Elemental and isotopic compositions of the hydrothermal sulfide on the East Pacific Rise near 13°N

Abstract

The mineralogical, elemental, and isotopic characteristics of a hydrothermal sulfide sample from one dredge station (12 degrees 42.30'N, 103 degrees 54.48' W, water depth 2655 m) on the East Pacific Rise near 13 degrees N were analyzed. The hydrothermal sulfide was composed mainly of sphalerite, chalcopyrite, and pyrite and was a Zn-rich sulfide; in layer ep-s-1, goethite formed by secondary oxidation was found. The concentrations of rare elements, such as Li (0.15 x 10(-6)-0.30 x 10(-6)), Be (0.01x10(-6)-0.05x10(-6)), Zr (73.8x10(-9)-1344x10(-9)), Nb (8.14x10(-9)-64.7x10(-9)), Hf (2.54x10(-9)-28.0x10(-9)), and Ta (0.203x10(-9)-1.21x10(-9)), were far lower in the hydrothermal sulfide than in the ocean crust, whereas the content of Au was higher and the contents of Co, Ni, Sr, Cs, Ba, Bi, and U were low. The correlations between Zn and Cr, Cd and Ga, Cu and P, P and In (R(2) > 0.8) were positive, whereas those between Zn and Fe, Cu, and Ba (R(2) > 0.8) were distinctly negative. From low-temperature mineral assemblages to high-temperature mineral assemblages, the spatial distributions of dispersive and rare elements (e.g. In, Li, Cs) in the hydrothermal sulfide displayed corresponding variations. The variations observed in some elements (e.g., Cd, Cs, P) are controlled by Zn, Fe, and Cu sulfides, respectively. Seafloor weathering accounts for the enrichment of V, Mn, and rare earth elements (REE) in the henna sulfide-oxidation layer that bears the secondary oxide mineral, leading to identical REE patterns for this layer (ep-s-1) and seawater. Seafloor weathering also distinctly affects the correlations between the element ratios of the hydrothermal sulfide. From high-temperature mineral assemblages to low-temperature mineral assemblages, Fe content and delta(34)S value of the hydrothermal sulfide increase gradually, and Zn content and lead isotopic ratios decrease gradually on the contrary, which indicate the influences of seawater on elements and the sulfur and lead isotopic compositions enhance gradually during the formation of hydrothermal sulfides.