Zeng Zhigang

Origin of a native sulfur chimney in the Kueishantao hydrothermal field, offshore northeast Taiwan

Analyses of rare earth and trace element concentrations of native sulfur samples from the Kueishantao hydrothermal field were performed at the Seafloor Hydrothermal Activity Laboratory of the Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences. Using an Elan DRC II ICP-MS, and combining the sulfur isotopic compositions of native sulfur samples, we studied the sources and formation of a native sulfur chimney. The results show, when comparing them with native sulfur from crater lakes and other volcanic areas, that the native sulfur content of this chimney is very high (99.96%), the rare earth element (REE) and trace element constituents of the chimney are very low (ΣREE<21×10−9), and the chondrite-normalized REE patterns of the native sulfur samples are similar to those of the Kueishantao andesite, implying that the interaction of subseafloor fluid-andesite at the Kueishantao hydrothermal field was of short duration. The sulfur isotopic compositions of the native sulfur samples reveal that the sulfur of the chimney, from H2S and SO2, originated by magmatic degassing and that the REEs and trace elements are mostly from the Kueishantao andesite and partly from seawater. Combining these results with an analysis of the thermodynamics, it is clear that from the relatively low temperature (<116°C), the oxygenated and acidic environment is favorable for formation of this native sulfur chimney in the Kueishantao hydrothermal field.

Formation of Fe-oxyhydroxides from the East Pacific Rise near latitude 13°N：Evidence from mineralogical and geochemical data

The mineralogical and geochemical characteristics of Fe-oxyhydroxide samples from one dredge station (long. 103°54.48′W, lat. 12°42.30′N, water depth 2655 m) on the East Pacific Rise near lat 13°N were analyzed by XRD, ICP-AES, and ICP-MS. Most Fe-oxyhydroxides are amorphous, with a few sphalerite microlites. In comparison with Fe-oxyhydroxides from other fields, the variable ranges in the chemical composition of Fe-oxyhydroxide samples are very narrow; their Fe, Si, and Mn contents were 39.90%, 8.92%, and 1.59%, respectively; they have high Cu (0.88%–1.85%) and Co (65×10−6−704×10−6) contents, and contain Co+Cu+Zn+Ni> 1.01%. The trace-element (As, Co, Ni, Cu, Zn, Ba, Sr) and major-element (Fe, Ca, Al, Mg) contents of these samples are in the range of hydrothermal sulfide from the East Pacific Rise near 13°N, reflecting that this type of Fe-oxyhydroxide constitutes a secondary oxidation product of hydrothermal sulfide. The Fe-oxyhydroxide samples from one dredge station on the East Pacific Rise near 13°N are lower in ΣREE (5.44×10−6–17.01×10−6), with a distinct negative Ce anomaly (0.12–0.28). The Fe-oxyhydroxide samples have similar chondrite-normalized rare-earth-element (REE) patterns to that of seawater, and they are very different from the REE composition characteristics of hydrothermal plume particles and hydrothermal fluids, showing that the REEs of Fe-oxyhydroxide are a major constituent of seawater and that the Fe-oxyhydroxides can become a sink of REE from seawater. The quick settling of hydrothermal plume particles resulted in the lower REE content and higher Mn content of these Fe-oxyhydroxides, which are captured in part of the V and P from seawater by adsorption. The Fe-oxyhydroxides from one dredge station on the East Pacific Rise near 13°N were formed by secondary oxidation in a low temperature, oxygenated environment. In comparison with the elemental (Zn, Cd, Pb, Fe, Co, Cu) average content of hydrothermal sulfide samples from the East Pacific Rise near 13°N, the Zn, Cd, and Pb contents of the Fe-oxyhydroxides are lower, and their Fe, Co, and Cu contents are higher.

