Role of H2O and CO2 in Silicate melt

H₂O and CO₂ are the two most important components of fluids in the upper mantle and crust, and their solubilities in silicate melts control many processes like generation, migration, saturation and crystallization, exsolution of bubbles, polymerization and combinedly the volcanic eruption.

• The solubility of H₂O and CO₂ in silicate melt is a complicated function of pressure, temperature and composition.

Water (H₂O)

The hydrogen of the H₂O molecule reacts with a bridging oxygen of the silicate network to form a hydroxyl (OH) molecule which is hydrogen bonded to a bridging oxygen. This reaction of the H₂O molecule causes depolymerization. Adding water to a silicate melt will, therefore decrease its viscosity. The viscosity of polymerized melts, such as acidic melts, decreases more significantly with water content than less polymerized (basic) melts.

Polymerized magmas (acidic) can dissolve large amount of water since they have more bridging oxygens. The solubility of water in magmas, therefore decreases with SiO₂ content due to the decrease in polymerization.

• acid magmas can contain more dissolved water than basic magmas.
• the highest water solubility is evidently characteristic of alkaline magmas.

• With decreasing pressure the solubility of water in a magma decreases because of the decrease in polymerization.
• As acidic magma rises through conduit the pressure decreases until the water can no longer be dissolved in the magma and exsolves as water vapor. The large expansion of vapor causes the explosive nature of eruption.

Carbon-di-oxide (CO₂)

Carbon-di-oxide (CO₂) dissolves in a magma by reacting with a non-bridging oxygen of the silicate network to form a carbonate molecule (CO₃^2-). The removal of the non-bridging oxygen causes polymerization of the network as shown and thus increases in viscosity. Because CO₂ solubility is dependent on non-bridging oxygens it is higher in depolymerization magmas.

The CO₂ content of basic magmas is, therefore, higher than acidic magmas.

• The CO₂ solubility in melts of basaltic composition is slightly higher than in granite melt whereae CO₂ solubility appears to increase with increaseing melt alkalinity.

A complication is in the presence of Al⁺³ as a network former (tetrahedra) since the carbonate ion formed by the reaction of CO₂ forms a complex with a divalent cation which can no longer balance the charge of the Al in the networl.

• In the case of Al-rich magmas CO₂ can cause depolymerization.