Chen-Feng You

Trace element behavior in hydrothermal experiments: Implications for fluid processes at shallow depths in subduction zones

Chemical evaluation of fluids affected during progressive water-sediment interactions provides critical information regarding the role of slab dehydration and/or crustal recycling in subduction zones. To place some constraints on geochemical processes during sediment subduction, reactions between decollement sediments and synthetic NaCl-CaCl2 solutions at 25–350°C and 800 bar were monitored in laboratory hydrothermal experiments using an autoclave apparatus. This is the first attempt in a single set of experiments to investigate the relative mobilities of many subduction zone volatiles and trace elements but, because of difficulties in conducting hydrothermal experiments on sediments at high P-T conditions, the experiments could only be designed for a shallow (∼ 10 km) depth. The experimental results demonstrate mobilization of volatiles (B and NH4) and incompatible elements (As, Be, Cs, Li, Pb, Rb) in hydrothermal fluids at relatively low temperatures (∼ 300°C). In addition, a limited fractionation of light from heavy rare earth elements (REEs) occurs under hydrothermal conditions. On the other hand, the high field strength elements (HFSEs) Cr, Hf, Nb, Ta, Ti, and Zr are not mobile in the reacted fluids. The observed behavior of volatiles and trace elements in hydrothermal fluids is similar to the observed enrichment in As, B, Cs, Li, Pb, Rb, and light REEs and depletion in HFSEs in arc magmas relative to magmas derived directly from the upper mantle. Thus, our work suggests a link between relative mobilities of trace elements in hydrothermal fluids and deep arc magma generation in subduction zones. The experimental results are highly consistent with the proposal that the addition of subduction zone hydrous fluids to the subarc mantle, which has been depleted by previous melting events, can produce the unique characteristics of arc magmas. Moreover, the results suggest that deeply subducted sediments may no longer have the composition necessary to generate the other distinct characteristics, such as the B-δ11 B and B-10Be systematics, of arc lavas. Finally, the mobilization of B, Cs, Pb, and light REEs relative to heavy REEs in the hydrothermal fluids fractionate the ratios of B/Be, B/Nb, Cs/Rb, Pb/Ce, La/Ba and LREE/HREE, which behave conservatively during normal magmatic processes. These results demonstrate that the composition of slab-derived fluids has great implications for the recycling of elements; not only in arc magmas but also in mantle plumes.

Mobilization of boron in convergent margins: Implications for the boron geochemical cycle

Analysis of total dissolved boron and boron isotopes for interstitial waters from recent Ocean Drilling Program drill cores in two accretionary prisms indicates an efficient return to the ocean of exchangeable boron of subducted sediments via pore-fluid expulsion. This source of boron helps to balance the ocean sinks, but a complete balance of the ocean budget requires deeply subducted boron to be returned efficiently to the oceans via explosive arc volcanism.

The partition of Be between soil and water

Abstract An extensive and systematic investigation of Be partition between solid and water was carried out using laboratory batch experiments with radioactive 7Be tracer. We found that: (1) Be is strongly held by the solid particles in natural environments under neutral conditions; (2) man-made δ-MnO2 and in situ weathering products of andesite have the highest Kd (> 106) while most mud, silt and clay minerals have Kd on the order of 105; (3) Kd is high under neutral or alkaline conditions but decreases rapidly by four orders of magnitude between pH 6 and 2; (4) Kd decreases with increasing concentration of suspended particles; (5) dissolved organic matter in Taiwan Strait surface water does not seem to greatly affect the value of adsorption Kd; (6) adsorption of Be onto soil is a two-stage process and the second stage is ∼ 1000 times slower than the first; (7) the reaction of surface ion exchange seems to proceed at a faster rate in river water than in seawater, probably because of the high ionic strength of the latter; (8) the activation barrier of sorption reaction for Be seems to be ∼ 10 kcal.mol−1 for river mud; and (9) the adsorption of Be onto soil seems to be a reversible process.

Experimental study of boron geochemistry: implications for fluid processes in subduction zones

A comprehensive experimental study, utilizing an autoclave hydrothermal apparatus with a 10B isotopic tracer, has been conducted to monitor the geochemical behavior of sediment B during early subduction zone processes. The partition coefficient of exchangeable B (KD) was determined over a temperature range of 25–350°C, at 800 bars and a water/rock ratio of 3-1.5 w/w. These KD are shown to be a complex function of temperature, pH, and possibly mineralogy. At low temperatures, KD is significantly high at ∼4 in contrast to the value of essentially zero at temperatures higher than ∼100°C. A KD of zero represents no B adsorption, implying efficient mobilization of exchangeable B at shallow depths during sediment subduction. Our experimental results demonstrate high mobilization of bulk B in sediments (both exchangeable and lattice bound) at elevated temperatures (200–350°C), in good agreement with previous observations of B in metasediments indicating progressive depletion during metamorphism. In addition, this study emphasizes the importance of a possible water/rock ratio dependence of B mobilization. In other words, the degree of sedimentary B mobilization in subduction zones strongly depends on the local thermal structure and porosity distribution. In low geothermal gradient areas, large amounts of porewater are expelled before significant B mobilization has occurred, so that some sedimentary B will survive and get into the deeper parts of the subduction zone. Our results imply that efficient mobilization of B from the subducted slab must occur and that arc magmatism recycles most of the remaining subducted B back to surface reservoirs. A reconsideration of the B budget in subduction zones provides critical information with respect to B sources and sinks in the ocean.

