PETROLOGICAL ANALYSES OF OPAQUE MINERAL ASSEMBLAGES TO CONSTRAIN THE FORMATION CONDITIONS OF SERPENTINIZED PORTIONS OF THE JOSEPHINE PERIDOTITE

Serpentinization of peridotite can produce some of the most reducing environments known in nature. During these serpentinizing reactions, oxygen fugacity levels (fO₂) in serpentinizing fluids drop to an extreme point where minor mineral phases such as awaruite (Ni₃Fe), native copper, and various nickel-sulfides become stable. Plotting minor mineral phases on a phase diagram can determine the temperature and chemistry of the serpentinizing fluids. Previously, the formation conditions of serpentinized portions of the Josephine Peridotite in Southern Oregon were not understood. Through analysis of eleven samples collected from a study area near Josephine Creek and Eight Dollar Mountain, Oregon, the formation conditions of the serpentinized portions of the Josephine Peridotite were determined. The analysis of the samples was conducted through a variety of methods including: x-ray diffraction (XRD), optical microscopy, reflected-light microscopy, and energy-dispersal x-ray spectroscopy (EDS). Plotting the results from these analyses on a phase diagram determined that the Josephine Peridotite formed at a temperature of 400 °C, and had activities of H_2(aq) ranging from log -0.60 H_2(aq) to log -0.63 H_2(aq) and activities of H₂S_(aq) ranging from log H₂S_(aq) -2.452 to log H₂S_(aq) -2.410. These values determined that the environment that serpentinized portions of the Josephine Peridotite was far more reducing than the serpentinizing fluids in the Logatchev and Rainbow hydrothermal fields found on the Mid-Atlantic Ridge.