Nadeau Presents History of Augustine Volcano’s Eruptions at AGU

Dr. Patricia Nadeau

Dr. Patricia Nadeau, Visiting Assistant Professor of Geological Sciences, presented her ongoing research at the American Geophysical Union Fall Meeting in New Orleans.

Nadeau also is expanding this work with two senior geology majors who will travel to New York over the semester break to collect their own data on Augustine’s eruptions.

The title of Nadeau’s AGU talk was “The evolution of Augustine Volcano’s magmatic system: Tales of mixing from an early dacite.”

Abstract: Though it has erupted exclusively intermediate-composition products in the Holocene, Augustine Volcano’s early history is marked by near-coeval Pleistocene eruptions of basalt and rhyolite, with no contemporaneous intermediate compositions recorded in the depositional record. Eruptions more recent than ~2 ka have been well-documented, as have the basaltic and rhyolitic deposits, but older Holocene material has not been studied in detail. Key to understanding the progression of Augustine’s eruptive behavior from bimodal to intermediate compositions is detailed investigation of such intervening units.

Here we investigate the phenocrysts and melt inclusions of the oldest recognized unit to have erupted following the rhyolite and basalt, a high-phosphorus dacite. The phenocrysts of the unit indicate a complex history of mixing, mingling, and heating. Plagioclase phenocrysts can be divided into two distinct Na-rich and Ca-rich populations, with further subdivisions based on patterns of rims and zoning. Quartz phenocrysts display intricate patterns of zonation, many of which have been truncated by resorption and subsequent further growth. Some quartz grains also exhibit growth halos of clinopyroxene needles. Amphiboles and orthopyroxene also show complex relationships, with Ca-rich amphiboles presenting as sub- to euhedral growth rims on orthopyroxene and cummingtonite cores. Each of these traits invokes some degree of disequilibrium likely brought on by mingling and mixing of magmas of different compositions, temperatures, and crystal assemblages. Melt inclusions in both quartz and plagioclase also provide evidence of at least one heating event following the onset of crystallization, as compositions are indicative of post-entrapment melting of host crystals.

Using distinct groupings of plagioclase, based on zoning patterns, and other core/rim mineralogical combinations, we aim to construct a timeline of separate stages of mixing. Diffusion chronometry of quartz zonation may yield additional information regarding the timing of some of the mixing events.

This work is part of a larger effort to understand the evolution of Augustine from basalt and rhyolite in its early stages to smaller, intermediate eruptions today, including the potential for future large rhyolitic eruptions.