Inclusion Barometry and Overstepping Isograd Reactions: How Close is the Approach to Equilibrium During Metamorphism?

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The aim of the research outlined in this proposal is to examine and quantify the extent to which metamorphic rocks approach equilibrium during transformation. It is an accepted fact of all metamorphic transformations that some degree of overstepping of a reaction boundary is required in order for reactions to proceed in a forward direction. That is, rocks that are perfectly equilibrated will experience no driving force for recrystallization and will thus remain unchanged. Hence the mere existence of different degrees of metamorphic recrystallization under different conditions of pressure and temperature in the crust is evidence that transformations have, indeed, taken place and thus some degree of overstepping of reactions is required. However, it is also generally accepted that the degree of disequilibrium (overstepping) is minor such that a close approach to equilibrium is maintained throughout a rock?s history. This is the traditional justification for employing equilibrium phase diagrams for the interpretation of metamorphic conditions.Part II. This proposal seeks to evaluate the validity of the assumption of a close approach to equilibrium during metamorphism. Preliminary data is presented in the proposal that indicates a substantial deviation from equilibrium for the nucleation of the common metamorphic mineral, garnet. The amount of overstepping inferred from these preliminary studies varies in different samples from a few degrees Celsius to over seventy degrees and 5000 bars of pressure. Even more significant, the inferred conditions of garnet nucleation are very similar to the conditions inferred for the maximum metamorphic temperature and pressure attained by the sample. The implication of this result is that, rather than undergoing a continuum of small changes along a trajectory of increasing pressure and temperature in the crust, the rock remained virtually unchanged until reaching conditions near the maximum pressure and temperature and then transformed through a cascade of reactions. This distinction is not merely academic. The assumption of a close approach to equilibrium during metamorphism has been the hallmark of metamorphic studies for over a century and nearly all of our understanding of the evolution of rocks during mountain building events and the inferred pressure and temperature conditions in the crust are based on this assumption. The preliminary results presented in this proposal imply that all of these previous inferences about conditions of metamorphism will need to be revised in light of these new findings. Furthermore, a cascade of metamorphic transformations occurring after significant overstepping has the potential to release copious amounts of fluids (e.g. H2O) into the crust, which has the potential for triggering substantial deformation. It is even suggested that deep, large magnitude earthquakes that are characteristic of subduction zones might be triggered by such a mechanism. This study is made possible by a new technique by which the pressure (and possibly temperature) at which a mineral such as garnet forms can be determined by measuring the residual pressure of an inclusion of quartz, epidote, or zircon in the garnet by Raman spectroscopy. The assumption required by this new approach is that mechanical equilibrium is attained between the garnet and the inclusion. Comparison of the results from this new method with those that are obtained by assuming chemical equilibrium (the traditional approach) provides a quantitative measure of the degree of overstepping. The proposed study will evaluate the validity of this new technique through a series of experiments, apply the approach to well-characterized samples from well-studied mountain belts (e.g. in central New England), and develop new mineral inclusion-host pairs to extend the method to additional rock types.