Partial melting produces liquid while some of the original rock remains solid. Its minerals and chemical components do not all enter the liquid in equal proportions. The extracted geological melt can therefore have a very different composition from its source, leaving a chemically modified residue.
Heating, adiabatic decompression melting or addition of a component such as water by flux melting can cross the relevant solidus. Melt fraction and extraction history influence both the resulting magma and the remaining mantle. Observing one erupted composition consequently does not uniquely determine the source rock or its temperature.
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