Collaborative Research: North Sister Volcano: A Window into Deep Crustal Processes in a Mafic Arc

University Of Oregon Eugene, Eugene OR

Investigators

Abstract

Johnston EAR-0230373 Crustal contamination of melts formed in subduction zones is widely accepted for arcs formed on silicic continental crust, but the potentially profound and likely cryptic effects of contamination by mafic crust are not well understood. The intellectual merit of the proposed work is the assessment of processes acting in the deep crust above a subduction zone that affect the magmas rising from the mantle. North Sister volcano of the Cascades volcanic arc is selected for this study because it is the least evolved (mafic) and oldest member of an arc volcano suite represented in the Three Sisters volcanoes, and lies in an unusually mafic segment of arc where the primitive, mantle contributions are constrained. The study's objectives are (1) to identify the compositional variety and proportion of regionally distinctive mantle-derived basalts that contributed to the construction of the basaltic andesite at North Sister; (2) to evaluate how these mantle inputs mix and interact with the deep crust; and (3) to constrain the depth at which the magmatic processes took place. To achieve these goals we will combine compositional and isotopic analysis of rocks selected to represent the age and mineralogical spectrum of lavas sampled during the course of mapping North Sister Volcano. Microanalysis of minerals and their melt inclusions will supplement the bulk rock analyses to discern magmatic histories. Experimental work on natural samples will reveal the pressure, temperature and water conditions under which the North Sister magmas formed. The broader impact of this work includes training in field, analytical and experimental skills of two graduate students, one of which is female and from an underrepresented groups in science. Two undergraduates will also be given research opportunities. The results of the study will be disseminated at national and international meetings and as papers in refereed journals. Technique development for high-pressure experimental work and melt inclusion analysis by laser ablation will contribute to scientific infrastructure. The project is a carefully crafted partnership between Oregon State and University of Oregon taking advantage of complementary facilities and investigator expertise. The proposed work is also of particular societal benefit as it addresses the history of a volcano that is in a particularly active part of the Cascades arc, where recently identified uplift has focused attention on potential volcanic hazards.