Philipp Ruprecht - Magma Chamber Dynamics

MFIX

RESEARCH

...Magma Mixing Recorded in Phenocryst Phases at Volcán Quizapu, Chile

...Phenocryst Zonation Patterns at El Misti Volcano, Peru, and the Andahua Monogenetic Cones, Peru

Magma Mixing Recorded in Phenocryst Phases at Volcán Quizapu, Chile

In my current Ph.D. project I am integrating several tools usually used independently to characterize the major physical and chemcial processes that modify evolved magmas in an continental arc setting. The physical and chemical evolution of a magmatic system is typically an interplay between convection vs. conduction, temperature-driven vs. compositionally-driven vs.volatile-driven processes. Key results to understand the dominant role of a subset of these processes is their given length- and time scale.

While the melt can hardly retain transient long term information of a magmatic system, the phenocryst phases act as natural recorders for the ambient physical and chemical conditions in their vicinity. Time and length scale information can be extracted from these phenocrysts using detailed geochemical and textural characterization. The different methods basically record on different "frequencies" and are therefore sensitive to a large set of magmatic processes that can range widely in terms of their length and time scale. The question now is: "How can we decompose this geochemical and textural signal into its fundamental modes?"

One way is to compare the data set with measured length and time scales from lab experiments. Another approach, the one I am using, is to develop numerical models for the "pure" processes that can be used to create and follow single phenocrysts and entire crystal populations during their space-time path. This way I am also able to extract valuable textural and geochemical information that can be compared to natural observations.

The approach described above is applied to the young volcanic system of Volcan Quizapu in the Central Chilean Andes. It displays a unique possibility to combine these geochemical and numerical approaches. Due to its simplicity the Volcan Quiapu system allows me to treat it as a natural laboratory. The picture (looking east) above shows the two main stratovolcanoes (Volcan Descabezado Grande, left, and Cerro Azul, right) located at ~35°S. Volcan Quizapu is a satallite vent on the northern flank of Cerro Azul (just below the low clouds in the picture between the two volcanoes) that erupted in 1846 AD five cubic kilometers of mainly dacitic lava. The flow field can be seen in the middleground of the picture. In 1932 AD a second erpution, this time explosive, covered the area - mainly to the east - with grey dacitic pumice. The volume of this eruption was comparable in size with about four cubic kilometers. Andesitic enclaves and mingled pumices are present in both eruptions indicating magma mixing prior to the eruption. Several key question can be investigated in this volcanic system:

1. Assuming the dacite magmas of the two eruptions were stored as a single batch in the shallow crust, how can the magmatic system be recharged with volatiles to change so dramatically in eruptive style within 86 years?

2. What is the fate of phenocryst phases (What do they record?) of the andesitic recharge magma that get incorporated into the dacitic host magma during the 1846 AD eruption, but remain in the magmatic system and get erupted 86 years later?

3. How do the different crystal populations (i.e. andesitic plagioclase vs. dacitic plagioclase) differ in terms of crystal residence times and what does that tell us about the plumbing system at Volcan Quizapu?

4. How do these crystal populations mix during the recharge? Is the mixing dominated by liquid-liquid mixing or does the andesitic enclaves get chilled and eroded subsequently in a sub-solidus fashion?

5. How does volatile exsolution from the andesitic inclusions affect the mingling and mixing of the two magmas?