Insights into mantle melting from graphical analysis of one-component systems

Decompression melting can be approximated as an isentropic (that is, reversible adiabatic) process. In such a process, specific entropy (S) and pressure (P) are the independent variables and equilibrium is achieved when the specific enthalpy (H) of the system reaches a minimum. We present a largely graphical analysis of decompression melting in one-component systems based on phase equilibria in H-P-S space. Although mantle sources contain more than one component, use of one-component model systems provides insights into several aspects of mantle melting that can be generalized to more complete and complex systems (for example, batch vs. fractional fusion; the influence of pressure-dependent solid-solid reactions on melting; melting of multilithologic mixtures such as peridotite plus eclogite; advection of heat and melting by rising magmas) and places these insights into the visualizable framework of simple phase diagrams