A Mössbauer study of pseudotachylytes: redox conditions during seismogenic faulting

The redox conditions during frictional melting provide information on the physical and chemical conditions during seismic slip in the crust. Here we examine frictional melts from five localities by analyzing host rocks and corresponding pseudotachylytes using Mössbauer spectroscopy. The faults examined are located at South Mountain, Arizona; Fort Foster, Maine (two localities); Long Ridge fault, North Carolina; and the Homestake shear zone, Colorado. The main iron-bearing phases in the pseudotachylytes are phyllosilicates (biotite, muscovite and clays) and iron oxides (magnetite and hematite) and minor pyrite. The ferrous/ferric ratios of the phyllosilicates in the host rocks are the same as those in the pseudotachylytes, except for the hematite-bearing pseudotachylyte from the Long Ridge fault, which is more oxidized. The magnetites in the host rocks and the corresponding pseudotachylytes have different ferric and ferrous iron distributions, which is attributed to different cation chemistry, rather than redox conditions. With the exception of the South Mountain locality, the ferric/ferrous ratios of the micas are interpreted to record the primary redox state of the pseudotachylyte melt as the calculated oxygen fugacities are consistent with magnetite and hematite equilibria. Pyrite-bearing pseudotachylytes plot ~0–1 log10 units above the fayalite-magnetite-quartz (FMQ) buffer. Magnetite-bearing pseudotachylytes plot ~2–4 log10 units above the FMQ buffer, and hematite-bearing pseudotachylytes plot 3.5 log10 units above the hematite-magnetite (HM) buffer. Hematite-bearing pseudotachylytes, together with previous oxygen isotope data, are inferred to represent melting in the presence of externally derived pressurized water. Other localities are inferred to represent melting under rock-buffered, closed system, conditions. If the localities studied are representative of seismogenic faulting, the calculated oxygen fugacities indicate that, in the system C–O–H–S, H2O and CO2 should be the dominant fluid species. This is the first detailed study of the redox state of pseudotachylytes.