Answer
Partial melting occurs when a rock undergoes melting, but only a portion of the rock actually turns into a liquid magma. The remaining solid portion, known as the residue, does not melt and retains its original composition. The resulting magma has a different composition from the rock it was derived from due to several key factors:
1. Different Melting Points: Minerals within a rock have different melting points. During partial melting, the minerals with lower melting points will melt and become part of the liquid magma, while the minerals with higher melting points remain in the solid residue. This selective melting leads to a separation of components and a change in the overall composition of the magma.
2. Incompatible Elements: Some elements are preferentially partitioned into the melt during partial melting, while others remain in the solid residue. These elements are referred to as incompatible elements. They tend to have a higher affinity for the liquid phase and are more easily incorporated into the melt compared to the solid minerals. As a result, the magma becomes enriched in these incompatible elements, altering its composition compared to the original rock.
3. Diffusion and Mass Transfer: During partial melting, there can be diffusion and mass transfer of components between the solid and liquid phases. This can lead to the exchange of elements and chemical reactions between the melt and the solid residue, further modifying the composition of the melt.
4. Fractional Crystallization: If the generated magma remains in contact with the solid residue, fractional crystallization can occur. As the magma cools, minerals with higher melting points may crystallize and separate from the remaining melt, resulting in further differentiation of the magma's composition.
These factors collectively contribute to the formation of a magma with a composition distinct from the rock it originated from. The degree of partial melting, the specific mineral assemblage, and the conditions of melting all influence the resulting magma's composition.
Work Step by Step
Partial melting occurs when a rock undergoes melting, but only a portion of the rock actually turns into a liquid magma. The remaining solid portion, known as the residue, does not melt and retains its original composition. The resulting magma has a different composition from the rock it was derived from due to several key factors:
1. Different Melting Points: Minerals within a rock have different melting points. During partial melting, the minerals with lower melting points will melt and become part of the liquid magma, while the minerals with higher melting points remain in the solid residue. This selective melting leads to a separation of components and a change in the overall composition of the magma.
2. Incompatible Elements: Some elements are preferentially partitioned into the melt during partial melting, while others remain in the solid residue. These elements are referred to as incompatible elements. They tend to have a higher affinity for the liquid phase and are more easily incorporated into the melt compared to the solid minerals. As a result, the magma becomes enriched in these incompatible elements, altering its composition compared to the original rock.
3. Diffusion and Mass Transfer: During partial melting, there can be diffusion and mass transfer of components between the solid and liquid phases. This can lead to the exchange of elements and chemical reactions between the melt and the solid residue, further modifying the composition of the melt.
4. Fractional Crystallization: If the generated magma remains in contact with the solid residue, fractional crystallization can occur. As the magma cools, minerals with higher melting points may crystallize and separate from the remaining melt, resulting in further differentiation of the magma's composition.
These factors collectively contribute to the formation of a magma with a composition distinct from the rock it originated from. The degree of partial melting, the specific mineral assemblage, and the conditions of melting all influence the resulting magma's composition.
