Field and experimental investigations demonstrate the chemistry of mid-ocean ridge hydrothermal vent fluids reflects fluid-mineral reaction at higher temperatures than those typically measured at the seafloor. To account for this and, in turn, be able to better constrain sub-seafloor hydrothermal processes, we have developed an empirical geothermometer based on the dissolved Fe/Mn ratio in high-temperature fluids. Using data from basalt alteration experiments, the relationship; T (°C)=331.24+112.41*log[Fe/Mn] has been calibrated between 350 and 450°C. The apparent Fe-Mn equilibrium demonstrated by the experimental data is in good agreement with natural vent fluids, suggesting broad applicability. When used in conjunction with constraints imposed by quartz solubility, associated sub-seafloor pressures can be estimated for basalt-hosted systems. As an example, this methodology is used to interpret new data from 13°N on the East Pacific Rise, where high-temperature fluids both enriched and depleted in chloride (339-646mmol/kg), relative to seawater, are actively venting within a close proximity. Accounting for these variable salinities, active phase separation is clearly taking place at 13°N, yet the fluid Fe/Mn ratios and the silica concentrations suggest equilibration at temperatures less than those coinciding with the two-phase region. These data show the chloride-enriched fluid reflects the highest temperature and pressure (∼432°C, 400bars) of equilibration, consistent with circulation near the top of the inferred magma chamber. This is in agreement with the elevated CO2 concentration relative to the chloride-depleted fluids. The noted temperature derived from the Fe/Mn geothermometer is higher than the critical temperature for a fluid of equivalent salinity. This carries the important implication that, despite being chloride-enriched relative to seawater, these fluids evolved as the vapor component of even higher salinity brine.
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We wish to thank the captain and crew of R/V Atlantis , the Alvin Group, and chief scientist Stefan Sievert for their assistance in planning and executing the acquisition of the EPR 13°N fluid samples. We also thank Erin Seyfried and Shijun Wu for their help with shipboard analyses and sampler preparation, as well as Rick Knurr for the shore-based analyses (IC and ICP-OES) of both the field and experimental samples. Thoughtful comments from Margaret Tivey, Laurence Coogan, Jeff Alt, and an anonymous reviewer, greatly improved the clarity and content of this publication. Financial support for this research was provided by NSF grants MGG-OCE 0549547 , 0751771 , 0813861 , 0961188 (W.E.S., K.D.).