'Pipe vesicles' in basalt: Trails left by dense immiscible melt droplets sinking through a viscous basal thermal boundary layer

摘要

Pipe vesicles, tubular cavities found in the basal parts of pahoehoe lava flows worldwide, constitute one of their fundamental primary structures. Like other vesicles, pipe vesicles have been ascribed to gas bubbles, whether rising buoyantly through viscous lava or trapped in an inward-moving solidification front. These models involve special and unrealistic conditions and still poorly explain characteristic features of pipe vesicles. In a new model proposed here, pipe vesicles are postulated to be hollow trails left by droplets of dense, immiscible iron-rich silicate or sulphide melt sinking through a viscous thermal boundary layer just above the lower chilled margin of a pahoehoe flow unit. Driven by gravity, the droplets sink vertically in static lava, but obliquely in mobile lava, such as lava undergoing gravitational spreading. Viscous and fluid dynamic forces tend to deform the sinking droplets and resist their motion. The characteristic inverted-Y, inverted-V and lambda (lambda) shapes of individual pipe vesicles reflect downward branching, with a large droplet breaking up in partially crystallized, highly polymerized basalt in the basal thermal boundary layer. Pipe vesicles do not yield lava flow directions even on a local (centimeters to meters) scale, but are indirect indicators of widespread liquid immiscibility processes in basalts and potential commercial Fe-Cu-Ni mineralization. As pipe vesicles are not vesicles (cavities left by gas escape), a new term is necessary, and the term "vermiform pahoehoe", having good descriptive power but no genetic connonations, is suggested for pahoehoe with "pipe vesicles".