Segregation structures in vapor-differentiated basaltic flows

Vesicle cylinders represent a spectacular kind of segregation structure inv olving residual liquids formed in situ during the cooling of lava flows. Th ese vertical pipes, commonly found within basalt flows typically 2-10 m thi ck, are interpreted as the product of a vapor-driven differentiation proces s. The olivine phenocrysts and the earliest generation of groundmass olivin es found in cylinder-bearing basalts appear to have been generally affected by magmatic oxidation, resulting in high-temperature iddingsite (HTI) alte ration. This feature is also observed within cylinder-fret basalt flows whi ch exhibit other kinds of vesicular segregation structures, such as vesicle -rich pegmatoid segregation sheets and/or segregation vesicles. Detailed te xtural, petrological, and geochemical characteristics of two types of cylin ders, three types of vesicle sheets, and five types of segregation vesicles are described, based on the study of 12 occurrences of HTI-bearing basalt flows from oceanic shield volcanoes or continental basalt plateaus. We prop ose a general classification of these segregation structures likely to deri ve from vapor differentiation. Flow thickness is probably the main factor i nfluencing their morphology. Finally, we suggest that the concomitant occur rence of olivine oxidation and vapor-differentiation effects results from t he late persistence of water oversaturation after eruption, perhaps due to a high rate of magma ascent.