Boron and halide systematics in submarine hydrothermal systems: Effects of phase separation and sedimentary contributions

Abstract Systematic studies of the distributions of B, δ 11 B, NH 4 , halides (Cl, Br, I) and trace alkalis (Li, Rb, Cs) in vent fluids, combined with experimental data on super- and subcritical phase separation, provide a method for separating the effects of interaction with basalts and/or sediments from those of phase separation. This allows a more general understanding of geochemical processes in submarine hydrothermal systems, especially where a connection with sediment is not otherwise obvious (e.g., Endeavour Segment, Juan de Fuca Ridge). Based on B and δ 11 B corrected for wallrock reactions, all published boron and chloride data from mid-ocean ridge systems (MOR) (e.g., 11°N, 13°N and 21°N of the East Pacific Rise), except for the Endeavour Segment, Juan de Fuca Ridge, are consistent with experimental phase separation data, suggesting a dominant control by the latter process. Fluids from sedimented ridge (SR) (e.g., Escanaba Trough and Guaymas Basin), and from back-arc basins (BAB) (e.g., Mariana Trough, Lau Basin and Okinawa Trough), when compared with mid-ocean ridge data, show expected effects of organic matter and/or sediment contributions. This is particularly noticeable from enhanced levels of Br, I, NH 4 , and trace alkali metal contents (such as Li, Rb and Cs). High B concentrations and elevated δ 11 B in Endeavour Segment can be explained by a small, but distinguishable contribution from sediments, which is confirmed by slightly enhanced levels of Br, I and NH 4 .

Kinetics and thermodynamics of adsorption for Cd on green manufactured nano-particles

Magnetic nano-particles CuFe(2)O(4) were successful manufactured from industrial sludge by combination of acid leaching, chemical exchange, and ferrite process. For the first time these recycled nano-particles were used as adsorbent to investigate the kinetics and thermodynamics for adsorption of Cd in aqueous solutions. These experimental results showed that Cd(2+) adsorption efficiency increased from 0.85 to 99.9% when pH increased from 2 to 6. The maximum adsorption capacity of Cd(2+) was found to be 17.54 mg g(-1) under the conditions at pH 6.0, contact time 30 min, and temperature 318 K. The pseudo-second-order kinetic model provides the best correlation with the experimental data compared to the pseudo-first-order model. The Langmuir model yields a better fitting than the Freundlich model for Cd(2+) adsorption on CuFe(2)O(4) nano-particles under investigated temperatures. The thermodynamic constants of the adsorption process were evaluated, ΔG°, ΔH° and ΔS° is -6.05 kJ mol(-1) (at 318 K), 0.71 kJ mol(-1), and 4.53 J mol(-1) K(-1), respectively. These results imply that Cd(2+) adsorption onto CuFe(2)O(4) is feasible, spontaneous and endothermic in nature.

Boron contents and isotopic compositions in pore waters: a new approach to determine temperature induced artifacts—geochemical implications

Abstract A comprehensive experimental study, utilizing a rocking autoclave hydrothermal apparatus with isotope tracers, was applied to evaluate the temperature of squeezing artifacts on B contents and isotopic compositions in pore waters. The partition coefficient (KD) was determined at temperatures from 25 ° to 350 °C, at 800 bars, and this information was applied to reconstruct pore water B and δ11B in ODP drill sites, where pH, T, and porosity are known. The partition coefficient of B is a function of temperature, pH, and sediment mineralogy. The solution pH exerts a dominant control at low temperatures; however, KD decreases to a value of essentially zero (compared to that of KD = ~ 3.5 at 25 °C) at high temperatures indicating no adsorption. Two empirical equations were derived to represent most of the available experimental results. For pelagic clay rich sediments, a KD = −3.84 − 0.020T + 0.88pH (R = 0.84; 1σ = 0.25) is established. For sediments that have experienced progressive metamorphism, a KD = −1.38 − 0.008T + 0.59pH (R = 0.81; 1σ = 0.37) can be applied. Similarly the effect on pore water δ11B can be corrected if the fractionation factors at different temperatures are assumed. The corrected B and δ11B in ODP Sites 671, 672, and 808 indicate significant mobilization of bulk B in sediment (exchangeable + lattice bound) at depth, especially near the decollement zone or other potential flow conduits. Tectonically expelled fluids from mud diapirs of Barbados Ridge Complex, hot springs of Rumsey Hills, California, and mud pot waters of the Salton Sea geothermal field, are enriched in B (up to 20 mM) with lower δ11B, supporting the argument of B mobilization as a result of fluid expulsion in accretionary prisms.

