, the pH values, B concentrations and B isotopic compositions show significant correlations with each other suggesting that the δ11B/B and pH/B ratios of hydrothermal plumes have stable values over the small distance form vent to plume (< 15 m). Thus the B concentrations and B isotopic compositions of hydrothermal plumes can be used to describe the diffusive processes governing the chemical compositions of hydrothermal plumes in the seawater environment.The water/rock ratios, based on the B concentrations and δ11B values, are between 1.96 and 3.63. The hydrothermal flux of boron from the yellow spring into the oceans is between 1.17 × 105 mol/yr and 1.32 × 105 mol/yr, and from the white spring it is between 6.69 × 104 mol/yr and 7.17 × 104 mol/yr, assuming that only andesites are present in the reaction zone.

Early formed high-Mg# olivine phenocrysts during evolution of MORB magmas usually host melt inclusions, which record important information about the early-stage evolution of magma. Five MORB samples from near East Pacific Rise (EPR) 13°N vary little in K/Ti (0.07–0.12), Tb/Lu (1.72–1.84) and Sm/Nd (0.310–0.332) and have similar REEs patterns, indicating that depleted upper mantle has similar mineral composition. Sixty-five initial melt inclusions derived by correcting olivine fractionation and “FeO-Loss” show averagely higher MgO contents than their host rocks. Melt inclusions have higher CaO/Al2O3 ratios than their host rocks, and these CaO/Al2O3 ratios are positively and negatively correlated with MgO and Na2O respectively, suggesting that these magmas have experienced high pressure crystallization of clinopyroxene. Average crystallization pressure, which is calculated based on the pressure dependence of clinopyroxene crystallization, is 0.83±0.25 GPa, and implys that these melt inclusions are averagely trapped in mantle depth of ∼24 km. These melt inclusions show negative correlations of Ca8/Al8 and Na8 with Fe8, and wider ranges of Ca8/Al8, Na8, Fe8 and K/Ti than their host rocks, suggesting that these melt inclusions formed by mixing magmas of different melting degrees and depths. According to the average value and ranges of Ca8/Al8, Na8, Fe8 and K/Ti, these magmas would necessitate other mixing ends in shallow crust except in upper mantle. The compositional diversity of melt inclusions in MORBs phenocrysts cannot always be used to indicate magma mixing and crystallization in shallow crust, and melt inclusions in high Mg# olivine formed under mantle pressure must be excluded in study of the magma process at crustal level. This study shows that, in EPR, MORBs have experienced mixing of magmas formed by different melting degrees and depths in the mantle.

Samples of Fe–Si–Mn oxyhydroxides were collected from the PACMANUS hydrothermal field, which lies in a young back-arc setting in the Eastern Manus Basin. The purpose of the study was to understand the origin and characteristics of Fe–Si–Mn oxyhydroxides associated with massive sulfides in a back-arc basin. The PACMANUS Fe–Si–Mn oxyhydroxides are composed of Fe oxyhydroxides and Mn oxyhydroxides with opal-A and nontronite; they have very low concentrations of trace elements (except for Ba, Mo, V and U) and rare earth elements, and they show REE distribution patterns with positive Eu anomalies and slight enrichments of LREEs. The Fe–Si–Mn oxyhydroxides appear to be precipitated mainly from hydrothermal fluid with limited seawater contamination, and scavenged trace metals are predominantly from the ambient seawater. The differences in the REE distribution patterns between the Fe-oxyhydroxide fraction and Mn-oxyhydroxide fraction originate from diagenetic processes. There are diverse filamentous microtextures resembling unique microbial populations, suggesting microbially-mediated mineralization during the precipitation of the Fe–Si–Mn oxyhydroxides.A possible genetic scenario for the formation of Fe–Si–Mn oxyhydroxides in the PACMANUS hydrothermal field is proposed: (1) precipitation of silica by the mixing of hydrothermal fluid with seawater at a diffuse vent, promoted by Fe-oxidizing bacteria and microbial mineralization; (2) rapid precipitation of Fe-oxyhydroxide from the hydrothermal fluid due to Fe2+ oxidation; (3) growth of Mn-oxyhydroxide partially encasing Fe-oxyhydroxide. Microbes act through the whole scenario. The Fe–Si–Mn oxyhydroxides have undergone changes as a consequence of fluctuating hydrothermal conditions and subsequent diagenetic degradation.Highlights► New data on Fe–Si–Mn oxyhydroxides from the PACMANUS hydrothermal field. ► Fe–Si–Mn oxyhydroxides appear to be precipitated mainly from hydrothermal fluid. ► Precipitation of the Fe–Si–Mn oxyhydroxides is partially controlled by microbes. ► REE distributions in Fe–Si–Mn oxyhydroxides have been affected by diagenetic processes.

We first report the trace and rare earth element compositions of native sulfur ball with sulfur contents varying from 97.08 wt.% to 99.85 wt.% from the Kueishantao hydrothermal field, off NE Taiwan. We then discuss the sources of trace and rare earth elements incorporated into the native sulfur ball during formation. Comparison of our results with native sulfur from crater lakes and other volcanic areas shows the sulfur content of native sulfur ball from the Kueishantao hydrothermal field is very high, and that the rare earth element (REE) and trace element constituents of the native sulfur balls are very low (∑REE < 35 ppb). In the native sulfur ball, V, Cr, Co, Ni, Nb, Rb, Cs, Ba, Pb, Th, U, Al, Ti and REE are mostly derived from andesite; Mg, K and Mn are mostly derived from seawater; and Fe, Cu, Zn and Ni are partly derived from magma. Based on the sulfur contents, trace and rare earth element compositions, and local environment, we suggest that the growth of the native sulfur ball is significantly slower than that of native sulfur chimneys, which results in the relatively higher contents of trace and rare earth element contents in the native sulfur ball than in the native sulfur chimneys from the Kueishantao hydrothermal field. Finally, we suggest a “glue pudding” growth model for understanding the origin of the native sulfur ball in the Kueishantao hydrothermal field, whereby the native sulfur ball forms from a mixture of oxygenated seawater and acidic, low-temperature hydrothermal fluid with H2S and SO2 gases, and is subsequently shaped by tidal and/or bottom currents.Research highlights► We first report the trace and rare earth element compositions of native sulfur ball. ► The growth of the native sulfur ball is significantly slower than that of chimneys. ► A “glue pudding” growth model for understanding the origin of the native sulfur ball.