Systematic variations in trace element composition of pyrites from the Xunmei hydrothermal field (26°S), Mid-Atlantic Ridge

Two zoned inactive chimney samples from the SMAR 26°S (Xunmei) hydrothermal field were studied petrographically and by in-situ LA-ICP-MS analysis. Morphologically different pyrites precipitated with increasing temperatures from the outermost chimney wall to intermediate zone, then to the inner zone, and finally to interstice pore fillings which represents the late mature stage. The distribution of trace elements in pyrites across the chimney indicates a strong dependence on time, temperature, and associated sulfide minerals. The variation of trace elements in different paragenetic stages of pyrite reveals that the hydrothermal system most likely evolved from low-temperature low-chloride liquid-dominated fluids (enriched in Zn, Cd, Tl, Ag, Pb, Mn, Mo, and V) to higher temperature, vapor-dominated fluids (Cu, Au, Te, and Bi), probably representing magmatic volatiles, and then to high-temperature fluids (Co and Se). In the waning stage of the hydrothermal system, circulating hot fluids in auxiliary conduits were depleted in most trace elements. LA-ICP-MS time-depth profiles reveal that Co, Se, and Mo are present mainly in lattice substitution, whereas Cu, Zn, Cd, Tl, Ag, Te, and Bi are related to micro-/nano-inclusions. Profiles for As, Pb, Au, and Sb can be either smooth or irregular, indicating both lattice substitutions and inclusions. Adsorbed films on pyrites control the distribution of V and Mo. To conclude, the behavior of trace elements is strongly associated with the fluid evolution during chimney growth, where trapping of micro-/nano-inclusions and surface adsorption are seen more frequently at the low-temperature stage, whereas lattice substitutions are dominant at elevated temperature stages.