Partial melting

Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the solid.

Partial melting occurs where the solidus and liquidus temperatures are different. For single minerals this can happen when they exhibit solid solution, for example in olivines between iron and magnesium. In rocks made up of several different minerals, some will melt at lower temperatures than others.

Partial melting is an important consideration in geology with respect to the chemical differentiation of crustal rocks. Virtually all rocks on Earth derive from material from the interior of the earth that has been partially melted. The main places where partial melting occurs are subduction zones, mid-ocean ridges and hotspots. In all these places partial melting is often associated with volcanism, although some melts do not make it to the surface. Partial melts are thought to play an important role in enriching old parts of the continental lithosphere in incompatible elements. Partial melts produced at depth move upwards due to the compaction of the surrounding matrix.

Gallery

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  Melting of a substance exhibiting solid solution, with a bulk composition of C_B. As the temperature rises, the solid follows the blue path, and begins melting at temperature T_A. The initial liquid produced has the composition C_L, and its composition follows the red path. At temperature T_B the whole solid has melted.

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  Diagram showing the physical processes inside the Earth that lead to the generation of magma. A to D are different plate tectonic settings.

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  Subduction in the Aleutian Islands by Robert R. Coats (1962).