Yin Xuebo

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.

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.

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.