Crustal melting and suprasolidus phase equilibria: From first principles to the state-of-the-art

Abstract

Partial melting is the fundamental process by which juvenile crust was produced from the mantle and subsequently reworked to become the stable, compositionally-differentiated continents on which we live and which host most of the elemental resources required by our modern technological society. Irreversible differentiation of the continental crust occurs principally through the production, segregation and migration of silica-rich (felsic) melts from deeper source rocks to shallower sinks where they erupt or, more commonly, crystallise as granite sensu lato. Here we provide for both novices and professionals a comprehensive but accessible account of the processes involved in crustal melting and suprasolidus phase equilibria from first principles to the forefront of modern research. To begin, we introduce the tectono-metamorphic context for crustal melting before considering the evidence at outcrop and in thin section for partial melting, then briefly review melt extraction from crustal rocks. As a prerequisite to understanding the physicochemical basis for crustal melting, we summarize the essential thermodynamics that underpin calculated phase equilibria, distinguish different types of melting reaction, and review the requirements for, methodology behind, and limitations of a phase equilibrium modelling approach based on equilibrium thermodynamics. We explain the various types of phase diagram used to investigate partial melting and assess open versus closed system processes, including internal and external buffering of H2O. Those crustal sources that partially melt to produce granite are considered in detail, namely basic rocks such as basalts and gabbros, clastic sedimentary rocks such as greywackes, siltstones and mudstones (pelites), and granites themselves. We concentrate mainly on intracrustal partial melting in convergent-margin settings, and on anatexis during exhumation of deeply-subducted continental crust from mantle depths. We discuss the behaviour of trace elements and accessory minerals during melting, and consider the implications for isotope geochemistry. To close we include a brief summary of some of the important points and offer a few suggestions for future lines of research on crustal melting.