Metasomatism of the peridotites from southern Mariana fore-arc: Trace element characteristics of clinopyroxene and amphibole

Modal composition and mineral composition of harzburgites from the southern Mariana fore-arc show that they are highly refractory. There are a few modals of clinopyroxene (0.7 vol %) in harzburgites. Two types of amphibole are found in these harzburgites: magnesiohornblende accompanied by clinopyroxene with higher Al2O3 content (>7%) and lower Mg#; tremolite around orthopyroxene with lower Al2O3 content (< 2%) and higher Mg#. Trace element of clinopyroxene and two types of amphibole are analyzed. Primitive mantle-normalised REE patterns for clinopyroxene and magnesio hornblende are very similar and both show HREE enrichment relative to LREE, while magnesiohornblende has higher content of trace element than clinopyroxene. The contents of trace element of tremolite are much lower than those of magnesiohornblende. Clinopyroxene shows enrichment of most of the trace element except HREE and Ti relative to clinopyroxene in abyssal peridotites. Petrology and trace element characteristic of clinopyroxene and two types of amphibole indicate that southern Mariana fore-arc harzburgites underwent two stages of metasomatism. The percolation of a hydrous melt led to mobility of Al, Ca, Fe, Mg, Na, and large amounts of trace element. LILE and LREE can be more active in hydrous melt than HREE and Ti, and the activities of most of the trace element except some of LILE are influenced by temperature and pressure.

Contribution of Magmatic Fluid to the Active Hydrothermal System in the JADE Field, Okinawa Trough: Evidence from Fluid Inclusions, Oxygen and Helium Isotopes

The Okinawa trough is an active, backarc-spreading basin in which hydrothermal fluids are venting from black and white smoker chimneys along a felsic volcanic ridge. The Okinawa seafloor hydrothermal precipitates are comparable to Kuroko-type massive sulfide deposits in many respects. Both systems occur in backarc settings and are hosted by a suite of bimodal volcanic rocks. They show mound-like features and contain predominantly Zn-Pb-Cu metal associations. Aqueous and CO2-rich hydrothermal fluids from the JADE field exhibit high trapping temperatures (up to 420°C), and moderate salinities (up to 8% NaCl equiv.). We provide evidence for the contribution of volatiles from a magmatic source to the actively forming submarine hydrothermal system. The following features in the JADE field support a magmatic component within this system: (1) the presence of a shallow-level felsic magma chamber 1-2 km beneath the seafloor; (2) anomalously high heat flow within the hydrothermal field; (3) a high content of CO2 and CH4 in the hydrothermal plume; (4) effervescence of volatile species (e.g., CO2, CH4, H2, N2,) in the hydrothermal fluid that forms independent discharges as CO2-rich fluid through CO2 hydrate pipes near black smoker chimneys; (5) helium isotope data demonstrating that these gases were derived from a mantle source; (6) high-temperatures and moderate to high fluid-inclusion salinities; (7) oxygen isotopic compositions of altered rocks and quartz indicating that mineralizing fluids have a δ18O composition consistent with a mixture of magmatic fluid and seawater.

Geochemical anomalies of hydrothermal plume at EPR 13°N

During the DY105-12, 14cruise (R/V DAYANG YIHAO, November 2003) on East Pacific Rise (EPR) 12–13°N, the submarine hydrothermal activity was investigated and the CTD hydrocast was carried out at EPR12°39′N–12°54′N. From the temperature anomalies and the concentrations of magnesium, chlorine, bromine in seawater samples, we discover that magnesium depletes 9.3%–22.4%, chlorine and bromine enrich 10.3%–28.7% and 10.7%–29.0% respectively relative to normal seawater at the stations which have chemistry anomalies, moreover temperature and chemistry anomalies are at the same layer. The depletion of magnesium in the plume may be caused by a fluid lacking of magnesium which rises after the hydrothermal fluid reaches the equilibrium with ambient seawater, the enrichment of chlorine and bromine might be the result of inputting later brine which is generated by phase separation due to hydrothermal activity. In addition, the Br/Cl ratio in the abnormal layers at the survey area is identical to that in seawater, which implies that halite dissolution (or precipitation) occurs neither when the fluid is vented nor when hydrothermal fluid entraining ambient seawater rises to form plume. From the abnormal instance at E55 station, it is very possible that there might exist a new hydrothermal vent site.

