INSIGHTS INTO MANTLE MELTING FROM GRAPHICAL ANALYSIS OF TWO-COMPONENT SYSTEMS

In previous work, we have shown how geological processes subject to alternative thermodynamic constraints — such as decompression melting at constant entropy, isenthalpic assimilation at constant pressure, and isochoric cooling inside a rigid inclusion host volume — can be visualized, quantified, and understood using a simplified, intuitive framework of one-component phase diagrams with suitably chosen axes matching the independent variables of the process.

Here we discuss extension of this approach to two-component systems that can capture the essential role of compositional differences between phases and among instances of a system in modulating the character and rate of mass and energy transfer during petrological processes. In many cases, all the essential insight needed to understand fully multi-component natural systems is present already in a suitable two-component model system. The outcome of an isenthalpic assimilation process in a binary model system, for example, is generally obvious from inspection of an enthalpy-composition diagram of the system. The challenge, however, is to maintain the visual clarity of a phase diagram representation when another dimension is added to the system. Frequently this is best accomplished through a series of snapshot diagrams or an animation that uses time as a third graphical dimension to illustrate the progress of a unidirectional process. For example, isentropic decompression melting is naturally visualized as an animated sequence of isobaric entropy vs. composition sections at progressively decreasing pressure. An isochoric cooling process, likewise, can be monitored with an animated sequence of isothermal volume vs. composition sections at progressively decreasing temperature. We will discuss constraints on melt productivity of isentropic decompression melting from homogeneous binary systems with solid solution, eutectic, and peritectic topologies. We will then consider the thermal coupling of chemically isolated subsystems representing lithologically heterogeneous sources ascending together on a common isentrope. Finally, we will demonstrate use of a practical on-line tool for selection and visualization of animated phase diagrams for use in teaching and research.