Magmatic water, also known as juvenile water, is an aqueous phase in equilibrium with minerals that have been dissolved by magma deep within the Earth’s crust, and is released to the atmosphere during a volcanic eruption. It plays a key role in assessing the crystallization of igneous rocks, particularly silicates, as well as the rheology and evolution of magmatic chambers. Magma is composed of minerals, rocks and volatile organic compounds (VOCs) in varying relative abundances.
Magmatic differentiation varies significantly based on various factors, most notably the presence of water. An abundance of VOCs within magma chambers decreases viscosity and leads to the formation of minerals bearing halogens, including Cl−, and OH− groups.
In addition, the relative abundance of VOCs varies within basaltic, andesitic, and rhyolitic magma chambers, leading to some volcanoes being exceedingly more explosive than others. Magmatic water is practically insoluble in silicate melts, but has demonstrated the highest solubility within rhyolitic melts. An abundance of magmatic water has been shown to lead to high-grade deformation, altering the amount of δ18O and δ2H within host rocks.
Precipitation of minerals is affected by water solubility within silicate melts, which typically exists as hydroxyl groups bound to Si4+ or Group 1 and Group 2 cations in concentrations ranging from approximately 6-7 wt. %., Specifically, the equilibrium of H2O and dissolved O2 yields hydroxides, where the Keq has been approximated between 0.1 and 0.3.
This inherent solubility is low, yet varies greatly dependent on the pressure of the system. Rhyolitic magmas have the highest solubility, ranging from approximately 0% at the surface to nearly 10% at 1100 °C and 5 kbar. Degassing occurs when hydrous magma is uplifted, gradually converting the dissolved water to aqueous phase. This aqueous phase is typically abundant in VOCs, metals (copper, lead, zinc, silver and gold), and Group 1 and Group 2 cations. Dependent on which cation the hydroxyl is bound to, it significantly impacts the properties of a volcanic eruption, particularly its explosiveness. During unusually high temperature and pressure conditions exceeding 374 °C and 218 bar, water enters a supercritical fluid state and becomes no longer a liquid or a gas.Magmatic water – Wikipedia