The origin of Mount St. Helens andesites

Abstract

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.

Investigation of long‐term geochemical variations and magmatic processes at Mount St. Helens

“…). This is in partial contrast to the model of Smith & Leeman () who argued that intermediate composition rocks at MSH were the result of mixing and fractional crystallization.…”

“…These melts are most likely generated from underplated meta‐gabbros at the base of the continental crust, as shallowly emplaced basaltic units would not be at a sufficient metamorphic grade to produce the typical adakitic geochemical characteristics. One additional aspect of our model that differs from that of Smith & Leeman (, ) is that the present model does not require mixing with a basaltic component to produce the andesites at MSH. If mixing is taking place, it is likely to be between a metamorphosed basalt/gabbroic melt and a slab MORB‐melt, as indicated by some of the geochemical data lying between those two end‐members (Fig.…”

“…Pallister et al . ; Smith & Leeman ). The compositions of MI reported here provide no evidence to suggest mixing of basaltic magmas during evolution of the MSH system.…”

“…Additionally, many of the studies on the Spirit Lake stages aside from the most recent eruptions have focused on bulk rock techniques (e.g. Smith & Leeman 1993).…”

Geochemistry, K–Ar geochronology and Sr–Nd–Pb isotope compositions of pitchstone in Gohado, southwestern Okcheon Belt, South Korea

“…On the basis of the available geochemical and Sr–Nd–Pb isotope data we interpret here that the pitchstone source magma is entirely of crustal derivation. The partial melting of crustal components is quite common in subduction‐related magmatic arcs (Smith & Leeman 1993; Morris & Creaser 1998). The Nd model ages indicate an Early Mesoproterozoic crustal residence age.…”