Reactions and diffusion during cooling

The above P-T-t paths do not cover all temperature-pressure histories. For example, all deep-seated rocks may be brought up rapidly by volcanic eruptions as xenoliths and hence may cool from a high temperature to room temperature rapidly. Furthermore, through drilling it is possible to obtain samples at high temperatures and pressures. 

Because of the complex thermal history, quantitative treatment of reaction kinetics, or the extent of the reaction as a function of time, must incorporate the dependence of the rate coefficient(s) with time. The treatment may be simple, as shown in Section 1.3.7, but more often is complicated for real geologic reactions. Some of the concepts are discussed below but the rigorous derivations are presented in later chapters. 

Consider a homogeneous reaction such as Fe-Mg order-disorder reaction in an 

The reaction rate constant and the diffusivity may depend weakly on pressure (see previous section). Because the temperature dependence is much more pronounced and temperature and pressure often co-vary, the temperature effect usually overwhelms the pressure effect. Therefore, there are various cooling rate indicators, but few direct decompression rate indicators have been developed based on geochemical kinetics. Rutherford and Hill (1993) developed a method to estimate the decompression (ascent) rate based on the width of the break-dovm rim of amphibole phenocryst due to dehydration. Indirectly, decompres- 

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