Geochemistry Of Igneous Processes


The magmatic processes involve, magma's generation and its evolution. There are several types of magma as silicate magma, carbonate magma, sulfide magma etc. It does contain several gases, liquids and also solids. It also contains almost all the elements in the periodic table. Oxygen is the major element present in them. The next abundant element is silica. Based on the abundance of silicon dioxide, they can be classified into several categories as acidic, intermediate, basic, and ultra basic. The acidic types do contain more than 65% of silicon dioxide. Where as the ultra basic one does posses about 45% of the above mentioned compound. The magmas generally originate at the lower crust and upper mantle.

They can also be classified based on the relationship among silicon dioxide, sodium oxide and potassium oxide. The volatile elements include water, carbon dioxide, etc. The density of magmas generally range from 2 to 3. Viscosity also does show wide variations depending on several factors such as composition, temperature, etc.

The magmas generated in the mantle are of basaltic type. The research has revealed that the mineral pyrolite can generate it in the mantle region due to the presence of carbon dioxide and water. Similarly, although the temperature is very low to generate it in the crustal region, the extrusions can generate magma in this region.

The magmas can evolve in several ways such as fractional crystallisation, mixing and assimilation.

The fractional crystallization does involve the separation of the early formed or crystallised minerals from the magma due to density variations between the solid and the residual liquid. For instance, as olivine gets separated first from the primary magma, the residual magma will be containing the lesser amounts of olivine unless there are some additions of olivine by any other processes.

As the name indicates, mixing does involve the mixing of several types of magma in the magma chamber. For instance, the mixing of basaltic and rhyolitic magma would lead to the evolution of andesitic, dacitic, etc types of magma.

Assimilation involves the chemical reactions between the erupting magma and the surrounding wall rocks. The magma which is reacting is known as the synttic magma. Since assimilation does involve the transfer of heat, this process would also lead to fractional crystallisation.

Based on the relationship between silicon dioxide, potassium oxide and sodium oxide, the magmas can be classified as alkaline, transitional, and sub alkaline types of magma. The alkaline types are rich in sodium and potassium. The sub alkaline types are generally rich in calcium.

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Tirumala Prasad