Bubble Collapse Structure: A Microstructural Record of Fluids, Bubble Formation and Collapse, and Mineralization in Pseudotachylyte

AbstractA complex microstructural feature has been discovered within centimeter-scale pseudotachylyte veins and is here termed “bubble collapse structure” (BCS). Three first-order features of BCSs are (1) a central quartz–dominated monocrystal or polycrystal, (2) a light-colored reaction halo, and (3) a generally radial but commonly curving pattern of opaque seams and/or elliptical quartz amygdules. Two populations of bubbles were principally responsible for forming BCSs. One was a population of millimeter-scale bubbles, and the second involved much more numerous small bubbles (mean diameter, μm). The first population likely formed from liquid water interstitial to the precursory cataclasite or from melting of hydrous minerals, whereas the second population formed by diffusional growth. BCSs began forming as the pseudotachylyte melt cooled and the pressure inside the bubbles decreased. As the large bubbles flattened into oblate ellipsoids parallel to the vein margins, melt flowed toward each collapsing bu...