Bubble collapse structure: A microstructural record of fluids, bubble formation and collapse, and mineralization in pseudotachylyte

摘要

A 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, 16±8 mm). 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 bubble, deforming the smaller bubbles into radially arranged prolate shapes. Microlites nucleated in the melt on bubble margins and grew radially, and several minerals (titanite, apatite) grew on the margins of the collapsing bubbles. The nearly solidified melt formed convex-inward bulges, marking the last viscous flow. On further cooling, fluids penetrated the pseudotachylyte vein, partly along curving paths controlled by stress fields, forming pressure solutionlike seams and triggering quartz precipitation within surviving vesicles. BCSs owe their origin to changing rheology and the formation of bubbles within the pseudotachylyte melt and the subsequent transition into vesicles and amygdules. BCSs allow insights into the premelting history and nucleation of water-rich precursory fault rocks, pseudotachylyte melt formation, cooling of the melt and accompanying volume loss, and the return of fluids into the fault zone during and after cooling.