Describe how partial melting of the mantle rock peridotite can generate a basaltic (mafic) magma.
Recall that igneous rocks are composed of a mixture of minerals and, therefore, tend to melt over a temperature range of at least 200°C (nearly 400°F). As rock begins to melt, the minerals with the lowest melting temperatures are the first to melt.
If melting continues, minerals with higher melting points begin to melt, and the composition of the melt steadily approaches the overall composition of the rock from which it was derived. Most often, however, melting is not complete. The incomplete melting of rocks is known as partial melting, a process that produces most magma.
Recall from Bowen’s reaction series that rocks with a granitic composition are composed of minerals with the lowest melting (crystallization) temperatures—namely, quartz and potassium feldspar.
Also note that as we move up Bowen’s reaction series, the minerals have progressively higher melting temperatures and that olivine, which is found at the top, has the highest melting point. When a rock undergoes partial melting, it will form a melt that is enriched in ions from minerals with the lowest melting temperatures, while the unmelted portion is composed of minerals with higher melting temperatures (Figure 1).
Separation of these two fractions yields a melt with a chemical composition that is richer in silica and nearer the felsic (granitic) end of the spectrum than the rock from which it formed. In general, partial melting of ultramafic rocks tends to yield mafic (basaltic) magmas, partial melting of mafic rocks generally yields intermediate (andesitic) magmas, and partial melting of intermediate rocks can generate felsic (granitic) magmas.
Formation of Basaltic Magma
Most magma that erupts at Earth’s surface is basaltic in composition and falls in a temperature range of 1000° to 1250°C. Experiments show that under the high-pressure conditions calculated for the upper mantle, partial melting of the ultramafic rock peridotite can generate a magma of basaltic composition. Further evidence that many basaltic magmas have a mantle source are the inclusions of the rock peridotite, which basaltic magmas often carry up to Earth’s surface from the mantle.
Basaltic (mafic) magmas that originate from partial melting of mantle rocks are called primary or primitive magmas because they have not yet evolved. Recall that partial melting that produces mantle-derived magmas may be triggered by a reduction in confining pressure during the process of decompression melting. This can occur, for example, where hot mantle rock ascends as part of slow-moving convective flow at mid-ocean ridges. Basaltic magmas are also generated at subduction zones, where water driven from the descending slab of oceanic crust promotes partial melting of the mantle rocks that lie above.
Formation of Andesitic and Granitic Magmas
If partial melting of mantle rocks generates most basaltic magmas, what is the source of the magma that crystallizes to form andesitic (intermediate) and granitic (felsic) rocks? Recall that silica-rich magmas erupt mainly along the continental margins. This is strong evidence that continental crust, which is thicker and has a lower density than oceanic crust, must play a role in generating these more highly evolved magmas.
One way andesitic magma can form is when a rising mantle-derived basaltic magma undergoes magmatic differentiation as it slowly makes its way through the continental crust. Recall from our discussion of Bowen’s reaction series that as basaltic magma solidifies, the silica-poor ferromagnesian minerals crystallize first.
If these iron-rich components are separated from the liquid by crystal settling, the remaining melt will have an andesitic composition.
Andesitic magmas can also form when rising basaltic magmas assimilate crustal rocks that tend to be silica rich.
Partial melting of basaltic rocks is yet another way in which at least some andesitic magmas are thought to be produced.
Although granitic magmas can be formed through magmatic differentiation of andesitic magmas, most granitic magmas probably form when hot basaltic magma ponds (becomes trapped because of its greater density) below continental crust (Figure 2).
When the heat from the hot basaltic magma partially melts the overlying crustal rocks, which are silica rich and have a much lower melting temperature, the result can be the production of large quantities of granitic magmas. This process is thought to have been responsible for the volcanic activity in and around Yellowstone National Park in the distant past.