The genesis of arc dacites: the case of Mount St. Helens, WA

  • Maren Wanke
  • Ozge Karakas
  • Olivier Bachmann
Original Paper


Throughout the last 35 ka, Mount St. Helens has been the most active volcano in the Cascade arc, but the origin of its voluminous dacites remains controversial. These dacites were traditionally interpreted as a result of melting metabasaltic lower crust. Yet, recent studies have challenged this view and suggested an origin dominated by differentiation of mafic magmas through assimilation-fractional crystallization (AFC) processes. To address this discrepancy on the origin of dacites at Mount St. Helens, we conduct an interdisciplinary study using a combination of thermal and geochemical modeling. Our results show that ~ 45% crystallization of a basaltic andesite parent reproduces the compositions of the dacites with a maximum of ~ 20–30% assimilation of lower crustal lithologies. Amphibole textures and compositions support such a differentiation trend in a polybaric mush system. Combined with recent geophysical imaging and experimental data, we suggest that Mount St. Helens dacites are generated by (1) mantle-derived arc magma evolving by AFC to intermediate compositions in a lower crustal magma reservoir and (2) ascent of these magmas to a mid to upper crustal reservoir, where they reach high crystallinity without significant further chemical differentiation, and are subject to frequent recharge that leave a clear mixing/mingling overprint.


Mount St. Helens Dacites Thermal model Lower crustal mush Amphibole AFC processes 



We thank John Pallister and Michael A. Clynne for their help in the field and a much-appreciated introduction to the Mount St. Helens volcanic system. Thoughtful comments by Dawnika Blatter, Michael A. Clynne, Bill Leeman, and an anonymous reviewer on an earlier version of this manuscript are gratefully acknowledged. We thank Peter Appel, Barbara Mader, and Marcel Guillong for their assistance during microprobe and laser analyses. We are grateful to Josef Dufek for support on the thermal modeling and to Peter Ulmer for many discussions on the topic of differentiation in polybaric plumbing system. This project has been supported by Swiss National Science Foundation Grants 200021_146268 and 200020_165501.

Supplementary material

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Supplementary material 2 (PDF 240 KB)


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© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Department of Earth Sciences, Institute of Geochemistry and PetrologyETH ZürichZurichSwitzerland

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