Short-circuit diffusion pathways

Grain boundaries can serve as fast diffusion pathways for vacancies and interstitial atoms when deformation takes place at elevated temperature. The same point can be made with regards to the interface between the matrix and particle phases. However, the particle interfaces that are widely distributed lack interconnectivity with each other, causing them to be less effective as short-circuit diffusion paths than are the more... 

Despite the fact that open-system behavior in mineral-isotopic systems is governed by a combination of volume and short-circuit diffusional processes, most thermochronologists make the simplifying assumption that volume diffusion alone controls the open- to closed-system transition that is so important to thermochronologic theory. There are a variety of reasons to believe that this assumption is reasonable. First, the stmcture of short-circuit pathways is such that they should be characterized by much faster diffusion than the intact crystal structure that surrounds them if so, then the rate of daughter isotope loss should be limited by the rate of diffusion out of intact domains and into short-circuit pathways. Second, the volumetric proportion of short-circuit pathways to intact domains is small in all but the most strongly deformed natural crystals, implying that their contribution to bulk... 

Fig. 9.1. Diffusion data revealing the alteration in diffusion rates as a result of short-circuit pathways provided by interfaces and other defects such as dislocations (adapted from Gjostein (1972)).

There are a number of processes that create fast pathways of exchange and effectively short-circuit volume diffusion into a crystal. Thus, the real world may be influenced by crystal defects and dislocations, mineral inclusions, exsolution lamellae, kink bands, microcracks, and other cryptic features (Fig. 12C). Diffusion is always active on a scale that can be modeled (Fig. 12B) and thus a world-view where all minerals are perfectly equilibrated and homogeneous (Fig. 12A) is generally a figment of imagination. In thermometry, these factors all potentially contribute to the compositions that are measured. Major advances have been made in determining when the macroscopic model world accurately predicts the microscopic real world situation. However, more work may be necessary to accurately deconvolute complex cases and tests should always be applied to evaluate thermometry. 

Imaging often reveals processes that reset stable isotope thermometers or that short-circuit pathways of exchange such that diffusion models do not apply. 

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