THREE KINDS OF METAMORPHIC PULSES: GEODYNAMIC, THERMODYNAMIC, KINETIC

Traditionally, metamorphism has been regarded and modeled as a slow, steady, tectonically paced process. However, new data and observations from the past decade have shown that this may not always be the case. Instead, metamorphism can be punctuated or even dominated by brief pulses. It may be that most of the physical and chemical manifestations of long-lived tectonometamorphic processes were created only during these brief pulses of activity. As evidence for such metamorphic pulses has grown, we must take care to note the differences and similarities in how we define “metamorphic pulses”, both in terms of how they are caused, and how they are manifested in the rock record. We recognize a “pulse” as any process, condition, or manifestation thereof that persists or dramatically accelerates for a brief period of time (relative to the background timescale of tectonic forcing). Here, we review some of the data and categorize metamorphic pulses into three groups: geodynamic pulses, thermodynamic pulses, kinetic pulses. A geodynamic pulse results from a rapid tectonic forcing that causes P,T, or X to change rapidly thus forcing a rapid petrologic manifestation. Advective heat transfer via magma emplacement, or vertical motion near shear zones, or any passage through sharp thermal gradients (such as may exist in subduction zones) are examples of scenarios that may create geodynamic pulses. A thermodynamic pulse results from equilibrium thermodynamic effects (i.e. growth of products, consumption of reactants) when a specific net-transfer reaction is crossed, or if reaction isopleths become narrowly spaced, even with slow and steady P-T change. A kinetic pulse can occur if a thermodynamically stable reaction has been overstepped due to some kinetic barrier. Once that barrier is removed or overcome via some kinetic trigger (perhaps the introduction of a catalyzing fluid), a rapid pulse of mineral growth can result. The three types of pulses have different implications for how we model and understand metamorphism and the driving tectonic forcings behind it.