Petrogenetic Processes Characterizing the Mount Bachelor, Oregon Magmatic System: Open- versus Closed-System Processes

Sara Elizabeth Johnson
August 2008

Abstract

Mount Bachelor volcanic chain (MBVC), located in central Oregon, is one of the larger basalt and basaltic-andesite edifices in central Oregon. Preliminary studies have defined how eruptions have changed composition with time, but a detailed assessment of magma chamber processes has not been conducted. To gain a more thorough understanding of the magmatic processes that have contributed to the observed compositional evolution, this study focuses on one of four eruptive episodes, specifically episode III, the most voluminous episode of the four. Magmatic processes are assessed by focusing on both whole-rock and single crystal data.

The geochemistry and textural diversity of twenty representative lavas were determined to elucidate the associated magmatic processes. Magmatic systems can be impacted by external influences, such as the incorporation of melt from the rock surrounding a magma chamber (assimilation), or the replenishment of magma from below (recharge). These processes result in open-system magma chambers and modify the original chemistry of magma. Closed-system processes such as crystal formation and separation can also significantly alter the magma chemistry.

The focus of this study was to determine which of these processes was most significant in the formation of the lavas erupted during episode III, utilizing both whole-rock and single crystal geochemical and isotopic analyses. Results suggest that crystal separation in a closed magmatic system generated the majority of the geochemical diversity observed. Radiogenic isotopic disequilibrium suggests, however, that heat released during crystallization resulted in melting of surrounding country rock, and thus the incorporation of that melt into the magma system. Evidence for magma recharge events is also observed in the isotopic analyses. Geochemical distinctions between units of episode III indicate the potential for several independent magmatic reservoirs to exist beneath the MBVC. Further work, specifically the incorporation of computer modeling, will allow documentation of the physical properties of the magma system and elucidate quantitative magma chamber models. This and additional work at the MBVC may lead to a better understanding of the magma systems located under the MBVC, as well as other volcanoes located in the central Oregon High Cascades region.

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