Clinopyroxene As a Quantitative Window Into Pre-Eruptive Magma Storage Conditions

The pre-eruptive pressure (depth) of magma storage beneath a volcano modulates the eruptive behaviour observed at the surface. Pressure controls the physical properties of magma such as the crystallinity via pressure-sensitive phase equilibria as well as the concentration of dissolved volatiles within the melt. Such parameters drastically affect how violently the volcano will erupt, and by extension the resultant societal and economic impact. However, it is increasingly apparent that one of the main ways that we recover pre-eruptive pressure from magmas, namely the link between the major element chemistry of the magmatic mineral clinopyroxene and pressure, is subject to large uncertainty. CANVAS will improve our understanding of the coupling between the chemical-structural variability of clinopyroxene and the crystallisation pressure. We will run a series of equilibrium crystallisation experiments on an Icelandic basalt starting material at atmospheric (1 atmosphere) and crustal (200 800 megapascals) pressures, employing thermal methods to grow large (>30 m) crystals. Experimental clinopyroxenes will be measured for major elements using electron probe microanalyser. The same crystals will be subject to synchrotron X-ray microdiffraction to establish crystal structure and electron density, refining the true (observed) distribution of cations within the crystal lattice. The true composition of natural clinopyroxene has been previously shown to differ from assumed chemistry. In doing so, we will perform the first large, systematic study of clinopyroxene at crustal pressures by synchrotron to our knowledge. This approach will allow us to robustly isolate experimental pressure effects from temperature and melt composition, refining the observable chemical and physical response of clinopyroxene. Our findings will significantly improve the robustness of pressure estimates from erupted magmas, a key data source to benchmark geophysical signals in active volcanoes.