BUBBLE FORMATION, COLLAPSE, AND INFILLING IN PSEUDOTACHYLYTE

A previously undescribed microstructure in pseudotachylyte, interpreted to result from the collapse of bubbles within the melt, is here reported. Each microstructure is centered on a typically elliptical quartz monocrystal or polycrystal up to nearly 1 mm² in area. Surrounding the quartz is a light-colored reaction halo, and extending through the halo and beyond is a radial pattern of opaque seams and elliptical quartz crystals. The features are largest in the cores of veins but most numerous between the core and vein margins. The quartz cores contain apatite, Fe, Cu, and Zn sulfides or sulfates and Fe oxides or hydroxides, and partial coatings of titanite. In some, a ring of fluid inclusions parallels the margin. Large features have elliptical or flattened quartz cores and show inward bulges of host pseudotachylyte containing microlites aligned in the bulging direction of bulging.

These microstructures are interpreted to have originated as bubbles. Two populations of bubbles suggest small bubbles (averaging 16+-8 microns) formed by diffusional growth, whereas large bubbles formed through melting of hydrous minerals or possibly from liquid water. Some radial fracturing owing to fluid overpressure within the bubbles may have produced the opaque seams. As the melt cooled, pressure decreased and the vesicles partially collapsed, producing a characteristic strain pattern.

After quenching, fluids first precipitated apatite and titanite, and later quartz. Fluid inclusion rings and sulfide or sulfate inclusions mark specific episodes of fluid flow or flow perturbations. These microstructures offer insights into aspects of fault evolution prior, during, and after melt generation along a fault.