Water contents of silicic to intermediate magmas

Abstract

Silicic to intermediate magmas have water contents that range from < 1 wt. % to around 7 wt. % depending on the character and depth of the magma source, type of melting reaction and the degree of differentiation. The condition of water saturation is rare, implying that crustal magma genesis generally takes place under fluid-absent conditions or that any fluid present must contain volatile species other than H 2O. The source and quantity of water present during partial melting profoundly affect the types of melting reactions, residues and magmas as well as subsequent magma evolution. For S-type granitic magmas, melting reactions involving muscovite breakdown in pelites yield low-temperature, “wet” melts that form veins, pockets and small plutons in regional metamorphic terranes. Biotite breakdown in pelites near the amphibolite-granulite facies transition yields hotter magmas with variable water contents. These are emplaced at medium to high levels. Under granulite facies conditions, biotite breakdown in quartzofeld-spathic rocks yields high-temperature, “dry” magmas emplaced as subvolcanic batholiths and volcanics.