MSA PRESIDENTAL ADDRESS: MODELING METAMORPHISM: ENERGY, FLUIDS AND FEEDBACKS

Over the past 100 years, observations of the regular and repeated spatial distribution patterns of minerals elucidated fundamental aspects of metamorphism. These systematic occurrences were generally associated with a heat source that provided the energy for mineralogic transformation but could not exclusively explain their spatial distribution. Recognition that fluids may be influential in some terranes provided an additional explanation for the diversity of occurrences and textures observed in contact metamorphic systems. Thus, thermal-chemical feedbacks were recognized but were difficult to evaluate.

Recently, three-dimensional, time-dependent computational models of heat and mass transport allow exploration of thermal, chemical and mechanical processes during contact metamorphism. These models incorporate increasing complexity in the number of variables being simulated (e.g. position and time dependent permeability, heterogeneous host rocks) and in the physics being modeled (e.g. latent heat release, hydrofracture). Not only can the importance of individual processes be discerned but the feedback amongst disparate processes explored. For example, as energy is redistributed from a cooling magma into the surrounding host rocks, the rocks transform to be in equilibrium with the new thermal environment. If sufficient permeability exists, thermal energy causes fluid density gradients that drive fluid flow, fluid flow advects heat and mass, and energy transport by fluids is dominant. Advective transport significantly alters heating rates in the host rocks, the duration of time rocks remain at high temperatures, and, consequently, mineral nucleation and growth. Because of the control that these parameters exert on textural development, subtle features related to fluids may be recorded in the crystal size distribution and in the location of isograds. This feedback can be observed as variations in the spatial patterns of mineral distribution.

Innovations in computational experimentation now allow investigation of complex thermal-mechanical-chemical feedback in metamorphic systems. Such experiments can be exploited to explore new phenomena, mined for the richness of data, used to predict testable hypotheses, and visualized to "see" metamorphism in action.