The Origin of Mount St. Helens Andesites

Mount St. Helens volcano has intermittently produced mainly dacitic products but occasionally erupted a more diverse suite of lavas including basalts and andesites. Petrogenetic relations between these magmas provide insight into the dynamics of the subjacent magma system. Mineralogical and geochemical features of representative lavas erupted during the past 2200 years can distinguish three basaltic and three andesitic variants. The mafic lavas include: (a) transitional, olivine + plagioclase basalts with low K2O and incompatible trace-element abundances: (b) calc-alkaline, olivine + plagioclase ± clinopyroxene basalts enriched in K 2O, TiO2, and incompatible trace elements: and (c) calc-alkaline, olivine + plagioclase basaltic andesites with incompatible trace-element contents transitional between the two basalt types. Intermediate lavas include (a) tholeiitic, two-pyroxene andesites, (b) calc-alkaline, plagioclase + two-pyroxenes ± olivine ± amphibole mafic andesites (56-59% wt.% SiO2), and (c) calc-alkaline, plagioclase + two-pyroxenes + amphibole high-silica andesites (61-62 wt.% SiO2). Eruption of these magmatic variants within the same eruptive phase implies the existence of different petrogenetic lineages, and that the plumbing system is sufficiently complex to simultaneously isolate and preserve numerous magma batches. It is unlikely that any of the andesites (or dacites) are derived by fractional crystallization of the recognized basaltic variants. Formation of the andesites simply by contamination (or assimilation-fractional crystallization) of basaltic magma is also improbable. More plausibly, the andesites represent mixing between basaltic and dacitic end-member magmas, each of which may be somewhat heterogeneous or vary in composition with time. In this model, efficient mixing must occur in some parts of the magma plumbing system, while some conduits or storage reservoirs must be effectively isolated