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 in silicate melts
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 exolve. 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.
Water in silicate melts at the high temperature and pressure conditions within the crust exists as a supercritical fluid rather than in a gaseous state (the critical point for water is at 374 °C and 218 bar).
Stable isotope data
Stable isotope studies of oxygen and hydrogen in igneous rocks indicate that the oxygen-18, δ18O, content is approximately 6 - 8 ‰ higher than standard mean ocean water (SMOW) while the deuterium, δ2H, content is 40 to 80 ‰ lower than SMOW. Water in equilibrium with igneous melts should bear the same isotopic signature for oxygen-18 and deuterium. Isotope data on hydrothermal solutions spatially associated with igneous intrusions should reflect this isotopic signature. However, isotopic studies of hydrothermal waters indicate that most bear the isotopic signature of meteoric water. Any magmatic waters in these hydrothermal solutions must have been swamped by the circulating meteoric groundwaters of the environment.
Fluid inclusions are microscopic bubbles of aqueous solutions which were trapped within crystals during crystallization and are considered as relic samples of the mineralizing waters. Analyses of the isotopic content of these trapped bubbles show a wide range of δ18O and δ2H content. All examined show an enrichment in 18O and depletion in 2H relative to SMOW and meteoric waters. Fluid inclusion data from a number of ore deposits plot directly on the magmatic water region of an δ18O vs δ2H plot.