Isentropic Melting Processes in the Mantle

Batch melting of ascending mantle can be approximated as an isentropic process, since on the time scale of melting heat flow into or out of source regions will typically be negligible and the process is slow enough to be close to reversible. Similarly, fractional fusion can be idealized as a series of incremental isentropic melting steps, although the entropy of the residue decreases in each step. Although actual melting processes (e.g., involving melt migration, diffusion, and convective boundary layers) must deviate to some extent from idealized isentropic conditions, modeling of mantle processes under the assumption of constant entropy is tractable from a thermodynamic perspective and leads to a number of insights. Here we present models of the productivity of isentropic pressure-release melting, consider the effect of solid-solid phase transitions on melting, and model deep crystal fractionation in ascending melts of the mantle.