@article {Pester2011,
	title = {{A new Fe/Mn geothermometer for hydrothermal systems: Implications for high-salinity fluids at 13{\textdegree}N on the East Pacific Rise}},
	journal = {Geochimica et Cosmochimica Acta},
	volume = {75},
	number = {24},
	year = {2011},
	pages = {7881{\textendash}7892},
	abstract = {Field and experimental investigations demonstrate the chemistry of mid-ocean ridge hydrothermal vent fluids reflects fluid{\textendash}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 ({\textdegree}C)~=~331.24~+~112.41*log[Fe/Mn] has been calibrated between 350 and 450~{\textdegree}C. The apparent Fe{\textendash}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{\textdegree}N on the East Pacific Rise, where high-temperature fluids both enriched and depleted in chloride (339{\textendash}646~mmol/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{\textdegree}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~{\textdegree}C, 400~bars) 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.},
	keywords = {HOV Alvin (Human Occupied Vehicle)},
	issn = {0016-7037},
	doi = {10.1016/j.gca.2011.08.043},
	url = {http://dx.doi.org/10.1016/j.gca.2011.08.043},
	author = {Pester, N J and Rough, M and Ding, K and Seyfried, W E}
}
@article {Seyfried2011,
	title = {{Vent fluid chemistry of the Rainbow Hydrothermal System (36 degrees N, MAR): Phase equilibria and in-situ pH controls on subseafloor alteration processes}},
	journal = {Geochimica et Cosmochimica Acta},
	volume = {75},
	number = {6},
	year = {2011},
	pages = {1574{\textendash}1593},
	abstract = {The Rainbow hydrothermal field is located at 36{\textdegree}13.8'N{\textendash}33{\textdegree}54.15'W at 2300~m depth on the western flank of a non-volcanic ridge between the South AMAR and AMAR segments of the Mid-Atlantic Ridge. The hydrothermal field consists of 10{\textendash}15 active chimneys that emit high-temperature (\~{}365~{\textdegree}C) fluid. In July 2008, vent fluids were sampled during cruise KNOX18RR, providing a rich dataset that extends in time information on subseafloor chemical and physical processes controlling vent fluid chemistry at Rainbow. Data suggest that the Mg concentration of the hydrothermal end-member is not zero, but rather 1.5{\textendash}2~mmol/kg. This surprising result may be caused by a combination of factors including moderately low dissolved silica, low pH, and elevated chloride of the hydrothermal fluid. Combining end-member Mg data with analogous data for dissolved Fe, Si, Al, Ca, and H2, permits calculation of mineral saturation states for minerals thought appropriate for ultramafic-hosted hydrothermal systems at temperatures and pressures in keeping with constraints imposed by field observations. These data indicate that chlorite solid solution, talc, and magnetite achieve saturation in Rainbow vent fluid at a similar pH(T,P) (400~{\textdegree}C, 500~bar) of approximately 4.95, while higher pH values are indicated for serpentine, suggesting that serpentine may not coexist with the former assemblage at depth at Rainbow. The high Fe/Mg ratio of the Rainbow vent fluid notwithstanding, the mole fraction of clinochlore and chamosite components of chlorite solid solution at depth are predicted to be 0.78 and 0.22, respectively. In situ pH measurements made at Rainbow vents are in good agreement with pH(T,P) values estimated from mineral solubility calculations, when the in situ pH data are adjusted for temperature and pressure. Calculations further indicate that pH(T,P) and dissolved H2 are extremely sensitive to changes in dissolved silica owing to constraints imposed by chlorite solid solution-fluid equilibria. Indeed, the predicted correlation between dissolved silica and H2 defines a trend that is in good agreement with vent fluid data from Rainbow and other high-temperature ultramafic-hosted hydrothermal systems. We speculate that the moderate concentrations of dissolved silica in vent fluids from these systems result from hydrothermal alteration of plagioclase and olivine in the form of subsurface gabbroic intrusions, which, in turn are variably replaced by chlorite~+~magnetite~+~talc~{\textpm}~tremolite, with important implications for pH lowering, dissolved sulfide concentrations, and metal mobility.},
	keywords = {ROV Jason (Remotely Operated Vehicle)},
	issn = {0016-7037},
	doi = {10.1016/j.gca.2011.01.001},
	url = {http://dx.doi.org/10.1016/j.gca.2011.01.001},
	author = {Seyfried, W E and Pester, N J and Ding, K and Rough, M}
}
