MAJOR AND TRACE ELEMENT LEACHING OF MOUNT MAZAMA TEPHRAS, OREGON

Mount Mazama erupted explosively about 7,700 years ago in the Central Cascades of Oregon, depositing tephra up to 1,000 km from the edifice. The composition of material erupted changed over time from andesite to rhyodacite. The purpose of this study was to characterize geochemical weathering of Mazama tephra deposits from multiple locations around Bend, Oregon. Nine soil pits were dug/augered about 110 km north of the eruptive center, containing 18 – 180 cm of Mazama tephra, with samples taken every 8 - 10 cm. For each sample, major and trace elements were analyzed using a portable XRF.

Previous work in this region on Quaternary and Tertiary andesite and dacite lava flows has shown no major element mobility and slight enrichments of mobile trace and REEs. Given the enhanced surface area and porosity of tephra deposits, a more dramatic weathering profile was expected. To evaluate this hypothesis, a portable XRF was used to sample the major element and trace element concentrations with respect to depth to determine if leaching of elements was occurring. Initially, a 600 μm size fraction of each sample was analyzed in replicate, and tau plots normalized to zirconium were created to evaluate cation enrichments and depletions. Subsequently, different size fractions were analyzed to observe if leaching occurred at different rates due to the size of the erupted material. Commonly, when studying weathering sequences of intermediate composition volcanics, one would expect to see a major element depletion of calcium > sodium > magnesium, whereas aluminum, iron, and titanium are typically immobile. Mobile trace elements include strontium, barium, and rubidium, with high field strength elements such as zirconium, yttrium, and niobium generally considered immobile. In the Mazama tephras studied, major element depletion was slight (Ca and Na), with the most pronounced depletions in the first 20 cm. Depletions in trace elements (Sr and Rb) were more prominent, again with the majority of leaching in the upper portions of the deposit. At depths >50 cm, there does not appear to be any elemental migration with respect to zirconium. The effects of tephra deposit thickness and grain size on element mobility and weathering are still being evaluated.