INTERPRETING CRYSTAL ZONATION PATTERNS AT MOUNT ST. HELENS

Mount St. Helens, Washington (MSH) is an unusual volcano in that it lies behind the primary volcanic arc of the Cascades and because it has erupted adakites in the past. The duration and origins the of adakitic magmatism are also poorly constrained. This study is aimed at investigating the magma chamber evolution by interpreting crystal zonation patterns from phenocrysts collected from different eruptive stages (Ape Canyon, Cougar, Swift Creek, and Spirit Lake). Fluctuations between temperature, pressure, volatile content, and elemental composition determine the chemical and physical makeup of resulting crystals. Thus, chemical zonation can provide an archive of the changes that occurred within the magma chamber prior to eruption. In particular, this study is interested in looking for evidence of magma mixing due to the recent suggestions that adakites and high-Mg andesites may be the result of such processes. Petrographic examination of the plagioclase crystals for different eruptive units is varied and complex. Some units display complex zonation patterns with clear evidence of dissolution events, but others display more continuous oscillatory zoning. Certain eruptive units also display optically homogenous crystals. Electron microprobe geochemical results mirror the petrographic observations.