Petrologic And Experimental Constraints On Magma Mingling And Ascent: Examples From Japan And Alaska

Abstract

This study investigates how magma is physically, chemically, and mineralogically modified in two regions in the shallow crust: the magma chamber and the conduit through which magma ascends from the chamber to the surface during eruption. Chapter 1 includes findings from 2 studies on how contrasting magmas interact in the chamber during basaltic replenishment events, which is performed by documenting different characteristics of mafic-to-intermediate enclaves and coexisting silicic host lavas from Unzen volcano, Japan. Two types of magmatic enclaves, referred to as Equigranular and Porphyritic, are easily distinguished texturally. Equigranular enclaves are andesitic, nonporphyritic, and consist of tabular shaped, medium grained microphenocrysts in a matrix glass in equilibrium with host dacite magma. Equigranular enclaves are a product of a more prolonged mixing and gradual crystallization at a slower cooling rate within the interior of mafic intrusions. Porphyritic enclaves display a wide range in composition from basalt to andesite and are porphyritic, where large resorbed plagioclase phenocrysts exist in a matrix of acicular crystals and glass. Porphyritic enclaves are produced when intruding basaltic magma engulfs melt and phenocrysts of resident silicic magma at their mutual interface. In response to the sudden compositional and temperature change in the surrounding melt, engulfed plagioclases develop resorption zones at their edges that are composed of a micron-sized network of glass inclusions and calcic plagioclase that is identical in composition to plagioclase microphenocrysts inherent to the enclave-forming magma. Over time, engulfed plagioclases are recycled back to the host as enclaves disaggregate. Chapter 2 describes results of a study that experimentally constrains the rate and manner in which reaction rims form on hornblende during decompression (magma ascent) using samples from Redoubt volcano, Alaska. Findings from this study show that the development of reaction rims depends on a balance between the rate of hornblende dissolution, which supplies the necessary components for reaction rims to the crystal-melt boundary, and the rate that dissolved components are transported away from the destabilized crystal to crystal faces of other pre-existing minerals. This reaction is strongly influenced by melt viscosity, which varies with changing water content, temperature, and pressure.