Field/experimental investigations demonstrate the chemistry of mid-ocean ridge
hydrothermal fluids reflects fluid-mineral reaction at temperatures higher than typically measured
at the seafloor. In order 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 °C 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. This
methodology is used to interpret new data from 13°N on the East Pacific Rise (EPR) and the
Lucky Strike Seamount on the Mid-Atlantic Ridge where high-temperature fluids both enriched
and depleted in chloride, relative to seawater, are actively venting within a close proximity.
Accounting for these variable salinities, phase separation is also a dominant process affecting the
chemistry of hydrothermal fluids; and vapor-liquid partition coefficients for accessory metals
(Na-normalized as NaCl dictates phase equilibria) have therefore been experimentally derived for
temperature between 360 and 460 °C, where those > Na represent increasing affinity for the
liquid phase. These data reveal the relationship Cu(I) < Na < Fe(II) < Zn < Ni(II) ≤ Mg ≤ Mn(II)
< Co(II) < Ca < Sr < Ba and a dependence on the cationic radii. The depth below seafloor of the
magmatic heat source at Lucky Strike is greater than that of faster spreading systems such as EPR
9-13°N, which host higher temperature vents with greater transition metal concentrations. The
combined experimental/field data suggest the resulting increased residence time in the up-flow
zone allows re-equilibration at temperatures lower than those resulting in phase separation. Fluid
chemistry measured in the immediate aftermath of eruptions at EPR 9-10°N conversely shows
large deviations from Fe/Mn equilibrium consistent with partitioning due to phase separation. The
eruptive data also exhibit that P-T conditions were sufficiently extreme that Fe is behaving in a
volatile manner observed in the experiments and in some instances subseafloor halite saturation may have occurred.
University of Minnesota Ph.D. dissertation. November 2012. Major: Geology. Advisor: William E. Seyfried, Jr. 1 computer file (PDF); ix, 130 pages.
Pester, Nicholas Jon.
The role of iron and manganese in elucidating the temperature of subseafloor hydrothermal reactions: insights from experimental and field data.
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