EVOLUTION OF THERMAL, MECHANICAL AND CHEMICAL PROCESSES DURING CONTACT METAMORPHISM

Magmas emplaced into the crust initiate transient thermal, mechanical and chemical processes. These processes evolve at different rates, interact with one another and determine the metamorphic effects observed over scales of single minerals to kilometers of rock. The nature of this mutual evolution can be considered as a system of coupled feedback relations. Because the output of one process effects the evolution of another, understanding the behavior of these feedbacks provides a realistic assessment of hydrothermal and metamorphic activity. For example, thermal energy dissipation increases temperatures in the host rocks and causes fluid density gradients that drive fluid flow; fluid flow advects mass and perturbs the local chemical system to larger affinities causing irreversible chemical reactions and consequent new mineral assemblages. These factors vary dynamically throughout a metamorphic cycle because the magnitude and timing are dependent on the interplay of advective and conductive heat transport. This, in turn, dictates the fundamental controls on mineral nucleation and growth and the scale of equilibration, through the rate at which the rocks traverse a reaction boundary, the heating rates (dT/dt) and the residence time that rocks remain at elevated temperatures.

A series of time-dependent 3D computational experiments of heat and mass transport explore these fundamental controls by altering a series of key parameters related to the host rock and the intrusion. Results indicate that the convective pulse heats rocks more slowly and maintains higher temperatures for longer times compared to heat transport by conduction. However,the initial rate of isotherm advance is similar because both flow regimes have an early diffusive pulse. These P-T-X-t events are recorded in the crystal size distribution of metamorphic minerals. Numerical approximation of these coupled dynamic processes remains a challenge but has the potential to provide new insights into the dynamic nature of complex metamorphic and hydrothermal activity.