Magmatic water

Magmatic water or juvenile water is water that exists within, and in equilibrium with, a magma or water-rich volatile fluids that are derived from a magma. This magmatic water is released to the atmosphere during a volcanic eruption. Magmatic water may also be released as hydrothermal fluids during the late stages of magmatic crystallization or solidification within the Earth's crust. The crystallization of hydroxyl bearing amphibole and mica minerals acts to contain part of the magmatic water within a solidified igneous rock. Ultimate sources of this magmatic water includes water and hydrous minerals in rocks melted during subduction as well as primordial water brought up from the deep mantle.

Water has limited solubility in silicate melts ranging from essentially zero at surface pressure to up to as much as 10% at 1100 °C and 5 kbar of pressure for a granitic melt. Solubility is lower for more mafic magmas. As the temperature and pressure drop during emplacement and cooling of the magma a separate aqueous phase will exsolve. This aqueous phase will be enriched in other volatile and silicate incompatible species such as the metals: copper, lead, zinc, silver and gold; alkalis and alkaline earths and others, including: lithium, beryllium, boron, rubidium; and volatiles: fluorine, chlorine and carbon dioxide.

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