Zinc stable isotopes in seafloor hydrothermal vent fluids and chinmeys

Many of the heaviest and lightest natural zinc (Zn) isotope ratios have been discovered in hydrothermal ore deposits. However, the processes responsible for fractionating Zn isotopes in hydrothermal systems are poorly understood. In order to better assess the total range of Zn isotopes in hydrothermal systems and to understand the factors which are responsible for this isotopic fractionation, we have measured Zn isotopes in seafloor hydrothermal fluids from numerous vents at 9-10 degrees N and 21 degrees N on the East Pacific Rise (EPR), the TAG hydrothermal field on the Mid-Atlantic Ridge, and in the Guaymas Basin. Fluid delta Zn-66 values measured at these sites range from +0.00 parts per thousand to +1.04 parts per thousand. Of the many physical and chemical parameters examined, only temperature was found to correlate with fluid delta Zn-66 values. Lower temperature fluids (<250 degrees C) had both heavier and more variable delta Zn-66 values compared to higher temperature fluids from the same hydrothermal fields. We suggest that subsurface cooling of hydrothermal fluids leads to precipitation of isotopically light sphalerite (Zn sulfide), and that this process is a primary cause of Zn isotope variation in hydrothermal fluids. Thermodynamic calculations carried out to determine saturation state of sphalerite in the vent fluids support this hypothesis with isotopically heaviest Zn found in fluids that were calculated to be saturated with respect to sphalerite. We have also measured Zn isotopes in chimney sulfides recovered from a high-temperature (383 degrees C) and a low-temperature (203 degrees C) vent at 9-10 degrees N on the EPR and, in both cases, found that the delta Zn-66 of chimney minerals was lighter or similar to the fluid 666 Zn. The first measurements of Zn isotopes in hydrothermal fluids have revealed large variations in hydrothermal fluid delta Zn-66, and suggest that subsurface Zn sulfide precipitation is a primary factor in causing variations in fluid delta Zn-66. By understanding how chemical processes that occur beneath the seafloor affect hydrothermal fluid 666 Zn, Zn isotopes may be used as a tracer for studying hydrothermal processes. (C) 2008 Elsevier B.V. All rights reserved.