Chemical and oxygen isotope composition of natural and artificial pseudotachylyte: role of water during frictional fusion

Pseudotachylyte samples from the Long Ridge fault, North Carolina, the Homestake shear zone, Colorado, the Fort Foster fault, Maine, and experimentally produced frictional melt were analyzed for major elements and O isotopes using an electron microprobe and laser microprobe, respectively. The pseudotachylytes display a δ18O range of +5.9–+10.7‰ (Standard Mean Ocean Water, SMOW) and are both isotopically lighter and heavier than their respective host rocks. In general, the isotopic composition of bulk pseudotachylyte (pt), in the absence of water–rock interaction, can be modeled asδ18Obulk pt=∑(Xfragδ18Ofrag)+∑(Xminδ18Omin)where Xfrag refers to the volume fraction of unmelted mineral fragments and Xmin refers to the volume fraction of minerals that undergo melting. Mineral-melt isotopic fractionation is considered to be small and less than 1‰. The majority of pseudotachylyte isotopic compositions can be explained as a disequilibrium partial melt of the protolith rock comprising melted feldspar and micas plus a small proportion (∼10%) of unmelted quartz clasts. In contrast, the Long Ridge fault samples are substantially depleted in 18O (by up to 4‰) relative to their host rock, which is best explained by closed system water–rock interaction of a pore fluid at 100°C with a melt at 1000°C and a water/rock weight ratio of 0.2. This fluid is also inferred to have caused oxidation of the pseudotachylyte melts, producing the hematite characteristic of this location.