Characteristics of Sr, Nd and Pb isotopic compositions of hydrothermal Si-Fe-Mn-oxyhydroxides at the PACMANUS hydrothermal field, Eastern Manus Basin

Si-Fe-Mn-oxyhydroxides dredged at the PACMANUS (Papua New Guinea–Australia–Canada–Manus) hydrothermal field, Eastern Manus Basin, have 87Sr/86Sr=0.708 079–0.708 581; εNd=5.149 833–6.534 826; 208Pb/204Pb=38.245–38.440; 207Pb/204Pb=15.503–15.560; 206Pb/204Pb=18.682–18.783. 87Sr/86Sr isotope ratios are relatively homogeneous and close to the value of the surrounding seawater (0.709 16). The content of Sr in the samples contributed by seawater was estimated to be 76.7%–83.1% of total amount. The mixing temperature of hydrothermal fluids and seawater were ranging from 53.2°C to 72.2°C and the hydrothermal activities were unstable when the samples precipitated. The eNd values of all the samples are positive, which differ from the values of ferromanganese nodules (crusts) with hydrogenic origin. Nd was mainly derived from substrate rocks leached by hydrothermal circulation and preserved the hydrothermal signature. Pb isotopic compositions of most samples show minor variability except Sample #9–2 that has relatively high values of Pb isotopes. The Pb may be derived from the Eastern Manus Basin rocks leached by the hydrothermal fluid. The slightly lower 208Pb/204Pb and 207Pb/204Pb values of the samples indicated that the hydrothermal circulation in PACMANUS was not entire and sufficient, or that hydrothermal circulation had transient changes in the past. Si-Fe-Mn-oxyhydroxides in the samples preserved the heterogeneities of local rocks.

Hydrothermal alteration of plagioclase microphenocrysts and glass in basalts from the East Pacific Rise near 13°N：An SEM-EDS study

The interactions of seafloor hydrothermal fluid with igneous rocks can result in leaching elements from the rocks, creating potential ore-forming fluids and influencing the chemical compositions of near-bottom seawater. The hydrothermal alteration of plagioclase microphenocrysts and basaltic glass in the pillow basalts from one dredge station (103°57.62″W, 12°50.55′N, water depth 2480 m) on the East Pacific Rise (EPR) near 13°N were analyzed using a scanning electron microscope (SEM) and energy dispersive X-ray spectrometry (EDS). The results show that the edges of the plagioclase microphenocrysts and the basaltic glass fragments are altered but the pyroxene and olivine microphenocrysts in the interior of the pillow basalts appear to be unaffected by the hydrothermal fluids. In addition, our results show that the chemical alteration at the rims of the plagioclase microphenocrysts and the edges of basaltic glass fragments can be divided into separate types of alteration. The chemical difference in hydrothermal alteration of the plagioclase microphenocrysts and the basaltic glass indicate that different degrees of hydrothermal fluid-solid phase interaction have taken place at the surface of the pillow basalts. If the degree of hydrothermal fluid-solid phase interaction is relatively minor, Si, Al, Ca and Na diffuse from the inside of the solid phase out and as a result these elements have a tendency to accumulate in the edge of the plagioclase microphenocrysts or basaltic glass. If the degree of hydrothermal fluid-solid phase interaction is relatively strong, Si, Al, Ca and Na also diffuse from the inside of solid phase out but these elements will have a relatively low concentration in the edge of the plagioclase microphenocrysts or basaltic glass. Based on the chemical variation observed in the edges of plagioclase microphenocrysts and basaltic glass, we estimate that the content of Si, Al and Fe in the edges of plagioclase microphenocrysts can have a variation of 10.69%, 17.59% and 109%, respectively. Similarly, the Si, Al and Fe concentrations in the edges of basaltic glass can have a variation of 9.79%, 16.30% and 37.83%, respectively, during the interaction of hydrothermal fluids and seafloor pillow basalt.