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Maximizing Microlites: Quantifying decompression-induced pyroxene crystallization

Jessica Bersson, Benjamin Andrews

Abstract

Microlites---tiny crystals that form during rapid decompression and/or cooling in magma---provide valuable records of magma ascent. Plagioclase crystals are frequently used to reconstruct decompression histories, but pyroxene microlites remain comparatively under-examined. This limits our understanding of their crystallization dynamics and potential as complementary "speedometers." This study focuses on pyroxene microlites, aiming to quantify their nucleation and growth rates in intermediate magmas during decompression, which are largely unknown. We investigate pyroxene microlites in natural eruptive products from various volcanic systems and through decompression experiments. Natural sample analyses, including 2D and 3D techniques, reveal microlite textures and growth patterns. Isothermal decompression experiments conducted using a cold-seal pressure vessel quantify pyroxene growth and nucleation rates as a function of decompression rate, supersaturation, and melt viscosity. Beyond interpreting ascent histories, understanding pyroxene microlite crystallization has significant implications for magma rheology and eruption dynamics. Syn-eruptive crystallization increases magma viscosity, directly influencing its mobility and eruptive style. By focusing on pyroxene microlites, this study provides a more nuanced understanding of magma ascent histories and the processes shaping volcanic eruptions.