Evolution of an active sea-floor massive sulphide deposit

Hydrothermal circulation at oceanic spreading ridges causes sea water to penetrate to depths of 2 to 3 km in the oceanic crust where it is heated to ∼400 °C before venting at spectacular ‘black smokers’. These hydrothermal systems exert a strong influence on ocean chemistry, yet their structure, longevity and magnitude remain largely unresolved. The active Transatlantic Geotraverse (TAG) deposit, at 26° N on the Mid-Atlantic Ridge, is one of the largest, oldest and most intensively studied of the massive sulphide mounds that accumulate beneath black-smoker fields. Here we report ages of sulphides and anhydrites from the recently drilled TAG substrate structures — determined from 234U–230Th systematics analysed by thermal ionization mass spectrometry. The new precise ages combined with existing data, show that the oldest material (11,000 to 37,000 years old) forms a layer across the centre of the deposit with younger material (2,300–7,800 years old) both above and below. This stratigraphy confirms that much of the sulphide and anhydrite are precipitated within the mound by mixing of entrained sea water with hydrothermal fluid. The age distribution is consistent with episodic activity of the hydrothermal system recurring at intervals of up to 2,000 years.

Hydrothermal alteration of hemi-pelagic sediments: experimental evaluation of geochemical processes in shallow subduction zones

Abstract Laboratory hydrothermal experiments provide unique information regarding the fate of volatile and/or incompatibles (e.g. B, Li, and As) during oceanic crust subduction. Examination of chemical redistribution between the subducted slab, mantle wedge, arc volcanics and overlying ocean water during subduction is critical to gain further insight into arc volcanism and chemical oceanic budgets. For instance, efficient mobilization of B at shallow depths may be a key aspect of its oceanic budget, and can help to explain the systematics of B-δ11B and B-10Be in arc lavas. Fluid–rock interactions at elevated temperatures and pressures in accretionary prisms were studied using a rocking autoclave hydrothermal apparatus to monitor sediment–porewater interaction over the range of 25 to 350°C, at 800 bars. Clay-rich hemi-pelagic sediments from the decollement zone of Ocean Drilling Program Site 808, Nankai Trough, were reacted with NaCl–CaCl2 solutions at water/rock ratios of ∼3.5 to 1.5 (w/w) to mimic alteration processes in the shallow subduction zone. Fluids were extracted at 25–50°C intervals and were analyzed for major and trace chemical constituents. The fluid chemistry changed significantly during the course of these experiments, but there was generally only minor modification of the solid phase; only B, Li, As, Br, and Pb are significantly depleted. During the heating cycle, dissolved Na, Mg and SO4 decreased sharply and NH4, SiO2, K, B, and Li increased at T⩾300°C. Calcium drops gradually at low temperatures, but concentrations rise sharply at T⩾300°C. Decomposition of organic matter, SO42− depletion, and Mg-fixation dominate at low temperatures; however, albitization of calcic plagioclase leads to marked Na depletions and Ca enrichments at T⩾300°C. Dissolved SiO2 remained below saturation with respect to quartz and amorphous silica throughout the entire experiment. B and Li mobilization with large isotopic fractionations occurred at low temperature. Exchangeable B (δ11B=∼15‰) is completely leached before reaching 150°C. Substantial O2 exchange between fluids and the solid phase occurred at T⩾200°C in the spiked experiment, where δ18O varies more than 100‰ in the fluids. During retrograde cooling, dissolved Mg, SO4, Ca, Si, K and Sr are released as a result of carbonate or anhydrite dissolution, and marked B re-adsorption occurred at temperatures below 60°C.

Treatment of complex heavy metal wastewater using a multi-staged ferrite process

Complete removal of heavy metal from complex heavy-metal wastewater (CHMW) requires advanced technology. This study investigated the feasibility of a multi-staged ferrite process (MSFP) for treating CHMW, containing Cd, Cu, Pb, Cr, Zn, Ag, Hg, Ni, Sn and Mn. Our experimental results showed that most of the supernatants after conventional single-step ferrite process could conform to the effluent standard of Environmental Protection Administration in Taiwan. However, the sludge could not satisfy the toxicity characteristic leaching procedure (TCLP) limits due to high Cd, Cu, and Pb concentrations. The performance of MSFP in removing heavy metals from wastewater was subsequently investigated and the parameters of three treating steps in MSFP were optimized under 70°C and 90°C at pH 9, and 80°C at pH 10. After the three-staged procedures, all heavy metals in supernatant and sludge could fulfill the contamination levels regulated by law. In addition, the sludge generated from the MSFP was examined by XRD and forms a stable spinel structure, which could be effectively separated by external magnetic field.