Effects of H 2 O and CO 2 on magma generation in the crust and mantle

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Abstract
The solution of H 2 O, CO 2 , and H 2 O–CO 2 mixtures in silicate magmas has many effects. Fusion temperatures are lowered, and the fusion and crystallization sequences may be modified. Hydrous minerals and carbonates may become stable within the fusion interval. Liquid compositions are changed as a function of dissolved volatile content, and as a function of H 2 O/CO 2 . The physical properties of magmas may be influenced markedly by dissolved volatile components. The presence of vapour bubbles arising from exsolution of volatile components during uprise or crystallization of magma may be responsible for physical and geochemical effects. These effects are outlined by examination of the phase relationships in several synthetic systems and rock–H 2 O–CO 2 systems. The results demonstrate that H 2 O is more influential than CO 2 in magma generation at crustal pressures. The presence of CO 2 , however, may influence the evolutionary history of a crustal magma body. In the mantle at depths of 75–80 km, the influence of CO 2 becomes as significant as that of H 2 O because CO 2 reacts with peridotite stabilizing calcic dolomite, and the solubility of CO 2 increases dramatically. The compositions of near-solidus mantle magmas are strongly influenced by H 2 O/CO 2 and by the distribution of carbonate, amphibole and phlogopite in the peridotite. The normal product of crustal anatexis is H 2 O-under-saturated granitic liquid. Tonalite liquids can-not be produced by crustal anatexis during normal regional metamorphism.