Fumarolic pipes in the Tshirege Member of the Bandelier Tuff on the Pajarito Plateau, Jemez Mountains, New Mexico

Abstract

The objective of this study is to demonstrate the relationships among devitrification, vapor phase alteration, localization of gas emanations into fumarolic pipes, and initial deformation of the ash flow sheet during cooling and lithification. Utilizing a unique and temporary exposure of the Tshirege Member of the Bandelier Tuff near Los Alamos, New Mexico, we identify several zones of distinctly preserved fossil fumarolic activity. The fumarolic zones vary in width from a few centimeters to more than a meter. Almost ubiquitously, these zones demonstrate finesdepletion, induration of the margins, upward-flaring geometries, and intense fracturing of overlying geologic units. The fumaroles were preferentially located on post welding, early formed cooling joints that vented to the surface after the vapor phase alteration stage. The pipes were regularly spaced at distances of approximately 4.5 m (N-S) to 7 m (E-W). In turn the pipes were covered by a surge deposit and overlying tuff which rapidly lithified. The overlying tuff was then brecciated during continued fumarolic pipe emissions. Geochemical evaluations confirm the presence of high-temperature mineral (scapolite) indicative of transport of hot volcanic gases through these zones. The pipe centers and walls are depleted in SiO 2 , and enriched in Al 2 O 3 and FeO. The overlying tuff breccia zones are enriched in Al 2 O 3 , FeO and MgO, and depleted in SiO 2 , NaO, and K 2 O. From comparison to other ignimbrite cooling histories, the fissures, fumaroles, and structures observed all likely formed in the first few decades after the deposition of the upper Tshirege subunits. This may have significant implications as to timing of initial cooling fractures and subsequent consolidation of gas emission pathways.

Spatial distribution of damage around faults in the Joe Lott Tuff Member of the Mount Belknap Volcanics, Utah: A mechanical analog for faulting in pyroclastic deposits on Mars

“…Cristobalite is a common high‐temperature alteration mineral found in silica‐rich tuffs [ Carlos et al , 1991; Caporuscio et al , 2012] and in basalts [ Naughton et al , 1976; Hamilton et al , 2008]. Occurrences of cristobalite in similar tuffs elsewhere have been attributed to hydrothermal or vapor‐phase alteration [ Naughton et al , 1976; Keith , 1991; Stimac et al , 1996; Shimizu and Aoki , 2011; Tempel et al , 2011; Wright et al , 2011; Caporuscio et al , 2012]. Previous studies have shown that percolation of meteoric water under ambient conditions is not a likely mechanism for cristobalite formation [ Matyskiela , 1997].…”

“…[23] Shortly following emplacement of the Joe Lott Tuff, an open synform, the Clear Creek downwarp, developed in the region of the study area [Callaghan and Parker, 1962]. Subsequently, between $16 Ma and $14 Ma, volcanism waned and basin-fill sedimentation was prevalent.…”

“…Subsequently, between $16 Ma and $14 Ma, volcanism waned and basin-fill sedimentation was prevalent. The Sevier River Formation was deposited on top of the Joe Lott Tuff and consists of poorly bedded, coarse-grained conglomerates to silts and clays, with rare interbedded air fall tuff [Callaghan and Parker, 1962;Rowley et al, 2002].…”