REDOX CONDITIONS DURING FRICTIONAL MELTING: A MOSSBAUER STUDY OF PSEUDOTACHYLYTES

Frictional heating during faulting has the potential to reduce strength by pressurizing water present in the fault zone, thereby triggering seismic slip. The observation that different pseudotachylytes contain accessory phases such a magnetite (Homestake shear zone, CO; South Mountain, AZ) hematite (Long Ridge fault, NC), and pyrite (Fort Foster fault, ME) suggests that the oxidation state of the frictional melt may record 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 the role of water during frictional melting. The main Fe-bearing phase in the pseudotachylytes identified by Mossbauer spectra are layer silicates, inferred to have formed during devitrification of the melt. Spectra also confirm the presence of magnetite or hematite. Relict and new magnetites can be distinguished by different 4-fold and 6-fold site occupancy by Fe. With the exception of one locality (Long Ridge fault) layer silicates in pseudotachylytes indicate ferric/ferrous ratios consistent with their host rocks, indicating closed system melting. Calculated oxygen fugacities (log fO2 bars=-15 to -7) are also consistent with the accessory phases present (hematite, magnetite or pyrite) at 1500K. The Long Ridge fault locality, which is characterized by hematite, displays higher ferric/ferrous ratios and higher calculated oxygen fugacities 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 (hematite) + H2 (gas). It is hypothesized that co-seismic hydrogen gas anomalies sometimes observed on active faults maybe caused by frictional melting at depth in the presence of water.