VARIATIONS IN THE OCCURRENCE AND MOBILITY OF ARSENIC IN ASH-FLOW TUFFS

Although volcanic rocks typically have only low to moderate arsenic concentrations, elevated levels of arsenic in ground waters have been associated with pyroclastic and volcaniclastic rocks in many areas of the world. The potential for arsenic leaching from these deposits is particularly problematic as they often comprise important water-bearing units in volcanic terrains. However, the role that chemical and mineralogical variations play in controlling the mobility of arsenic from pyroclastic rocks is largely unexplored.

We present data from chemical, X-ray diffraction, and X-ray absorption spectroscopy analysis of rhyolitic ash-flow tuffs collected from throughout Oregon and elsewhere, representing continental arcs and extensional settings in back arc regions. Samples span a range of primary compositions, and type and degree of devitrification and chemical weathering. Where possible, glassy and devitrified and or fresh and weathered samples were obtained from the same unit in the same locality. The results indicate that arsenic in minimally altered glassy tuffs is present predominantly as As(III) while arsenic in altered tuffs is present predominantly as As(V). Initial results of whole rock and leachate analyses suggest that unweathered (glassy or devitrified) tuffs contain higher concentrations of arsenic, but a greater percentage of the total arsenic present is leached from tuffs altered by chemical weathering or devitrification. Arsenic expelled from the weathering of primary glassy groundmass may occupy a number of secondary host phases, including oxyhydroxides, illite and smectite clays, and zeolites such as clinoptilolite. The relative mobility of arsenic from these phases as a function of solution chemistry is currently being investigated.