REDOX CONDITIONS DURING FRICTIONAL MELTING: A MOSSBAUER STUDY OF PSEUDOTACHYLYTES

Water maybe important in controlling fault strength during seismic slip. Frictional heating during faulting has the potential to reduce strength by pressurizing water present in the fault zone, thereby promoting additional slip. Frictional melts may record the presence or absence of water at the time of rupture. The observation that different pseudotachylytes (fault-related frictional melts) contain different accessory phases such a magnetite (Homestake shear zone, Colorado; South Mountain, Arizona), hematite (Long Ridge fault, North Carolina), and pyrite (Fort Foster fault, Maine) suggests that the oxidation state of the frictional melt may reflect the oxygen fugacity (and hence fluid pressure) during melting. This study presents a Mossbauer study of pseudotachylytes and their host rocks from four localities with a view to examining Fe+3/Fe+2 ratios in the melts. The main Fe-bearing phase in the pseudotachylytes are layer silicates inferred to have formed during devitrification of the melt. With the exception of one locality (Long Ridge fault) the ferric/ferrous ratios of the melts are similar to their host rocks, indicating dry closed system melting. Calculated oxygen fugacities (log fO2 bars=-14 to -5) are also consistent with the equilibrium buffer curves (MH, FMQ and QIF) at 1200K. These data indicate the original ferric/ferrous ratios of the melts are preserved in the pseudotachylyte matrix. The Long Ridge fault locality, which is characterized by hematite, displays higher ferric/ferrous ratios and higher oxygen fugacities (by several log units) compared to its host. This locality also displays oxygen isotope depletion (up to 4 per mil) compared to the host, indicating interaction with shallow water, possibly by a reaction such as: 2FeO + H2O=Fe2O3 + H2 gas. It is hypothesized that co-seismic hydrogen gas anomalies sometimes observed on active faults are due to frictional melting at depth in the presence of water.