Immiscible sulfide droplets in pseudotachylyte: Evidence for high temperature (> 1200 °C) melts

Microprobe analysis indicates that nearly all droplets are pyrrhotite. The disappearance of pyrite indicates that melt temperatures must have exceeded 750 °C, but other indications suggest that the melt temperature must have been much higher. The extremely common presence of pyrrhotite droplets suggests that pyrrhotite from the protolith melted, requiring a minimum melt temperature of ∼ 1200 °C. In some samples, evidence for fluid-rich bubbles, and possible silicate spherules indicates three coexisting immiscible phases within the silicate melt. The presence of sulfide droplets appears to be common, especially in relatively low oxygen-fugacity melts that formed at shallow crustal levels. This can provide a good textural marker of melting and therefore of pseudotachylyte formation, especially where other indications of melting (i.e., high temperature microlites, vesicles, etc.) are lacking, and illustrates the extreme temperatures possible along frictionally sliding surfaces during seismic events.