Relations between thermal history and secondary structures of ignimbrites exclusive of rheomorphism

Abstract

Newly calculated model density profiles of ignimbrites also provide model thermal histories, which serve as a framework to help analyze the development of cooling joints, devitrification textures, and lithophysal cavities. Only those parts of sections of Rattlesnake Tuff in central Oregon that resided >600 °C for at least two years show devitrification. Low tensile strength means that joints form by thermoelastic contraction after cooling by as little as 25 °C. This means that columnar joints formed in as little time as a few weeks after deposition of the Aravaipa Tuff in SE Arizona. Compaction is rapid as well, so both columnar jointing and compaction are complete before the onset of devitrification in deposits <40 m thick. A section of Rattlesnake Tuff shows stratabound occurrences of devitrified spots and cavities; devitrification appears to have begun at scattered spots. Most spots are bounded by crescentic cavities in formerly ductile shards. The equation for conductive cooling of a spherical heat source shows that for rock volumes larger than ~15 cm diameter, the rate of cooling is insufficient to prevent heating by latent heat by as much as 12 °C. Inflation therefore appears to have been caused by vapor released by devitrification and its slight adiabatic expansion. Eventual wholesale devitrification of matrix resulted in more-devitrified spots scattered in less devitrified matrix. Lithophysal cavities in the Rattlesnake Tuff and in the Peach Springs Tuff in NW Arizona are more abundant in lower density horizons, showing that cavity growth is favored in permeable zones between impermeable horizons. The Peach Springs Tuff and other deposits described in the literature have horizontal joints that formed during cooling and whose origin has yet to be deterministically modeled. The longest joints are most closely spaced in the zone of cavities and formed after columnar joints. It is inferred that after formation, each vertical column responds independently to evolving stresses; at this time, asperities together with gas pressures accompanying cavity formation result in horizontal joints. Columnar joints do not completely relieve growing gas pressures during devitrification, likely because of sealing by wholesale inflation of the rock mass and by mineral deposition. Some devitrified horizons of Rattlesnake Tuff and Peach Springs Tuff are densely fractured, resulting in a rubbly surface. These small fractures show zigzag traces, bifurcations, abrupt terminations, and pinchand- swell walls, similar to ductile fractures described by Eichhubl (2004). Their origin is interpreted to be the result of tensile stress due to porosity increase during later stages of devitrification. Closely adjacent sections of Rattlesnake Tuff at one site differ by 18% in thickness, reflecting buried paleotopography. Each section is devitrified. The thicker profile has a zone of lithophysal cavities near its base; the thinner profile has few cavities. The greater thickness translates to only 0.18 MPa additional lithostatic pressure at the base of the thicker profile, which serves to emphasize the subtle differences in initial conditions that can lead to formation of cavities.