Petrological analyses of opaque mineral assemblages to constrain the formation conditions of the Josephine Peridotite

Event Center BC

The serpentinizing reaction of olivine and pyroxene with seawater creates some of the most reducing environments known in nature. Serpentinization of peridotite can produce low enough fO2 values that stabilize metals such as awaruite (Ni3Fe) and copper, as well as various nickel-sulfides. By applying the mineral phases present within a rock to phase equilibria, the pressure, temperature, and thermodynamic conditions of the serpentinizing fluids can be determined. Previously, the formation conditions for the Josephine Peridotite in Oregon and California were unknown. Careful analyses of eleven samples from the Josephine Peridotite using x-ray diffraction, optical microscopy, reflected-light microscopy, and energy-dispersal x-ray spectroscopy have determined the formation conditions of the hydrothermal fluid that serpentinized the Josephine Peridotite. These analyses have shown that the Josephine Peridotite formed at temperatures of 400 °C, at pressures of 50 MPa, had activities of H2(aq) ranging from log -0.60 H2(aq) to log -0.63 H2(aq) and activities of H2S(aq) ranging from log H2S(aq) -2.452 to log H2S(aq) -2.410. These results show that the hydrothermal vents that serpentinized the Josephine Peridotite were much more reducing than found at the Logatchev and Rainbow hydrothermal fields found on the Mid-Atlantic Ridge.